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Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Mendel Cooper

<thegrendel.abs@gmail.com>

10 Mar 2014

Revision History

Revision 6.5 05 Apr 2012 Revised by: mc

'TUNGSTENBERRY' release

Revision 6.6 27 Nov 2012 Revised by: mc

'YTTERBIUMBERRY' release

Revision 10 10 Mar 2014 Revised by: mc

'PUBLICDOMAIN' release

This tutorial assumes no previous knowledge of scripting or

programming, yet progresses rapidly toward an intermediate/advanced

level of instruction . . . all the while sneaking in little nuggets

of UNIX® wisdom and lore. It serves as a textbook, a manual for

self-study, and as a reference and source of knowledge on shell

scripting techniques. The exercises and heavily-commented examples

invite active reader participation, under the premise that the only

way to really learn scripting is to write scripts.

This book is suitable for classroom use as a general introduction to

programming concepts.

This document is herewith granted to the Public Domain. No copyright!

________________________________________________________________

Dedication

For Anita, the source of all the magic

Table of Contents

Part 1. Introduction

1. Shell Programming!

2. Starting Off With a Sha-Bang

Part 2. Basics

3. Special Characters

4. Introduction to Variables and Parameters

5. Quoting

6. Exit and Exit Status

7. Tests

8. Operations and Related Topics

Part 3. Beyond the Basics

9. Another Look at Variables

10. Manipulating Variables

11. Loops and Branches

12. Command Substitution

13. Arithmetic Expansion

14. Recess Time

Part 4. Commands

15. Internal Commands and Builtins

16. External Filters, Programs and Commands

17. System and Administrative Commands

Part 5. Advanced Topics

18. Regular Expressions

19. Here Documents

20. I/O Redirection

21. Subshells

22. Restricted Shells

23. Process Substitution

24. Functions

25. Aliases

26. List Constructs

27. Arrays

28. Indirect References

29. /dev and /proc

30. Network Programming

31. Of Zeros and Nulls

32. Debugging

33. Options

34. Gotchas

35. Scripting With Style

36. Miscellany

37. Bash, versions 2, 3, and 4

38. Endnotes

38.1. Author's Note

38.2. About the Author

38.3. Where to Go For Help

38.4. Tools Used to Produce This Book

38.5. Credits

38.6. Disclaimer

Bibliography

A. Contributed Scripts

B. Reference Cards

C. A Sed and Awk Micro-Primer

C.1. Sed

C.2. Awk

D. Parsing and Managing Pathnames

E. Exit Codes With Special Meanings

F. A Detailed Introduction to I/O and I/O Redirection

G. Command-Line Options

G.1. Standard Command-Line Options

G.2. Bash Command-Line Options

H. Important Files

I. Important System Directories

J. An Introduction to Programmable Completion

K. Localization

L. History Commands

M. Sample .bashrc and .bash_profile Files

N. Converting DOS Batch Files to Shell Scripts

O. Exercises

O.1. Analyzing Scripts

O.2. Writing Scripts

P. Revision History

Q. Download and Mirror Sites

R. To Do List

S. Copyright

T. ASCII Table

Index

List of Tables

8-1. Operator Precedence

15-1. Job identifiers

33-1. Bash options

36-1. Numbers representing colors in Escape Sequences

B-1. Special Shell Variables

B-2. TEST Operators: Binary Comparison

B-3. TEST Operators: Files

B-4. Parameter Substitution and Expansion

B-5. String Operations

B-6. Miscellaneous Constructs

C-1. Basic sed operators

C-2. Examples of sed operators

E-1. Reserved Exit Codes

N-1. Batch file keywords / variables / operators, and their shell

equivalents

N-2. DOS commands and their UNIX equivalents

P-1. Revision History

List of Examples

2-1. cleanup: A script to clean up log files in /var/log

2-2. cleanup: An improved clean-up script

2-3. cleanup: An enhanced and generalized version of above scripts.

3-1. Code blocks and I/O redirection

3-2. Saving the output of a code block to a file

3-3. Running a loop in the background

3-4. Backup of all files changed in last day

4-1. Variable assignment and substitution

4-2. Plain Variable Assignment

4-3. Variable Assignment, plain and fancy

4-4. Integer or string?

4-5. Positional Parameters

4-6. wh, whois domain name lookup

4-7. Using shift

5-1. Echoing Weird Variables

5-2. Escaped Characters

5-3. Detecting key-presses

6-1. exit / exit status

6-2. Negating a condition using !

7-1. What is truth?

7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[

7-3. Arithmetic Tests using (( ))

7-4. Testing for broken links

7-5. Arithmetic and string comparisons

7-6. Testing whether a string is null

7-7. zmore

8-1. Greatest common divisor

8-2. Using Arithmetic Operations

8-3. Compound Condition Tests Using && and ||

8-4. Representation of numerical constants

8-5. C-style manipulation of variables

9-1. $IFS and whitespace

9-2. Timed Input

9-3. Once more, timed input

9-4. Timed read

9-5. Am I root?

9-6. arglist: Listing arguments with $* and $@

9-7. Inconsistent $* and $@ behavior

9-8. $* and $@ when $IFS is empty

9-9. Underscore variable

9-10. Using declare to type variables

9-11. Generating random numbers

9-12. Picking a random card from a deck

9-13. Brownian Motion Simulation

9-14. Random between values

9-15. Rolling a single die with RANDOM

9-16. Reseeding RANDOM

9-17. Pseudorandom numbers, using awk

10-1. Inserting a blank line between paragraphs in a text file

10-2. Generating an 8-character "random" string

10-3. Converting graphic file formats, with filename change

10-4. Converting streaming audio files to ogg

10-5. Emulating getopt

10-6. Alternate ways of extracting and locating substrings

10-7. Using parameter substitution and error messages

10-8. Parameter substitution and "usage" messages

10-9. Length of a variable

10-10. Pattern matching in parameter substitution

10-11. Renaming file extensions:

10-12. Using pattern matching to parse arbitrary strings

10-13. Matching patterns at prefix or suffix of string

11-1. Simple for loops

11-2. for loop with two parameters in each [list] element

11-3. Fileinfo: operating on a file list contained in a variable

11-4. Operating on a parameterized file list

11-5. Operating on files with a for loop

11-6. Missing in [list] in a for loop

11-7. Generating the [list] in a for loop with command substitution

11-8. A grep replacement for binary files

11-9. Listing all users on the system

11-10. Checking all the binaries in a directory for authorship

11-11. Listing the symbolic links in a directory

11-12. Symbolic links in a directory, saved to a file

11-13. A C-style for loop

11-14. Using efax in batch mode

11-15. Simple while loop

11-16. Another while loop

11-17. while loop with multiple conditions

11-18. C-style syntax in a while loop

11-19. until loop

11-20. Nested Loop

11-21. Effects of break and continue in a loop

11-22. Breaking out of multiple loop levels

11-23. Continuing at a higher loop level

11-24. Using continue N in an actual task

11-25. Using case

11-26. Creating menus using case

11-27. Using command substitution to generate the case variable

11-28. Simple string matching

11-29. Checking for alphabetic input

11-30. Creating menus using select

11-31. Creating menus using select in a function

12-1. Stupid script tricks

12-2. Generating a variable from a loop

12-3. Finding anagrams

15-1. A script that spawns multiple instances of itself

15-2. printf in action

15-3. Variable assignment, using read

15-4. What happens when read has no variable

15-5. Multi-line input to read

15-6. Detecting the arrow keys

15-7. Using read with file redirection

15-8. Problems reading from a pipe

15-9. Changing the current working directory

15-10. Letting let do arithmetic.

15-11. Showing the effect of eval

15-12. Using eval to select among variables

15-13. Echoing the command-line parameters

15-14. Forcing a log-off

15-15. A version of rot13

15-16. Using set with positional parameters

15-17. Reversing the positional parameters

15-18. Reassigning the positional parameters

15-19. "Unsetting" a variable

15-20. Using export to pass a variable to an embedded awk script

15-21. Using getopts to read the options/arguments passed to a script

15-22. "Including" a data file

15-23. A (useless) script that sources itself

15-24. Effects of exec

15-25. A script that exec's itself

15-26. Waiting for a process to finish before proceeding

15-27. A script that kills itself

16-1. Using ls to create a table of contents for burning a CDR disk

16-2. Hello or Good-bye

16-3. Badname, eliminate file names in current directory containing

bad characters and whitespace.

16-4. Deleting a file by its inode number

16-5. Logfile: Using xargs to monitor system log

16-6. Copying files in current directory to another

16-7. Killing processes by name

16-8. Word frequency analysis using xargs

16-9. Using expr

16-10. Using date

16-11. Date calculations

16-12. Word Frequency Analysis

16-13. Which files are scripts?

16-14. Generating 10-digit random numbers

16-15. Using tail to monitor the system log

16-16. Printing out the From lines in stored e-mail messages

16-17. Emulating grep in a script

16-18. Crossword puzzle solver

16-19. Looking up definitions in Webster's 1913 Dictionary

16-20. Checking words in a list for validity

16-21. toupper: Transforms a file to all uppercase.

16-22. lowercase: Changes all filenames in working directory to

lowercase.

16-23. du: DOS to UNIX text file conversion.

16-24. rot13: ultra-weak encryption.

16-25. Generating "Crypto-Quote" Puzzles

16-26. Formatted file listing.

16-27. Using column to format a directory listing

16-28. nl: A self-numbering script.

16-29. manview: Viewing formatted manpages

16-30. Using cpio to move a directory tree

16-31. Unpacking an rpm archive

16-32. Stripping comments from C program files

16-33. Exploring /usr/X11R6/bin

16-34. An "improved" strings command

16-35. Using cmp to compare two files within a script.

16-36. basename and dirname

16-37. A script that copies itself in sections

16-38. Checking file integrity

16-39. Uudecoding encoded files

16-40. Finding out where to report a spammer

16-41. Analyzing a spam domain

16-42. Getting a stock quote

16-43. Updating FC4

16-44. Using ssh

16-45. A script that mails itself

16-46. Generating prime numbers

16-47. Monthly Payment on a Mortgage

16-48. Base Conversion

16-49. Invoking bc using a here document

16-50. Calculating PI

16-51. Converting a decimal number to hexadecimal

16-52. Factoring

16-53. Calculating the hypotenuse of a triangle

16-54. Using seq to generate loop arguments

16-55. Letter Count"

16-56. Using getopt to parse command-line options

16-57. A script that copies itself

16-58. Exercising dd

16-59. Capturing Keystrokes

16-60. Preparing a bootable SD card for the Raspberry Pi

16-61. Securely deleting a file

16-62. Filename generator

16-63. Converting meters to miles

16-64. Using m4

17-1. Setting a new password

17-2. Setting an erase character

17-3. secret password: Turning off terminal echoing

17-4. Keypress detection

17-5. Checking a remote server for identd

17-6. pidof helps kill a process

17-7. Checking a CD image

17-8. Creating a filesystem in a file

17-9. Adding a new hard drive

17-10. Using umask to hide an output file from prying eyes

17-11. Backlight: changes the brightness of the (laptop) screen

backlight

17-12. killall, from /etc/rc.d/init.d

19-1. broadcast: Sends message to everyone logged in

19-2. dummyfile: Creates a 2-line dummy file

19-3. Multi-line message using cat

19-4. Multi-line message, with tabs suppressed

19-5. Here document with replaceable parameters

19-6. Upload a file pair to Sunsite incoming directory

19-7. Parameter substitution turned off

19-8. A script that generates another script

19-9. Here documents and functions

19-10. "Anonymous" Here Document

19-11. Commenting out a block of code

19-12. A self-documenting script

19-13. Prepending a line to a file

19-14. Parsing a mailbox

20-1. Redirecting stdin using exec

20-2. Redirecting stdout using exec

20-3. Redirecting both stdin and stdout in the same script with exec

20-4. Avoiding a subshell

20-5. Redirected while loop

20-6. Alternate form of redirected while loop

20-7. Redirected until loop

20-8. Redirected for loop

20-9. Redirected for loop (both stdin and stdout redirected)

20-10. Redirected if/then test

20-11. Data file names.data for above examples

20-12. Logging events

21-1. Variable scope in a subshell

21-2. List User Profiles

21-3. Running parallel processes in subshells

22-1. Running a script in restricted mode

23-1. Code block redirection without forking

23-2. Redirecting the output of process substitution into a loop.

24-1. Simple functions

24-2. Function Taking Parameters

24-3. Functions and command-line args passed to the script

24-4. Passing an indirect reference to a function

24-5. Dereferencing a parameter passed to a function

24-6. Again, dereferencing a parameter passed to a function

24-7. Maximum of two numbers

24-8. Converting numbers to Roman numerals

24-9. Testing large return values in a function

24-10. Comparing two large integers

24-11. Real name from username

24-12. Local variable visibility

24-13. Demonstration of a simple recursive function

24-14. Another simple demonstration

24-15. Recursion, using a local variable

24-16. The Fibonacci Sequence

24-17. The Towers of Hanoi

25-1. Aliases within a script

25-2. unalias: Setting and unsetting an alias

26-1. Using an and list to test for command-line arguments

26-2. Another command-line arg test using an and list

26-3. Using or lists in combination with an and list

27-1. Simple array usage

27-2. Formatting a poem

27-3. Various array operations

27-4. String operations on arrays

27-5. Loading the contents of a script into an array

27-6. Some special properties of arrays

27-7. Of empty arrays and empty elements

27-8. Initializing arrays

27-9. Copying and concatenating arrays

27-10. More on concatenating arrays

27-11. The Bubble Sort

27-12. Embedded arrays and indirect references

27-13. The Sieve of Eratosthenes

27-14. The Sieve of Eratosthenes, Optimized

27-15. Emulating a push-down stack

27-16. Complex array application: Exploring a weird mathematical

series

27-17. Simulating a two-dimensional array, then tilting it

28-1. Indirect Variable References

28-2. Passing an indirect reference to awk

29-1. Using /dev/tcp for troubleshooting

29-2. Playing music

29-3. Finding the process associated with a PID

29-4. On-line connect status

30-1. Print the server environment

30-2. IP addresses

31-1. Hiding the cookie jar

31-2. Setting up a swapfile using /dev/zero

31-3. Creating a ramdisk

32-1. A buggy script

32-2. Missing keyword

32-3. test24: another buggy script

32-4. Testing a condition with an assert

32-5. Trapping at exit

32-6. Cleaning up after Control-C

32-7. A Simple Implementation of a Progress Bar

32-8. Tracing a variable

32-9. Running multiple processes (on an SMP box)

34-1. Numerical and string comparison are not equivalent

34-2. Subshell Pitfalls

34-3. Piping the output of echo to a read

36-1. shell wrapper

36-2. A slightly more complex shell wrapper

36-3. A generic shell wrapper that writes to a logfile

36-4. A shell wrapper around an awk script

36-5. A shell wrapper around another awk script

36-6. Perl embedded in a Bash script

36-7. Bash and Perl scripts combined

36-8. Python embedded in a Bash script

36-9. A script that speaks

36-10. A (useless) script that recursively calls itself

36-11. A (useful) script that recursively calls itself

36-12. Another (useful) script that recursively calls itself

36-13. A "colorized" address database

36-14. Drawing a box

36-15. Echoing colored text

36-16. A "horserace" game

36-17. A Progress Bar

36-18. Return value trickery

36-19. Even more return value trickery

36-20. Passing and returning arrays

36-21. Fun with anagrams

36-22. Widgets invoked from a shell script

36-23. Test Suite

37-1. String expansion

37-2. Indirect variable references - the new way

37-3. Simple database application, using indirect variable

referencing

37-4. Using arrays and other miscellaneous trickery to deal four

random hands from a deck of cards

37-5. A simple address database

37-6. A somewhat more elaborate address database

37-7. Testing characters

37-8. Reading N characters

37-9. Using a here document to set a variable

37-10. Piping input to a read

37-11. Negative array indices

37-12. Negative parameter in string-extraction construct

A-1. mailformat: Formatting an e-mail message

A-2. rn: A simple-minded file renaming utility

A-3. blank-rename: Renames filenames containing blanks

A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted

password

A-5. copy-cd: Copying a data CD

A-6. Collatz series

A-7. days-between: Days between two dates

A-8. Making a dictionary

A-9. Soundex conversion

A-10. Game of Life

A-11. Data file for Game of Life

A-12. behead: Removing mail and news message headers

A-13. password: Generating random 8-character passwords

A-14. fifo: Making daily backups, using named pipes

A-15. Generating prime numbers using the modulo operator

A-16. tree: Displaying a directory tree

A-17. tree2: Alternate directory tree script

A-18. string functions: C-style string functions

A-19. Directory information

A-20. Library of hash functions

A-21. Colorizing text using hash functions

A-22. More on hash functions

A-23. Mounting USB keychain storage devices

A-24. Converting to HTML

A-25. Preserving weblogs

A-26. Protecting literal strings

A-27. Unprotecting literal strings

A-28. Spammer Identification

A-29. Spammer Hunt

A-30. Making wget easier to use

A-31. A podcasting script

A-32. Nightly backup to a firewire HD

A-33. An expanded cd command

A-34. A soundcard setup script

A-35. Locating split paragraphs in a text file

A-36. Insertion sort

A-37. Standard Deviation

A-38. A pad file generator for shareware authors

A-39. A man page editor

A-40. Petals Around the Rose

A-41. Quacky: a Perquackey-type word game

A-42. Nim

A-43. A command-line stopwatch

A-44. An all-purpose shell scripting homework assignment solution

A-45. The Knight's Tour

A-46. Magic Squares

A-47. Fifteen Puzzle

A-48. The Towers of Hanoi, graphic version

A-49. The Towers of Hanoi, alternate graphic version

A-50. An alternate version of the getopt-simple.sh script

A-51. The version of the UseGetOpt.sh example used in the Tab

Expansion appendix

A-52. Cycling through all the possible color backgrounds

A-53. Morse Code Practice

A-54. Base64 encoding/decoding

A-55. Inserting text in a file using sed

A-56. The Gronsfeld Cipher

A-57. Bingo Number Generator

A-58. Basics Reviewed

A-59. Testing execution times of various commands

A-60. Associative arrays vs. conventional arrays (execution times)

C-1. Counting Letter Occurrences

J-1. Completion script for UseGetOpt.sh

M-1. Sample .bashrc file

M-2. .bash_profile file

N-1. VIEWDATA.BAT: DOS Batch File

N-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT

T-1. A script that generates an ASCII table

T-2. Another ASCII table script

T-3. A third ASCII table script, using awk

Part 1. Introduction

Script: A writing; a written document. [Obs.]

--Webster's Dictionary, 1913 ed.

The shell is a command interpreter. More than just the insulating

layer between the operating system kernel and the user, it's also a

fairly powerful programming language. A shell program, called a

script, is an easy-to-use tool for building applications by "gluing

together" system calls, tools, utilities, and compiled binaries.

Virtually the entire repertoire of UNIX commands, utilities, and

tools is available for invocation by a shell script. If that were not

enough, internal shell commands, such as testing and loop constructs,

lend additional power and flexibility to scripts. Shell scripts are

especially well suited for administrative system tasks and other

routine repetitive tasks not requiring the bells and whistles of a

full-blown tightly structured programming language.

Table of Contents

1. Shell Programming!

2. Starting Off With a Sha-Bang

2.1. Invoking the script

2.2. Preliminary Exercises

________________________________________________________________

Chapter 1. Shell Programming!

No programming language is perfect. There is not even a single best

language; there are only languages well suited or perhaps poorly

suited for particular purposes.

--Herbert Mayer

A working knowledge of shell scripting is essential to anyone wishing

to become reasonably proficient at system administration, even if

they do not anticipate ever having to actually write a script.

Consider that as a Linux machine boots up, it executes the shell

scripts in /etc/rc.d to restore the system configuration and set up

services. A detailed understanding of these startup scripts is

important for analyzing the behavior of a system, and possibly

modifying it.

The craft of scripting is not hard to master, since scripts can be

built in bite-sized sections and there is only a fairly small set of

shell-specific operators and options [1] to learn. The syntax is

simple -- even austere -- similar to that of invoking and chaining

together utilities at the command line, and there are only a few

"rules" governing their use. Most short scripts work right the first

time, and debugging even the longer ones is straightforward.

In the early days of personal computing, the BASIC language en

abled

anyone reasonably computer proficient to write programs on an

early

generation of microcomputers. Decades later, the Bash scriptin

language enables anyone with a rudimentary knowledge of Linux

UNIX to do the same on modern machines.

We now have miniaturized single-board computers with amazing

capabilities, such as the [http://www.raspberrypi.org/]

Raspberry Pi.

Bash scripting provides a way to explore the capabilities of t

hese

fascinating devices.

A shell script is a quick-and-dirty method of prototyping a complex

application. Getting even a limited subset of the functionality to

work in a script is often a useful first stage in project

development. In this way, the structure of the application can be

tested and tinkered with, and the major pitfalls found before

proceeding to the final coding in C, C++, Java, Perl, or Python.

Shell scripting hearkens back to the classic UNIX philosophy of

breaking complex projects into simpler subtasks, of chaining together

components and utilities. Many consider this a better, or at least

more esthetically pleasing approach to problem solving than using one

of the new generation of high-powered all-in-one languages, such as

Perl, which attempt to be all things to all people, but at the cost

of forcing you to alter your thinking processes to fit the tool.

According to Herbert Mayer, "a useful language needs arrays,

pointers, and a generic mechanism for building data structures." By

these criteria, shell scripting falls somewhat short of being

"useful." Or, perhaps not. . . .

When not to use shell scripts

* Resource-intensive tasks, especially where speed is a factor

(sorting, hashing, recursion [2] ...)

* Procedures involving heavy-duty math operations, especially

floating point arithmetic, arbitrary precision calculations, or

complex numbers (use C++ or FORTRAN instead)

* Cross-platform portability required (use C or Java instead)

* Complex applications, where structured programming is a necessity

(type-checking of variables, function prototypes, etc.)

* Mission-critical applications upon which you are betting the

future of the company

* Situations where security is important, where you need to

guarantee the integrity of your system and protect against

intrusion, cracking, and vandalism

* Project consists of subcomponents with interlocking dependencies

* Extensive file operations required (Bash is limited to serial

file access, and that only in a particularly clumsy and

inefficient line-by-line fashion.)

* Need native support for multi-dimensional arrays

* Need data structures, such as linked lists or trees

* Need to generate / manipulate graphics or GUIs

* Need direct access to system hardware or external peripherals

* Need port or socket I/O

* Need to use libraries or interface with legacy code

* Proprietary, closed-source applications (Shell scripts put the

source code right out in the open for all the world to see.)

If any of the above applies, consider a more powerful scripting

language -- perhaps Perl, Tcl, Python, Ruby -- or possibly a compiled

language such as C, C++, or Java. Even then, prototyping the

application as a shell script might still be a useful development

step.

We will be using Bash, an acronym [3] for "Bourne-Again shell" and a

pun on Stephen Bourne's now classic Bourne shell. Bash has become a

de facto standard for shell scripting on most flavors of UNIX. Most

of the principles this book covers apply equally well to scripting

with other shells, such as the Korn Shell, from which Bash derives

some of its features, [4] and the C Shell and its variants. (Note

that C Shell programming is not recommended due to certain inherent

problems, as pointed out in an October, 1993 Usenet post by Tom

Christiansen.)

What follows is a tutorial on shell scripting. It relies heavily on

examples to illustrate various features of the shell. The example

scripts work -- they've been tested, insofar as possible -- and some

of them are even useful in real life. The reader can play with the

actual working code of the examples in the source archive

(scriptname.sh or scriptname.bash), [5] give them execute permission

(chmod u+rx scriptname), then run them to see what happens. Should

the source archive not be available, then cut-and-paste from the

[http://www.tldp.org/LDP/abs/abs-guide.html.tar.gz] HTML or

[http://bash.deta.in/abs-guide.pdf] pdf rendered versions. Be aware

that some of the scripts presented here introduce features before

they are explained, and this may require the reader to temporarily

skip ahead for enlightenment.

Unless otherwise noted, [mailto:thegrendel.abs@gmail.com] the author

of this book wrote the example scripts that follow.

His countenance was bold and bashed not.

--Edmund Spenser

________________________________________________________________

Chapter 2. Starting Off With a Sha-Bang

Shell programming is a 1950s juke box . . .

--Larry Wall

In the simplest case, a script is nothing more than a list of system

commands stored in a file. At the very least, this saves the effort

of retyping that particular sequence of commands each time it is

invoked.

Example 2-1. cleanup: A script to clean up log files in /var/log

Cleanup

Run as root, of course.

cd /var/log

cat /dev/null > messages

cat /dev/null > wtmp

echo "Log files cleaned up."

There is nothing unusual here, only a set of commands that could just

as easily have been invoked one by one from the command-line on the

console or in a terminal window. The advantages of placing the

commands in a script go far beyond not having to retype them time and

again. The script becomes a program -- a tool -- and it can easily be

modified or customized for a particular application.

Example 2-2. cleanup: An improved clean-up script

!/bin/bash

Proper header for a Bash script.

Cleanup, version 2

Run as root, of course.

Insert code here to print error message and exit if not root.

LOG_DIR=/var/log

Variables are better than hard-coded values.

cd $LOG_DIR

cat /dev/null > messages

cat /dev/null > wtmp

echo "Logs cleaned up."

exit # The right and proper method of "exiting" from a script.

# A bare "exit" (no parameter) returns the exit status

#+ of the preceding command.

Now that's beginning to look like a real script. But we can go even

farther . . .

Example 2-3. cleanup: An enhanced and generalized version of above

scripts.

!/bin/bash

Cleanup, version 3

Warning:

-------

This script uses quite a number of features that will be explained

+ later on.

By the time you've finished the first half of the book,

+ there should be nothing mysterious about it.

LOG_DIR=/var/log

ROOT_UID=0 # Only users with $UID 0 have root privileges.

LINES=50 # Default number of lines saved.

E_XCD=86 # Can't change directory?

E_NOTROOT=87 # Non-root exit error.

Run as root, of course.

if [ "$UID" -ne "$ROOT_UID" ]

then

echo "Must be root to run this script."

exit $E_NOTROOT

if [ -n "$1" ]

Test whether command-line argument is present (non-empty).

then

lines=$1

else

lines=$LINES # Default, if not specified on command-line.

Stephane Chazelas suggests the following,

+ as a better way of checking command-line arguments,

+ but this is still a bit advanced for this stage of the tutorial.

E_WRONGARGS=85 # Non-numerical argument (bad argument format).

case "$1" in

"" ) lines=50;;

[!0-9]) echo "Usage: `basename $0` lines-to-cleanup";

exit $E_WRONGARGS;;

* ) lines=$1;;

esac

* Skip ahead to "Loops" chapter to decipher all this.

cd $LOG_DIR

if [ `pwd` != "$LOG_DIR" ] # or if [ "$PWD" != "$LOG_DIR" ]

# Not in /var/log?

then

echo "Can't change to $LOG_DIR."

exit $E_XCD

fi # Doublecheck if in right directory before messing with log file.

Far more efficient is:

cd /var/log || {

echo "Cannot change to necessary directory." >&2

exit $E_XCD;

}

tail -n $lines messages > mesg.temp # Save last section of message log file.

mv mesg.temp messages # Rename it as system log file.

cat /dev/null > messages

* No longer needed, as the above method is safer.

cat /dev/null > wtmp # ': > wtmp' and '> wtmp' have the same effect.

echo "Log files cleaned up."

Note that there are other log files in /var/log not affected

+ by this script.

exit 0

A zero return value from the script upon exit indicates success

+ to the shell.

Since you may not wish to wipe out the entire system log, this

version of the script keeps the last section of the message log

intact. You will constantly discover ways of fine-tuning previously

written scripts for increased effectiveness.

* * *

The sha-bang ( #!) [6] at the head of a script tells your system that

this file is a set of commands to be fed to the command interpreter

indicated. The #! is actually a two-byte [7] magic number, a special

marker that designates a file type, or in this case an executable

shell script (type man magic for more details on this fascinating

topic). Immediately following the sha-bang is a path name. This is

the path to the program that interprets the commands in the script,

whether it be a shell, a programming language, or a utility. This

command interpreter then executes the commands in the script,

starting at the top (the line following the sha-bang line), and

ignoring comments. [8]

!/bin/sh

!/bin/bash

!/usr/bin/perl

!/usr/bin/tcl

!/bin/sed -f

!/bin/awk -f

Each of the above script header lines calls a different command

interpreter, be it /bin/sh, the default shell (bash in a Linux

system) or otherwise. [9] Using #!/bin/sh, the default Bourne shell

in most commercial variants of UNIX, makes the script portable to

non-Linux machines, though you sacrifice Bash-specific features. The

script will, however, conform to the POSIX [10] sh standard.

Note that the path given at the "sha-bang" must be correct, otherwise

an error message -- usually "Command not found." -- will be the only

result of running the script. [11]

#! can be omitted if the script consists only of a set of generic

system commands, using no internal shell directives. The second

example, above, requires the initial #!, since the variable

assignment line, lines=50, uses a shell-specific construct. [12] Note

again that #!/bin/sh invokes the default shell interpreter, which

defaults to /bin/bash on a Linux machine.

Tip

This tutorial encourages a modular approach to constructing a script.

Make note of and collect "boilerplate" code snippets that might be

useful in future scripts. Eventually you will build quite an

extensive library of nifty routines. As an example, the following

script prolog tests whether the script has been invoked with the

correct number of parameters.

E_WRONG_ARGS=85

script_parameters="-a -h -m -z"

-a = all, -h = help, etc.

if [ $# -ne $Number_of_expected_args ]

then

echo "Usage: `basename $0` $script_parameters"

# `basename $0` is the script's filename.

exit $E_WRONG_ARGS

Many times, you will write a script that carries out one particular

task. The first script in this chapter is an example. Later, it might

occur to you to generalize the script to do other, similar tasks.

Replacing the literal ("hard-wired") constants by variables is a step

in that direction, as is replacing repetitive code blocks by

functions.

________________________________________________________________

2.1. Invoking the script

Having written the script, you can invoke it by sh scriptname, [13]

or alternatively bash scriptname. (Not recommended is using sh

<scriptname, since this effectively disables reading from stdin

within the script.) Much more convenient is to make the script itself

directly executable with a chmod.

Either:

chmod 555 scriptname (gives everyone read/execute permission)

[14]

chmod +rx scriptname (gives everyone read/execute permission)

chmod u+rx scriptname (gives only the script owner

read/execute permission)

Having made the script executable, you may now test it by

./scriptname. [15] If it begins with a "sha-bang" line, invoking the

script calls the correct command interpreter to run it.

As a final step, after testing and debugging, you would likely want

to move it to /usr/local/bin (as root, of course), to make the script

available to yourself and all other users as a systemwide executable.

The script could then be invoked by simply typing scriptname [ENTER]

from the command-line.

________________________________________________________________

2.2. Preliminary Exercises

1. System administrators often write scripts to automate common

tasks. Give several instances where such scripts would be useful.

2. Write a script that upon invocation shows the time and date,

lists all logged-in users, and gives the system uptime. The

script then saves this information to a logfile.

Part 2. Basics

Table of Contents

3. Special Characters

4. Introduction to Variables and Parameters

4.1. Variable Substitution

4.2. Variable Assignment

4.3. Bash Variables Are Untyped

4.4. Special Variable Types

5. Quoting

5.1. Quoting Variables

5.2. Escaping

6. Exit and Exit Status

7. Tests

7.1. Test Constructs

7.2. File test operators

7.3. Other Comparison Operators

7.4. Nested if/then Condition Tests

7.5. Testing Your Knowledge of Tests

8. Operations and Related Topics

8.1. Operators

8.2. Numerical Constants

8.3. The Double-Parentheses Construct

8.4. Operator Precedence

________________________________________________________________

Chapter 3. Special Characters

What makes a character special? If it has a meaning beyond its

literal meaning, a meta-meaning, then we refer to it as a special

character. Along with commands and keywords, special characters are

building blocks of Bash scripts.

Special Characters Found In Scripts and Elsewhere

Comments. Lines beginning with a # (with the exception of #!)

are comments and will not be executed.

This line is a comment.

Comments may also occur following the end of a command.

echo "A comment will follow." # Comment here.

^ Note whitespace before #

Comments may also follow whitespace at the beginning of a

line.

# A tab precedes this comment.

Comments may even be embedded within a pipe.

initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\

Delete lines containing '#' comment character.

sed -e 's/\./\. /g' -e 's/_/_ /g'` )

Excerpted from life.sh script

Caution

A command may not follow a comment on the same line. There is no

method of terminating the comment, in order for "live code" to begin

on the same line. Use a new line for the next command.

Note

Of course, a quoted or an escaped # in an echo statement does not

begin a comment. Likewise, a # appears in certain

parameter-substitution constructs and in numerical constant

expressions.

echo "The # here does not begin a comment."

echo 'The # here does not begin a comment.'

echo The \# here does not begin a comment.

echo The # here begins a comment.

echo ${PATH#*:} # Parameter substitution, not a comment.

echo $(( 2#101011 )) # Base conversion, not a comment.

Thanks, S.C.

The standard quoting and escape characters (" ' \) escape the #.

Certain pattern matching operations also use the #.

;

Command separator [semicolon]. Permits putting two or more

commands on the same line.

echo hello; echo there

if [ -x "$filename" ]; then # Note the space after the semicolon.

+ ^^

echo "File $filename exists."; cp $filename $filename.bak

else # ^^

echo "File $filename not found."; touch $filename

fi; echo "File test complete."

Note that the ";" sometimes needs to be escaped.

;;

Terminator in a case option [double semicolon].

case "$variable" in

abc) echo "\$variable = abc" ;;

xyz) echo "\$variable = xyz" ;;

esac

;;&, ;&

Terminators in a case option (version 4+ of Bash).

"dot" command [period]. Equivalent to source (see Example

15-22). This is a bash builtin.

"dot", as a component of a filename. When working with

filenames, a leading dot is the prefix of a "hidden" file, a

file that an ls will not normally show.

bash$ touch .hidden-file

bash$ ls -l

total 10

-rw-r--r-- 1 bozo 4034 Jul 18 22:04 data1.addressbook

-rw-r--r-- 1 bozo 4602 May 25 13:58 data1.addressbook.bak

-rw-r--r-- 1 bozo 877 Dec 17 2000 employment.addressbook

bash$ ls -al

total 14

drwxrwxr-x 2 bozo bozo 1024 Aug 29 20:54 ./

drwx------ 52 bozo bozo 3072 Aug 29 20:51 ../

-rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.addressbook

-rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.addressbook.bak

-rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.addressbook

-rw-rw-r-- 1 bozo bozo 0 Aug 29 20:54 .hidden-file

When considering directory names, a single dot represents the

current working directory, and two dots denote the parent

directory.

bash$ pwd

/home/bozo/projects

bash$ cd .

bash$ pwd

/home/bozo/projects

bash$ cd ..

bash$ pwd

/home/bozo/

The dot often appears as the destination (directory) of a file

movement command, in this context meaning current directory.

bash$ cp /home/bozo/current_work/junk/* .

Copy all the "junk" files to $PWD.

"dot" character match. When matching characters, as part of a

regular expression, a "dot" matches a single character.

partial quoting [double quote]. "STRING" preserves (from

interpretation) most of the special characters within STRING.

See Chapter 5.

full quoting [single quote]. 'STRING' preserves all special

characters within STRING. This is a stronger form of quoting

than "STRING". See Chapter 5.

comma operator. The comma operator [16] links together a

series of arithmetic operations. All are evaluated, but only

the last one is returned.

let "t2 = ((a = 9, 15 / 3))"

Set "a = 9" and "t2 = 15 / 3"

The comma operator can also concatenate strings.

for file in /{,usr/}bin/*calc

^ Find all executable files ending in "calc"

+ in /bin and /usr/bin directories.

if [ -x "$file" ]

then

echo $file

done

/bin/ipcalc

/usr/bin/kcalc

/usr/bin/oidcalc

/usr/bin/oocalc

Thank you, Rory Winston, for pointing this out.

,, ,

Lowercase conversion in parameter substitution (added in

version 4 of Bash).

escape [backslash]. A quoting mechanism for single characters.

\X escapes the character X. This has the effect of "quoting"

X, equivalent to 'X'. The \ may be used to quote " and ', so

they are expressed literally.

See Chapter 5 for an in-depth explanation of escaped

characters.

Filename path separator [forward slash]. Separates the

components of a filename (as in /home/bozo/projects/Makefile).

This is also the division arithmetic operator.

command substitution. The `command` construct makes available

the output of command for assignment to a variable. This is

also known as backquotes or backticks.

null command [colon]. This is the shell equivalent of a "NOP"

(no op, a do-nothing operation). It may be considered a

synonym for the shell builtin true. The ":" command is itself

a Bash builtin, and its exit status is true (0).

echo $? # 0

Endless loop:

while :

operation-1

operation-2

...

operation-n

done

Same as:

while true

do

...

done

Placeholder in if/then test:

if condition

then : # Do nothing and branch ahead

else # Or else ...

take-some-action

Provide a placeholder where a binary operation is expected,

see Example 8-2 and default parameters.

: ${username=`whoami`}

${username=`whoami`} Gives an error without the leading :

unless "username" is a command or builtin...

: ${1?"Usage: $0 ARGUMENT"} # From "usage-message.sh example script.

Provide a placeholder where a command is expected in a here

document. See Example 19-10.

Evaluate string of variables using parameter substitution (as

in Example 10-7).

: ${HOSTNAME?} ${USER?} ${MAIL?}

Prints error message

+ if one or more of essential environmental variables not set.

Variable expansion / substring replacement.

In combination with the > redirection operator, truncates a

file to zero length, without changing its permissions. If the

file did not previously exist, creates it.

: > data.xxx # File "data.xxx" now empty.

Same effect as cat /dev/null >data.xxx

However, this does not fork a new process, since ":" is a builtin.

The name of this function is :

This is not portable behavior, and therefore not a recommended

practice. In fact, more recent releases of Bash do not permit

this usage. An underscore _ works, though.

A colon can serve as a placeholder in an otherwise empty

function.

not_empty ()

{

} # Contains a : (null command), and so is not empty.

reverse (or negate) the sense of a test or exit status [bang].

The ! operator inverts the exit status of the command to which

it is applied (see Example 6-2). It also inverts the meaning

of a test operator. This can, for example, change the sense of

equal ( = ) to not-equal ( != ). The ! operator is a Bash

keyword.

In a different context, the ! also appears in indirect

variable references.

In yet another context, from the command line, the ! invokes

the Bash history mechanism (see Appendix L). Note that within

a script, the history mechanism is disabled.

wild card [asterisk]. The * character serves as a "wild card"

for filename expansion in globbing. By itself, it matches

every filename in a given directory.

bash$ echo *

abs-book.sgml add-drive.sh agram.sh alias.sh

The * also represents any number (or zero) characters in a

regular expression.

arithmetic operator. In the context of arithmetic operations,

the * denotes multiplication.

** A double asterisk can represent the exponentiation operator

or extended file-match globbing.

test operator. Within certain expressions, the ? indicates a

test for a condition.

In a double-parentheses construct, the ? can serve as an

element of a C-style trinary operator. [17]

condition?result-if-true:result-if-false

(( var0 = var1<98?9:21 ))

^ ^

if [ "$var1" -lt 98 ]

then

var0=9

else

var0=21

fi

In a parameter substitution expression, the ? tests whether a

variable has been set.

wild card. The ? character serves as a single-character "wild

card" for filename expansion in globbing, as well as

representing one character in an extended regular expression.

Variable substitution (contents of a variable).

var1=5

var2=23skidoo

echo $var1 # 5

echo $var2 # 23skidoo

A $ prefixing a variable name indicates the value the variable

holds.

end-of-line. In a regular expression, a "$" addresses the end

of a line of text.

${}

Parameter substitution.

... '

Quoted string expansion. This construct expands single or

multiple escaped octal or hex values into ASCII [18] or

Unicode characters.

$*, $@

positional parameters.

exit status variable. The $? variable holds the exit status of

a command, a function, or of the script itself.

process ID variable. The $ variable holds the process ID [19]

of the script in which it appears.

()

command group.

(a=hello; echo $a)

Important

A listing of commands within parentheses starts a subshell.

Variables inside parentheses, within the subshell, are not visible to

the rest of the script. The parent process, the script, cannot read

variables created in the child process, the subshell.

a=123

( a=321; )

echo "a = $a" # a = 123

"a" within parentheses acts like a local variable.

array initialization.

Array=(element1 element2 element3)

{xxx,yyy,zzz,...}

Brace expansion.

echo \"{These,words,are,quoted}\" # " prefix and suffix

"These" "words" "are" "quoted"

cat {file1,file2,file3} > combined_file

Concatenates the files file1, file2, and file3 into combined_file.

cp file22.{txt,backup}

Copies "file22.txt" to "file22.backup"

A command may act upon a comma-separated list of file specs

within braces. [20] Filename expansion (globbing) applies to

the file specs between the braces.

Caution

No spaces allowed within the braces unless the spaces are quoted or

escaped.

echo {file1,file2}\ :{\ A," B",' C'}

file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C

{a..z}

Extended Brace expansion.

echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z

Echoes characters between a and z.

echo {0..3} # 0 1 2 3

Echoes characters between 0 and 3.

base64_charset=( {A..Z} {a..z} {0..9} + / = )

Initializing an array, using extended brace expansion.

From vladz's "base64.sh" example script.

The {a..z} extended brace expansion construction is a feature

introduced in version 3 of Bash.

{}

Block of code [curly brackets]. Also referred to as an inline

group, this construct, in effect, creates an anonymous

function (a function without a name). However, unlike in a

"standard" function, the variables inside a code block remain

visible to the remainder of the script.

bash$ { local a;

a=123; }

bash: local: can only be used in a

function

a=123

{ a=321; }

echo "a = $a" # a = 321 (value inside code block)

Thanks, S.C.

The code block enclosed in braces may have I/O redirected to

and from it.

Example 3-1. Code blocks and I/O redirection

!/bin/bash

Reading lines in /etc/fstab.

File=/etc/fstab

{

read line1

read line2

} < $File

echo "First line in $File is:"

echo "$line1"

echo

echo "Second line in $File is:"

echo "$line2"

exit 0

Now, how do you parse the separate fields of each line?

Hint: use awk, or . . .

. . . Hans-Joerg Diers suggests using the "set" Bash builtin.

Example 3-2. Saving the output of a code block to a file

!/bin/bash

rpm-check.sh

Queries an rpm file for description, listing,

+ and whether it can be installed.

Saves output to a file.

This script illustrates using a code block.

SUCCESS=0

E_NOARGS=65

if [ -z "$1" ]

then

echo "Usage: `basename $0` rpm-file"

exit $E_NOARGS

{ # Begin code block.

echo

echo "Archive Description:"

rpm -qpi $1 # Query description.

echo

echo "Archive Listing:"

rpm -qpl $1 # Query listing.

echo

rpm -i --test $1 # Query whether rpm file can be installed.

if [ "$?" -eq $SUCCESS ]

then

echo "$1 can be installed."

else

echo "$1 cannot be installed."

echo # End code block.

} > "$1.test" # Redirects output of everything in block to file.

echo "Results of rpm test in file $1.test"

See rpm man page for explanation of options.

exit 0

Note

Unlike a command group within (parentheses), as above, a code block

enclosed by {braces} will not normally launch a subshell. [21]

It is possible to iterate a code block using a non-standard for-loop.

{}

placeholder for text. Used after xargs -i (replace strings

option). The {} double curly brackets are a placeholder for

output text.

ls . | xargs -i -t cp ./{} $1

^^ ^^

From "ex42.sh" (copydir.sh) example.

{} \;

pathname. Mostly used in find constructs. This is not a shell

builtin.

Definition: A pathname is a filename that includes the complete path.

As an example, /home/bozo/Notes/Thursday/schedule.txt. This is

sometimes referred to as the absolute path.

Note

The ";" ends the -exec option of a find command sequence. It needs to

be escaped to protect it from interpretation by the shell.

[ ]

test.

Test expression between [ ]. Note that [ is part of the shell

builtin test (and a synonym for it), not a link to the

external command /usr/bin/test.

[[ ]]

test.

Test expression between [[ ]]. More flexible than the

single-bracket [ ] test, this is a shell keyword.

See the discussion on the [[ ... ]] construct.

[ ]

array element.

In the context of an array, brackets set off the numbering of

each element of that array.

Array[1]=slot_1

echo ${Array[1]}

[ ]

range of characters.

As part of a regular expression, brackets delineate a range of

characters to match.

$[ ... ]

integer expansion.

Evaluate integer expression between $[ ].

a=3

b=7

echo $[$a+$b] # 10

echo $[$a*$b] # 21

Note that this usage is deprecated, and has been replaced by

the (( ... )) construct.

(( ))

integer expansion.

Expand and evaluate integer expression between (( )).

See the discussion on the (( ... )) construct.

> &> >& >> < <>

redirection.

scriptname >filename redirects the output of scriptname to

file filename. Overwrite filename if it already exists.

command &>filename redirects both the stdout and the stderr of

command to filename.

Note

This is useful for suppressing output when testing for a condition.

For example, let us test whether a certain command exists.

bash$ type bogus_command &>/dev/null

bash$ echo $?

Or in a script:

command_test () { type "$1" &>/dev/null; }

^

cmd=rmdir # Legitimate command.

command_test $cmd; echo $? # 0

cmd=bogus_command # Illegitimate command

command_test $cmd; echo $? # 1

command >&2 redirects stdout of command to stderr.

scriptname >>filename appends the output of scriptname to file

filename. If filename does not already exist, it is created.

[i]<>filename opens file filename for reading and writing, and

assigns file descriptor i to it. If filename does not exist,

it is created.

process substitution.

(command)>

<(command)

In a different context, the "<" and ">" characters act as

string comparison operators.

In yet another context, the "<" and ">" characters act as

integer comparison operators. See also Example 16-9.

redirection used in a here document.

<<<

redirection used in a here string.

<, >

ASCII comparison.

veg1=carrots

veg2=tomatoes

if [[ "$veg1" < "$veg2" ]]

then

echo "Although $veg1 precede $veg2 in the dictionary,"

echo -n "this does not necessarily imply anything "

echo "about my culinary preferences."

else

echo "What kind of dictionary are you using, anyhow?"

\<, \>

word boundary in a regular expression.

bash$ grep '\<the\>' textfile

pipe. Passes the output (stdout) of a previous command to the

input (stdin) of the next one, or to the shell. This is a

method of chaining commands together.

echo ls -l | sh

Passes the output of "echo ls -l" to the shell,

+ with the same result as a simple "ls -l".

cat *.lst | sort | uniq

Merges and sorts all ".lst" files, then deletes duplicate lines.

A pipe, as a classic method of interprocess communication, sends the

stdout of one process to the stdin of another. In a typical case, a

command, such as cat or echo, pipes a stream of data to a filter, a

command that transforms its input for processing. [22]

cat $filename1 $filename2 | grep $search_word

For an interesting note on the complexity of using UNIX pipes, see

the UNIX FAQ, Part 3.

The output of a command or commands may be piped to a script.

!/bin/bash

uppercase.sh : Changes input to uppercase.

tr 'a-z' 'A-Z'

Letter ranges must be quoted

+ to prevent filename generation from single-letter filenames.

exit 0

Now, let us pipe the output of ls -l to this script.

bash$ ls -l | ./uppercase.sh

-RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT

-RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT

-RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE

Note

The stdout of each process in a pipe must be read as the stdin of the

next. If this is not the case, the data stream will block, and the

pipe will not behave as expected.

cat file1 file2 | ls -l | sort

The output from "cat file1 file2" disappears.

A pipe runs as a child process, and therefore cannot alter script

variables.

variable="initial_value"

echo "new_value" | read variable

echo "variable = $variable" # variable = initial_value

If one of the commands in the pipe aborts, this prematurely

terminates execution of the pipe. Called a broken pipe, this

condition sends a SIGPIPE signal.

force redirection (even if the noclobber option is set). This

will forcibly overwrite an existing file.

OR logical operator. In a test construct, the || operator

causes a return of 0 (success) if either of the linked test

conditions is true.

Run job in background. A command followed by an & will run in

the background.

bash$ sleep 10 &

[1] 850

[1]+ Done sleep 10

Within a script, commands and even loops may run in the

background.

Example 3-3. Running a loop in the background

!/bin/bash

background-loop.sh

for i in 1 2 3 4 5 6 7 8 9 10 # First loop.

echo -n "$i "

done & # Run this loop in background.

# Will sometimes execute after second loop.

echo # This 'echo' sometimes will not display.

for i in 11 12 13 14 15 16 17 18 19 20 # Second loop.

echo -n "$i "

done

echo # This 'echo' sometimes will not display.

======================================================

The expected output from the script:

1 2 3 4 5 6 7 8 9 10

11 12 13 14 15 16 17 18 19 20

Sometimes, though, you get:

11 12 13 14 15 16 17 18 19 20

1 2 3 4 5 6 7 8 9 10 bozo $

(The second 'echo' doesn't execute. Why?)

Occasionally also:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

(The first 'echo' doesn't execute. Why?)

Very rarely something like:

11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20

The foreground loop preempts the background one.

exit 0

Nasimuddin Ansari suggests adding sleep 1

+ after the echo -n "$i" in lines 6 and 14,

+ for some real fun.

Caution

A command run in the background within a script may cause the script

to hang, waiting for a keystroke. Fortunately, there is a remedy for

this.

AND logical operator. In a test construct, the && operator

causes a return of 0 (success) only if both the linked test

conditions are true.

option, prefix. Option flag for a command or filter. Prefix

for an operator. Prefix for a default parameter in parameter

substitution.

COMMAND -[Option1][Option2][...]

ls -al

sort -dfu $filename

if [ $file1 -ot $file2 ]

then # ^

echo "File $file1 is older than $file2."

if [ "$a" -eq "$b" ]

then # ^

echo "$a is equal to $b."

if [ "$c" -eq 24 -a "$d" -eq 47 ]

then # ^ ^

echo "$c equals 24 and $d equals 47."

param2=${param1:-$DEFAULTVAL}

^

The double-dash -- prefixes long (verbatim) options to

commands.

sort --ignore-leading-blanks

Used with a Bash builtin, it means the end of options to that

particular command.

Tip

This provides a handy means of removing files whose names begin with

a dash.

bash$ ls -l

-rw-r--r-- 1 bozo bozo 0 Nov 25 12:29 -badname

bash$ rm -- -badname

bash$ ls -l

total 0

The double-dash is also used in conjunction with set.

set -- $variable (as in Example 15-18)

redirection from/to stdin or stdout [dash].

bash$ cat -

abc

...

Ctl-D

As expected, cat - echoes stdin, in this case keyboarded user

input, to stdout. But, does I/O redirection using - have

real-world applications?

(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)

Move entire file tree from one directory to another

[courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change]

1) cd /source/directory

Source directory, where the files to be moved are.

2) &&

"And-list": if the 'cd' operation successful,

then execute the next command.

3) tar cf - .

The 'c' option 'tar' archiving command creates a new archive,

the 'f' (file) option, followed by '-' designates the target file

as stdout, and do it in current directory tree ('.').

4) |

Piped to ...

5) ( ... )

a subshell

6) cd /dest/directory

Change to the destination directory.

7) &&

"And-list", as above

8) tar xpvf -

Unarchive ('x'), preserve ownership and file permissions ('p'),

and send verbose messages to stdout ('v'),

reading data from stdin ('f' followed by '-').

Note that 'x' is a command, and 'p', 'v', 'f' are options.

Whew!

More elegant than, but equivalent to:

cd source/directory

tar cf - . | (cd ../dest/directory; tar xpvf -)

Also having same effect:

cp -a /source/directory/* /dest/directory

Or:

cp -a /source/directory/* /source/directory/.[^.]* /dest/directory

If there are hidden files in /source/directory.

bunzip2 -c linux-2.6.16.tar.bz2 | tar xvf -

--uncompress tar file-- | --then pass it to "tar"--

If "tar" has not been patched to handle "bunzip2",

+ this needs to be done in two discrete steps, using a pipe.

The purpose of the exercise is to unarchive "bzipped" kernel source.

Note that in this context the "-" is not itself a Bash

operator, but rather an option recognized by certain UNIX

utilities that write to stdout, such as tar, cat, etc.

bash$ echo "whatever" | cat -

whatever

Where a filename is expected, - redirects output to stdout

(sometimes seen with tar cf), or accepts input from stdin,

rather than from a file. This is a method of using a

file-oriented utility as a filter in a pipe.

bash$ file

Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...

By itself on the command-line, file fails with an error

message.

Add a "-" for a more useful result. This causes the shell to

await user input.

bash$ file -

abc

standard input: ASCII text

bash$ file -

!/bin/bash

standard input: Bourne-Again shell script text executable

Now the command accepts input from stdin and analyzes it.

The "-" can be used to pipe stdout to other commands. This

permits such stunts as prepending lines to a file.

Using diff to compare a file with a section of another:

grep Linux file1 | diff file2 -

Finally, a real-world example using - with tar.

Example 3-4. Backup of all files changed in last day

!/bin/bash

Backs up all files in current directory modified within last 24 hours

+ in a "tarball" (tarred and gzipped file).

BACKUPFILE=backup-$(date +%m-%d-%Y)

Embeds date in backup filename.

Thanks, Joshua Tschida, for the idea.

archive=${1:-$BACKUPFILE}

If no backup-archive filename specified on command-line,

+ it will default to "backup-MM-DD-YYYY.tar.gz."

tar cvf - `find . -mtime -1 -type f -print` > $archive.tar

gzip $archive.tar

echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"."

Stephane Chazelas points out that the above code will fail

+ if there are too many files found

+ or if any filenames contain blank characters.

He suggests the following alternatives:

-------------------------------------------------------------------

find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar"

using the GNU version of "find".

find . -mtime -1 -type f -exec tar rvf "$archive.tar" '{}' \;

portable to other UNIX flavors, but much slower.

-------------------------------------------------------------------

exit 0

Caution

Filenames beginning with "-" may cause problems when coupled with the

"-" redirection operator. A script should check for this and add an

appropriate prefix to such filenames, for example ./-FILENAME,

$PWD/-FILENAME, or $PATHNAME/-FILENAME.

If the value of a variable begins with a -, this may likewise create

problems.

var="-n"

echo $var

Has the effect of "echo -n", and outputs nothing.

previous working directory. A cd - command changes to the

previous working directory. This uses the $OLDPWD

environmental variable.

Caution

Do not confuse the "-" used in this sense with the "-" redirection

operator just discussed. The interpretation of the "-" depends on the

context in which it appears.

Minus. Minus sign in an arithmetic operation.

Equals. Assignment operator

a=28

echo $a # 28

In a different context, the "=" is a string comparison

operator.

Plus. Addition arithmetic operator.

In a different context, the + is a Regular Expression

operator.

Option. Option flag for a command or filter.

Certain commands and builtins use the + to enable certain

options and the - to disable them. In parameter substitution,

the + prefixes an alternate value that a variable expands to.

modulo. Modulo (remainder of a division) arithmetic operation.

let "z = 5 % 3"

echo $z # 2

In a different context, the % is a pattern matching operator.

home directory [tilde]. This corresponds to the $HOME internal

variable. ~bozo is bozo's home directory, and ls ~bozo lists

the contents of it. ~/ is the current user's home directory,

and ls ~/ lists the contents of it.

bash$ echo ~bozo

/home/bozo

bash$ echo ~

/home/bozo

bash$ echo ~/

/home/bozo/

bash$ echo ~:

/home/bozo:

bash$ echo ~nonexistent-user

~nonexistent-user

current working directory. This corresponds to the $PWD

internal variable.

previous working directory. This corresponds to the $OLDPWD

internal variable.

regular expression match. This operator was introduced with

version 3 of Bash.

beginning-of-line. In a regular expression, a "^" addresses

the beginning of a line of text.

^, ^^

Uppercase conversion in parameter substitution (added in

version 4 of Bash).

Control Characters

change the behavior of the terminal or text display. A control

character is a CONTROL + key combination (pressed

simultaneously). A control character may also be written in

octal or hexadecimal notation, following an escape.

Control characters are not normally useful inside a script.

+ Ctl-A

Moves cursor to beginning of line of text (on the

command-line).

+ Ctl-B

Backspace (nondestructive).

+ Ctl-C

Break. Terminate a foreground job.

+ Ctl-D

Log out from a shell (similar to exit).

EOF (end-of-file). This also terminates input from stdin.

When typing text on the console or in an xterm window, Ctl-D

erases the character under the cursor. When there are no

characters present, Ctl-D logs out of the session, as

expected. In an xterm window, this has the effect of closing

the window.

+ Ctl-E

Moves cursor to end of line of text (on the command-line).

+ Ctl-F

Moves cursor forward one character position (on the

command-line).

+ Ctl-G

BEL. On some old-time teletype terminals, this would

actually ring a bell. In an xterm it might beep.

+ Ctl-H

Rubout (destructive backspace). Erases characters the cursor

backs over while backspacing.

!/bin/bash

Embedding Ctl-H in a string.

a="^H^H" # Two Ctl-H's -- backspaces

# ctl-V ctl-H, using vi/vim

echo "abcdef" # abcdef

echo

echo -n "abcdef$a " # abcd f

Space at end ^ ^ Backspaces twice.

echo

echo -n "abcdef$a" # abcdef

No space at end ^ Doesn't backspace (why?).

# Results may not be quite as expected.

echo; echo

Constantin Hagemeier suggests trying:

a= \010\010'

a= \b\b'

a= \x08\x08'

But, this does not change the results.

Now, try this.

rubout="^H^H^H^H^H" # 5 x Ctl-H.

echo -n "12345678"

sleep 2

echo -n "$rubout"

sleep 2

+ Ctl-I

Horizontal tab.

+ Ctl-J

Newline (line feed). In a script, may also be expressed in

octal notation -- '\012' or in hexadecimal -- '\x0a'.

+ Ctl-K

Vertical tab.

When typing text on the console or in an xterm window, Ctl-K

erases from the character under the cursor to end of line.

Within a script, Ctl-K may behave differently, as in Lee Lee

Maschmeyer's example, below.

+ Ctl-L

Formfeed (clear the terminal screen). In a terminal, this

has the same effect as the clear command. When sent to a

printer, a Ctl-L causes an advance to end of the paper

sheet.

+ Ctl-M

Carriage return.

!/bin/bash

Thank you, Lee Maschmeyer, for this example.

read -n 1 -s -p \

Control-M leaves cursor at beginning of this line. Press Enter. \x0d'

# Of course, '0d' is the hex equivalent of Control-M.

echo >&2 # The '-s' makes anything typed silent,

#+ so it is necessary to go to new line explicitly.

read -n 1 -s -p Control-J leaves cursor on next line. \x0a'

# '0a' is the hex equivalent of Control-J, linefeed.

echo >&2

read -n 1 -s -p And Control-K\x0bgoes straight down.'

echo >&2 # Control-K is vertical tab.

A better example of the effect of a vertical tab is:

var= \x0aThis is the bottom line\x0bThis is the top line\x0a'

echo "$var"

This works the same way as the above example. However:

echo "$var" | col

This causes the right end of the line to be higher than the left end.

It also explains why we started and ended with a line feed --

+ to avoid a garbled screen.

As Lee Maschmeyer explains:

--------------------------

In the [first vertical tab example] . . . the vertical tab

+ makes the printing go straight down without a carriage return.

This is true only on devices, such as the Linux console,

+ that can't go "backward."

The real purpose of VT is to go straight UP, not down.

It can be used to print superscripts on a printer.

The col utility can be used to emulate the proper behavior of VT.

exit 0

+ Ctl-N

Erases a line of text recalled from history buffer [23] (on

the command-line).

+ Ctl-O

Issues a newline (on the command-line).

+ Ctl-P

Recalls last command from history buffer (on the

command-line).

+ Ctl-Q

Resume (XON).

This resumes stdin in a terminal.

+ Ctl-R

Backwards search for text in history buffer (on the

command-line).

+ Ctl-S

Suspend (XOFF).

This freezes stdin in a terminal. (Use Ctl-Q to restore

input.)

+ Ctl-T

Reverses the position of the character the cursor is on with

the previous character (on the command-line).

+ Ctl-U

Erase a line of input, from the cursor backward to beginning

of line. In some settings, Ctl-U erases the entire line of

input, regardless of cursor position.

+ Ctl-V

When inputting text, Ctl-V permits inserting control

characters. For example, the following two are equivalent:

echo -e '\x0a'

echo <Ctl-V><Ctl-J>

Ctl-V is primarily useful from within a text editor.

+ Ctl-W

When typing text on the console or in an xterm window, Ctl-W

erases from the character under the cursor backwards to the

first instance of whitespace. In some settings, Ctl-W erases

backwards to first non-alphanumeric character.

+ Ctl-X

In certain word processing programs, Cuts highlighted text

and copies to clipboard.

+ Ctl-Y

Pastes back text previously erased (with Ctl-U or Ctl-W).

+ Ctl-Z

Pauses a foreground job.

Substitute operation in certain word processing

applications.

EOF (end-of-file) character in the MSDOS filesystem.

Whitespace

functions as a separator between commands and/or variables.

Whitespace consists of either spaces, tabs, blank lines, or

any combination thereof. [24] In some contexts, such as

variable assignment, whitespace is not permitted, and results

in a syntax error.

Blank lines have no effect on the action of a script, and are

therefore useful for visually separating functional sections.

$IFS, the special variable separating fields of input to

certain commands. It defaults to whitespace.

Definition: A field is a discrete chunk of data expressed as a string

of consecutive characters. Separating each field from adjacent fields

is either whitespace or some other designated character (often

determined by the $IFS). In some contexts, a field may be called a

record.

To preserve whitespace within a string or in a variable, use

quoting.

UNIX filters can target and operate on whitespace using the

POSIX character class [:space:].

________________________________________________________________

Chapter 4. Introduction to Variables and Parameters

Variables are how programming and scripting languages represent data.

A variable is nothing more than a label, a name assigned to a

location or set of locations in computer memory holding an item of

data.

Variables appear in arithmetic operations and manipulation of

quantities, and in string parsing.

________________________________________________________________

4.1. Variable Substitution

The name of a variable is a placeholder for its value, the data it

holds. Referencing (retrieving) its value is called variable

substitution.

Let us carefully distinguish between the name of a variable

and its value. If variable1 is the name of a variable, then

$variable1 is a reference to its value, the data item it

contains. [25]

bash$ variable1=23

bash$ echo variable1

variable1

bash$ echo $variable1

The only times a variable appears "naked" -- without the $

prefix -- is when declared or assigned, when unset, when

exported, in an arithmetic expression within double

parentheses (( ... )), or in the special case of a variable

representing a signal (see Example 32-5). Assignment may be

with an = (as in var1=27), in a read statement, and at the

head of a loop (for var2 in 1 2 3).

Enclosing a referenced value in double quotes (" ... ") does

not interfere with variable substitution. This is called

partial quoting, sometimes referred to as "weak quoting."

Using single quotes (' ... ') causes the variable name to be

used literally, and no substitution will take place. This is

full quoting, sometimes referred to as 'strong quoting.' See

Chapter 5 for a detailed discussion.

Note that $variable is actually a simplified form of

${variable}. In contexts where the $variable syntax causes an

error, the longer form may work (see Section 10.2, below).

Example 4-1. Variable assignment and substitution

!/bin/bash

ex9.sh

Variables: assignment and substitution

a=375

hello=$a

^ ^

-------------------------------------------------------------------------

No space permitted on either side of = sign when initializing variables.

What happens if there is a space?

"VARIABLE =value"

^

% Script tries to run "VARIABLE" command with one argument, "=value".

"VARIABLE= value"

^

% Script tries to run "value" command with

+ the environmental variable "VARIABLE" set to "".

-------------------------------------------------------------------------

echo hello # hello

Not a variable reference, just the string "hello" ...

echo $hello # 375

^ This is a variable reference.

echo ${hello} # 375

Likewise a variable reference, as above.

Quoting . . .

echo "$hello" # 375

echo "${hello}" # 375

echo

hello="A B C D"

echo $hello # A B C D

echo "$hello" # A B C D

As we see, echo $hello and echo "$hello" give different results.

=======================================

Quoting a variable preserves whitespace.

=======================================

echo

echo '$hello' # $hello

^ ^

Variable referencing disabled (escaped) by single quotes,

+ which causes the "$" to be interpreted literally.

Notice the effect of different types of quoting.

hello= # Setting it to a null value.

echo "\$hello (null value) = $hello" # $hello (null value) =

Note that setting a variable to a null value is not the same as

+ unsetting it, although the end result is the same (see below).

--------------------------------------------------------------

It is permissible to set multiple variables on the same line,

+ if separated by white space.

Caution, this may reduce legibility, and may not be portable.

var1=21 var2=22 var3=$V3

echo

echo "var1=$var1 var2=$var2 var3=$var3"

May cause problems with legacy versions of "sh" . . .

--------------------------------------------------------------

echo; echo

numbers="one two three"

^ ^

other_numbers="1 2 3"

^ ^

If there is whitespace embedded within a variable,

+ then quotes are necessary.

other_numbers=1 2 3 # Gives an error message.

echo "numbers = $numbers"

echo "other_numbers = $other_numbers" # other_numbers = 1 2 3

Escaping the whitespace also works.

mixed_bag=2\ ---\ Whatever

^ ^ Space after escape (\).

echo "$mixed_bag" # 2 --- Whatever

echo; echo

echo "uninitialized_variable = $uninitialized_variable"

Uninitialized variable has null value (no value at all!).

uninitialized_variable= # Declaring, but not initializing it --

#+ same as setting it to a null value, as above.

echo "uninitialized_variable = $uninitialized_variable"

# It still has a null value.

uninitialized_variable=23 # Set it.

unset uninitialized_variable # Unset it.

echo "uninitialized_variable = $uninitialized_variable"

# uninitialized_variable =

# It still has a null value.

echo

exit 0

Caution

An uninitialized variable has a "null" value -- no assigned value at

all (not zero!).

if [ -z "$unassigned" ]

then

echo "\$unassigned is NULL."

fi # $unassigned is NULL.

Using a variable before assigning a value to it may cause problems.

It is nevertheless possible to perform arithmetic operations on an

uninitialized variable.

echo "$uninitialized" # (blank line)

let "uninitialized += 5" # Add 5 to it.

echo "$uninitialized" # 5

Conclusion:

An uninitialized variable has no value,

+ however it evaluates as 0 in an arithmetic operation.

!/bin/bash

Naked variables

echo

When is a variable "naked", i.e., lacking the ' in front?

When it is being assigned, rather than referenced.

Assignment

a=879

echo "The value of \"a\" is $a."

Assignment using 'let'

let a=16+5

echo "The value of \"a\" is now $a."

echo

In a 'for' loop (really, a type of disguised assignment):

echo -n "Values of \"a\" in the loop are: "

for a in 7 8 9 11

echo -n "$a "

done

echo

In a 'read' statement (also a type of assignment):

echo -n "Enter \"a\" "

read a

echo "The value of \"a\" is now $a."

echo

exit 0

Example 4-3. Variable Assignment, plain and fancy

!/bin/bash

a=23 # Simple case

echo $a

b=$a

echo $b

Now, getting a little bit fancier (command substitution).

a=`echo Hello!` # Assigns result of 'echo' command to 'a' ...

echo $a

Note that including an exclamation mark (!) within a

+ command substitution construct will not work from the command-line,

+ since this triggers the Bash "history mechanism."

Inside a script, however, the history functions are disabled by default.

a=`ls -l` # Assigns result of 'ls -l' command to 'a'

echo $a # Unquoted, however, it removes tabs and newlines.

echo

echo "$a" # The quoted variable preserves whitespace.

# (See the chapter on "Quoting.")

exit 0

Variable assignment using the $(...) mechanism (a newer method

than backquotes). This is likewise a form of command

substitution.

From /etc/rc.d/rc.local

R=$(cat /etc/redhat-release)

arch=$(uname -m)

________________________________________________________________

4.3. Bash Variables Are Untyped

Unlike many other programming languages, Bash does not segregate its

variables by "type." Essentially, Bash variables are character

strings, but, depending on context, Bash permits arithmetic

operations and comparisons on variables. The determining factor is

whether the value of a variable contains only digits.

Example 4-4. Integer or string?

!/bin/bash

int-or-string.sh

a=2334 # Integer.

let "a += 1"

echo "a = $a " # a = 2335

echo # Integer, still.

b=${a/23/BB} # Substitute "BB" for "23".

# This transforms $b into a string.

echo "b = $b" # b = BB35

declare -i b # Declaring it an integer doesn't help.

echo "b = $b" # b = BB35

let "b += 1" # BB35 + 1

echo "b = $b" # b = 1

echo # Bash sets the "integer value" of a string to 0.

c=BB34

echo "c = $c" # c = BB34

d=${c/BB/23} # Substitute "23" for "BB".

# This makes $d an integer.

echo "d = $d" # d = 2334

let "d += 1" # 2334 + 1

echo "d = $d" # d = 2335

echo

What about null variables?

e='' # ... Or e="" ... Or e=

echo "e = $e" # e =

let "e += 1" # Arithmetic operations allowed on a null variable?

echo "e = $e" # e = 1

echo # Null variable transformed into an integer.

What about undeclared variables?

echo "f = $f" # f =

let "f += 1" # Arithmetic operations allowed?

echo "f = $f" # f = 1

echo # Undeclared variable transformed into an integer.

However ...

let "f /= $undecl_var" # Divide by zero?

let: f /= : syntax error: operand expected (error token is " ")

Syntax error! Variable $undecl_var is not set to zero here!

But still ...

let "f /= 0"

let: f /= 0: division by 0 (error token is "0")

Expected behavior.

Bash (usually) sets the "integer value" of null to zero

+ when performing an arithmetic operation.

But, don't try this at home, folks!

It's undocumented and probably non-portable behavior.

Conclusion: Variables in Bash are untyped,

+ with all attendant consequences.

exit $?

Untyped variables are both a blessing and a curse. They permit more

flexibility in scripting and make it easier to grind out lines of

code (and give you enough rope to hang yourself!). However, they

likewise permit subtle errors to creep in and encourage sloppy

programming habits.

To lighten the burden of keeping track of variable types in a script,

Bash does permit declaring variables.

________________________________________________________________

4.4. Special Variable Types

Local variables

Variables visible only within a code block or function (see

also local variables in functions)

Environmental variables

Variables that affect the behavior of the shell and user

interface

Note

In a more general context, each process has an "environment", that

is, a group of variables that the process may reference. In this

sense, the shell behaves like any other process.

Every time a shell starts, it creates shell variables that correspond

to its own environmental variables. Updating or adding new

environmental variables causes the shell to update its environment,

and all the shell's child processes (the commands it executes)

inherit this environment.

Caution

The space allotted to the environment is limited. Creating too many

environmental variables or ones that use up excessive space may cause

problems.

bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"

bash$ du

bash: /usr/bin/du: Argument list too long

Note: this "error" has been fixed, as of kernel version 2.6.23.

(Thank you, Stéphane Chazelas for the clarification, and for

providing the above example.)

If a script sets environmental variables, they need to be

"exported," that is, reported to the environment local to the

script. This is the function of the export command.

Note

A script can export variables only to child processes, that is, only

to commands or processes which that particular script initiates. A

script invoked from the command-line cannot export variables back to

the command-line environment. Child processes cannot export variables

back to the parent processes that spawned them.

Definition: A child process is a subprocess launched by another

process, its parent.

Positional parameters

Arguments passed to the script from the command line [26] :

$0, $1, $2, $3 . . .

$0 is the name of the script itself, $1 is the first argument,

$2 the second, $3 the third, and so forth. [27] After $9, the

arguments must be enclosed in brackets, for example, ${10},

${11}, ${12}.

The special variables $* and $@ denote all the positional

parameters.

Example 4-5. Positional Parameters

!/bin/bash

Call this script with at least 10 parameters, for example

./scriptname 1 2 3 4 5 6 7 8 9 10

MINPARAMS=10

echo

echo "The name of this script is \"$0\"."

Adds ./ for current directory

echo "The name of this script is \"`basename $0`\"."

Strips out path name info (see 'basename')

echo

if [ -n "$1" ] # Tested variable is quoted.

then

echo "Parameter #1 is $1" # Need quotes to escape #

if [ -n "$2" ]

then

echo "Parameter #2 is $2"

if [ -n "$3" ]

then

echo "Parameter #3 is $3"

...

if [ -n "${10}" ] # Parameters > $9 must be enclosed in {brackets}.

then

echo "Parameter #10 is ${10}"

echo "-----------------------------------"

echo "All the command-line parameters are: "$*""

if [ $# -lt "$MINPARAMS" ]

then

echo

echo "This script needs at least $MINPARAMS command-line arguments!"

echo

exit 0

Bracket notation for positional parameters leads to a fairly

simple way of referencing the last argument passed to a script

on the command-line. This also requires indirect referencing.

args=$# # Number of args passed.

lastarg=${!args}

Note: This is an indirect reference to $args ...

Or: lastarg=${!#} (Thanks, Chris Monson.)

This is an indirect reference to the $# variable.

Note that lastarg=${!$#} doesn't work.

Some scripts can perform different operations, depending on

which name they are invoked with. For this to work, the script

needs to check $0, the name it was invoked by. [28] There must

also exist symbolic links to all the alternate names of the

script. See Example 16-2.

Tip

If a script expects a command-line parameter but is invoked without

one, this may cause a null variable assignment, generally an

undesirable result. One way to prevent this is to append an extra

character to both sides of the assignment statement using the

expected positional parameter.

variable1_=$1_ # Rather than variable1=$1

This will prevent an error, even if positional parameter is absent.

critical_argument01=$variable1_

The extra character can be stripped off later, like so.

variable1=${variable1_/_/}

Side effects only if $variable1_ begins with an underscore.

This uses one of the parameter substitution templates discussed later.

(Leaving out the replacement pattern results in a deletion.)

A more straightforward way of dealing with this is

+ to simply test whether expected positional parameters have been passed.

if [ -z $1 ]

then

exit $E_MISSING_POS_PARAM

However, as Fabian Kreutz points out,

+ the above method may have unexpected side-effects.

A better method is parameter substitution:

${1:-$DefaultVal}

See the "Parameter Substition" section

+ in the "Variables Revisited" chapter.

---

Example 4-6. wh, whois domain name lookup

!/bin/bash

ex18.sh

Does a 'whois domain-name' lookup on any of 3 alternate servers:

ripe.net, cw.net, radb.net

Place this script -- renamed 'wh' -- in /usr/local/bin

Requires symbolic links:

ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe

ln -s /usr/local/bin/wh /usr/local/bin/wh-apnic

ln -s /usr/local/bin/wh /usr/local/bin/wh-tucows

E_NOARGS=75

if [ -z "$1" ]

then

echo "Usage: `basename $0` [domain-name]"

exit $E_NOARGS

Check script name and call proper server.

case `basename $0` in # Or: case ${0##*/} in

"wh" ) whois $1@whois.tucows.com;;

"wh-ripe" ) whois $1@whois.ripe.net;;

"wh-apnic" ) whois $1@whois.apnic.net;;

"wh-cw" ) whois $1@whois.cw.net;;

* ) echo "Usage: `basename $0` [domain-name]";;

esac

exit $?

---

The shift command reassigns the positional parameters, in

effect shifting them to the left one notch.

$1 <--- $2, $2 <--- $3, $3 <--- $4, etc.

The old $1 disappears, but $0 (the script name) does not

change. If you use a large number of positional parameters to

a script, shift lets you access those past 10, although

{bracket} notation also permits this.

Example 4-7. Using shift

!/bin/bash

shft.sh: Using 'shift' to step through all the positional parameters.

Name this script something like shft.sh,

+ and invoke it with some parameters.

+ For example:

sh shft.sh a b c def 83 barndoor

until [ -z "$1" ] # Until all parameters used up . . .

echo -n "$1 "

shift

done

echo # Extra linefeed.

But, what happens to the "used-up" parameters?

echo "$2"

Nothing echoes!

When $2 shifts into $1 (and there is no $3 to shift into $2)

+ then $2 remains empty.

So, it is not a parameter copy, but a move.

exit

+ alternative method of stepping through the positional params.

The shift command can take a numerical parameter indicating

how many positions to shift.

!/bin/bash

shift-past.sh

shift 3 # Shift 3 positions.

n=3; shift $n

Has the same effect.

echo "$1"

exit 0

======================== #

$ sh shift-past.sh 1 2 3 4 5

However, as Eleni Fragkiadaki, points out,

+ attempting a 'shift' past the number of

+ positional parameters ($#) returns an exit status of 1,

+ and the positional parameters themselves do not change.

This means possibly getting stuck in an endless loop. . . .

For example:

until [ -z "$1" ]

do

echo -n "$1 "

shift 20 # If less than 20 pos params,

done #+ then loop never ends!

When in doubt, add a sanity check. . . .

shift 20 || break

^^^^^^^^

Note

The shift command works in a similar fashion on parameters passed to

a function. See Example 36-18.

________________________________________________________________

Chapter 5. Quoting

Quoting means just that, bracketing a string in quotes. This has the

effect of protecting special characters in the string from

reinterpretation or expansion by the shell or shell script. (A

character is "special" if it has an interpretation other than its

literal meaning. For example, the asterisk * represents a wild card

character in globbing and Regular Expressions).

bash$ ls -l [Vv]*

-rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT

-rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh

-rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh

bash$ ls -l '[Vv]*'

ls: [Vv]*: No such file or directory

In everyday speech or writing, when we "quote" a phrase, we set it

apart and give it special meaning. In a Bash script, when we quote a

string, we set it apart and protect its literal meaning.

Certain programs and utilities reinterpret or expand special

characters in a quoted string. An important use of quoting is

protecting a command-line parameter from the shell, but still letting

the calling program expand it.

bash$ grep '[Ff]irst' *.txt

file1.txt:This is the first line of file1.txt.

file2.txt:This is the First line of file2.txt.

Note that the unquoted grep [Ff]irst *.txt works under the Bash

shell. [29]

Quoting can also suppress echo's "appetite" for newlines.

bash$ echo $(ls -l)

total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 1

2:57 u.sh

bash$ echo "$(ls -l)"

total 8

-rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh

-rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh

________________________________________________________________

5.1. Quoting Variables

When referencing a variable, it is generally advisable to enclose its

name in double quotes. This prevents reinterpretation of all special

characters within the quoted string -- except $, ` (backquote), and \

(escape). [30] Keeping $ as a special character within double quotes

permits referencing a quoted variable ("$variable"), that is,

replacing the variable with its value (see Example 4-1, above).

Use double quotes to prevent word splitting. [31] An argument

enclosed in double quotes presents itself as a single word, even if

it contains whitespace separators.

List="one two three"

for a in $List # Splits the variable in parts at whitespace.

echo "$a"

done

one

two

three

echo "---"

for a in "$List" # Preserves whitespace in a single variable.

do # ^ ^

echo "$a"

done

one two three

A more elaborate example:

variable1="a variable containing five words"

COMMAND This is $variable1 # Executes COMMAND with 7 arguments:

"This" "is" "a" "variable" "containing" "five" "words"

COMMAND "This is $variable1" # Executes COMMAND with 1 argument:

"This is a variable containing five words"

variable2="" # Empty.

COMMAND $variable2 $variable2 $variable2

# Executes COMMAND with no arguments.

COMMAND "$variable2" "$variable2" "$variable2"

# Executes COMMAND with 3 empty arguments.

COMMAND "$variable2 $variable2 $variable2"

# Executes COMMAND with 1 argument (2 spaces).

Thanks, Stéphane Chazelas.

Tip

Enclosing the arguments to an echo statement in double quotes is

necessary only when word splitting or preservation of whitespace is

an issue.

Example 5-1. Echoing Weird Variables

!/bin/bash

weirdvars.sh: Echoing weird variables.

echo

var="'(]\\{}\$\""

echo $var # '(]\{}$"

echo "$var" # '(]\{}$" Doesn't make a difference.

echo

IFS='\'

echo $var # '(] {}$" \ converted to space. Why?

echo "$var" # '(]\{}$"

Examples above supplied by Stephane Chazelas.

echo

var2="\\\\\""

echo $var2 # "

echo "$var2" # \\"

echo

But ... var2="\\\\"" is illegal. Why?

var3='\\\\'

echo "$var3" # \\\\

Strong quoting works, though.

************************************************************ #

As the first example above shows, nesting quotes is permitted.

echo "$(echo '"')" # "

^ ^

At times this comes in useful.

var1="Two bits"

echo "\$var1 = "$var1"" # $var1 = Two bits

^ ^

Or, as Chris Hiestand points out ...

if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]]

^ ^ ^ ^ ^ ^ ^ ^

then

...

************************************************************ #

Single quotes (' ') operate similarly to double quotes, but do not

permit referencing variables, since the special meaning of $ is

turned off. Within single quotes, every special character except '

gets interpreted literally. Consider single quotes ("full quoting")

to be a stricter method of quoting than double quotes ("partial

quoting").

Note

Since even the escape character (\) gets a literal interpretation

within single quotes, trying to enclose a single quote within single

quotes will not yield the expected result.

echo "Why can't I write 's between single quotes"

echo

The roundabout method.

echo 'Why can'\''t I write '"'"'s between single quotes'

|-------| |----------| |-----------------------|

Three single-quoted strings, with escaped and quoted single quotes between.

This example courtesy of Stéphane Chazelas.

________________________________________________________________

5.2. Escaping

Escaping is a method of quoting single characters. The escape (\)

preceding a character tells the shell to interpret that character

literally.

Caution

With certain commands and utilities, such as echo and sed, escaping a

character may have the opposite effect - it can toggle on a special

meaning for that character.

Special meanings of certain escaped characters

used with echo and sed

means newline

means return

means tab

means vertical tab

means backspace

means alert (beep or flash)

\0xx

translates to the octal ASCII equivalent of 0nn, where nn is a

string of digits

Important

The ... ' quoted string-expansion construct is a mechanism that

uses escaped octal or hex values to assign ASCII characters to

variables, e.g., quote= \042'.

Example 5-2. Escaped Characters

!/bin/bash

escaped.sh: escaped characters

First, let's show some basic escaped-character usage. ###

Escaping a newline.

------------------

echo ""

echo "This will print

as two lines."

This will print

as two lines.

echo "This will print \

as one line."

This will print as one line.

echo; echo

echo "============="

echo "\v\v\v\v" # Prints \v\v\v\v literally.

Use the -e option with 'echo' to print escaped characters.

echo "============="

echo "VERTICAL TABS"

echo -e "\v\v\v\v" # Prints 4 vertical tabs.

echo "=============="

echo "QUOTATION MARK"

echo -e "\042" # Prints " (quote, octal ASCII character 42).

echo "=============="

The \X' construct makes the -e option unnecessary.

echo; echo "NEWLINE and (maybe) BEEP"

echo \n' # Newline.

echo \a' # Alert (beep).

# May only flash, not beep, depending on terminal.

We have seen \nnn" string expansion, and now . . .

=================================================================== #

Version 2 of Bash introduced the \nnn' string expansion construct.

=================================================================== #

echo "Introducing the \$\' ... \' string-expansion construct . . . "

echo ". . . featuring more quotation marks."

echo \t \042 \t' # Quote (") framed by tabs.

Note that '\nnn' is an octal value.

It also works with hexadecimal values, in an \xhhh' construct.

echo \t \x22 \t' # Quote (") framed by tabs.

Thank you, Greg Keraunen, for pointing this out.

Earlier Bash versions allowed '\x022'.

echo

Assigning ASCII characters to a variable.

----------------------------------------

quote= \042' # " assigned to a variable.

echo "$quote Quoted string $quote and this lies outside the quotes."

echo

Concatenating ASCII chars in a variable.

triple_underline= \137\137\137' # 137 is octal ASCII code for '_'.

echo "$triple_underline UNDERLINE $triple_underline"

echo

ABC= \101\102\103\010' # 101, 102, 103 are octal A, B, C.

echo $ABC

echo

escape= \033' # 033 is octal for escape.

echo "\"escape\" echoes as $escape"

no visible output.

echo

exit 0

A more elaborate example:

Example 5-3. Detecting key-presses

!/bin/bash

Author: Sigurd Solaas, 20 Apr 2011

Used in ABS Guide with permission.

Requires version 4.2+ of Bash.

key="no value yet"

while true; do

clear

echo "Bash Extra Keys Demo. Keys to try:"

echo

echo "* Insert, Delete, Home, End, Page_Up and Page_Down"

echo "* The four arrow keys"

echo "* Tab, enter, escape, and space key"

echo "* The letter and number keys, etc."

echo

echo " d = show date/time"

echo " q = quit"

echo "================================"

echo

# Convert the separate home-key to home-key_num_7:

if [ "$key" = \x1b\x4f\x48' ]; then

key= \x1b\x5b\x31\x7e'

# Quoted string-expansion construct.

# Convert the separate end-key to end-key_num_1.

if [ "$key" = \x1b\x4f\x46' ]; then

key= \x1b\x5b\x34\x7e'

case "$key" in

\x1b\x5b\x32\x7e') # Insert

echo Insert Key

;;

\x1b\x5b\x33\x7e') # Delete

echo Delete Key

;;

\x1b\x5b\x31\x7e') # Home_key_num_7

echo Home Key

;;

\x1b\x5b\x34\x7e') # End_key_num_1

echo End Key

;;

\x1b\x5b\x35\x7e') # Page_Up

echo Page_Up

;;

\x1b\x5b\x36\x7e') # Page_Down

echo Page_Down

;;

\x1b\x5b\x41') # Up_arrow

echo Up arrow

;;

\x1b\x5b\x42') # Down_arrow

echo Down arrow

;;

\x1b\x5b\x43') # Right_arrow

echo Right arrow

;;

\x1b\x5b\x44') # Left_arrow

echo Left arrow

;;

\x09') # Tab

echo Tab Key

;;

\x0a') # Enter

echo Enter Key

;;

\x1b') # Escape

echo Escape Key

;;

\x20') # Space

echo Space Key

;;

date

;;

echo Time to quit...

echo

exit 0

;;

echo You pressed: \'"$key"\'

;;

esac

echo

echo "================================"

unset K1 K2 K3

read -s -N1 -p "Press a key: "

K1="$REPLY"

read -s -N2 -t 0.001

K2="$REPLY"

read -s -N1 -t 0.001

K3="$REPLY"

key="$K1$K2$K3"

done

exit $?

Whereas . . .

echo "\" # Invokes secondary prompt from the command-line.

# In a script, gives an error message.

However . . .

echo '\' # Results in \

Note

The behavior of \ depends on whether it is escaped, strong-quoted,

weak-quoted, or appearing within command substitution or a here

document.

# Simple escaping and quoting

echo \z # z

echo \\z # \z

echo '\z' # \z

echo '\\z' # \\z

echo "\z" # \z

echo "\\z" # \z

# Command substitution

echo `echo \z` # z

echo `echo \\z` # z

echo `echo \\\z` # \z

echo `echo \\\\z` # \z

echo `echo \\\\\\z` # \z

echo `echo \\\\\\\z` # \\z

echo `echo "\z"` # \z

echo `echo "\\z"` # \z

# Here document

cat <<EOF

EOF # \z

cat <<EOF

\\z

EOF # \z

These examples supplied by Stéphane Chazelas.

Elements of a string assigned to a variable may be escaped, but the

escape character alone may not be assigned to a variable.

variable=\

echo "$variable"

Will not work - gives an error message:

test.sh: : command not found

A "naked" escape cannot safely be assigned to a variable.

What actually happens here is that the "\" escapes the newline and

+ the effect is variable=echo "$variable"

+ invalid variable assignment

variable=\

23skidoo

echo "$variable" # 23skidoo

# This works, since the second line

#+ is a valid variable assignment.

variable=\

\^ escape followed by space

echo "$variable" # space

variable=\\

echo "$variable" # \

variable=\\\

echo "$variable"

Will not work - gives an error message:

test.sh: \: command not found

First escape escapes second one, but the third one is left "naked",

+ with same result as first instance, above.

variable=\\\\

echo "$variable" # \\

# Second and fourth escapes escaped.

# This is o.k.

Escaping a space can prevent word splitting in a command's argument

list.

file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7"

List of files as argument(s) to a command.

Add two files to the list, and list all.

ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list

echo "------------------------------------------------------------------------

What happens if we escape a couple of spaces?

ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list

Error: the first three files concatenated into a single argument to 'ls -l'

because the two escaped spaces prevent argument (word) splitting.

The escape also provides a means of writing a multi-line command.

Normally, each separate line constitutes a different command, but an

escape at the end of a line escapes the newline character, and the

command sequence continues on to the next line.

(cd /source/directory && tar cf - . ) | \

(cd /dest/directory && tar xpvf -)

Repeating Alan Cox's directory tree copy command,

but split into two lines for increased legibility.

As an alternative:

tar cf - -C /source/directory . |

tar xpvf - -C /dest/directory

See note below.

(Thanks, Stéphane Chazelas.)

Note

If a script line ends with a |, a pipe character, then a \, an

escape, is not strictly necessary. It is, however, good programming

practice to always escape the end of a line of code that continues to

the following line.

echo "foo

bar"

foo

bar

echo

echo 'foo

bar' # No difference yet.

foo

bar

echo

echo foo\

bar # Newline escaped.

foobar

echo

echo "foo\

bar" # Same here, as \ still interpreted as escape within weak quotes.

foobar

echo

echo 'foo\

bar' # Escape character \ taken literally because of strong quoting.

foo\

bar

Examples suggested by Stéphane Chazelas.

________________________________________________________________

Chapter 6. Exit and Exit Status

... there are dark corners in the Bourne shell, and people use all of

them.

--Chet Ramey

The exit command terminates a script, just as in a C program. It can

also return a value, which is available to the script's parent

process.

Every command returns an exit status (sometimes referred to as a

return status or exit code). A successful command returns a 0, while

an unsuccessful one returns a non-zero value that usually can be

interpreted as an error code. Well-behaved UNIX commands, programs,

and utilities return a 0 exit code upon successful completion, though

there are some exceptions.

Likewise, functions within a script and the script itself return an

exit status. The last command executed in the function or script

determines the exit status. Within a script, an exit nnn command may

be used to deliver an nnn exit status to the shell (nnn must be an

integer in the 0 - 255 range).

Note

When a script ends with an exit that has no parameter, the exit

status of the script is the exit status of the last command executed

in the script (previous to the exit).

!/bin/bash

COMMAND_1

. . .

COMMAND_LAST

Will exit with status of last command.

exit

The equivalent of a bare exit is exit $? or even just omitting the

exit.

!/bin/bash

COMMAND_1

. . .

COMMAND_LAST

Will exit with status of last command.

exit $?

!/bin/bash

COMMAND1

. . .

COMMAND_LAST

Will exit with status of last command.

$? reads the exit status of the last command executed. After a

function returns, $? gives the exit status of the last command

executed in the function. This is Bash's way of giving functions a

"return value." [32]

Following the execution of a pipe, a $? gives the exit status of the

last command executed.

After a script terminates, a $? from the command-line gives the exit

status of the script, that is, the last command executed in the

script, which is, by convention, 0 on success or an integer in the

range 1 - 255 on error.

Example 6-1. exit / exit status

!/bin/bash

echo hello

echo $? # Exit status 0 returned because command executed successfully.

lskdf # Unrecognized command.

echo $? # Non-zero exit status returned -- command failed to execute.

echo

exit 113 # Will return 113 to shell.

# To verify this, type "echo $?" after script terminates.

By convention, an 'exit 0' indicates success,

+ while a non-zero exit value means an error or anomalous condition.

See the "Exit Codes With Special Meanings" appendix.

$? is especially useful for testing the result of a command in a

script (see Example 16-35 and Example 16-20).

Note

The !, the logical not qualifier, reverses the outcome of a test or

command, and this affects its exit status.

Example 6-2. Negating a condition using !

true # The "true" builtin.

echo "exit status of \"true\" = $?" # 0

! true

echo "exit status of \"! true\" = $?" # 1

Note that the "!" needs a space between it and the command.

!true leads to a "command not found" error

The '!' operator prefixing a command invokes the Bash history mechanism.

true

!true

No error this time, but no negation either.

It just repeats the previous command (true).

=========================================================== #

Preceding a _pipe_ with ! inverts the exit status returned.

ls | bogus_command # bash: bogus_command: command not found

echo $? # 127

! ls | bogus_command # bash: bogus_command: command not found

echo $? # 0

Note that the ! does not change the execution of the pipe.

Only the exit status changes.

=========================================================== #

Thanks, Stéphane Chazelas and Kristopher Newsome.

Caution

Certain exit status codes have reserved meanings and should not be

user-specified in a script.

________________________________________________________________

Chapter 7. Tests

Every reasonably complete programming language can test for a

condition, then act according to the result of the test. Bash has the

test command, various bracket and parenthesis operators, and the

if/then construct.

________________________________________________________________

7.1. Test Constructs

* An if/then construct tests whether the exit status of a list of

commands is 0 (since 0 means "success" by UNIX convention), and

if so, executes one or more commands.

* There exists a dedicated command called [ (left bracket special

character). It is a synonym for test, and a builtin for

efficiency reasons. This command considers its arguments as

comparison expressions or file tests and returns an exit status

corresponding to the result of the comparison (0 for true, 1 for

false).

* With version 2.02, Bash introduced the [[ ... ]] extended test

command, which performs comparisons in a manner more familiar to

programmers from other languages. Note that [[ is a keyword, not

a command.

Bash sees [[ $a -lt $b ]] as a single element, which returns an

exit status.

* The (( ... )) and let ... constructs return an exit status,

according to whether the arithmetic expressions they evaluate

expand to a non-zero value. These arithmetic-expansion constructs

may therefore be used to perform arithmetic comparisons.

(( 0 && 1 )) # Logical AND

echo $? # 1 ***

And so ...

let "num = (( 0 && 1 ))"

echo $num # 0

But ...

let "num = (( 0 && 1 ))"

echo $? # 1 ***

(( 200 || 11 )) # Logical OR

echo $? # 0 ***

...

let "num = (( 200 || 11 ))"

echo $num # 1

let "num = (( 200 || 11 ))"

echo $? # 0 ***

(( 200 | 11 )) # Bitwise OR

echo $? # 0 ***

...

let "num = (( 200 | 11 ))"

echo $num # 203

let "num = (( 200 | 11 ))"

echo $? # 0 ***

The "let" construct returns the same exit status

+ as the double-parentheses arithmetic expansion.

Caution

Again, note that the exit status of an arithmetic expression is not

an error value.

var=-2 && (( var+=2 ))

echo $? # 1

var=-2 && (( var+=2 )) && echo $var

# Will not echo $var!

* An if can test any command, not just conditions enclosed within

brackets.

if cmp a b &> /dev/null # Suppress output.

then echo "Files a and b are identical."

else echo "Files a and b differ."

The very useful "if-grep" construct:

-----------------------------------

if grep -q Bash file

then echo "File contains at least one occurrence of Bash."

word=Linux

letter_sequence=inu

if echo "$word" | grep -q "$letter_sequence"

The "-q" option to grep suppresses output.

then

echo "$letter_sequence found in $word"

else

echo "$letter_sequence not found in $word"

if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED

then echo "Command succeeded."

else echo "Command failed."

* These last two examples courtesy of Stéphane Chazelas.

Example 7-1. What is truth?

!/bin/bash

Tip:

If you're unsure how a certain condition might evaluate,

+ test it in an if-test.

echo

echo "Testing \"0\""

if [ 0 ] # zero

then

echo "0 is true."

else # Or else ...

echo "0 is false."

fi # 0 is true.

echo

echo "Testing \"1\""

if [ 1 ] # one

then

echo "1 is true."

else

echo "1 is false."

fi # 1 is true.

echo

echo "Testing \"-1\""

if [ -1 ] # minus one

then

echo "-1 is true."

else

echo "-1 is false."

fi # -1 is true.

echo

echo "Testing \"NULL\""

if [ ] # NULL (empty condition)

then

echo "NULL is true."

else

echo "NULL is false."

fi # NULL is false.

echo

echo "Testing \"xyz\""

if [ xyz ] # string

then

echo "Random string is true."

else

echo "Random string is false."

fi # Random string is true.

echo

echo "Testing \"\$xyz\""

if [ $xyz ] # Tests if $xyz is null, but...

# it's only an uninitialized variable.

then

echo "Uninitialized variable is true."

else

echo "Uninitialized variable is false."

fi # Uninitialized variable is false.

echo

echo "Testing \"-n \$xyz\""

if [ -n "$xyz" ] # More pedantically correct.

then

echo "Uninitialized variable is true."

else

echo "Uninitialized variable is false."

fi # Uninitialized variable is false.

echo

xyz= # Initialized, but set to null value.

echo "Testing \"-n \$xyz\""

if [ -n "$xyz" ]

then

echo "Null variable is true."

else

echo "Null variable is false."

fi # Null variable is false.

echo

When is "false" true?

echo "Testing \"false\""

if [ "false" ] # It seems that "false" is just a string ...

then

echo "\"false\" is true." #+ and it tests true.

else

echo "\"false\" is false."

fi # "false" is true.

echo

echo "Testing \"\$false\"" # Again, uninitialized variable.

if [ "$false" ]

then

echo "\"\$false\" is true."

else

echo "\"\$false\" is false."

fi # "$false" is false.

# Now, we get the expected result.

What would happen if we tested the uninitialized variable "$true"?

echo

exit 0

Exercise. Explain the behavior of Example 7-1, above.

if [ condition-true ]

then

command 1

command 2

...

else # Or else ...

# Adds default code block executing if original condition tests false.

command 3

command 4

...

Note

When if and then are on same line in a condition test, a semicolon

must terminate the if statement. Both if and then are keywords.

Keywords (or commands) begin statements, and before a new statement

on the same line begins, the old one must terminate.

if [ -x "$filename" ]; then

Else if and elif

elif

elif is a contraction for else if. The effect is to nest an

inner if/then construct within an outer one.

if [ condition1 ]

then

command1

command2

command3

elif [ condition2 ]

Same as else if

then

command4

command5

else

default-command

The if test condition-true construct is the exact equivalent of if [

condition-true ]. As it happens, the left bracket, [ , is a token

[33] which invokes the test command. The closing right bracket, ] ,

in an if/test should not therefore be strictly necessary, however

newer versions of Bash require it.

Note

The test command is a Bash builtin which tests file types and

compares strings. Therefore, in a Bash script, test does not call the

external /usr/bin/test binary, which is part of the sh-utils package.

Likewise, [ does not call /usr/bin/[, which is linked to

/usr/bin/test.

bash$ type test

test is a shell builtin

bash$ type '['

[ is a shell builtin

bash$ type '[['

[[ is a shell keyword

bash$ type ']]'

]] is a shell keyword

bash$ type ']'

bash: type: ]: not found

If, for some reason, you wish to use /usr/bin/test in a Bash script,

then specify it by full pathname.

Example 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[

!/bin/bash

echo

if test -z "$1"

then

echo "No command-line arguments."

else

echo "First command-line argument is $1."

echo

if /usr/bin/test -z "$1" # Equivalent to "test" builtin.

^^^^^^^^^^^^^ # Specifying full pathname.

then

echo "No command-line arguments."

else

echo "First command-line argument is $1."

echo

if [ -z "$1" ] # Functionally identical to above code blocks.

if [ -z "$1" should work, but...

+ Bash responds to a missing close-bracket with an error message.

then

echo "No command-line arguments."

else

echo "First command-line argument is $1."

echo

if /usr/bin/[ -z "$1" ] # Again, functionally identical to above.

if /usr/bin/[ -z "$1" # Works, but gives an error message.

# Note:

This has been fixed in Bash, version 3.x.

then

echo "No command-line arguments."

else

echo "First command-line argument is $1."

echo

exit 0

The [[ ]] construct is the more versatile Bash version of [ ]. This

is the extended test command, adopted from ksh88.

* * *

No filename expansion or word splitting takes place between [[ and

]], but there is parameter expansion and command substitution.

file=/etc/passwd

if [[ -e $file ]]

then

echo "Password file exists."

Using the [[ ... ]] test construct, rather than [ ... ] can prevent

many logic errors in scripts. For example, the &&, ||, <, and >

operators work within a [[ ]] test, despite giving an error within a

[ ] construct.

Arithmetic evaluation of octal / hexadecimal constants takes place

automatically within a [[ ... ]] construct.

[[ Octal and hexadecimal evaluation ]]

Thank you, Moritz Gronbach, for pointing this out.

decimal=15

octal=017 # = 15 (decimal)

hex=0x0f # = 15 (decimal)

if [ "$decimal" -eq "$octal" ]

then

echo "$decimal equals $octal"

else

echo "$decimal is not equal to $octal" # 15 is not equal to 017

fi # Doesn't evaluate within [ single brackets ]!

if [[ "$decimal" -eq "$octal" ]]

then

echo "$decimal equals $octal" # 15 equals 017

else

echo "$decimal is not equal to $octal"

fi # Evaluates within [[ double brackets ]]!

if [[ "$decimal" -eq "$hex" ]]

then

echo "$decimal equals $hex" # 15 equals 0x0f

else

echo "$decimal is not equal to $hex"

fi # [[ $hexadecimal ]] also evaluates!

Note

Following an if, neither the test command nor the test brackets ( [ ]

or [[ ]] ) are strictly necessary.

dir=/home/bozo

if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message.

echo "Now in $dir."

else

echo "Can't change to $dir."

The "if COMMAND" construct returns the exit status of COMMAND.

Similarly, a condition within test brackets may stand alone without

an if, when used in combination with a list construct.

var1=20

var2=22

[ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"

home=/home/bozo

[ -d "$home" ] || echo "$home directory does not exist."

The (( )) construct expands and evaluates an arithmetic expression.

If the expression evaluates as zero, it returns an exit status of 1,

or "false". A non-zero expression returns an exit status of 0, or

"true". This is in marked contrast to using the test and [ ]

constructs previously discussed.

Example 7-3. Arithmetic Tests using (( ))

!/bin/bash

arith-tests.sh

Arithmetic tests.

The (( ... )) construct evaluates and tests numerical expressions.

Exit status opposite from [ ... ] construct!

(( 0 ))

echo "Exit status of \"(( 0 ))\" is $?." # 1

(( 1 ))

echo "Exit status of \"(( 1 ))\" is $?." # 0

(( 5 > 4 )) # true

echo "Exit status of \"(( 5 > 4 ))\" is $?." # 0

(( 5 > 9 )) # false

echo "Exit status of \"(( 5 > 9 ))\" is $?." # 1

(( 5 == 5 )) # true

echo "Exit status of \"(( 5 == 5 ))\" is $?." # 0

(( 5 = 5 )) gives an error message.

(( 5 - 5 )) # 0

echo "Exit status of \"(( 5 - 5 ))\" is $?." # 1

(( 5 / 4 )) # Division o.k.

echo "Exit status of \"(( 5 / 4 ))\" is $?." # 0

(( 1 / 2 )) # Division result < 1.

echo "Exit status of \"(( 1 / 2 ))\" is $?." # Rounded off to 0.

# 1

(( 1 / 0 )) 2>/dev/null # Illegal division by 0.

^^^^^^^^^^^

echo "Exit status of \"(( 1 / 0 ))\" is $?." # 1

What effect does the "2>/dev/null" have?

What would happen if it were removed?

Try removing it, then rerunning the script.

======================================= #

(( ... )) also useful in an if-then test.

var1=5

var2=4

if (( var1 > var2 ))

then #^ ^ Note: Not $var1, $var2. Why?

echo "$var1 is greater than $var2"

fi # 5 is greater than 4

exit 0

________________________________________________________________

7.2. File test operators

Returns true if...

-e

file exists

-a

file exists

This is identical in effect to -e. It has been "deprecated,"

[34] and its use is discouraged.

-f

file is a regular file (not a directory or device file)

-s

file is not zero size

-d

file is a directory

-b

file is a block device

-c

file is a character device

device0="/dev/sda2" # / (root directory)

if [ -b "$device0" ]

then

echo "$device0 is a block device."

/dev/sda2 is a block device.

device1="/dev/ttyS1" # PCMCIA modem card.

if [ -c "$device1" ]

then

echo "$device1 is a character device."

/dev/ttyS1 is a character device.

-p

file is a pipe

function show_input_type()

{

[ -p /dev/fd/0 ] && echo PIPE || echo STDIN

}

show_input_type "Input" # STDIN

echo "Input" | show_input_type # PIPE

This example courtesy of Carl Anderson.

-h

file is a symbolic link

-L

file is a symbolic link

-S

file is a socket

-t

file (descriptor) is associated with a terminal device

This test option may be used to check whether the stdin [ -t 0

] or stdout [ -t 1 ] in a given script is a terminal.

-r

file has read permission (for the user running the test)

-w

file has write permission (for the user running the test)

-x

file has execute permission (for the user running the test)

-g

set-group-id (sgid) flag set on file or directory

If a directory has the sgid flag set, then a file created

within that directory belongs to the group that owns the

directory, not necessarily to the group of the user who

created the file. This may be useful for a directory shared by

a workgroup.

-u

set-user-id (suid) flag set on file

A binary owned by root with set-user-id flag set runs with

root privileges, even when an ordinary user invokes it. [35]

This is useful for executables (such as pppd and cdrecord)

that need to access system hardware. Lacking the suid flag,

these binaries could not be invoked by a non-root user.

-rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd

A file with the suid flag set shows an s in its permissions.

-k

sticky bit set

Commonly known as the sticky bit, the save-text-mode flag is a

special type of file permission. If a file has this flag set,

that file will be kept in cache memory, for quicker access.

[36] If set on a directory, it restricts write permission.

Setting the sticky bit adds a t to the permissions on the file

or directory listing. This restricts altering or deleting

specific files in that directory to the owner of those files.

drwxrwxrwt 7 root 1024 May 19 21:26 tmp/

If a user does not own a directory that has the sticky bit

set, but has write permission in that directory, she can only

delete those files that she owns in it. This keeps users from

inadvertently overwriting or deleting each other's files in a

publicly accessible directory, such as /tmp. (The owner of the

directory or root can, of course, delete or rename files

there.)

-O

you are owner of file

-G

group-id of file same as yours

-N

file modified since it was last read

f1 -nt f2

file f1 is newer than f2

f1 -ot f2

file f1 is older than f2

f1 -ef f2

files f1 and f2 are hard links to the same file

"not" -- reverses the sense of the tests above (returns true

if condition absent).

Example 7-4. Testing for broken links

!/bin/bash

broken-link.sh

Written by Lee bigelow <ligelowbee@yahoo.com>

Used in ABS Guide with permission.

A pure shell script to find dead symlinks and output them quoted

+ so they can be fed to xargs and dealt with :)

+ eg. sh broken-link.sh /somedir /someotherdir|xargs rm

This, however, is a better method:

find "somedir" -type l -print0|\

xargs -r0 file|\

grep "broken symbolic"|

sed -e 's/^\|: broken symbolic.$/"/g'

+ but that wouldn't be pure Bash, now would it.

Caution: beware the /proc file system and any circular links!

If no args are passed to the script set directories-to-search

+ to current directory. Otherwise set the directories-to-search

+ to the args passed.

[ $# -eq 0 ] && directorys=`pwd` || directorys=$@

Setup the function linkchk to check the directory it is passed

+ for files that are links and don't exist, then print them quoted.

If one of the elements in the directory is a subdirectory then

+ send that subdirectory to the linkcheck function.

linkchk () {

for element in $1/*; do

[ -h "$element" -a ! -e "$element" ] && echo \"$element\"

[ -d "$element" ] && linkchk $element

# Of course, '-h' tests for symbolic link, '-d' for directory.

done

}

Send each arg that was passed to the script to the linkchk() function

+ if it is a valid directoy. If not, then print the error message

+ and usage info.

for directory in $directorys; do

if [ -d $directory ]

then linkchk $directory

else

echo "$directory is not a directory"

echo "Usage: $0 dir1 dir2 ..."

done

exit $?

Example 31-1, Example 11-8, Example 11-3, Example 31-3, and Example

A-1 also illustrate uses of the file test operators.

________________________________________________________________

7.3. Other Comparison Operators

A binary comparison operator compares two variables or quantities.

Note that integer and string comparison use a different set of

operators.

integer comparison

-eq

is equal to

if [ "$a" -eq "$b" ]

-ne

is not equal to

if [ "$a" -ne "$b" ]

-gt

is greater than

if [ "$a" -gt "$b" ]

-ge

is greater than or equal to

if [ "$a" -ge "$b" ]

-lt

is less than

if [ "$a" -lt "$b" ]

-le

is less than or equal to

if [ "$a" -le "$b" ]

is less than (within double parentheses)

(("$a" < "$b"))

is less than or equal to (within double parentheses)

(("$a" <= "$b"))

is greater than (within double parentheses)

(("$a" > "$b"))

is greater than or equal to (within double parentheses)

(("$a" >= "$b"))

string comparison

is equal to

if [ "$a" = "$b" ]

Caution

Note the whitespace framing the =.

if [ "$a"="$b" ] is not equivalent to the above.

is equal to

if [ "$a" == "$b" ]

This is a synonym for =.

Note

The == comparison operator behaves differently within a

double-brackets test than within single brackets.

[[ $a == z* ]] # True if $a starts with an "z" (pattern matching).

[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

[ $a == z* ] # File globbing and word splitting take place.

[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).

Thanks, Stéphane Chazelas

is not equal to

if [ "$a" != "$b" ]

This operator uses pattern matching within a [[ ... ]]

construct.

is less than, in ASCII alphabetical order

if [[ "$a" < "$b" ]]

if [ "$a" \< "$b" ]

Note that the "<" needs to be escaped within a [ ] construct.

is greater than, in ASCII alphabetical order

if [[ "$a" > "$b" ]]

if [ "$a" \> "$b" ]

Note that the ">" needs to be escaped within a [ ] construct.

See Example 27-11 for an application of this comparison

operator.

-z

string is null, that is, has zero length

String='' # Zero-length ("null") string variable.

if [ -z "$String" ]

then

echo "\$String is null."

else

echo "\$String is NOT null."

fi # $String is null.

-n

string is not null.

Caution

The -n test requires that the string be quoted within the test

brackets. Using an unquoted string with ! -z, or even just the

unquoted string alone within test brackets (see Example 7-6) normally

works, however, this is an unsafe practice. Always quote a tested

string. [37]

Example 7-5. Arithmetic and string comparisons

!/bin/bash

a=4

b=5

Here "a" and "b" can be treated either as integers or strings.

There is some blurring between the arithmetic and string comparisons,

+ since Bash variables are not strongly typed.

Bash permits integer operations and comparisons on variables

+ whose value consists of all-integer characters.

Caution advised, however.

echo

if [ "$a" -ne "$b" ]

then

echo "$a is not equal to $b"

echo "(arithmetic comparison)"

echo

if [ "$a" != "$b" ]

then

echo "$a is not equal to $b."

echo "(string comparison)"

# "4" != "5"

# ASCII 52 != ASCII 53

In this particular instance, both "-ne" and "!=" work.

echo

exit 0

Example 7-6. Testing whether a string is null

!/bin/bash

str-test.sh: Testing null strings and unquoted strings,

+ but not strings and sealing wax, not to mention cabbages and kings . . .

Using if [ ... ]

If a string has not been initialized, it has no defined value.

This state is called "null" (not the same as zero!).

if [ -n $string1 ] # string1 has not been declared or initialized.

then

echo "String \"string1\" is not null."

else

echo "String \"string1\" is null."

fi # Wrong result.

Shows $string1 as not null, although it was not initialized.

echo

Let's try it again.

if [ -n "$string1" ] # This time, $string1 is quoted.

then

echo "String \"string1\" is not null."

else

echo "String \"string1\" is null."

fi # Quote strings within test brackets!

echo

if [ $string1 ] # This time, $string1 stands naked.

then

echo "String \"string1\" is not null."

else

echo "String \"string1\" is null."

fi # This works fine.

The [ ... ] test operator alone detects whether the string is null.

However it is good practice to quote it (if [ "$string1" ]).

As Stephane Chazelas points out,

if [ $string1 ] has one argument, "]"

if [ "$string1" ] has two arguments, the empty "$string1" and "]"

echo

string1=initialized

if [ $string1 ] # Again, $string1 stands unquoted.

then

echo "String \"string1\" is not null."

else

echo "String \"string1\" is null."

fi # Again, gives correct result.

Still, it is better to quote it ("$string1"), because . . .

string1="a = b"

if [ $string1 ] # Again, $string1 stands unquoted.

then

echo "String \"string1\" is not null."

else

echo "String \"string1\" is null."

fi # Not quoting "$string1" now gives wrong result!

exit 0 # Thank you, also, Florian Wisser, for the "heads-up".

Example 7-7. zmore

!/bin/bash

zmore

View gzipped files with 'more' filter.

E_NOARGS=85

E_NOTFOUND=86

E_NOTGZIP=87

if [ $# -eq 0 ] # same effect as: if [ -z "$1" ]

$1 can exist, but be empty: zmore "" arg2 arg3

then

echo "Usage: `basename $0` filename" >&2

# Error message to stderr.

exit $E_NOARGS

# Returns 85 as exit status of script (error code).

filename=$1

if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces.

then

echo "File $filename not found!" >&2 # Error message to stderr.

exit $E_NOTFOUND

if [ ${filename##*.} != "gz" ]

Using bracket in variable substitution.

then

echo "File $1 is not a gzipped file!"

exit $E_NOTGZIP

zcat $1 | more

Uses the 'more' filter.

May substitute 'less' if desired.

exit $? # Script returns exit status of pipe.

Actually "exit $?" is unnecessary, as the script will, in any case,

+ return the exit status of the last command executed.

compound comparison

-a

logical and

exp1 -a exp2 returns true if both exp1 and exp2 are true.

-o

logical or

exp1 -o exp2 returns true if either exp1 or exp2 is true.

These are similar to the Bash comparison operators && and ||, used

within double brackets.

[[ condition1 && condition2 ]]

The -o and -a operators work with the test command or occur within

single test brackets.

if [ "$expr1" -a "$expr2" ]

then

echo "Both expr1 and expr2 are true."

else

echo "Either expr1 or expr2 is false."

Caution

But, as rihad points out:

[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ] # true

[ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ] # (no output)

^^^^^^^ False condition. So far, everything as expected.

However ...

[ 1 -eq 2 -a -n "`echo true 1>&2`" ] # true

^^^^^^^ False condition. So, why "true" output?

Is it because both condition clauses within brackets evaluate?

[[ 1 -eq 2 && -n "`echo true 1>&2`" ]] # (no output)

No, that's not it.

Apparently && and || "short-circuit" while -a and -o do not.

Refer to Example 8-3, Example 27-17, and Example A-29 to see compound

comparison operators in action.

________________________________________________________________

7.4. Nested if/then Condition Tests

Condition tests using the if/then construct may be nested. The net

result is equivalent to using the && compound comparison operator.

a=3

if [ "$a" -gt 0 ]

then

if [ "$a" -lt 5 ]

then

echo "The value of \"a\" lies somewhere between 0 and 5."

Same result as:

if [ "$a" -gt 0 ] && [ "$a" -lt 5 ]

then

echo "The value of \"a\" lies somewhere between 0 and 5."

Example 37-4 and Example 17-11 demonstrate nested if/then condition

tests.

________________________________________________________________

7.5. Testing Your Knowledge of Tests

The systemwide xinitrc file can be used to launch the X server. This

file contains quite a number of if/then tests. The following is

excerpted from an "ancient" version of xinitrc (Red Hat 7.1, or

thereabouts).

if [ -f $HOME/.Xclients ]; then

exec $HOME/.Xclients

elif [ -f /etc/X11/xinit/Xclients ]; then

exec /etc/X11/xinit/Xclients

else

# failsafe settings. Although we should never get here

# (we provide fallbacks in Xclients as well) it can't hurt.

xclock -geometry 100x100-5+5 &

xterm -geometry 80x50-50+150 &

if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then

netscape /usr/share/doc/HTML/index.html &

Explain the test constructs in the above snippet, then examine an

updated version of the file, /etc/X11/xinit/xinitrc, and analyze the

if/then test constructs there. You may need to refer ahead to the

discussions of grep, sed, and regular expressions.

________________________________________________________________

Chapter 8. Operations and Related Topics

8.1. Operators

assignment

variable assignment

Initializing or changing the value of a variable

All-purpose assignment operator, which works for both

arithmetic and string assignments.

var=27

category=minerals # No spaces allowed after the "=".

Caution

Do not confuse the "=" assignment operator with the = test operator.

= as a test operator

if [ "$string1" = "$string2" ]

then

command

if [ "X$string1" = "X$string2" ] is safer,

+ to prevent an error message should one of the variables be empty.

(The prepended "X" characters cancel out.)

arithmetic operators

plus

minus

multiplication

division

exponentiation

Bash, version 2.02, introduced the "**" exponentiation operator.

let "z=5**3" # 5 * 5 * 5

echo "z = $z" # z = 125

modulo, or mod (returns the remainder of an integer division

operation)

bash$ expr 5 % 3

5/3 = 1, with remainder 2

This operator finds use in, among other things, generating

numbers within a specific range (see Example 9-11 and Example

9-15) and formatting program output (see Example 27-16 and

Example A-6). It can even be used to generate prime numbers,

(see Example A-15). Modulo turns up surprisingly often in

numerical recipes.

Example 8-1. Greatest common divisor

!/bin/bash

gcd.sh: greatest common divisor

Uses Euclid's algorithm

The "greatest common divisor" (gcd) of two integers

+ is the largest integer that will divide both, leaving no remainder.

Euclid's algorithm uses successive division.

In each pass,

+ dividend <--- divisor

+ divisor <--- remainder

+ until remainder = 0.

The gcd = dividend, on the final pass.

For an excellent discussion of Euclid's algorithm, see

+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm.

------------------------------------------------------

Argument check

ARGS=2

E_BADARGS=85

if [ $# -ne "$ARGS" ]

then

echo "Usage: `basename $0` first-number second-number"

exit $E_BADARGS

------------------------------------------------------

gcd ()

{

dividend=$1 # Arbitrary assignment.

divisor=$2 #! It doesn't matter which of the two is larger.

# Why not?

remainder=1 # If an uninitialized variable is used inside

#+ test brackets, an error message results.

until [ "$remainder" -eq 0 ]

do # ^^^^^^^^^^ Must be previously initialized!

let "remainder = $dividend % $divisor"

dividend=$divisor # Now repeat with 2 smallest numbers.

divisor=$remainder

done # Euclid's algorithm

} # Last $dividend is the gcd.

gcd $1 $2

echo; echo "GCD of $1 and $2 = $dividend"; echo

Exercises :

---------

1) Check command-line arguments to make sure they are integers,

+ and exit the script with an appropriate error message if not.

2) Rewrite the gcd () function to use local variables.

exit 0

plus-equal (increment variable by a constant) [38]

let "var += 5" results in var being incremented by 5.

minus-equal (decrement variable by a constant)

times-equal (multiply variable by a constant)

let "var *= 4" results in var being multiplied by 4.

slash-equal (divide variable by a constant)

mod-equal (remainder of dividing variable by a constant)

Arithmetic operators often occur in an expr or let expression.

Example 8-2. Using Arithmetic Operations

!/bin/bash

Counting to 11 in 10 different ways.

n=1; echo -n "$n "

let "n = $n + 1" # let "n = n + 1" also works.

echo -n "$n "

: $((n = $n + 1))

":" necessary because otherwise Bash attempts

+ to interpret "$((n = $n + 1))" as a command.

echo -n "$n "

(( n = n + 1 ))

A simpler alternative to the method above.

Thanks, David Lombard, for pointing this out.

echo -n "$n "

n=$(($n + 1))

echo -n "$n "

: $[ n = $n + 1 ]

":" necessary because otherwise Bash attempts

+ to interpret "$[ n = $n + 1 ]" as a command.

Works even if "n" was initialized as a string.

echo -n "$n "

n=$[ $n + 1 ]

Works even if "n" was initialized as a string.

* Avoid this type of construct, since it is obsolete and nonportable.

Thanks, Stephane Chazelas.

echo -n "$n "

Now for C-style increment operators.

Thanks, Frank Wang, for pointing this out.

let "n++" # let "++n" also works.

echo -n "$n "

(( n++ )) # (( ++n )) also works.

echo -n "$n "

: $(( n++ )) # : $(( ++n )) also works.

echo -n "$n "

: $[ n++ ] # : $[ ++n ] also works

echo -n "$n "

echo

exit 0

Note

Integer variables in older versions of Bash were signed long (32-bit)

integers, in the range of -2147483648 to 2147483647. An operation

that took a variable outside these limits gave an erroneous result.

echo $BASH_VERSION # 1.14

a=2147483646

echo "a = $a" # a = 2147483646

let "a+=1" # Increment "a".

echo "a = $a" # a = 2147483647

let "a+=1" # increment "a" again, past the limit.

echo "a = $a" # a = -2147483648

# ERROR: out of range,

# + and the leftmost bit, the sign bit,

# + has been set, making the result negative.

As of version >= 2.05b, Bash supports 64-bit integers.

Caution

Bash does not understand floating point arithmetic. It treats numbers

containing a decimal point as strings.

a=1.5

let "b = $a + 1.3" # Error.

t2.sh: let: b = 1.5 + 1.3: syntax error in expression

(error token is ".5 + 1.3")

echo "b = $b" # b=1

Use bc in scripts that that need floating point calculations or math

library functions.

bitwise operators. The bitwise operators seldom make an appearance in

shell scripts. Their chief use seems to be manipulating and testing

values read from ports or sockets. "Bit flipping" is more relevant to

compiled languages, such as C and C++, which provide direct access to

system hardware. However, see vladz's ingenious use of bitwise

operators in his base64.sh (Example A-54) script.

bitwise operators

bitwise left shift (multiplies by 2 for each shift position)

<<=

left-shift-equal

let "var <<= 2" results in var left-shifted 2 bits (multiplied

by 4)

bitwise right shift (divides by 2 for each shift position)

>>=

right-shift-equal (inverse of <<=)

bitwise AND

bitwise AND-equal

bitwise OR

bitwise OR-equal

bitwise NOT

bitwise XOR

bitwise XOR-equal

logical (boolean) operators

NOT

if [ ! -f $FILENAME ]

then

...

AND

if [ $condition1 ] && [ $condition2 ]

Same as: if [ $condition1 -a $condition2 ]

Returns true if both condition1 and condition2 hold true...

if [[ $condition1 && $condition2 ]] # Also works.

Note that && operator not permitted inside brackets

+ of [ ... ] construct.

Note

&& may also be used, depending on context, in an and list to

concatenate commands.

if [ $condition1 ] || [ $condition2 ]

Same as: if [ $condition1 -o $condition2 ]

Returns true if either condition1 or condition2 holds true...

if [[ $condition1 || $condition2 ]] # Also works.

Note that || operator not permitted inside brackets

+ of a [ ... ] construct.

Note

Bash tests the exit status of each statement linked with a logical

operator.

Example 8-3. Compound Condition Tests Using && and ||

!/bin/bash

a=24

b=47

if [ "$a" -eq 24 ] && [ "$b" -eq 47 ]

then

echo "Test #1 succeeds."

else

echo "Test #1 fails."

ERROR: if [ "$a" -eq 24 && "$b" -eq 47 ]

+ attempts to execute ' [ "$a" -eq 24 '

+ and fails to finding matching ']'.

Note: if [[ $a -eq 24 && $b -eq 24 ]] works.

The double-bracket if-test is more flexible

+ than the single-bracket version.

(The "&&" has a different meaning in line 17 than in line 6.)

Thanks, Stephane Chazelas, for pointing this out.

if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]

then

echo "Test #2 succeeds."

else

echo "Test #2 fails."

The -a and -o options provide

+ an alternative compound condition test.

Thanks to Patrick Callahan for pointing this out.

if [ "$a" -eq 24 -a "$b" -eq 47 ]

then

echo "Test #3 succeeds."

else

echo "Test #3 fails."

if [ "$a" -eq 98 -o "$b" -eq 47 ]

then

echo "Test #4 succeeds."

else

echo "Test #4 fails."

a=rhino

b=crocodile

if [ "$a" = rhino ] && [ "$b" = crocodile ]

then

echo "Test #5 succeeds."

else

echo "Test #5 fails."

exit 0

The && and || operators also find use in an arithmetic

context.

bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))

1 0 1 0

miscellaneous operators

Comma operator

The comma operator chains together two or more arithmetic

operations. All the operations are evaluated (with possible

side effects. [39]

let "t1 = ((5 + 3, 7 - 1, 15 - 4))"

echo "t1 = $t1" ^^^^^^ # t1 = 11

Here t1 is set to the result of the last operation. Why?

let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2".

echo "t2 = $t2 a = $a" # t2 = 5 a = 9

The comma operator finds use mainly in for loops. See Example

11-13.

________________________________________________________________

8.2. Numerical Constants

A shell script interprets a number as decimal (base 10), unless that

number has a special prefix or notation. A number preceded by a 0 is

octal (base 8). A number preceded by 0x is hexadecimal (base 16). A

number with an embedded # evaluates as BASE#NUMBER (with range and

notational restrictions).

Example 8-4. Representation of numerical constants

!/bin/bash

numbers.sh: Representation of numbers in different bases.

Decimal: the default

let "dec = 32"

echo "decimal number = $dec" # 32

Nothing out of the ordinary here.

Octal: numbers preceded by '0' (zero)

let "oct = 032"

echo "octal number = $oct" # 26

Expresses result in decimal.

--------- ------ -- -------

Hexadecimal: numbers preceded by '0x' or '0X'

let "hex = 0x32"

echo "hexadecimal number = $hex" # 50

echo $((0x9abc)) # 39612

^^ ^^ double-parentheses arithmetic expansion/evaluation

Expresses result in decimal.

Other bases: BASE#NUMBER

BASE between 2 and 64.

NUMBER must use symbols within the BASE range, see below.

let "bin = 2#111100111001101"

echo "binary number = $bin" # 31181

let "b32 = 32#77"

echo "base-32 number = $b32" # 231

let "b64 = 64#@_"

echo "base-64 number = $b64" # 4031

This notation only works for a limited range (2 - 64) of ASCII characters.

10 digits + 26 lowercase characters + 26 uppercase characters + @ + _

echo

echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))

# 1295 170 44822 3375

Important note:

--------------

Using a digit out of range of the specified base notation

+ gives an error message.

let "bad_oct = 081"

(Partial) error message output:

bad_oct = 081: value too great for base (error token is "081")

Octal numbers use only digits in the range 0 - 7.

exit $? # Exit value = 1 (error)

Thanks, Rich Bartell and Stephane Chazelas, for clarification.

________________________________________________________________

8.3. The Double-Parentheses Construct

Similar to the let command, the (( ... )) construct permits

arithmetic expansion and evaluation. In its simplest form, a=$(( 5 +

3 )) would set a to 5 + 3, or 8. However, this double-parentheses

construct is also a mechanism for allowing C-style manipulation of

variables in Bash, for example, (( var++ )).

Example 8-5. C-style manipulation of variables

!/bin/bash

c-vars.sh

Manipulating a variable, C-style, using the (( ... )) construct.

echo

(( a = 23 )) # Setting a value, C-style,

#+ with spaces on both sides of the "=".

echo "a (initial value) = $a" # 23

(( a++ )) # Post-increment 'a', C-style.

echo "a (after a++) = $a" # 24

(( a-- )) # Post-decrement 'a', C-style.

echo "a (after a--) = $a" # 23

(( ++a )) # Pre-increment 'a', C-style.

echo "a (after ++a) = $a" # 24

(( --a )) # Pre-decrement 'a', C-style.

echo "a (after --a) = $a" # 23

echo

Note that, as in C, pre- and post-decrement operators

+ have different side-effects.

n=1; let --n && echo "True" || echo "False" # False

n=1; let n-- && echo "True" || echo "False" # True

Thanks, Jeroen Domburg.

echo

(( t = a<45?7:11 )) # C-style trinary operator.

^ ^ ^

echo "If a < 45, then t = 7, else t = 11." # a = 23

echo "t = $t " # t = 7

echo

-----------------

Easter Egg alert!

-----------------

Chet Ramey seems to have snuck a bunch of undocumented C-style

+ constructs into Bash (actually adapted from ksh, pretty much).

In the Bash docs, Ramey calls (( ... )) shell arithmetic,

+ but it goes far beyond that.

Sorry, Chet, the secret is out.

These work only with version 2.04 or later of Bash.

exit

This looks rather daunting at first glance.

Separate the conditions:

while [ -n "$remaining" -a "$retry" -gt 0 ]; do

--condition 1-- ^^ --condition 2-

If variable "$remaining" is not zero length

+ AND (-a)

+ variable "$retry" is greater-than zero

+ then

+ the [ expresion-within-condition-brackets ] returns success (0)

+ and the while-loop executes an iteration.

==============================================================

Evaluate "condition 1" and "condition 2" before

+ ANDing them. Why? Because the AND (-a) has a lower precedence

+ than the -n and -gt operators,

+ and therefore gets evaluated last.

if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then

Again, separate the conditions:

if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then

--condition 1--------- ^^ --condition 2-----

If file "/etc/sysconfig/i18n" exists

+ AND (-a)

+ variable $NOLOCALE is zero length

+ then

+ the [ test-expresion-within-condition-brackets ] returns success (0)

+ and the commands following execute.

As before, the AND (-a) gets evaluated last

+ because it has the lowest precedence of the operators within

+ the test brackets.

==============================================================

Note:

${NOLOCALE:-} is a parameter expansion that seems redundant.

But, if $NOLOCALE has not been declared, it gets set to null,

+ in effect declaring it.

This makes a difference in some contexts.

Tip

To avoid confusion or error in a complex sequence of test operators,

break up the sequence into bracketed sections.

if [ "$v1" -gt "$v2" -o "$v1" -lt "$v2" -a -e "$filename" ]

Unclear what's going on here...

if [[ "$v1" -gt "$v2" ]] || [[ "$v1" -lt "$v2" ]] && [[ -e "$filename" ]]

Much better -- the condition tests are grouped in logical sections.

Part 3. Beyond the Basics

Table of Contents

9. Another Look at Variables

9.1. Internal Variables

9.2. Typing variables: declare or typeset

9.3. $RANDOM: generate random integer

10. Manipulating Variables

10.1. Manipulating Strings

10.2. Parameter Substitution

11. Loops and Branches

11.1. Loops

11.2. Nested Loops

11.3. Loop Control

11.4. Testing and Branching

12. Command Substitution

13. Arithmetic Expansion

14. Recess Time

________________________________________________________________

Chapter 9. Another Look at Variables

Used properly, variables can add power and flexibility to scripts.

This requires learning their subtleties and nuances.

________________________________________________________________

9.1. Internal Variables

Builtin variables:

variables affecting bash script behavior

$BASH

The path to the Bash binary itself

bash$ echo $BASH

/bin/bash

$BASH_ENV

An environmental variable pointing to a Bash startup file to

be read when a script is invoked

$BASH_SUBSHELL

A variable indicating the subshell level. This is a new

addition to Bash, version 3.

See Example 21-1 for usage.

$BASHPID

Process ID of the current instance of Bash. This is not the

same as the $ variable, but it often gives the same result.

bash4$ echo $

11015

bash4$ echo $BASHPID

11015

bash4$ ps ax | grep bash4

11015 pts/2 R 0:00 bash4

But ...

!/bin/bash4

echo "\$\$ outside of subshell = $" # 9602

echo "\$BASH_SUBSHELL outside of subshell = $BASH_SUBSHELL" # 0

echo "\$BASHPID outside of subshell = $BASHPID" # 9602

echo

( echo "\$\$ inside of subshell = $" # 9602

echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL" # 1

echo "\$BASHPID inside of subshell = $BASHPID" ) # 9603

# Note that $ returns PID of parent process.

$BASH_VERSINFO[n]

A 6-element array containing version information about the

installed release of Bash. This is similar to $BASH_VERSION,

below, but a bit more detailed.

Bash version info:

for n in 0 1 2 3 4 5

echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"

done

BASH_VERSINFO[0] = 3 # Major version no.

BASH_VERSINFO[1] = 00 # Minor version no.

BASH_VERSINFO[2] = 14 # Patch level.

BASH_VERSINFO[3] = 1 # Build version.

BASH_VERSINFO[4] = release # Release status.

BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture

# (same as $MACHTYPE).

$BASH_VERSION

The version of Bash installed on the system

bash$ echo $BASH_VERSION

3.2.25(1)-release

tcsh% echo $BASH_VERSION

BASH_VERSION: Undefined variable.

Checking $BASH_VERSION is a good method of determining which

shell is running. $SHELL does not necessarily give the correct

answer.

$CDPATH

A colon-separated list of search paths available to the cd

command, similar in function to the $PATH variable for

binaries. The $CDPATH variable may be set in the local

~/.bashrc file.

bash$ cd bash-doc

bash: cd: bash-doc: No such file or directory

bash$ CDPATH=/usr/share/doc

bash$ cd bash-doc

/usr/share/doc/bash-doc

bash$ echo $PWD

/usr/share/doc/bash-doc

$DIRSTACK

The top value in the directory stack [41] (affected by pushd

and popd)

This builtin variable corresponds to the dirs command, however

dirs shows the entire contents of the directory stack.

$EDITOR

The default editor invoked by a script, usually vi or emacs.

$EUID

"effective" user ID number

Identification number of whatever identity the current user

has assumed, perhaps by means of su.

Caution

The $EUID is not necessarily the same as the $UID.

$FUNCNAME

Name of the current function

xyz23 ()

{

echo "$FUNCNAME now executing." # xyz23 now executing.

}

xyz23

echo "FUNCNAME = $FUNCNAME" # FUNCNAME =

# Null value outside a function.

!/bin/bash

ifs.sh

var1="a+b+c"

var2="d-e-f"

var3="g,h,i"

IFS=+

The plus sign will be interpreted as a separator.

echo $var1 # a b c

echo $var2 # d-e-f

echo $var3 # g,h,i

echo

IFS="-"

The plus sign reverts to default interpretation.

The minus sign will be interpreted as a separator.

echo $var1 # a+b+c

echo $var2 # d e f

echo $var3 # g,h,i

echo

IFS=","

The comma will be interpreted as a separator.

The minus sign reverts to default interpretation.

echo $var1 # a+b+c

echo $var2 # d-e-f

echo $var3 # g h i

echo

IFS=" "

The space character will be interpreted as a separator.

The comma reverts to default interpretation.

echo $var1 # a+b+c

echo $var2 # d-e-f

echo $var3 # g,h,i

======================================================== #

However ...

$IFS treats whitespace differently than other characters.

output_args_one_per_line()

{

for arg

echo "[$arg]"

done # ^ ^ Embed within brackets, for your viewing pleasure.

}

echo; echo "IFS=\" \""

echo "-------"

IFS=" "

var=" a b c "

^ ^^ ^^^

output_args_one_per_line $var # output_args_one_per_line `echo " a b c "`

[a]

[b]

[c]

echo; echo "IFS=:"

echo "-----"

IFS=:

var=":a::b:c:::" # Same pattern as above,

^ ^^ ^^^ #+ but substituting ":" for " " ...

output_args_one_per_line $var

[]

[a]

[]

[b]

[c]

[]

Note "empty" brackets.

The same thing happens with the "FS" field separator in awk.

echo

exit

(Many thanks, Stéphane Chazelas, for clarification and above

examples.)

See also Example 16-41, Example 11-8, and Example 19-14 for

instructive examples of using $IFS.

$IGNOREEOF

Ignore EOF: how many end-of-files (control-D) the shell will

ignore before logging out.

$LC_COLLATE

Often set in the .bashrc or /etc/profile files, this variable

controls collation order in filename expansion and pattern

matching. If mishandled, LC_COLLATE can cause unexpected

results in filename globbing.

Note

As of version 2.05 of Bash, filename globbing no longer distinguishes

between lowercase and uppercase letters in a character range between

brackets. For example, ls [A-M]* would match both File1.txt and

file1.txt. To revert to the customary behavior of bracket matching,

set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or

~/.bashrc.

$LC_CTYPE

This internal variable controls character interpretation in

globbing and pattern matching.

$LINENO

This variable is the line number of the shell script in which

this variable appears. It has significance only within the

script in which it appears, and is chiefly useful for

debugging purposes.

* BEGIN DEBUG BLOCK *

last_cmd_arg=$_ # Save it.

echo "At line number $LINENO, variable \"v1\" = $v1"

echo "Last command argument processed = $last_cmd_arg"

* END DEBUG BLOCK *

$MACHTYPE

machine type

Identifies the system hardware.

bash$ echo $MACHTYPE

i686

$OLDPWD

Old working directory ("OLD-Print-Working-Directory", previous

directory you were in).

$OSTYPE

operating system type

bash$ echo $OSTYPE

linux

$PATH

Path to binaries, usually /usr/bin/, /usr/X11R6/bin/,

/usr/local/bin, etc.

When given a command, the shell automatically does a hash

table search on the directories listed in the path for the

executable. The path is stored in the environmental variable,

$PATH, a list of directories, separated by colons. Normally,

the system stores the $PATH definition in /etc/profile and/or

~/.bashrc (see Appendix H).

bash$ echo $PATH

/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin

PATH=${PATH}:/opt/bin appends the /opt/bin directory to the

current path. In a script, it may be expedient to temporarily

add a directory to the path in this way. When the script

exits, this restores the original $PATH (a child process, such

as a script, may not change the environment of the parent

process, the shell).

Note

The current "working directory", ./, is usually omitted from the

$PATH as a security measure.

$PIPESTATUS

Array variable holding exit status(es) of last executed

foreground pipe.

bash$ echo $PIPESTATUS

bash$ ls -al | bogus_command

bash: bogus_command: command not found

bash$ echo ${PIPESTATUS[1]}

127

bash$ ls -al | bogus_command

bash: bogus_command: command not found

bash$ echo $?

127

The members of the $PIPESTATUS array hold the exit status of

each respective command executed in a pipe. $PIPESTATUS[0]

holds the exit status of the first command in the pipe,

$PIPESTATUS[1] the exit status of the second command, and so

on.

Caution

The $PIPESTATUS variable may contain an erroneous 0 value in a login

shell (in releases prior to 3.0 of Bash).

tcsh% bash

bash$ who | grep nobody | sort

bash$ echo ${PIPESTATUS[*]}

The above lines contained in a script would produce the expected 0 1

0 output.

Thank you, Wayne Pollock for pointing this out and supplying the

above example.

Note

The $PIPESTATUS variable gives unexpected results in some contexts.

bash$ echo $BASH_VERSION

3.00.14(1)-release

bash$ $ ls | bogus_command | wc

bash: bogus_command: command not found

0 0 0

bash$ echo ${PIPESTATUS[@]}

141 127 0

Chet Ramey attributes the above output to the behavior of ls. If ls

writes to a pipe whose output is not read, then SIGPIPE kills it, and

its exit status is 141. Otherwise its exit status is 0, as expected.

This likewise is the case for tr.

Note

$PIPESTATUS is a "volatile" variable. It needs to be captured

immediately after the pipe in question, before any other command

intervenes.

bash$ $ ls | bogus_command | wc

bash: bogus_command: command not found

0 0 0

bash$ echo ${PIPESTATUS[@]}

0 127 0

bash$ echo ${PIPESTATUS[@]}

Note

The pipefail option may be useful in cases where $PIPESTATUS does not

give the desired information.

$PPID

The $PPID of a process is the process ID (pid) of its parent

process. [42]

Compare this with the pidof command.

$PROMPT_COMMAND

A variable holding a command to be executed just before the

primary prompt, $PS1 is to be displayed.

$PS1

This is the main prompt, seen at the command-line.

$PS2

The secondary prompt, seen when additional input is expected.

It displays as ">".

$PS3

The tertiary prompt, displayed in a select loop (see Example

11-30).

$PS4

The quartenary prompt, shown at the beginning of each line of

output when invoking a script with the -x [verbose trace]

option. It displays as "+".

As a debugging aid, it may be useful to embed diagnostic

information in $PS4.

P4='$(read time junk < /proc/$/schedstat; echo "@@@ $time @@@ " )'

Per suggestion by Erik Brandsberg.

set -x

Various commands follow ...

$PWD

Working directory (directory you are in at the time)

This is the analog to the pwd builtin command.

!/bin/bash

E_WRONG_DIRECTORY=85

clear # Clear the screen.

TargetDirectory=/home/bozo/projects/GreatAmericanNovel

cd $TargetDirectory

echo "Deleting stale files in $TargetDirectory."

if [ "$PWD" != "$TargetDirectory" ]

then # Keep from wiping out wrong directory by accident.

echo "Wrong directory!"

echo "In $PWD, rather than $TargetDirectory!"

echo "Bailing out!"

exit $E_WRONG_DIRECTORY

rm -rf *

rm .[A-Za-z0-9]* # Delete dotfiles.

rm -f .[^.]* ..?* to remove filenames beginning with multiple dots.

(shopt -s dotglob; rm -f *) will also work.

Thanks, S.C. for pointing this out.

A filename (`basename`) may contain all characters in the 0 - 255 range,

+ except "/".

Deleting files beginning with weird characters, such as -

+ is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname)

result=$? # Result of delete operations. If successful = 0.

echo

ls -al # Any files left?

echo "Done."

echo "Old files deleted in $TargetDirectory."

echo

Various other operations here, as necessary.

exit $result

$REPLY

The default value when a variable is not supplied to read.

Also applicable to select menus, but only supplies the item

number of the variable chosen, not the value of the variable

itself.

!/bin/bash

reply.sh

REPLY is the default value for a 'read' command.

echo

echo -n "What is your favorite vegetable? "

read

echo "Your favorite vegetable is $REPLY."

REPLY holds the value of last "read" if and only if

+ no variable supplied.

echo

echo -n "What is your favorite fruit? "

read fruit

echo "Your favorite fruit is $fruit."

echo "but..."

echo "Value of \$REPLY is still $REPLY."

$REPLY is still set to its previous value because

+ the variable $fruit absorbed the new "read" value.

echo

exit 0

$SECONDS

The number of seconds the script has been running.

!/bin/bash

TIME_LIMIT=10

INTERVAL=1

echo

echo "Hit Control-C to exit before $TIME_LIMIT seconds."

echo

while [ "$SECONDS" -le "$TIME_LIMIT" ]

do # $SECONDS is an internal shell variable.

if [ "$SECONDS" -eq 1 ]

then

units=second

else

units=seconds

echo "This script has been running $SECONDS $units."

# On a slow or overburdened machine, the script may skip a count

#+ every once in a while.

sleep $INTERVAL

done

echo -e "\a" # Beep!

exit 0

$SHELLOPTS

The list of enabled shell options, a readonly variable.

bash$ echo $SHELLOPTS

braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs

$SHLVL

Shell level, how deeply Bash is nested. [43] If, at the

command-line, $SHLVL is 1, then in a script it will increment

to 2.

Note

This variable is not affected by subshells. Use $BASH_SUBSHELL when

you need an indication of subshell nesting.

$TMOUT

If the $TMOUT environmental variable is set to a non-zero

value time, then the shell prompt will time out after $time

seconds. This will cause a logout.

As of version 2.05b of Bash, it is now possible to use $TMOUT

in a script in combination with read.

Works in scripts for Bash, versions 2.05b and later.

TMOUT=3 # Prompt times out at three seconds.

echo "What is your favorite song?"

echo "Quickly now, you only have $TMOUT seconds to answer!"

read song

if [ -z "$song" ]

then

song="(no answer)"

# Default response.

echo "Your favorite song is $song."

There are other, more complex, ways of implementing timed

input in a script. One alternative is to set up a timing loop

to signal the script when it times out. This also requires a

signal handling routine to trap (see Example 32-5) the

interrupt generated by the timing loop (whew!).

Example 9-2. Timed Input

!/bin/bash

timed-input.sh

TMOUT=3 Also works, as of newer versions of Bash.

TIMER_INTERRUPT=14

TIMELIMIT=3 # Three seconds in this instance.

# May be set to different value.

PrintAnswer()

{

if [ "$answer" = TIMEOUT ]

then

echo $answer

else # Don't want to mix up the two instances.

echo "Your favorite veggie is $answer"

kill $! # Kills no-longer-needed TimerOn function

#+ running in background.

# $! is PID of last job running in background.

}

TimerOn()

{

sleep $TIMELIMIT && kill -s 14 $ &

# Waits 3 seconds, then sends sigalarm to script.

}

Int14Vector()

{

answer="TIMEOUT"

PrintAnswer

exit $TIMER_INTERRUPT

}

trap Int14Vector $TIMER_INTERRUPT

Timer interrupt (14) subverted for our purposes.

echo "What is your favorite vegetable "

TimerOn

read answer

PrintAnswer

Admittedly, this is a kludgy implementation of timed input.

However, the "-t" option to "read" simplifies this task.

See the "t-out.sh" script.

However, what about timing not just single user input,

+ but an entire script?

If you need something really elegant ...

+ consider writing the application in C or C++,

+ using appropriate library functions, such as 'alarm' and 'setitimer.'

exit 0

An alternative is using stty.

Example 9-3. Once more, timed input

!/bin/bash

timeout.sh

Written by Stephane Chazelas,

+ and modified by the document author.

INTERVAL=5 # timeout interval

timedout_read() {

timeout=$1

varname=$2

old_tty_settings=`stty -g`

stty -icanon min 0 time ${timeout}0

eval read $varname # or just read $varname

stty "$old_tty_settings"

# See man page for "stty."

}

echo; echo -n "What's your name? Quick! "

timedout_read $INTERVAL your_name

This may not work on every terminal type.

The maximum timeout depends on the terminal.

+ (it is often 25.5 seconds).

echo

if [ ! -z "$your_name" ] # If name input before timeout ...

then

echo "Your name is $your_name."

else

echo "Timed out."

echo

The behavior of this script differs somewhat from "timed-input.sh."

At each keystroke, the counter resets.

exit 0

Perhaps the simplest method is using the -t option to read.

Example 9-4. Timed read

!/bin/bash

t-out.sh [time-out]

Inspired by a suggestion from "syngin seven" (thanks).

TIMELIMIT=4 # 4 seconds

read -t $TIMELIMIT variable <&1

^^^

In this instance, "<&1" is needed for Bash 1.x and 2.x,

but unnecessary for Bash 3+.

echo

if [ -z "$variable" ] # Is null?

then

echo "Timed out, variable still unset."

else

echo "variable = $variable"

exit 0

$UID

User ID number

Current user's user identification number, as recorded in

/etc/passwd

This is the current user's real id, even if she has

temporarily assumed another identity through su. $UID is a

readonly variable, not subject to change from the command line

or within a script, and is the counterpart to the id builtin.

Example 9-5. Am I root?

!/bin/bash

am-i-root.sh: Am I root or not?

ROOT_UID=0 # Root has $UID 0.

if [ "$UID" -eq "$ROOT_UID" ] # Will the real "root" please stand up?

then

echo "You are root."

else

echo "You are just an ordinary user (but mom loves you just the same)."

exit 0

============================================================= #

Code below will not execute, because the script already exited.

An alternate method of getting to the root of matters:

ROOTUSER_NAME=root

username=`id -nu` # Or... username=`whoami`

if [ "$username" = "$ROOTUSER_NAME" ]

then

echo "Rooty, toot, toot. You are root."

else

echo "You are just a regular fella."

!/bin/bash

arglist.sh

Invoke this script with several arguments, such as "one two three" ...

E_BADARGS=85

if [ ! -n "$1" ]

then

echo "Usage: `basename $0` argument1 argument2 etc."

exit $E_BADARGS

echo

index=1 # Initialize count.

echo "Listing args with \"\$*\":"

for arg in "$*" # Doesn't work properly if "$*" isn't quoted.

echo "Arg #$index = $arg"

let "index+=1"

done # $* sees all arguments as single word.

echo "Entire arg list seen as single word."

echo

index=1 # Reset count.

# What happens if you forget to do this?

echo "Listing args with \"\$@\":"

for arg in "$@"

echo "Arg #$index = $arg"

let "index+=1"

done # $@ sees arguments as separate words.

echo "Arg list seen as separate words."

echo

index=1 # Reset count.

echo "Listing args with \$* (unquoted):"

for arg in $*

echo "Arg #$index = $arg"

let "index+=1"

done # Unquoted $* sees arguments as separate words.

echo "Arg list seen as separate words."

exit 0

Following a shift, the $@ holds the remaining command-line

parameters, lacking the previous $1, which was lost.

!/bin/bash

Invoke with ./scriptname 1 2 3 4 5

echo "$@" # 1 2 3 4 5

shift

echo "$@" # 2 3 4 5

shift

echo "$@" # 3 4 5

Each "shift" loses parameter $1.

"$@" then contains the remaining parameters.

The $@ special parameter finds use as a tool for filtering

input into shell scripts. The cat "$@" construction accepts

input to a script either from stdin or from files given as

parameters to the script. See Example 16-24 and Example 16-25.

Caution

The $* and $@ parameters sometimes display inconsistent and puzzling

behavior, depending on the setting of $IFS.

Example 9-7. Inconsistent $* and $@ behavior

!/bin/bash

Erratic behavior of the "$*" and "$@" internal Bash variables,

+ depending on whether or not they are quoted.

Demonstrates inconsistent handling of word splitting and linefeeds.

set -- "First one" "second" "third:one" "" "Fifth: :one"

Setting the script arguments, $1, $2, $3, etc.

echo

echo 'IFS unchanged, using "$*"'

c=0

for i in "$*" # quoted

do echo "$((c+=1)): [$i]" # This line remains the same in every instance.

# Echo args.

done

echo ---

echo 'IFS unchanged, using $*'

c=0

for i in $* # unquoted

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS unchanged, using "$@"'

c=0

for i in "$@"

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS unchanged, using $@'

c=0

for i in $@

do echo "$((c+=1)): [$i]"

done

echo ---

IFS=:

echo 'IFS=":", using "$*"'

c=0

for i in "$*"

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using $*'

c=0

for i in $*

do echo "$((c+=1)): [$i]"

done

echo ---

var=$*

echo 'IFS=":", using "$var" (var=$*)'

c=0

for i in "$var"

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using $var (var=$*)'

c=0

for i in $var

do echo "$((c+=1)): [$i]"

done

echo ---

var="$*"

echo 'IFS=":", using $var (var="$*")'

c=0

for i in $var

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using "$var" (var="$*")'

c=0

for i in "$var"

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using "$@"'

c=0

for i in "$@"

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using $@'

c=0

for i in $@

do echo "$((c+=1)): [$i]"

done

echo ---

var=$@

echo 'IFS=":", using $var (var=$@)'

c=0

for i in $var

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using "$var" (var=$@)'

c=0

for i in "$var"

do echo "$((c+=1)): [$i]"

done

echo ---

var="$@"

echo 'IFS=":", using "$var" (var="$@")'

c=0

for i in "$var"

do echo "$((c+=1)): [$i]"

done

echo ---

echo 'IFS=":", using $var (var="$@")'

c=0

for i in $var

do echo "$((c+=1)): [$i]"

done

echo

Try this script with ksh or zsh -y.

exit 0

This example script written by Stephane Chazelas,

+ and slightly modified by the document author.

Note

The $@ and $* parameters differ only when between double quotes.

Example 9-8. $* and $@ when $IFS is empty

!/bin/bash

If $IFS set, but empty,

+ then "$*" and "$@" do not echo positional params as expected.

mecho () # Echo positional parameters.

{

echo "$1,$2,$3";

}

IFS="" # Set, but empty.

set a b c # Positional parameters.

mecho "$*" # abc,,

^^

mecho $* # a,b,c

mecho $@ # a,b,c

mecho "$@" # a,b,c

The behavior of $* and $@ when $IFS is empty depends

+ on which Bash or sh version being run.

It is therefore inadvisable to depend on this "feature" in a script.

Thanks, Stephane Chazelas.

exit

Other Special Parameters

Flags passed to script (using set). See Example 15-16.

Caution

This was originally a ksh construct adopted into Bash, and

unfortunately it does not seem to work reliably in Bash scripts. One

possible use for it is to have a script self-test whether it is

interactive.

PID (process ID) of last job run in background

LOG=$0.log

COMMAND1="sleep 100"

echo "Logging PIDs background commands for script: $0" >> "$LOG"

So they can be monitored, and killed as necessary.

echo >> "$LOG"

Logging commands.

echo -n "PID of \"$COMMAND1\": " >> "$LOG"

${COMMAND1} &

echo $! >> "$LOG"

PID of "sleep 100": 1506

Thank you, Jacques Lederer, for suggesting this.

Using $! for job control:

possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }

Forces completion of an ill-behaved program.

Useful, for example, in init scripts.

Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.

Or, alternately:

This example by Matthew Sage.

Used with permission.

TIMEOUT=30 # Timeout value in seconds

count=0

possibly_hanging_job & {

while ((count < TIMEOUT )); do

eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))'

# /proc is where information about running processes is found.

# "-d" tests whether it exists (whether directory exists).

# So, we're waiting for the job in question to show up.

((count++))

sleep 1

done

eval '[ -d "/proc/$!" ] && kill -15 $!'

# If the hanging job is running, kill it.

}

-------------------------------------------------------------- #

However, this may not not work as specified if another process

+ begins to run after the "hanging_job" . . .

In such a case, the wrong job may be killed.

Ariel Meragelman suggests the following fix.

TIMEOUT=30

count=0

Timeout value in seconds

possibly_hanging_job & {

while ((count < TIMEOUT )); do

eval '[ ! -d "/proc/$lastjob" ] && ((count = TIMEOUT))'

lastjob=$!

((count++))

sleep 1

done

eval '[ -d "/proc/$lastjob" ] && kill -15 $lastjob'

}

exit

Special variable set to final argument of previous command

executed.

Example 9-9. Underscore variable

!/bin/bash

echo $_ # /bin/bash

# Just called /bin/bash to run the script.

# Note that this will vary according to

#+ how the script is invoked.

du >/dev/null # So no output from command.

echo $_ # du

ls -al >/dev/null # So no output from command.

echo $_ # -al (last argument)

echo $_ # :

Exit status of a command, function, or the script itself (see

Example 24-7)

Process ID (PID) of the script itself. [45] The $ variable

often finds use in scripts to construct "unique" temp file

names (see Example 32-6, Example 16-31, and Example 15-27).

This is usually simpler than invoking mktemp.

________________________________________________________________

9.2. Typing variables: declare or typeset

The declare or typeset builtins, which are exact synonyms, permit

modifying the properties of variables. This is a very weak form of

the typing [46] available in certain programming languages. The

declare command is specific to version 2 or later of Bash. The

typeset command also works in ksh scripts.

declare/typeset options

-r readonly

(declare -r var1 works the same as readonly var1)

This is the rough equivalent of the C const type qualifier. An

attempt to change the value of a readonly variable fails with

an error message.

declare -r var1=1

echo "var1 = $var1" # var1 = 1

(( var1++ )) # x.sh: line 4: var1: readonly variable

-i integer

declare -i number

The script will treat subsequent occurrences of "number" as an integer.

number=3

echo "Number = $number" # Number = 3

number=three

echo "Number = $number" # Number = 0

Tries to evaluate the string "three" as an integer.

Certain arithmetic operations are permitted for declared

integer variables without the need for expr or let.

n=6/3

echo "n = $n" # n = 6/3

declare -i n

n=6/3

echo "n = $n" # n = 2

-a array

declare -a indices

The variable indices will be treated as an array.

-f function(s)

declare -f

A declare -f line with no arguments in a script causes a

listing of all the functions previously defined in that

script.

declare -f function_name

A declare -f function_name in a script lists just the function

named.

-x export

declare -x var3

This declares a variable as available for exporting outside

the environment of the script itself.

-x var=$value

declare -x var3=373

The declare command permits assigning a value to a variable in

the same statement as setting its properties.

Example 9-10. Using declare to type variables

!/bin/bash

func1 ()

{

echo This is a function.

}

declare -f # Lists the function above.

echo

declare -i var1 # var1 is an integer.

var1=2367

echo "var1 declared as $var1"

var1=var1+1 # Integer declaration eliminates the need for 'let'.

echo "var1 incremented by 1 is $var1."

Attempt to change variable declared as integer.

echo "Attempting to change var1 to floating point value, 2367.1."

var1=2367.1 # Results in error message, with no change to variable.

echo "var1 is still $var1"

echo

declare -r var2=13.36 # 'declare' permits setting a variable property

#+ and simultaneously assigning it a value.

echo "var2 declared as $var2" # Attempt to change readonly variable.

var2=13.37 # Generates error message, and exit from script.

echo "var2 is still $var2" # This line will not execute.

exit 0 # Script will not exit here.

Caution

Using the declare builtin restricts the scope of a variable.

foo ()

{

FOO="bar"

}

bar ()

{

foo

echo $FOO

}

bar # Prints bar.

However . . .

foo (){

declare FOO="bar"

}

bar ()

{

foo

echo $FOO

}

bar # Prints nothing.

Thank you, Michael Iatrou, for pointing this out.

________________________________________________________________

9.2.1. Another use for declare

The declare command can be helpful in identifying variables,

environmental or otherwise. This can be especially useful with

arrays.

bash$ declare | grep HOME

HOME=/home/bozo

bash$ zzy=68

bash$ declare | grep zzy

zzy=68

bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green")

bash$ echo ${Colors[@]}

purple reddish-orange light green

bash$ declare | grep Colors

Colors=([0]="purple" [1]="reddish-orange" [2]="light green")

________________________________________________________________

9.3. $RANDOM: generate random integer

Anyone who attempts to generate random numbers by deterministic means

is, of course, living in a state of sin.

--John von Neumann

$RANDOM is an internal Bash function (not a constant) that returns a

pseudorandom [47] integer in the range 0 - 32767. It should not be

used to generate an encryption key.

Example 9-11. Generating random numbers

!/bin/bash

$RANDOM returns a different random integer at each invocation.

Nominal range: 0 - 32767 (signed 16-bit integer).

MAXCOUNT=10

count=1

echo

echo "$MAXCOUNT random numbers:"

echo "-----------------"

while [ "$count" -le $MAXCOUNT ] # Generate 10 ($MAXCOUNT) random integer

number=$RANDOM

echo $number

let "count += 1" # Increment count.

done

echo "-----------------"

If you need a random int within a certain range, use the 'modulo' operator.

This returns the remainder of a division operation.

RANGE=500

echo

number=$RANDOM

let "number %= $RANGE"

^^

echo "Random number less than $RANGE --- $number"

echo

If you need a random integer greater than a lower bound,

+ then set up a test to discard all numbers below that.

FLOOR=200

number=0 #initialize

while [ "$number" -le $FLOOR ]

number=$RANDOM

done

echo "Random number greater than $FLOOR --- $number"

echo

# Let's examine a simple alternative to the above loop, namely

# let "number = $RANDOM + $FLOOR"

# That would eliminate the while-loop and run faster.

# But, there might be a problem with that. What is it?

Combine above two techniques to retrieve random number between two limits.

number=0 #initialize

while [ "$number" -le $FLOOR ]

number=$RANDOM

let "number %= $RANGE" # Scales $number down within $RANGE.

done

echo "Random number between $FLOOR and $RANGE --- $number"

echo

Generate binary choice, that is, "true" or "false" value.

BINARY=2

T=1

number=$RANDOM

let "number %= $BINARY"

Note that let "number >>= 14" gives a better random distribution

+ (right shifts out everything except last binary digit).

if [ "$number" -eq $T ]

then

echo "TRUE"

else

echo "FALSE"

echo

Generate a toss of the dice.

SPOTS=6 # Modulo 6 gives range 0 - 5.

# Incrementing by 1 gives desired range of 1 - 6.

# Thanks, Paulo Marcel Coelho Aragao, for the simplification.

die1=0

die2=0

Would it be better to just set SPOTS=7 and not add 1? Why or why not?

Tosses each die separately, and so gives correct odds.

let "die1 = $RANDOM % $SPOTS +1" # Roll first one.

let "die2 = $RANDOM % $SPOTS +1" # Roll second one.

# Which arithmetic operation, above, has greater precedence --

#+ modulo (%) or addition (+)?

let "throw = $die1 + $die2"

echo "Throw of the dice = $throw"

echo

exit 0

Example 9-12. Picking a random card from a deck

!/bin/bash

pick-card.sh

This is an example of choosing random elements of an array.

Pick a card, any card.

Suites="Clubs

Diamonds

Hearts

Spades"

Denominations="2

Jack

Queen

King

Ace"

Note variables spread over multiple lines.

suite=($Suites) # Read into array variable.

denomination=($Denominations)

num_suites=${#suite[*]} # Count how many elements.

num_denominations=${#denomination[*]}

echo -n "${denomination[$((RANDOM%num_denominations))]} of "

echo ${suite[$((RANDOM%num_suites))]}

$bozo sh pick-cards.sh

Jack of Clubs

Thank you, "jipe," for pointing out this use of $RANDOM.

exit 0

Example 9-13. Brownian Motion Simulation

!/bin/bash

brownian.sh

Author: Mendel Cooper

Reldate: 10/26/07

License: GPL3

----------------------------------------------------------------

This script models Brownian motion:

+ the random wanderings of tiny particles in a fluid,

+ as they are buffeted by random currents and collisions.

+ This is colloquially known as the "Drunkard's Walk."

It can also be considered as a stripped-down simulation of a

+ Galton Board, a slanted board with a pattern of pegs,

+ down which rolls a succession of marbles, one at a time.

+ At the bottom is a row of slots or catch basins in which

+ the marbles come to rest at the end of their journey.

Think of it as a kind of bare-bones Pachinko game.

As you see by running the script,

+ most of the marbles cluster around the center slot.

+ This is consistent with the expected binomial distribution.

As a Galton Board simulation, the script

+ disregards such parameters as

+ board tilt-angle, rolling friction of the marbles,

+ angles of impact, and elasticity of the pegs.

To what extent does this affect the accuracy of the simulation?

----------------------------------------------------------------

PASSES=500 # Number of particle interactions / marbles.

ROWS=10 # Number of "collisions" (or horiz. peg rows).

RANGE=3 # 0 - 2 output range from $RANDOM.

POS=0 # Left/right position.

RANDOM=$ # Seeds the random number generator from PID

#+ of script.

declare -a Slots # Array holding cumulative results of passes.

NUMSLOTS=21 # Number of slots at bottom of board.

Initialize_Slots () { # Zero out all elements of the array.

for i in $( seq $NUMSLOTS )

Slots[$i]=0

done

echo # Blank line at beginning of run.

}

Show_Slots () {

echo; echo

echo -n " "

for i in $( seq $NUMSLOTS ) # Pretty-print array elements.

printf "%3d" ${Slots[$i]} # Allot three spaces per result.

done

echo # Row of slots:

echo " |__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|"

echo " ||"

echo # Note that if the count within any particular slot exceeds 99,

#+ it messes up the display.

# Running only(!) 500 passes usually avoids this.

}

Move () { # Move one unit right / left, or stay put.

Move=$RANDOM # How random is $RANDOM? Well, let's see ...

let "Move %= RANGE" # Normalize into range of 0 - 2.

case "$Move" in

0 ) ;; # Do nothing, i.e., stay in place.

1 ) ((POS--));; # Left.

2 ) ((POS++));; # Right.

* ) echo -n "Error ";; # Anomaly! (Should never occur.)

esac

}

Play () { # Single pass (inner loop).

i=0

while [ "$i" -lt "$ROWS" ] # One event per row.

Move

((i++));

done

SHIFT=11 # Why 11, and not 10?

let "POS += $SHIFT" # Shift "zero position" to center.

(( Slots[$POS]++ )) # DEBUG: echo $POS

echo -n "$POS "

}

Run () { # Outer loop.

p=0

while [ "$p" -lt "$PASSES" ]

Play

(( p++ ))

POS=0 # Reset to zero. Why?

done

}

--------------

main ()

Initialize_Slots

Run

Show_Slots

--------------

exit $?

Exercises:

---------

1) Show the results in a vertical bar graph, or as an alternative,

+ a scattergram.

2) Alter the script to use /dev/urandom instead of $RANDOM.

Will this make the results more random?

3) Provide some sort of "animation" or graphic output

for each marble played.

Jipe points out a set of techniques for generating random numbers

within a range.

Generate random number between 6 and 30.

rnumber=$((RANDOM%25+6))

Generate random number in the same 6 - 30 range,

+ but the number must be evenly divisible by 3.

rnumber=$(((RANDOM%30/3+1)*3))

Note that this will not work all the time.

It fails if $RANDOM%30 returns 0.

Frank Wang suggests the following alternative:

rnumber=$(( RANDOM%27/3*3+6 ))

Bill Gradwohl came up with an improved formula that works for

positive numbers.

rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min))

Here Bill presents a versatile function that returns a random number

between two specified values.

Example 9-14. Random between values

!/bin/bash

random-between.sh

Random number between two specified values.

Script by Bill Gradwohl, with minor modifications by the document author.

Corrections in lines 187 and 189 by Anthony Le Clezio.

Used with permission.

randomBetween() {

# Generates a positive or negative random number

#+ between $min and $max

#+ and divisible by $divisibleBy.

# Gives a "reasonably random" distribution of return values.

# Bill Gradwohl - Oct 1, 2003

syntax() {

# Function embedded within function.

echo

echo "Syntax: randomBetween [min] [max] [multiple]"

echo

echo -n "Expects up to 3 passed parameters, "

echo "but all are completely optional."

echo "min is the minimum value"

echo "max is the maximum value"

echo -n "multiple specifies that the answer must be "

echo "a multiple of this value."

echo " i.e. answer must be evenly divisible by this number."

echo

echo "If any value is missing, defaults area supplied as: 0 32767 1"

echo -n "Successful completion returns 0, "

echo "unsuccessful completion returns"

echo "function syntax and 1."

echo -n "The answer is returned in the global variable "

echo "randomBetweenAnswer"

echo -n "Negative values for any passed parameter are "

echo "handled correctly."

}

local min=${1:-0}

local max=${2:-32767}

local divisibleBy=${3:-1}

# Default values assigned, in case parameters not passed to function.

local x

local spread

# Let's make sure the divisibleBy value is positive.

[ ${divisibleBy} -lt 0 ] && divisibleBy=$((0-divisibleBy))

# Sanity check.

if [ $# -gt 3 -o ${divisibleBy} -eq 0 -o ${min} -eq ${max} ]; then

syntax

return 1

# See if the min and max are reversed.

if [ ${min} -gt ${max} ]; then

# Swap them.

x=${min}

min=${max}

max=${x}

# If min is itself not evenly divisible by $divisibleBy,

#+ then fix the min to be within range.

if [ $((min/divisibleBy*divisibleBy)) -ne ${min} ]; then

if [ ${min} -lt 0 ]; then

min=$((min/divisibleBy*divisibleBy))

else

min=$((((min/divisibleBy)+1)*divisibleBy))

# If max is itself not evenly divisible by $divisibleBy,

#+ then fix the max to be within range.

if [ $((max/divisibleBy*divisibleBy)) -ne ${max} ]; then

if [ ${max} -lt 0 ]; then

max=$((((max/divisibleBy)-1)*divisibleBy))

else

max=$((max/divisibleBy*divisibleBy))

# ---------------------------------------------------------------------

# Now, to do the real work.

# Note that to get a proper distribution for the end points,

#+ the range of random values has to be allowed to go between

#+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1.

# The slight increase will produce the proper distribution for the

#+ end points.

# Changing the formula to use abs(max-min)+1 will still produce

#+ correct answers, but the randomness of those answers is faulty in

#+ that the number of times the end points ($min and $max) are returned

#+ is considerably lower than when the correct formula is used.

# ---------------------------------------------------------------------

spread=$((max-min))

# Omair Eshkenazi points out that this test is unnecessary,

#+ since max and min have already been switched around.

[ ${spread} -lt 0 ] && spread=$((0-spread))

let spread+=divisibleBy

randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min))

return 0

# However, Paulo Marcel Coelho Aragao points out that

#+ when $max and $min are not divisible by $divisibleBy,

#+ the formula fails.

# He suggests instead the following formula:

# rnumber = $(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy))

}

Let's test the function.

min=-14

max=20

divisibleBy=3

Generate an array of expected answers and check to make sure we get

+ at least one of each answer if we loop long enough.

declare -a answer

minimum=${min}

maximum=${max}

if [ $((minimum/divisibleBy*divisibleBy)) -ne ${minimum} ]; then

if [ ${minimum} -lt 0 ]; then

minimum=$((minimum/divisibleBy*divisibleBy))

else

minimum=$((((minimum/divisibleBy)+1)*divisibleBy))

# If max is itself not evenly divisible by $divisibleBy,

#+ then fix the max to be within range.

if [ $((maximum/divisibleBy*divisibleBy)) -ne ${maximum} ]; then

if [ ${maximum} -lt 0 ]; then

maximum=$((((maximum/divisibleBy)-1)*divisibleBy))

else

maximum=$((maximum/divisibleBy*divisibleBy))

We need to generate only positive array subscripts,

+ so we need a displacement that that will guarantee

+ positive results.

disp=$((0-minimum))

for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do

answer[i+disp]=0

done

Now loop a large number of times to see what we get.

loopIt=1000 # The script author suggests 100000,

#+ but that takes a good long while.

for ((i=0; i<${loopIt}; ++i)); do

# Note that we are specifying min and max in reversed order here to

#+ make the function correct for this case.

randomBetween ${max} ${min} ${divisibleBy}

# Report an error if an answer is unexpected.

[ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ]

&& echo MIN or MAX error - ${randomBetweenAnswer}!

[ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] \

&& echo DIVISIBLE BY error - ${randomBetweenAnswer}!

# Store the answer away statistically.

answer[randomBetweenAnswer+disp]=$((answer[randomBetweenAnswer+disp]+1))

done

Let's check the results

for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do

[ ${answer[i+disp]} -eq 0 ] \

&& echo "We never got an answer of $i." \

|| echo "${i} occurred ${answer[i+disp]} times."

done

exit 0

Just how random is $RANDOM? The best way to test this is to write a

script that tracks the distribution of "random" numbers generated by

$RANDOM. Let's roll a $RANDOM die a few times . . .

Example 9-15. Rolling a single die with RANDOM

!/bin/bash

How random is RANDOM?

RANDOM=$ # Reseed the random number generator using script process ID.

PIPS=6 # A die has 6 pips.

MAXTHROWS=600 # Increase this if you have nothing better to do with your tim

throw=0 # Number of times the dice have been cast.

ones=0 # Must initialize counts to zero,

twos=0 #+ since an uninitialized variable is null, NOT zero.

threes=0

fours=0

fives=0

sixes=0

print_result ()

{

echo

echo "ones = $ones"

echo "twos = $twos"

echo "threes = $threes"

echo "fours = $fours"

echo "fives = $fives"

echo "sixes = $sixes"

echo

}

update_count()

{

case "$1" in

0) ((ones++));; # Since a die has no "zero", this corresponds to 1.

1) ((twos++));; # And this to 2.

2) ((threes++));; # And so forth.

3) ((fours++));;

4) ((fives++));;

5) ((sixes++));;

esac

}

echo

while [ "$throw" -lt "$MAXTHROWS" ]

let "die1 = RANDOM % $PIPS"

update_count $die1

let "throw += 1"

done

print_result

exit $?

The scores should distribute evenly, assuming RANDOM is random.

With $MAXTHROWS at 600, all should cluster around 100,

+ plus-or-minus 20 or so.

Keep in mind that RANDOM is a pseudorandom generator,

+ and not a spectacularly good one at that.

Randomness is a deep and complex subject.

Sufficiently long "random" sequences may exhibit

+ chaotic and other "non-random" behavior.

Exercise (easy):

---------------

Rewrite this script to flip a coin 1000 times.

Choices are "HEADS" and "TAILS."

As we have seen in the last example, it is best to reseed the RANDOM

generator each time it is invoked. Using the same seed for RANDOM

repeats the same series of numbers. [48] (This mirrors the behavior

of the random() function in C.)

Example 9-16. Reseeding RANDOM

!/bin/bash

seeding-random.sh: Seeding the RANDOM variable.

v 1.1, reldate 09 Feb 2013

MAXCOUNT=25 # How many numbers to generate.

SEED=

random_numbers ()

{

local count=0

local number

while [ "$count" -lt "$MAXCOUNT" ]

number=$RANDOM

echo -n "$number "

let "count++"

done

}

echo; echo

SEED=1

RANDOM=$SEED # Setting RANDOM seeds the random number generator.

echo "Random seed = $SEED"

random_numbers

RANDOM=$SEED # Same seed for RANDOM . . .

echo; echo "Again, with same random seed ..."

echo "Random seed = $SEED"

random_numbers # . . . reproduces the exact same number series.

# When is it useful to duplicate a "random" series?

echo; echo

SEED=2

RANDOM=$SEED # Trying again, but with a different seed . . .

echo "Random seed = $SEED"

random_numbers # . . . gives a different number series.

echo; echo

RANDOM=$ seeds RANDOM from process id of script.

It is also possible to seed RANDOM from 'time' or 'date' commands.

Getting fancy...

SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }'| sed s/^0*//)

Pseudo-random output fetched

+ from /dev/urandom (system pseudo-random device-file),

+ then converted to line of printable (octal) numbers by "od",

+ then "awk" retrieves just one number for SEED,

+ finally "sed" removes any leading zeros.

RANDOM=$SEED

echo "Random seed = $SEED"

random_numbers

echo; echo

exit 0

Note

The /dev/urandom pseudo-device file provides a method of generating

much more "random" pseudorandom numbers than the $RANDOM variable. dd

if=/dev/urandom of=targetfile bs=1 count=XX creates a file of

well-scattered pseudorandom numbers. However, assigning these numbers

to a variable in a script requires a workaround, such as filtering

through od (as in above example, Example 16-14, and Example A-36), or

even piping to md5sum (see Example 36-16).

There are also other ways to generate pseudorandom numbers in a

script. Awk provides a convenient means of doing this.

Example 9-17. Pseudorandom numbers, using awk

!/bin/bash

random2.sh: Returns a pseudorandom number in the range 0 - 1,

+ to 6 decimal places. For example: 0.822725

Uses the awk rand() function.

AWKSCRIPT=' { srand(); print rand() } '

Command(s)/parameters passed to awk

Note that srand() reseeds awk's random number generator.

echo -n "Random number between 0 and 1 = "

echo | awk "$AWKSCRIPT"

What happens if you leave out the 'echo'?

exit 0

Exercises:

---------

1) Using a loop construct, print out 10 different random numbers.

(Hint: you must reseed the srand() function with a different seed

+ in each pass through the loop. What happens if you omit this?)

2) Using an integer multiplier as a scaling factor, generate random numbers

+ in the range of 10 to 100.

3) Same as exercise #2, above, but generate random integers this time.

The date command also lends itself to generating pseudorandom integer

sequences.

________________________________________________________________

Chapter 10. Manipulating Variables

10.1. Manipulating Strings

Bash supports a surprising number of string manipulation operations.

Unfortunately, these tools lack a unified focus. Some are a subset of

parameter substitution, and others fall under the functionality of

the UNIX expr command. This results in inconsistent command syntax

and overlap of functionality, not to mention confusion.

String Length

${#string}

expr length $string

These are the equivalent of strlen() in C.

expr "$string" : '.*'

stringZ=abcABC123ABCabc

echo ${#stringZ} # 15

echo `expr length $stringZ` # 15

echo `expr "$stringZ" : '.*'` # 15

Example 10-1. Inserting a blank line between paragraphs in a text

file

!/bin/bash

paragraph-space.sh

Ver. 2.1, Reldate 29Jul12 [fixup]

Inserts a blank line between paragraphs of a single-spaced text file.

Usage: $0 <FILENAME

MINLEN=60 # Change this value? It's a judgment call.

Assume lines shorter than $MINLEN characters ending in a period

+ terminate a paragraph. See exercises below.

while read line # For as many lines as the input file has ...

echo "$line" # Output the line itself.

len=${#line}

if [[ "$len" -lt "$MINLEN" && "$line" =~ [*{\.}]$ ]]

if [[ "$len" -lt "$MINLEN" && "$line" =~ \[*\.\] ]]

An update to Bash broke the previous version of this script. Ouch!

Thank you, Halim Srama, for pointing this out and suggesting a fix.

then echo # Add a blank line immediately

fi #+ after a short line terminated by a period.

done

exit

Exercises:

---------

1) The script usually inserts a blank line at the end

+ of the target file. Fix this.

2) Line 17 only considers periods as sentence terminators.

Modify this to include other common end-of-sentence characters,

+ such as ?, !, and ".

Length of Matching Substring at Beginning of String

expr match "$string" '$substring'

$substring is a regular expression.

expr "$string" : '$substring'

$substring is a regular expression.

stringZ=abcABC123ABCabc

|------|

12345678

echo `expr match "$stringZ" 'abc[A-Z]*.2'` # 8

echo `expr "$stringZ" : 'abc[A-Z]*.2'` # 8

Index

expr index $string $substring

Numerical position in $string of first character in $substring

that matches.

stringZ=abcABC123ABCabc

123456 ...

echo `expr index "$stringZ" C12` # 6

# C position.

echo `expr index "$stringZ" 1c` # 3

'c' (in #3 position) matches before '1'.

This is the near equivalent of strchr() in C.

Substring Extraction

${string:position}

Extracts substring from $string at $position.

If the $string parameter is "*" or "@", then this extracts the

positional parameters, [49] starting at $position.

${string:position:length}

Extracts $length characters of substring from $string at

$position.

stringZ=abcABC123ABCabc

0123456789.....

0-based indexing.

echo ${stringZ:0} # abcABC123ABCabc

echo ${stringZ:1} # bcABC123ABCabc

echo ${stringZ:7} # 23ABCabc

echo ${stringZ:7:3} # 23A

# Three characters of substring.

Is it possible to index from the right end of the string?

echo ${stringZ:-4} # abcABC123ABCabc

Defaults to full string, as in ${parameter:-default}.

However . . .

echo ${stringZ:(-4)} # Cabc

echo ${stringZ: -4} # Cabc

Now, it works.

Parentheses or added space "escape" the position parameter.

Thank you, Dan Jacobson, for pointing this out.

The position and length arguments can be "parameterized," that

is, represented as a variable, rather than as a numerical

constant.

Example 10-2. Generating an 8-character "random" string

!/bin/bash

rand-string.sh

Generating an 8-character "random" string.

if [ -n "$1" ] # If command-line argument present,

then #+ then set start-string to it.

str0="$1"

else # Else use PID of script as start-string.

str0="$"

POS=2 # Starting from position 2 in the string.

LEN=8 # Extract eight characters.

str1=$( echo "$str0" | md5sum | md5sum )

Doubly scramble ^^^^^^ ^^^^^^

+ by piping and repiping to md5sum.

randstring="${str1:$POS:$LEN}"

Can parameterize ^^^^ ^^^^

echo "$randstring"

exit $?

bozo$ ./rand-string.sh my-password

1bdd88c4

No, this is is not recommended

+ as a method of generating hack-proof passwords.

If the $string parameter is "*" or "@", then this extracts a

maximum of $length positional parameters, starting at

$position.

echo ${*:2} # Echoes second and following positional parameters.

echo ${@:2} # Same as above.

echo ${*:2:3} # Echoes three positional parameters, starting at second.

expr substr $string $position $length

Extracts $length characters from $string starting at

$position.

stringZ=abcABC123ABCabc

123456789......

1-based indexing.

echo `expr substr $stringZ 1 2` # ab

echo `expr substr $stringZ 4 3` # ABC

expr match "$string" '$$substring$'

Extracts $substring at beginning of $string, where $substring

is a regular expression.

expr "$string" : '$$substring$'

Extracts $substring at beginning of $string, where $substring

is a regular expression.

stringZ=abcABC123ABCabc

=======

echo `expr match "$stringZ" '$.[b-c]*[A-Z]..[0-9]$'` # abcABC1

echo `expr "$stringZ" : '$.[b-c]*[A-Z]..[0-9]$'` # abcABC1

echo `expr "$stringZ" : '$.......$'` # abcABC1

All of the above forms give an identical result.

expr match "$string" '.*$$substring$'

Extracts $substring at end of $string, where $substring is a

regular expression.

expr "$string" : '.*$$substring$'

Extracts $substring at end of $string, where $substring is a

regular expression.

stringZ=abcABC123ABCabc

======

echo `expr match "$stringZ" '.*$[A-C][A-C][A-C][a-c]*$'` # ABCabc

echo `expr "$stringZ" : '.*$......$'` # ABCabc

Substring Removal

${string#substring}

Deletes shortest match of $substring from front of $string.

${string##substring}

Deletes longest match of $substring from front of $string.

stringZ=abcABC123ABCabc

|----| shortest

|----------| longest

echo ${stringZ#a*C} # 123ABCabc

Strip out shortest match between 'a' and 'C'.

echo ${stringZ##a*C} # abc

Strip out longest match between 'a' and 'C'.

You can parameterize the substrings.

X='a*C'

echo ${stringZ#$X} # 123ABCabc

echo ${stringZ##$X} # abc

# As above.

${string%substring}

Deletes shortest match of $substring from back of $string.

For example:

Rename all filenames in $PWD with "TXT" suffix to a "txt" suffix.

For example, "file1.TXT" becomes "file1.txt" . . .

SUFF=TXT

suff=txt

for i in $(ls *.$SUFF)

mv -f $i ${i%.$SUFF}.$suff

# Leave unchanged everything *except* the shortest pattern match

#+ starting from the right-hand-side of the variable $i . . .

done ### This could be condensed into a "one-liner" if desired.

Thank you, Rory Winston.

${string%%substring}

Deletes longest match of $substring from back of $string.

stringZ=abcABC123ABCabc

|| shortest

|------------| longest

echo ${stringZ%b*c} # abcABC123ABCa

Strip out shortest match between 'b' and 'c', from back of $stringZ.

echo ${stringZ%%b*c} # a

Strip out longest match between 'b' and 'c', from back of $stringZ.

This operator is useful for generating filenames.

Example 10-3. Converting graphic file formats, with filename

change

!/bin/bash

cvt.sh:

Converts all the MacPaint image files in a directory to "pbm" format.

Uses the "macptopbm" binary from the "netpbm" package,

+ which is maintained by Brian Henderson (bryanh@giraffe-data.com).

Netpbm is a standard part of most Linux distros.

OPERATION=macptopbm

SUFFIX=pbm # New filename suffix.

if [ -n "$1" ]

then

directory=$1 # If directory name given as a script argument...

else

directory=$PWD # Otherwise use current working directory.

Assumes all files in the target directory are MacPaint image files,

+ with a ".mac" filename suffix.

for file in $directory/* # Filename globbing.

filename=${file%.*c} # Strip ".mac" suffix off filename

#+ ('.*c' matches everything

#+ between '.' and 'c', inclusive).

$OPERATION $file > "$filename.$SUFFIX"

# Redirect conversion to new filename.

rm -f $file # Delete original files after converting.

echo "$filename.$SUFFIX" # Log what is happening to stdout.

done

exit 0

Exercise:

--------

As it stands, this script converts all the files in the current

+ working directory.

Modify it to work only on files with a ".mac" suffix.

* And here's another way to do it. * #

!/bin/bash

Batch convert into different graphic formats.

Assumes imagemagick installed (standard in most Linux distros).

INFMT=png # Can be tif, jpg, gif, etc.

OUTFMT=pdf # Can be tif, jpg, gif, pdf, etc.

for pic in *"$INFMT"

p2=$(ls "$pic" | sed -e s/\.$INFMT//)

# echo $p2

convert "$pic" $p2.$OUTFMT

done

exit $?

Example 10-4. Converting streaming audio files to ogg

!/bin/bash

ra2ogg.sh: Convert streaming audio files (*.ra) to ogg.

Uses the "mplayer" media player program:

http://www.mplayerhq.hu/homepage

Uses the "ogg" library and "oggenc":

http://www.xiph.org/

This script may need appropriate codecs installed, such as sipr.so ...

Possibly also the compat-libstdc++ package.

OFILEPREF=${1%%ra} # Strip off the "ra" suffix.

OFILESUFF=wav # Suffix for wav file.

OUTFILE="$OFILEPREF""$OFILESUFF"

E_NOARGS=85

if [ -z "$1" ] # Must specify a filename to convert.

then

echo "Usage: `basename $0` [filename]"

exit $E_NOARGS

mplayer "$1" -ao pcm:file=$OUTFILE

oggenc "$OUTFILE" # Correct file extension automatically added by oggenc.

rm "$OUTFILE" # Delete intermediate *.wav file.

# If you want to keep it, comment out above line.

exit $?

Note:

----

On a Website, simply clicking on a *.ram streaming audio file

+ usually only downloads the URL of the actual *.ra audio file.

You can then use "wget" or something similar

+ to download the *.ra file itself.

Exercises:

---------

As is, this script converts only *.ra filenames.

Add flexibility by permitting use of *.ram and other filenames.

If you're really ambitious, expand the script

+ to do automatic downloads and conversions of streaming audio files.

Given a URL, batch download streaming audio files (using "wget")

+ and convert them on the fly.

A simple emulation of getopt using substring-extraction

constructs.

Example 10-5. Emulating getopt

!/bin/bash

getopt-simple.sh

Author: Chris Morgan

Used in the ABS Guide with permission.

getopt_simple()

{

echo "getopt_simple()"

echo "Parameters are '$*'"

until [ -z "$1" ]

echo "Processing parameter of: '$1'"

if [ ${1:0:1} = '/' ]

then

tmp=${1:1} # Strip off leading '/' . . .

parameter=${tmp%%=*} # Extract name.

value=${tmp##*=} # Extract value.

echo "Parameter: '$parameter', value: '$value'"

eval $parameter=$value

shift

done

}

Pass all options to getopt_simple().

getopt_simple $*

echo "test is '$test'"

echo "test2 is '$test2'"

exit 0 # See also, UseGetOpt.sh, a modified version of this script.

---

sh getopt_example.sh /test=value1 /test2=value2

Parameters are '/test=value1 /test2=value2'

Processing parameter of: '/test=value1'

Parameter: 'test', value: 'value1'

Processing parameter of: '/test2=value2'

Parameter: 'test2', value: 'value2'

test is 'value1'

test2 is 'value2'

Substring Replacement

${string/substring/replacement}

Replace first match of $substring with $replacement. [50]

${string//substring/replacement}

Replace all matches of $substring with $replacement.

stringZ=abcABC123ABCabc

echo ${stringZ/abc/xyz} # xyzABC123ABCabc

# Replaces first match of 'abc' with 'xyz'.

echo ${stringZ//abc/xyz} # xyzABC123ABCxyz

# Replaces all matches of 'abc' with # 'xyz'.

echo ---------------

echo "$stringZ" # abcABC123ABCabc

echo ---------------

# The string itself is not altered!

Can the match and replacement strings be parameterized?

match=abc

repl=000

echo ${stringZ/$match/$repl} # 000ABC123ABCabc

^ ^ ^^^

echo ${stringZ//$match/$repl} # 000ABC123ABC000

Yes! ^ ^ ^^^ ^^^

echo

What happens if no $replacement string is supplied?

echo ${stringZ/abc} # ABC123ABCabc

echo ${stringZ//abc} # ABC123ABC

A simple deletion takes place.

${string/#substring/replacement}

If $substring matches front end of $string, substitute

$replacement for $substring.

${string/%substring/replacement}

If $substring matches back end of $string, substitute

$replacement for $substring.

stringZ=abcABC123ABCabc

echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc

# Replaces front-end match of 'abc' with 'XY

Z'.

echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ

# Replaces back-end match of 'abc' with 'XYZ

________________________________________________________________

10.1.1. Manipulating strings using awk

A Bash script may invoke the string manipulation facilities of awk as

an alternative to using its built-in operations.

Example 10-6. Alternate ways of extracting and locating substrings

!/bin/bash

substring-extraction.sh

String=23skidoo1

012345678 Bash

123456789 awk

Note different string indexing system:

Bash numbers first character of string as 0.

Awk numbers first character of string as 1.

echo ${String:2:4} # position 3 (0-1-2), 4 characters long

# skid

The awk equivalent of ${string:pos:length} is substr(string,pos,length).

echo | awk '

{ print substr("'"${String}"'",3,4) # skid

}

Piping an empty "echo" to awk gives it dummy input,

+ and thus makes it unnecessary to supply a filename.

echo "----"

And likewise:

echo | awk '

{ print index("'"${String}"'", "skid") # 3

} # (skid starts at position 3)

' # The awk equivalent of "expr index" ...

exit 0

________________________________________________________________

10.1.2. Further Reference

For more on string manipulation in scripts, refer to Section 10.2 and

the relevant section of the expr command listing.

Script examples:

1. Example 16-9

2. Example 10-9

3. Example 10-10

4. Example 10-11

5. Example 10-13

6. Example A-36

7. Example A-41

________________________________________________________________

10.2. Parameter Substitution

Manipulating and/or expanding variables

${parameter}

Same as $parameter, i.e., value of the variable parameter. In

certain contexts, only the less ambiguous ${parameter} form

works.

May be used for concatenating variables with strings.

your_id=${USER}-on-${HOSTNAME}

echo "$your_id"

echo "Old \$PATH = $PATH"

PATH=${PATH}:/opt/bin # Add /opt/bin to $PATH for duration of script.

echo "New \$PATH = $PATH"

${parameter-default}, ${parameter:-default}

If parameter not set, use default.

var1=1

var2=2

var3 is unset.

echo ${var1-$var2} # 1

echo ${var3-$var2} # 2

^ Note the $ prefix.

echo ${username-`whoami`}

Echoes the result of `whoami`, if variable $username is still unset.

Note

${parameter-default} and ${parameter:-default} are almost equivalent.

The extra : makes a difference only when parameter has been declared,

but is null.

!/bin/bash

param-sub.sh

Whether a variable has been declared

+ affects triggering of the default option

+ even if the variable is null.

username0=

echo "username0 has been declared, but is set to null."

echo "username0 = ${username0-`whoami`}"

Will not echo.

echo

echo username1 has not been declared.

echo "username1 = ${username1-`whoami`}"

Will echo.

username2=

echo "username2 has been declared, but is set to null."

echo "username2 = ${username2:-`whoami`}"

^

Will echo because of :- rather than just - in condition test.

Compare to first instance, above.

Once again:

variable=

variable has been declared, but is set to null.

echo "${variable-0}" # (no output)

echo "${variable:-1}" # 1

^

unset variable

echo "${variable-2}" # 2

echo "${variable:-3}" # 3

exit 0

The default parameter construct finds use in providing

"missing" command-line arguments in scripts.

DEFAULT_FILENAME=generic.data

filename=${1:-$DEFAULT_FILENAME}

If not otherwise specified, the following command block operates

+ on the file "generic.data".

Begin-Command-Block

...

End-Command-Block

From "hanoi2.bash" example:

DISKS=${1:-E_NOPARAM} # Must specify how many disks.

Set $DISKS to $1 command-line-parameter,

+ or to $E_NOPARAM if that is unset.

See also Example 3-4, Example 31-2, and Example A-6.

Compare this method with using an and list to supply a default

command-line argument.

${parameter=default}, ${parameter:=default}

If parameter not set, set it to default.

Both forms nearly equivalent. The : makes a difference only

when $parameter has been declared and is null, [51] as above.

echo ${var=abc} # abc

echo ${var=xyz} # abc

$var had already been set to abc, so it did not change.

${parameter+alt_value}, ${parameter:+alt_value}

If parameter set, use alt_value, else use null string.

Both forms nearly equivalent. The : makes a difference only

when parameter has been declared and is null, see below.

echo "###### \${parameter+alt_value} ########"

echo

a=${param1+xyz}

echo "a = $a" # a =

param2=

a=${param2+xyz}

echo "a = $a" # a = xyz

param3=123

a=${param3+xyz}

echo "a = $a" # a = xyz

echo

echo "###### \${parameter:+alt_value} ########"

echo

a=${param4:+xyz}

echo "a = $a" # a =

param5=

a=${param5:+xyz}

echo "a = $a" # a =

Different result from a=${param5+xyz}

param6=123

a=${param6:+xyz}

echo "a = $a" # a = xyz

${parameter?err_msg}, ${parameter:?err_msg}

If parameter set, use it, else print err_msg and abort the

script with an exit status of 1.

Both forms nearly equivalent. The : makes a difference only

when parameter has been declared and is null, as above.

Example 10-7. Using parameter substitution and error messages

!/bin/bash

Check some of the system's environmental variables.

This is good preventative maintenance.

If, for example, $USER, the name of the person at the console, is not set,

+ the machine will not recognize you.

: ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}

echo

echo "Name of the machine is $HOSTNAME."

echo "You are $USER."

echo "Your home directory is $HOME."

echo "Your mail INBOX is located in $MAIL."

echo

echo "If you are reading this message,"

echo "critical environmental variables have been set."

echo

------------------------------------------------------

The ${variablename?} construction can also check

+ for variables set within the script.

ThisVariable=Value-of-ThisVariable

Note, by the way, that string variables may be set

+ to characters disallowed in their names.

: ${ThisVariable?}

echo "Value of ThisVariable is $ThisVariable".

echo; echo

: ${ZZXy23AB?"ZZXy23AB has not been set."}

Since ZZXy23AB has not been set,

+ then the script terminates with an error message.

You can specify the error message.

: ${variablename?"ERROR MESSAGE"}

Same result with: dummy_variable=${ZZXy23AB?}

dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}

echo ${ZZXy23AB?} >/dev/null

Compare these methods of checking whether a variable has been set

+ with "set -u" . . .

echo "You will not see this message, because script already terminated."

HERE=0

exit $HERE # Will NOT exit here.

In fact, this script will return an exit status (echo $?) of 1.

Example 10-8. Parameter substitution and "usage" messages

!/bin/bash

usage-message.sh

: ${1?"Usage: $0 ARGUMENT"}

Script exits here if command-line parameter absent,

+ with following error message.

usage-message.sh: 1: Usage: usage-message.sh ARGUMENT

echo "These two lines echo only if command-line parameter given."

echo "command-line parameter = \"$1\""

exit 0 # Will exit here only if command-line parameter present.

Check the exit status, both with and without command-line parameter.

If command-line parameter present, then "$?" is 0.

If not, then "$?" is 1.

Parameter substitution and/or expansion. The following expressions

are the complement to the match in expr string operations (see

Example 16-9). These particular ones are used mostly in parsing file

path names.

Variable length / Substring removal

${#var}

String length (number of characters in $var). For an array,

${#array} is the length of the first element in the array.

Note

Exceptions:

+ ${#*} and ${#@} give the number of positional parameters.

+ For an array, ${#array[*]} and ${#array[@]} give the number

of elements in the array.

Example 10-9. Length of a variable

!/bin/bash

length.sh

E_NO_ARGS=65

if [ $# -eq 0 ] # Must have command-line args to demo script.

then

echo "Please invoke this script with one or more command-line arguments."

exit $E_NO_ARGS

var01=abcdEFGH28ij

echo "var01 = ${var01}"

echo "Length of var01 = ${#var01}"

Now, let's try embedding a space.

var02="abcd EFGH28ij"

echo "var02 = ${var02}"

echo "Length of var02 = ${#var02}"

echo "Number of command-line arguments passed to script = ${#@}"

echo "Number of command-line arguments passed to script = ${#*}"

exit 0

${var#Pattern}, ${var##Pattern}

${var#Pattern} Remove from $var the shortest part of $Pattern

that matches the front end of $var.

${var##Pattern} Remove from $var the longest part of $Pattern

that matches the front end of $var.

A usage illustration from Example A-7:

Function from "days-between.sh" example.

Strips leading zero(s) from argument passed.

strip_leading_zero () # Strip possible leading zero(s)

{ #+ from argument passed.

return=${1#0} # The "1" refers to "$1" -- passed arg.

} # The "0" is what to remove from "$1" -- strips zeros.

Manfred Schwarb's more elaborate variation of the above:

strip_leading_zero2 () # Strip possible leading zero(s), since otherwise

{ # Bash will interpret such numbers as octal values.

shopt -s extglob # Turn on extended globbing.

local val=${1##+(0)} # Use local variable, longest matching series of 0's.

shopt -u extglob # Turn off extended globbing.

_strip_leading_zero2=${val:-0}

# If input was 0, return 0 instead of "".

}

Another usage illustration:

echo `basename $PWD` # Basename of current working directory.

echo "${PWD##*/}" # Basename of current working directory.

echo

echo `basename $0` # Name of script.

echo $0 # Name of script.

echo "${0##*/}" # Name of script.

echo

filename=test.data

echo "${filename##*.}" # data

# Extension of filename.

${var%Pattern}, ${var%%Pattern}

${var%Pattern} Remove from $var the shortest part of $Pattern

that matches the back end of $var.

${var%%Pattern} Remove from $var the longest part of $Pattern

that matches the back end of $var.

Version 2 of Bash added additional options.

Example 10-10. Pattern matching in parameter substitution

!/bin/bash

patt-matching.sh

Pattern matching using the # ## % %% parameter substitution operators.

var1=abcd12345abc6789

pattern1=a*c # * (wild card) matches everything between a - c.

echo

echo "var1 = $var1" # abcd12345abc6789

echo "var1 = ${var1}" # abcd12345abc6789

# (alternate form)

echo "Number of characters in ${var1} = ${#var1}"

echo

echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c')

echo "--------------"

echo '${var1#$pattern1} =' "${var1#$pattern1}" # d12345abc6789

Shortest possible match, strips out first 3 characters abcd12345abc6789

^^^^^ |-|

echo '${var1##$pattern1} =' "${var1##$pattern1}" # 6789

Longest possible match, strips out first 12 characters abcd12345abc6789

^^^^^ |----------|

echo; echo; echo

pattern2=b*9 # everything between 'b' and '9'

echo "var1 = $var1" # Still abcd12345abc6789

echo

echo "pattern2 = $pattern2"

echo "--------------"

echo '${var1%pattern2} =' "${var1%$pattern2}" # abcd12345a

Shortest possible match, strips out last 6 characters abcd12345abc6789

^^^^ |----|

echo '${var1%%pattern2} =' "${var1%%$pattern2}" # a

Longest possible match, strips out last 12 characters abcd12345abc6789

^^^^ |-------------|

Remember, # and ## work from the left end (beginning) of string,

% and %% work from the right end.

echo

exit 0

Example 10-11. Renaming file extensions:

!/bin/bash

rfe.sh: Renaming file extensions.

rfe old_extension new_extension

Example:

To rename all .gif files in working directory to .jpg,

rfe gif jpg

E_BADARGS=65

case $# in

0|1) # The vertical bar means "or" in this context.

echo "Usage: `basename $0` old_file_suffix new_file_suffix"

exit $E_BADARGS # If 0 or 1 arg, then bail out.

;;

esac

for filename in *.$1

Traverse list of files ending with 1st argument.

mv $filename ${filename%$1}$2

# Strip off part of filename matching 1st argument,

#+ then append 2nd argument.

done

exit 0

Variable expansion / Substring replacement

These constructs have been adopted from ksh.

${var:pos}

Variable var expanded, starting from offset pos.

${var:pos:len}

Expansion to a max of len characters of variable var, from

offset pos. See Example A-13 for an example of the creative

use of this operator.

${var/Pattern/Replacement}

First match of Pattern, within var replaced with Replacement.

If Replacement is omitted, then the first match of Pattern is

replaced by nothing, that is, deleted.

${var//Pattern/Replacement}

Global replacement. All matches of Pattern, within var

replaced with Replacement.

As above, if Replacement is omitted, then all occurrences of

Pattern are replaced by nothing, that is, deleted.

Example 10-12. Using pattern matching to parse arbitrary

strings

!/bin/bash

var1=abcd-1234-defg

echo "var1 = $var1"

t=${var1#*-*}

echo "var1 (with everything, up to and including first - stripped out) = $t"

t=${var1#*-} works just the same,

+ since # matches the shortest string,

+ and * matches everything preceding, including an empty string.

(Thanks, Stephane Chazelas, for pointing this out.)

t=${var1##*-*}

echo "If var1 contains a \"-\", returns empty string... var1 = $t"

t=${var1%*-*}

echo "var1 (with everything from the last - on stripped out) = $t"

echo

-------------------------------------------

path_name=/home/bozo/ideas/thoughts.for.today

-------------------------------------------

echo "path_name = $path_name"

t=${path_name##/*/}

echo "path_name, stripped of prefixes = $t"

Same effect as t=`basename $path_name` in this particular case.

t=${path_name%/}; t=${t##*/} is a more general solution,

+ but still fails sometimes.

If $path_name ends with a newline, then `basename $path_name` will not work

+ but the above expression will.

(Thanks, S.C.)

t=${path_name%/*.*}

Same effect as t=`dirname $path_name`

echo "path_name, stripped of suffixes = $t"

These will fail in some cases, such as "../", "/foo////", # "foo/", "/".

Removing suffixes, especially when the basename has no suffix,

+ but the dirname does, also complicates matters.

(Thanks, S.C.)

echo

t=${path_name:11}

echo "$path_name, with first 11 chars stripped off = $t"

t=${path_name:11:5}

echo "$path_name, with first 11 chars stripped off, length 5 = $t"

echo

t=${path_name/bozo/clown}

echo "$path_name with \"bozo\" replaced by \"clown\" = $t"

t=${path_name/today/}

echo "$path_name with \"today\" deleted = $t"

t=${path_name//o/O}

echo "$path_name with all o's capitalized = $t"

t=${path_name//o/}

echo "$path_name with all o's deleted = $t"

exit 0

${var/#Pattern/Replacement}

If prefix of var matches Pattern, then substitute Replacement

for Pattern.

${var/%Pattern/Replacement}

If suffix of var matches Pattern, then substitute Replacement

for Pattern.

Example 10-13. Matching patterns at prefix or suffix of string

!/bin/bash

var-match.sh:

Demo of pattern replacement at prefix / suffix of string.

v0=abc1234zip1234abc # Original variable.

echo "v0 = $v0" # abc1234zip1234abc

echo

Match at prefix (beginning) of string.

v1=${v0/#abc/ABCDEF} # abc1234zip1234abc

# |-|

echo "v1 = $v1" # ABCDEF1234zip1234abc

# |----|

Match at suffix (end) of string.

v2=${v0/%abc/ABCDEF} # abc1234zip123abc

# |-|

echo "v2 = $v2" # abc1234zip1234ABCDEF

# |----|

echo

----------------------------------------------------

Must match at beginning / end of string,

+ otherwise no replacement results.

----------------------------------------------------

v3=${v0/#123/000} # Matches, but not at beginning.

echo "v3 = $v3" # abc1234zip1234abc

# NO REPLACEMENT.

v4=${v0/%123/000} # Matches, but not at end.

echo "v4 = $v4" # abc1234zip1234abc

# NO REPLACEMENT.

exit 0

${!varprefix*}, ${!varprefix@}

Matches names of all previously declared variables beginning

with varprefix.

This is a variation on indirect reference, but with a * or @.

Bash, version 2.04, adds this feature.

xyz23=whatever

xyz24=

a=${!xyz*} # Expands to *names* of declared variables

^ ^ ^ + beginning with "xyz".

echo "a = $a" # a = xyz23 xyz24

a=${!xyz@} # Same as above.

echo "a = $a" # a = xyz23 xyz24

echo "---"

abc23=something_else

b=${!abc*}

echo "b = $b" # b = abc23

c=${!b} # Now, the more familiar type of indirect reference.

echo $c # something_else

________________________________________________________________

Chapter 11. Loops and Branches

What needs this iteration, woman?

--Shakespeare, Othello

Operations on code blocks are the key to structured and organized

shell scripts. Looping and branching constructs provide the tools for

accomplishing this.

________________________________________________________________

11.1. Loops

A loop is a block of code that iterates [52] a list of commands as

long as the loop control condition is true.

for loops

for arg in [list]

This is the basic looping construct. It differs significantly

from its C counterpart.

for arg in [list]

command(s)...

done

Note

During each pass through the loop, arg takes on the value of each

successive variable in the list.

for arg in "$var1" "$var2" "$var3" ... "$varN"

In pass 1 of the loop, arg = $var1

In pass 2 of the loop, arg = $var2

In pass 3 of the loop, arg = $var3

...

In pass N of the loop, arg = $varN

Arguments in [list] quoted to prevent possible word splitting.

The argument list may contain wild cards.

If do is on same line as for, there needs to be a semicolon

after list.

for arg in [list] ; do

Example 11-1. Simple for loops

!/bin/bash

Listing the planets.

for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

echo $planet # Each planet on a separate line.

done

echo; echo

for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"

# All planets on same line.

# Entire 'list' enclosed in quotes creates a single variable.

# Why? Whitespace incorporated into the variable.

echo $planet

done

echo; echo "Whoops! Pluto is no longer a planet!"

exit 0

Each [list] element may contain multiple parameters. This is

useful when processing parameters in groups. In such cases,

use the set command (see Example 15-16) to force parsing of

each [list] element and assignment of each component to the

positional parameters.

Example 11-2. for loop with two parameters in each [list]

element

!/bin/bash

Planets revisited.

Associate the name of each planet with its distance from the sun.

for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483"

set -- $planet # Parses variable "planet"

#+ and sets positional parameters.

# The "--" prevents nasty surprises if $planet is null or

#+ begins with a dash.

# May need to save original positional parameters,

#+ since they get overwritten.

# One way of doing this is to use an array,

# original_params=("$@")

echo "$1 $2,000,000 miles from the sun"

#-------two tabs---concatenate zeroes onto parameter $2

done

(Thanks, S.C., for additional clarification.)

exit 0

A variable may supply the [list] in a for loop.

Example 11-3. Fileinfo: operating on a file list contained in

a variable

!/bin/bash

fileinfo.sh

FILES="/usr/sbin/accept

/usr/sbin/pwck

/usr/sbin/chroot

/usr/bin/fakefile

/sbin/badblocks

/sbin/ypbind" # List of files you are curious about.

# Threw in a dummy file, /usr/bin/fakefile.

echo

for file in $FILES

if [ ! -e "$file" ] # Check if file exists.

then

echo "$file does not exist."; echo

continue # On to next.

ls -l $file | awk '{ print $8 " file size: " $5 }' # Print 2 fields

whatis `basename $file` # File info.

# Note that the whatis database needs to have been set up for this to work.

# To do this, as root run /usr/bin/makewhatis.

echo

done

exit 0

The [list] in a for loop may be parameterized.

Example 11-4. Operating on a parameterized file list

!/bin/bash

filename="*txt"

for file in $filename

echo "Contents of $file"

echo "---"

cat "$file"

echo

done

If the [list] in a for loop contains wild cards (* and ?) used

in filename expansion, then globbing takes place.

Example 11-5. Operating on files with a for loop

!/bin/bash

list-glob.sh: Generating [list] in a for-loop, using "globbing" ...

Globbing = filename expansion.

echo

for file in *

^ Bash performs filename expansion

+ on expressions that globbing recognizes.

ls -l "$file" # Lists all files in $PWD (current directory).

# Recall that the wild card character "*" matches every filename,

#+ however, in "globbing," it doesn't match dot-files.

# If the pattern matches no file, it is expanded to itself.

# To prevent this, set the nullglob option

#+ (shopt -s nullglob).

# Thanks, S.C.

done

echo; echo

for file in [jx]*

rm -f $file # Removes only files beginning with "j" or "x" in $PWD.

echo "Removed file \"$file\"".

done

echo

exit 0

Omitting the in [list] part of a for loop causes the loop to

operate on $@ -- the positional parameters. A particularly

clever illustration of this is Example A-15. See also Example

15-17.

Example 11-6. Missing in [list] in a for loop

!/bin/bash

Invoke this script both with and without arguments,

+ and see what happens.

for a

echo -n "$a "

done

The 'in list' missing, therefore the loop operates on '$@'

+ (command-line argument list, including whitespace).

echo

exit 0

It is possible to use command substitution to generate the

[list] in a for loop. See also Example 16-54, Example 11-11

and Example 16-48.

Example 11-7. Generating the [list] in a for loop with command

substitution

!/bin/bash

for-loopcmd.sh: for-loop with [list]

+ generated by command substitution.

NUMBERS="9 7 3 8 37.53"

for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53

echo -n "$number "

done

echo

exit 0

Here is a somewhat more complex example of using command

substitution to create the [list].

Example 11-8. A grep replacement for binary files

!/bin/bash

bin-grep.sh: Locates matching strings in a binary file.

A "grep" replacement for binary files.

Similar effect to "grep -a"

E_BADARGS=65

E_NOFILE=66

if [ $# -ne 2 ]

then

echo "Usage: `basename $0` search_string filename"

exit $E_BADARGS

if [ ! -f "$2" ]

then

echo "File \"$2\" does not exist."

exit $E_NOFILE

IFS= \012' # Per suggestion of Anton Filippov.

# was: IFS="\n"

for word in $( strings "$2" | grep "$1" )

The "strings" command lists strings in binary files.

Output then piped to "grep", which tests for desired string.

echo $word

done

As S.C. points out, lines 23 - 30 could be replaced with the simpler

strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'

Try something like "./bin-grep.sh mem /bin/ls"

+ to exercise this script.

exit 0

More of the same.

Example 11-9. Listing all users on the system

!/bin/bash

userlist.sh

PASSWORD_FILE=/etc/passwd

n=1 # User number

for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )

Field separator = : ^^^^^^

Print first field ^^^^^^^^

Get input from password file /etc/passwd ^^^^^^^^^^^^^^^^^

echo "USER #$n = $name"

let "n += 1"

done

USER #1 = root

USER #2 = bin

USER #3 = daemon

...

USER #33 = bozo

exit $?

Discussion:

----------

How is it that an ordinary user, or a script run by same,

+ can read /etc/passwd? (Hint: Check the /etc/passwd file permissions.)

Is this a security hole? Why or why not?

Yet another example of the [list] resulting from command

substitution.

Example 11-10. Checking all the binaries in a directory for

authorship

!/bin/bash

findstring.sh:

Find a particular string in the binaries in a specified directory.

directory=/usr/bin/

fstring="Free Software Foundation" # See which files come from the FSF.

for file in $( find $directory -type f -name '*' | sort )

strings -f $file | grep "$fstring" | sed -e "s%$directory%%"

# In the "sed" expression,

#+ it is necessary to substitute for the normal "/" delimiter

#+ because "/" happens to be one of the characters filtered out.

# Failure to do so gives an error message. (Try it.)

done

exit $?

Exercise (easy):

---------------

Convert this script to take command-line parameters

+ for $directory and $fstring.

A final example of [list] / command substitution, but this

time the "command" is a function.

generate_list ()

{

echo "one two three"

}

for word in $(generate_list) # Let "word" grab output of function.

echo "$word"

done

one

two

three

The output of a for loop may be piped to a command or

commands.

Example 11-11. Listing the symbolic links in a directory

!/bin/bash

symlinks.sh: Lists symbolic links in a directory.

directory=${1-`pwd`}

Defaults to current working directory,

+ if not otherwise specified.

Equivalent to code block below.

----------------------------------------------------------

ARGS=1 # Expect one command-line argument.

if [ $# -ne "$ARGS" ] # If not 1 arg...

then

directory=`pwd` # current working directory

else

directory=$1

fi

----------------------------------------------------------

echo "symbolic links in directory \"$directory\""

for file in "$( find $directory -type l )" # -type l = symbolic links

echo "$file"

done | sort # Otherwise file list is unsorted

Strictly speaking, a loop isn't really necessary here,

+ since the output of the "find" command is expanded into a single word.

However, it's easy to understand and illustrative this way.

As Dominik 'Aeneas' Schnitzer points out,

+ failing to quote $( find $directory -type l )

+ will choke on filenames with embedded whitespace.

containing whitespace.

exit 0

--------------------------------------------------------

Jean Helou proposes the following alternative:

echo "symbolic links in directory \"$directory\""

Backup of the current IFS. One can never be too cautious.

OLDIFS=$IFS

IFS=:

for file in $(find $directory -type l -printf "%p$IFS")

do # ^^^^^^^^^^^^^^^^

echo "$file"

done|sort

And, James "Mike" Conley suggests modifying Helou's code thusly:

OLDIFS=$IFS

IFS='' # Null IFS means no word breaks

for file in $( find $directory -type l )

echo $file

done | sort

This works in the "pathological" case of a directory name having

+ an embedded colon.

"This also fixes the pathological case of the directory name having

+ a colon (or space in earlier example) as well."

The stdout of a loop may be redirected to a file, as this

slight modification to the previous example shows.

Example 11-12. Symbolic links in a directory, saved to a file

!/bin/bash

symlinks.sh: Lists symbolic links in a directory.

OUTFILE=symlinks.list # save-file

directory=${1-`pwd`}

Defaults to current working directory,

+ if not otherwise specified.

echo "symbolic links in directory \"$directory\"" > "$OUTFILE"

echo "---------------------------" >> "$OUTFILE"

for file in "$( find $directory -type l )" # -type l = symbolic links

echo "$file"

done | sort >> "$OUTFILE" # stdout of loop

^^^^^^^^^^^^^ redirected to save file.

echo "Output file = $OUTFILE"

exit $?

There is an alternative syntax to a for loop that will look

very familiar to C programmers. This requires double

parentheses.

Example 11-13. A C-style for loop

!/bin/bash

Multiple ways to count up to 10.

echo

Standard syntax.

for a in 1 2 3 4 5 6 7 8 9 10

echo -n "$a "

done

echo; echo

+==========================================+

Using "seq" ...

for a in `seq 10`

echo -n "$a "

done

echo; echo

+==========================================+

Using brace expansion ...

Bash, version 3+.

for a in {1..10}

echo -n "$a "

done

echo; echo

+==========================================+

Now, let's do the same, using C-like syntax.

LIMIT=10

for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and naked "LIMIT"

echo -n "$a "

done # A construct borrowed from ksh93.

echo; echo

+=========================================================================+

Let's use the C "comma operator" to increment two variables simultaneously.

for ((a=1, b=1; a <= LIMIT ; a++, b++))

do # The comma concatenates operations.

echo -n "$a-$b "

done

echo; echo

exit 0

See also Example 27-16, Example 27-17, and Example A-6.

---

Now, a for loop used in a "real-life" context.

Example 11-14. Using efax in batch mode

!/bin/bash

Faxing (must have 'efax' package installed).

EXPECTED_ARGS=2

E_BADARGS=85

MODEM_PORT="/dev/ttyS2" # May be different on your machine.

^^^^^ PCMCIA modem card default port.

if [ $# -ne $EXPECTED_ARGS ]

Check for proper number of command-line args.

then

echo "Usage: `basename $0` phone# text-file"

exit $E_BADARGS

if [ ! -f "$2" ]

then

echo "File $2 is not a text file."

# File is not a regular file, or does not exist.

exit $E_BADARGS

fax make $2 # Create fax-formatted files from text files.

for file in $(ls $2.0*) # Concatenate the converted files.

# Uses wild card (filename "globbing")

#+ in variable list.

fil="$fil $file"

done

efax -d "$MODEM_PORT" -t "T$1" $fil # Finally, do the work.

Trying adding -o1 if above line fails.

As S.C. points out, the for-loop can be eliminated with

efax -d /dev/ttyS2 -o1 -t "T$1" $2.0*

+ but it's not quite as instructive [grin].

exit $? # Also, efax sends diagnostic messages to stdout.

Note

The keywords do and done delineate the for-loop command block.

However, these may, in certain contexts, be omitted by framing the

command block within curly brackets

for((n=1; n<=10; n++))

No do!

{

echo -n "* $n *"

}

No done!

Outputs:

* 1 2 3 4 5 6 7 8 9 ** 10 *

And, echo $? returns 0, so Bash does not register an error.

echo

But, note that in a classic for-loop: for n in [list] ...

+ a terminal semicolon is required.

for n in 1 2 3

{ echo -n "$n "; }

^

Thank you, YongYe, for pointing this out.

while

This construct tests for a condition at the top of a loop, and

keeps looping as long as that condition is true (returns a 0

exit status). In contrast to a for loop, a while loop finds

use in situations where the number of loop repetitions is not

known beforehand.

while [ condition ]

command(s)...

done

The bracket construct in a while loop is nothing more than our

old friend, the test brackets used in an if/then test. In

fact, a while loop can legally use the more versatile

double-brackets construct (while [[ condition ]]).

As is the case with for loops, placing the do on the same line

as the condition test requires a semicolon.

while [ condition ] ; do

Note that the test brackets are not mandatory in a while loop.

See, for example, the getopts construct.

Example 11-15. Simple while loop

!/bin/bash

var0=0

LIMIT=10

while [ "$var0" -lt "$LIMIT" ]

^ ^

Spaces, because these are "test-brackets" . . .

echo -n "$var0 " # -n suppresses newline.

# ^ Space, to separate printed out numbers.

var0=`expr $var0 + 1` # var0=$(($var0+1)) also works.

# var0=$((var0 + 1)) also works.

# let "var0 += 1" also works.

done # Various other methods also work.

echo

exit 0

Example 11-16. Another while loop

!/bin/bash

echo

# Equivalent to:

while [ "$var1" != "end" ] # while test "$var1" != "end"

echo "Input variable #1 (end to exit) "

read var1 # Not 'read $var1' (why?).

echo "variable #1 = $var1" # Need quotes because of "#" . . .

# If input is 'end', echoes it here.

# Does not test for termination condition until top of loop.

echo

done

exit 0

A while loop may have multiple conditions. Only the final

condition determines when the loop terminates. This

necessitates a slightly different loop syntax, however.

Example 11-17. while loop with multiple conditions

!/bin/bash

var1=unset

previous=$var1

while echo "previous-variable = $previous"

echo

previous=$var1

[ "$var1" != end ] # Keeps track of what $var1 was previously.

# Four conditions on *while*, but only the final one controls loop.

# The *last* exit status is the one that counts.

echo "Input variable #1 (end to exit) "

read var1

echo "variable #1 = $var1"

done

Try to figure out how this all works.

It's a wee bit tricky.

exit 0

As with a for loop, a while loop may employ C-style syntax by

using the double-parentheses construct (see also Example 8-5).

Example 11-18. C-style syntax in a while loop

!/bin/bash

wh-loopc.sh: Count to 10 in a "while" loop.

LIMIT=10 # 10 iterations.

a=1

while [ "$a" -le $LIMIT ]

echo -n "$a "

let "a+=1"

done # No surprises, so far.

echo; echo

+=================================================================+

Now, we'll repeat with C-like syntax.

((a = 1)) # a=1

Double parentheses permit space when setting a variable, as in C.

while (( a <= LIMIT )) # Double parentheses,

do #+ and no "$" preceding variables.

echo -n "$a "

((a += 1)) # let "a+=1"

# Yes, indeed.

# Double parentheses permit incrementing a variable with C-like syntax.

done

echo

C and Java programmers can feel right at home in Bash.

exit 0

Inside its test brackets, a while loop can call a function.

t=0

condition ()

{

((t++))

if [ $t -lt 5 ]

then

return 0 # true

else

return 1 # false

}

while condition

^^^^^^^^^

Function call -- four loop iterations.

echo "Still going: t = $t"

done

Still going: t = 1

Still going: t = 2

Still going: t = 3

Still going: t = 4

Similar to the if-test construct, a while loop can omit the test

brackets.

while condition

command(s) ...

done

By coupling the power of the read command with a while loop,

we get the handy while read construct, useful for reading and

parsing files.

cat $filename | # Supply input from a file.

while read line # As long as there is another line to read ...

...

done

=========== Snippet from "sd.sh" example script ========== #

while read value # Read one data point at a time.

rt=$(echo "scale=$SC; $rt + $value" | bc)

(( ct++ ))

done

am=$(echo "scale=$SC; $rt / $ct" | bc)

echo $am; return $ct # This function "returns" TWO values!

# Caution: This little trick will not work if $ct > 255!

# To handle a larger number of data points,

#+ simply comment out the "return $ct" above.

} <"$datafile" # Feed in data file.

Note

A while loop may have its stdin redirected to a file by a < at its

end.

A while loop may have its stdin supplied by a pipe.

until

This construct tests for a condition at the top of a loop, and

keeps looping as long as that condition is false (opposite of

while loop).

until [ condition-is-true ]

command(s)...

done

Note that an until loop tests for the terminating condition at

the top of the loop, differing from a similar construct in

some programming languages.

As is the case with for loops, placing the do on the same line

as the condition test requires a semicolon.

until [ condition-is-true ] ; do

Example 11-19. until loop

!/bin/bash

END_CONDITION=end

until [ "$var1" = "$END_CONDITION" ]

Tests condition here, at top of loop.

echo "Input variable #1 "

echo "($END_CONDITION to exit)"

read var1

echo "variable #1 = $var1"

echo

done

--- #

As with "for" and "while" loops,

+ an "until" loop permits C-like test constructs.

LIMIT=10

var=0

until (( var > LIMIT ))

do # ^^ ^ ^ ^^ No brackets, no $ prefixing variables.

echo -n "$var "

(( var++ ))

done # 0 1 2 3 4 5 6 7 8 9 10

exit 0

How to choose between a for loop or a while loop or until loop? In C,

you would typically use a for loop when the number of loop iterations

is known beforehand. With Bash, however, the situation is fuzzier.

The Bash for loop is more loosely structured and more flexible than

its equivalent in other languages. Therefore, feel free to use

whatever type of loop gets the job done in the simplest way.

________________________________________________________________

11.2. Nested Loops

A nested loop is a loop within a loop, an inner loop within the body

of an outer one. How this works is that the first pass of the outer

loop triggers the inner loop, which executes to completion. Then the

second pass of the outer loop triggers the inner loop again. This

repeats until the outer loop finishes. Of course, a break within

either the inner or outer loop would interrupt this process.

Example 11-20. Nested Loop

!/bin/bash

nested-loop.sh: Nested "for" loops.

outer=1 # Set outer loop counter.

Beginning of outer loop.

for a in 1 2 3 4 5

echo "Pass $outer in outer loop."

echo "---------------------"

inner=1 # Reset inner loop counter.

# ===============================================

# Beginning of inner loop.

for b in 1 2 3 4 5

echo "Pass $inner in inner loop."

let "inner+=1" # Increment inner loop counter.

done

# End of inner loop.

# ===============================================

let "outer+=1" # Increment outer loop counter.

echo # Space between output blocks in pass of outer loop.

done

End of outer loop.

exit 0

See Example 27-11 for an illustration of nested while loops, and

Example 27-13 to see a while loop nested inside an until loop.

________________________________________________________________

11.3. Loop Control

Tournez cent tours, tournez mille tours,

Tournez souvent et tournez toujours . . .

--Verlaine, "Chevaux de bois"

Commands affecting loop behavior

break, continue

The break and continue loop control commands [53] correspond

exactly to their counterparts in other programming languages.

The break command terminates the loop (breaks out of it),

while continue causes a jump to the next iteration of the

loop, skipping all the remaining commands in that particular

loop cycle.

Example 11-21. Effects of break and continue in a loop

!/bin/bash

LIMIT=19 # Upper limit

echo

echo "Printing Numbers 1 through 20 (but not 3 and 11)."

a=0

while [ $a -le "$LIMIT" ]

a=$(($a+1))

if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11.

then

continue # Skip rest of this particular loop iteration.

echo -n "$a " # This will not execute for 3 and 11.

done

Exercise:

Why does the loop print up to 20?

echo; echo

echo Printing Numbers 1 through 20, but something happens after 2.

Same loop, but substituting 'break' for 'continue'.

a=0

while [ "$a" -le "$LIMIT" ]

a=$(($a+1))

if [ "$a" -gt 2 ]

then

break # Skip entire rest of loop.

echo -n "$a "

done

echo; echo; echo

exit 0

The break command may optionally take a parameter. A plain

break terminates only the innermost loop in which it is

embedded, but a break N breaks out of N levels of loop.

Example 11-22. Breaking out of multiple loop levels

!/bin/bash

break-levels.sh: Breaking out of loops.

"break N" breaks out of N level loops.

for outerloop in 1 2 3 4 5

echo -n "Group $outerloop: "

# --------------------------------------------------------

for innerloop in 1 2 3 4 5

echo -n "$innerloop "

if [ "$innerloop" -eq 3 ]

then

break # Try break 2 to see what happens.

# ("Breaks" out of both inner and outer loops.)

done

# --------------------------------------------------------

echo

done

echo

exit 0

The continue command, similar to break, optionally takes a

parameter. A plain continue cuts short the current iteration

within its loop and begins the next. A continue N terminates

all remaining iterations at its loop level and continues with

the next iteration at the loop, N levels above.

Example 11-23. Continuing at a higher loop level

!/bin/bash

The "continue N" command, continuing at the Nth level loop.

for outer in I II III IV V # outer loop

echo; echo -n "Group $outer: "

# --------------------------------------------------------------------

for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop

if [[ "$inner" -eq 7 && "$outer" = "III" ]]

then

continue 2 # Continue at loop on 2nd level, that is "outer loop".

# Replace above line with a simple "continue"

# to see normal loop behavior.

echo -n "$inner " # 7 8 9 10 will not echo on "Group III."

done

# --------------------------------------------------------------------

done

echo; echo

Exercise:

Come up with a meaningful use for "continue N" in a script.

exit 0

Example 11-24. Using continue N in an actual task

Albert Reiner gives an example of how to use "continue N":

---------------------------------------------------------

Suppose I have a large number of jobs that need to be run, with

+ any data that is to be treated in files of a given name pattern

+ in a directory. There are several machines that access

+ this directory, and I want to distribute the work over these

+ different boxen.

Then I usually nohup something like the following on every box:

while true

for n in .iso.*

[ "$n" = ".iso.opts" ] && continue

beta=${n#.iso.}

[ -r .Iso.$beta ] && continue

[ -r .lock.$beta ] && sleep 10 && continue

lockfile -r0 .lock.$beta || continue

echo -n "$beta: " `date`

run-isotherm $beta

date

ls -alF .Iso.$beta

[ -r .Iso.$beta ] && rm -f .lock.$beta

continue 2

done

break

done

exit 0

The details, in particular the sleep N, are particular to my

+ application, but the general pattern is:

while true

for job in {pattern}

{job already done or running} && continue

{mark job as running, do job, mark job as done}

continue 2

done

break # Or something like `sleep 600' to avoid termination.

done

This way the script will stop only when there are no more jobs to do

+ (including jobs that were added during runtime). Through the use

+ of appropriate lockfiles it can be run on several machines

+ concurrently without duplication of calculations [which run a couple

+ of hours in my case, so I really want to avoid this]. Also, as search

+ always starts again from the beginning, one can encode priorities in

+ the file names. Of course, one could also do this without `continue 2',

+ but then one would have to actually check whether or not some job

+ was done (so that we should immediately look for the next job) or not

+ (in which case we terminate or sleep for a long time before checking

+ for a new job).

Caution

The continue N construct is difficult to understand and tricky to use

in any meaningful context. It is probably best avoided.

________________________________________________________________

11.4. Testing and Branching

The case and select constructs are technically not loops, since they

do not iterate the execution of a code block. Like loops, however,

they direct program flow according to conditions at the top or bottom

of the block.

Controlling program flow in a code block

case (in) / esac

The case construct is the shell scripting analog to switch in

C/C++. It permits branching to one of a number of code blocks,

depending on condition tests. It serves as a kind of shorthand

for multiple if/then/else statements and is an appropriate

tool for creating menus.

case "$variable" in

"$condition1" )

command...

;;

"$condition2" )

command...

;;

esac

Note

+ Quoting the variables is not mandatory, since word splitting

does not take place.

+ Each test line ends with a right paren ). [54]

+ Each condition block ends with a double semicolon ;;.

+ If a condition tests true, then the associated commands

execute and the case block terminates.

+ The entire case block ends with an esac (case spelled

backwards).

Example 11-25. Using case

!/bin/bash

Testing ranges of characters.

echo; echo "Hit a key, then hit return."

read Keypress

case "$Keypress" in

[[:lower:]] ) echo "Lowercase letter";;

[[:upper:]] ) echo "Uppercase letter";;

[0-9] ) echo "Digit";;

* ) echo "Punctuation, whitespace, or other";;

esac # Allows ranges of characters in [square brackets],

#+ or POSIX ranges in [[double square brackets.

In the first version of this example,

+ the tests for lowercase and uppercase characters were

+ [a-z] and [A-Z].

This no longer works in certain locales and/or Linux distros.

POSIX is more portable.

Thanks to Frank Wang for pointing this out.

Exercise:

--------

As the script stands, it accepts a single keystroke, then terminates.

Change the script so it accepts repeated input,

+ reports on each keystroke, and terminates only when "X" is hit.

Hint: enclose everything in a "while" loop.

exit 0

Example 11-26. Creating menus using case

!/bin/bash

Crude address database

clear # Clear the screen.

echo " Contact List"

echo " ------- ----"

echo "Choose one of the following persons:"

echo

echo "[E]vans, Roland"

echo "[J]ones, Mildred"

echo "[S]mith, Julie"

echo "[Z]ane, Morris"

echo

read person

case "$person" in

Note variable is quoted.

"E" | "e" )

# Accept upper or lowercase input.

echo

echo "Roland Evans"

echo "4321 Flash Dr."

echo "Hardscrabble, CO 80753"

echo "(303) 734-9874"

echo "(303) 734-9892 fax"

echo "revans@zzy.net"

echo "Business partner & old friend"

;;

Note double semicolon to terminate each option.

"J" | "j" )

echo

echo "Mildred Jones"

echo "249 E. 7th St., Apt. 19"

echo "New York, NY 10009"

echo "(212) 533-2814"

echo "(212) 533-9972 fax"

echo "milliej@loisaida.com"

echo "Ex-girlfriend"

echo "Birthday: Feb. 11"

;;

Add info for Smith & Zane later.

* )

# Default option.

# Empty input (hitting RETURN) fits here, too.

echo

echo "Not yet in database."

;;

esac

echo

Exercise:

--------

Change the script so it accepts multiple inputs,

+ instead of terminating after displaying just one address.

exit 0

An exceptionally clever use of case involves testing for

command-line parameters.

! /bin/bash

case "$1" in

"") echo "Usage: ${0##*/} <filename>"; exit $E_PARAM;;

# No command-line parameters,

# or first parameter empty.

Note that ${0##*/} is ${var##pattern} param substitution.

# Net result is $0.

-*) FILENAME=./$1;; # If filename passed as argument ($1)

#+ starts with a dash,

#+ replace it with ./$1

#+ so further commands don't interpret it

#+ as an option.

* ) FILENAME=$1;; # Otherwise, $1.

esac

Here is a more straightforward example of command-line

parameter handling:

! /bin/bash

while [ $# -gt 0 ]; do # Until you run out of parameters . . .

case "$1" in

-d|--debug)

# "-d" or "--debug" parameter?

DEBUG=1

;;

-c|--conf)

CONFFILE="$2"

shift

if [ ! -f $CONFFILE ]; then

echo "Error: Supplied file doesn't exist!"

exit $E_CONFFILE # File not found error.

;;

esac

shift # Check next set of parameters.

done

From Stefano Falsetto's "Log2Rot" script,

+ part of his "rottlog" package.

Used with permission.

Example 11-27. Using command substitution to generate the case

variable

!/bin/bash

case-cmd.sh: Using command substitution to generate a "case" variable.

case $( arch ) in # $( arch ) returns machine architecture.

# Equivalent to 'uname -m' ...

i386 ) echo "80386-based machine";;

i486 ) echo "80486-based machine";;

i586 ) echo "Pentium-based machine";;

i686 ) echo "Pentium2+-based machine";;

* ) echo "Other type of machine";;

esac

exit 0

A case construct can filter strings for globbing patterns.

Example 11-28. Simple string matching

!/bin/bash

match-string.sh: Simple string matching

using a 'case' construct.

match_string ()

{ # Exact string match.

MATCH=0

E_NOMATCH=90

PARAMS=2 # Function requires 2 arguments.

E_BAD_PARAMS=91

[ $# -eq $PARAMS ] || return $E_BAD_PARAMS

case "$1" in

"$2") return $MATCH;;

* ) return $E_NOMATCH;;

esac

}

a=one

b=two

c=three

d=two

match_string $a # wrong number of parameters

echo $? # 91

match_string $a $b # no match

echo $? # 90

match_string $b $d # match

echo $? # 0

exit 0

Example 11-29. Checking for alphabetic input

!/bin/bash

isalpha.sh: Using a "case" structure to filter a string.

SUCCESS=0

FAILURE=1 # Was FAILURE=-1,

#+ but Bash no longer allows negative return value.

isalpha () # Tests whether *first character* of input string is alphabetic.

{

if [ -z "$1" ] # No argument passed?

then

return $FAILURE

case "$1" in

[a-zA-Z]*) return $SUCCESS;; # Begins with a letter?

* ) return $FAILURE;;

esac

} # Compare this with "isalpha ()" function in C.

isalpha2 () # Tests whether *entire string* is alphabetic.

{

[ $# -eq 1 ] || return $FAILURE

case $1 in

*[!a-zA-Z]*|"") return $FAILURE;;

*) return $SUCCESS;;

esac

}

isdigit () # Tests whether *entire string* is numerical.

{ # In other words, tests for integer variable.

[ $# -eq 1 ] || return $FAILURE

case $1 in

*[!0-9]*|"") return $FAILURE;;

*) return $SUCCESS;;

esac

}

check_var () # Front-end to isalpha ().

{

if isalpha "$@"

then

echo "\"$*\" begins with an alpha character."

if isalpha2 "$@"

then # No point in testing if first char is non-alpha.

echo "\"$*\" contains only alpha characters."

else

echo "\"$*\" contains at least one non-alpha character."

else

echo "\"$*\" begins with a non-alpha character."

# Also "non-alpha" if no argument passed.

echo

}

digit_check () # Front-end to isdigit ().

{

if isdigit "$@"

then

echo "\"$*\" contains only digits [0 - 9]."

else

echo "\"$*\" has at least one non-digit character."

echo

}

a=23skidoo

b=H3llo

c=-What?

d=What?

e=$(echo $b) # Command substitution.

f=AbcDef

g=27234

h=27a34

i=27.34

check_var $a

check_var $b

check_var $c

check_var $d

check_var $e

check_var $f

check_var # No argument passed, so what happens?

digit_check $g

digit_check $h

digit_check $i

exit 0 # Script improved by S.C.

Exercise:

--------

Write an 'isfloat ()' function that tests for floating point numbers.

Hint: The function duplicates 'isdigit ()',

+ but adds a test for a mandatory decimal point.

select

The select construct, adopted from the Korn Shell, is yet

another tool for building menus.

select variable [in list]

command...

break

done

This prompts the user to enter one of the choices presented in

the variable list. Note that select uses the $PS3 prompt (#? )

by default, but this may be changed.

Example 11-30. Creating menus using select

!/bin/bash

PS3='Choose your favorite vegetable: ' # Sets the prompt string.

# Otherwise it defaults to #? .

echo

select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"

echo

echo "Your favorite veggie is $vegetable."

echo "Yuck!"

echo

break # What happens if there is no 'break' here?

done

exit

Exercise:

--------

Fix this script to accept user input not specified in

+ the "select" statement.

For example, if the user inputs "peas,"

+ the script would respond "Sorry. That is not on the menu."

If in list is omitted, then select uses the list of command

line arguments ($@) passed to the script or the function

containing the select construct.

Compare this to the behavior of a

for variable [in list]

construct with the in list omitted.

Example 11-31. Creating menus using select in a function

!/bin/bash

PS3='Choose your favorite vegetable: '

echo

choice_of()

{

select vegetable

[in list] omitted, so 'select' uses arguments passed to function.

echo

echo "Your favorite veggie is $vegetable."

echo "Yuck!"

echo

break

done

}

choice_of beans rice carrots radishes rutabaga spinach

$1 $2 $3 $4 $5 $6

passed to choice_of() function

exit 0

S. C. points out that "arg list too long" error might result.

Better is xargs rm -- < filename

( -- covers those cases where "filename" begins with a "-" )

textfile_listing=`ls *.txt`

Variable contains names of all *.txt files in current working directory.

echo $textfile_listing

textfile_listing2=$(ls *.txt) # The alternative form of command substitution

echo $textfile_listing2

Same result.

A possible problem with putting a list of files into a single string

is that a newline may creep in.

A safer way to assign a list of files to a parameter is with an array.

shopt -s nullglob # If no match, filename expands to nothing.

textfile_listing=( *.txt )

Thanks, S.C.

Note

Command substitution invokes a subshell.

Caution

Command substitution may result in word splitting.

COMMAND `echo a b` # 2 args: a and b

COMMAND "`echo a b`" # 1 arg: "a b"

COMMAND `echo` # no arg

COMMAND "`echo`" # one empty arg

Thanks, S.C.

Even when there is no word splitting, command substitution can remove

trailing newlines.

cd "`pwd`" # This should always work.

However...

mkdir 'dir with trailing newline

cd 'dir with trailing newline

cd "`pwd`" # Error message:

bash: cd: /tmp/file with trailing newline: No such file or directory

cd "$PWD" # Works fine.

old_tty_setting=$(stty -g) # Save old terminal setting.

echo "Hit a key "

stty -icanon -echo # Disable "canonical" mode for terminal.

# Also, disable *local* echo.

key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress.

stty "$old_tty_setting" # Restore old setting.

echo "You hit ${#key} key." # ${#variable} = number of characters in $variabl

Hit any key except RETURN, and the output is "You hit 1 key."

Hit RETURN, and it's "You hit 0 key."

The newline gets eaten in the command substitution.

Code snippet by Stéphane Chazelas.

Caution

Using echo to output an unquoted variable set with command

substitution removes trailing newlines characters from the output of

the reassigned command(s). This can cause unpleasant surprises.

dir_listing=`ls -l`

echo $dir_listing # unquoted

Expecting a nicely ordered directory listing.

However, what you get is:

total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo

bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh

The newlines disappeared.

echo "$dir_listing" # quoted

-rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt

-rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh

-rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh

Command substitution even permits setting a variable to the contents

of a file, using either redirection or the cat command.

variable1=`<file1` # Set "variable1" to contents of "file1".

variable2=`cat file2` # Set "variable2" to contents of "file2".

# This, however, forks a new process,

#+ so the line of code executes slower than the above

version.

Note that the variables may contain embedded whitespace,

+ or even (horrors), control characters.

It is not necessary to explicitly assign a variable.

echo "` <$0`" # Echoes the script itself to stdout.

Excerpts from system file, /etc/rc.d/rc.sysinit

+ (on a Red Hat Linux installation)

if [ -f /fsckoptions ]; then

fsckoptions=`cat /fsckoptions`

...

if [ -e "/proc/ide/${disk[$device]}/media" ] ; then

hdmedia=`cat /proc/ide/${disk[$device]}/media`

...

if [ ! -n "`uname -r | grep -- "-"`" ]; then

ktag="`cat /proc/version`"

...

if [ $usb = "1" ]; then

sleep 5

mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Pr

ot=02"`

kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot

=01"`

...

Caution

Do not set a variable to the contents of a long text file unless you

have a very good reason for doing so. Do not set a variable to the

contents of a binary file, even as a joke.

Example 12-1. Stupid script tricks

!/bin/bash

stupid-script-tricks.sh: Don't try this at home, folks.

From "Stupid Script Tricks," Volume I.

exit 99 ### Comment out this line if you dare.

dangerous_variable=`cat /boot/vmlinuz` # The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"

string-length of $dangerous_variable = 794151

(Newer kernels are bigger.)

Does not give same count as 'wc -c /boot/vmlinuz'.

echo "$dangerous_variable"

Don't try this! It would hang the script.

The document author is aware of no useful applications for

+ setting a variable to the contents of a binary file.

exit 0

Notice that a buffer overrun does not occur. This is one instance

where an interpreted language, such as Bash, provides more protection

from programmer mistakes than a compiled language.

Command substitution permits setting a variable to the output of a

loop. The key to this is grabbing the output of an echo command

within the loop.

Example 12-2. Generating a variable from a loop

!/bin/bash

csubloop.sh: Setting a variable to the output of a loop.

variable1=`for i in 1 2 3 4 5

echo -n "$i" # The 'echo' command is critical

done` #+ to command substitution here.

echo "variable1 = $variable1" # variable1 = 12345

i=0

variable2=`while [ "$i" -lt 10 ]

echo -n "$i" # Again, the necessary 'echo'.

let "i += 1" # Increment.

done`

echo "variable2 = $variable2" # variable2 = 0123456789

Demonstrates that it's possible to embed a loop

+ inside a variable declaration.

exit 0

Command substitution makes it possible to extend the toolset

available to Bash. It is simply a matter of writing a program or

script that outputs to stdout (like a well-behaved UNIX tool should)

and assigning that output to a variable.

include <stdio.h>

/* "Hello, world." C program */

int main()

{

printf( "Hello, world.\n" );

return (0);

}

bash$ gcc -o hello hello.c

!/bin/bash

hello.sh

greeting=`./hello`

echo $greeting

bash$ sh hello.sh

Hello, world.

Note

The $(...) form has superseded backticks for command substitution.

output=$(sed -n /"$1"/p $file) # From "grp.sh" example.

Setting a variable to the contents of a text file.

File_contents1=$(cat $file1)

File_contents2=$(<$file2) # Bash permits this also.

The $(...) form of command substitution treats a double backslash in

a different way than `...`.

bash$ echo `echo \\`

bash$ echo $(echo \\)

The $(...) form of command substitution permits nesting. [57]

word_count=$( wc -w $(echo * | awk '{print $8}') )

Or, for something a bit more elaborate . . .

Example 12-3. Finding anagrams

!/bin/bash

agram2.sh

Example of nested command substitution.

Uses "anagram" utility

+ that is part of the author's "yawl" word list package.

http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz

http://bash.deta.in/yawl-0.3.2.tar.gz

E_NOARGS=86

E_BADARG=87

MINLEN=7

if [ -z "$1" ]

then

echo "Usage $0 LETTERSET"

exit $E_NOARGS # Script needs a command-line argument.

elif [ ${#1} -lt $MINLEN ]

then

echo "Argument must have at least $MINLEN letters."

exit $E_BADARG

FILTER='.......' # Must have at least 7 letters.

1234567

Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )

$( $( nested command sub. ) )

( array assignment )

echo

echo "${#Anagrams[*]} 7+ letter anagrams found"

echo

echo ${Anagrams[0]} # First anagram.

echo ${Anagrams[1]} # Second anagram.

# Etc.

echo "${Anagrams[*]}" # To list all the anagrams in a single line . . .

Look ahead to the Arrays chapter for enlightenment on

+ what's going on here.

$((EXPRESSION)) is arithmetic expansion. # Not to be confused with

#+ command substitution.

You may also use operations within double parentheses without assignment.

n=0

echo "n = $n" # n = 0

(( n += 1 )) # Increment.

(( $n += 1 )) is incorrect!

echo "n = $n" # n = 1

let z=z+3

let "z += 3" # Quotes permit the use of spaces in variable assignment.

# The 'let' operator actually performs arithmetic evaluation,

#+ rather than expansion.

Examples of arithmetic expansion in scripts:

1. Example 16-9

2. Example 11-15

3. Example 27-1

4. Example 27-11

5. Example A-16

________________________________________________________________

Chapter 14. Recess Time

This bizarre little intermission gives the reader a chance to relax

and maybe laugh a bit.

Fellow Linux user, greetings! You are reading something which

will bring you luck and good fortune. Just e-mail a copy of

this document to 10 of your friends. Before making the copies,

send a 100-line Bash script to the first person on the list

at the bottom of this letter. Then delete their name and add

yours to the bottom of the list.

Don't break the chain! Make the copies within 48 hours.

Wilfred P. of Brooklyn failed to send out his ten copies and

woke the next morning to find his job description changed

to "COBOL programmer." Howard L. of Newport News sent

out his ten copies and within a month had enough hardware

to build a 100-node Beowulf cluster dedicated to playing

Tuxracer. Amelia V. of Chicago laughed at this letter

and broke the chain. Shortly thereafter, a fire broke out

in her terminal and she now spends her days writing

documentation for MS Windows.

Don't break the chain! Send out your ten copies today!

Courtesy 'NIX "fortune cookies", with some alterations and many

apologies

Part 4. Commands

Mastering the commands on your Linux machine is an indispensable

prelude to writing effective shell scripts.

This section covers the following commands:

* . (See also source)

* ac

* adduser

* agetty

* agrep

* ar

* arch

* at

* autoload

* awk (See also Using awk for math operations)

* badblocks

* banner

* basename

* batch

* bc

* bg

* bind

* bison

* builtin

* bzgrep

* bzip2

* cal

* caller

* cat

* cd

* chattr

* chfn

* chgrp

* chkconfig

* chmod

* chown

* chroot

* cksum

* clear

* clock

* cmp

* col

* colrm

* column

* comm

* command

* compgen

* complete

* compress

* coproc

* cp

* cpio

* cron

* crypt

* csplit

* cu

* cut

* date

* dc

* dd

* debugfs

* declare

* depmod

* df

* dialog

* diff

* diff3

* diffstat

* dig

* dirname

* dirs

* disown

* dmesg

* doexec

* dos2unix

* du

* dump

* dumpe2fs

* e2fsck

* echo

* egrep

* enable

* enscript

* env

* eqn

* eval

* exec

* exit (Related topic: exit status)

* expand

* export

* expr

* factor

* false

* fdformat

* fdisk

* fg

* fgrep

* file

* find

* finger

* flex

* flock

* fmt

* fold

* free

* fsck

* ftp

* fuser

* getfacl

* getopt

* getopts

* gettext

* getty

* gnome-mount

* grep

* groff

* groupmod

* groups (Related topic: the $GROUPS variable)

* gs

* gzip

* halt

* hash

* hdparm

* head

* help

* hexdump

* host

* hostid

* hostname (Related topic: the $HOSTNAME variable)

* hwclock

* iconv

* id (Related topic: the $UID variable)

* ifconfig

* info

* infocmp

* init

* insmod

* install

* ip

* ipcalc

* iptables

* iwconfig

* jobs

* join

* jot

* kill

* killall

* last

* lastcomm

* lastlog

* ldd

* less

* let

* lex

* lid

* ln

* locate

* lockfile

* logger

* logname

* logout

* logrotate

* look

* losetup

* lp

* ls

* lsdev

* lsmod

* lsof

* lspci

* lsusb

* ltrace

* lynx

* lzcat

* lzma

* m4

* mail

* mailstats

* mailto

* make

* MAKEDEV

* man

* mapfile

* mcookie

* md5sum

* merge

* mesg

* mimencode

* mkbootdisk

* mkdir

* mkdosfs

* mke2fs

* mkfifo

* mkisofs

* mknod

* mkswap

* mktemp

* mmencode

* modinfo

* modprobe

* more

* mount

* msgfmt

* mv

* nc

* netconfig

* netstat

* newgrp

* nice

* nl

* nm

* nmap

* nohup

* nslookup

* objdump

* od

* openssl

* passwd

* paste

* patch (Related topic: diff)

* pathchk

* pax

* pgrep

* pidof

* ping

* pkill

* popd

* pr

* printenv

* printf

* procinfo

* ps

* pstree

* ptx

* pushd

* pwd (Related topic: the $PWD variable)

* quota

* rcp

* rdev

* rdist

* read

* readelf

* readlink

* readonly

* reboot

* recode

* renice

* reset

* resize

* restore

* rev

* rlogin

* rm

* rmdir

* rmmod

* route

* rpm

* rpm2cpio

* rsh

* rsync

* runlevel

* run-parts

* rx

* rz

* sar

* scp

* script

* sdiff

* sed

* seq

* service

* set

* setfacl

* setquota

* setserial

* setterm

* sha1sum

* shar

* shopt

* shred

* shutdown

* size

* skill

* sleep

* slocate

* snice

* sort

* source

* sox

* split

* sq

* ssh

* stat

* strace

* strings

* strip

* stty

* su

* sudo

* sum

* suspend

* swapoff

* swapon

* sx

* sync

* sz

* tac

* tail

* tar

* tbl

* tcpdump

* tee

* telinit

* telnet

* Tex

* texexec

* time

* times

* tmpwatch

* top

* touch

* tput

* tr

* traceroute

* true

* tset

* tsort

* tty

* tune2fs

* type

* typeset

* ulimit

* umask

* umount

* uname

* unarc

* unarj

* uncompress

* unexpand

* uniq

* units

* unlzma

* unrar

* unset

* unsq

* unzip

* uptime

* usbmodules

* useradd

* userdel

* usermod

* users

* usleep

* uucp

* uudecode

* uuencode

* uux

* vacation

* vdir

* vmstat

* vrfy

* w

* wait

* wall

* watch

* wc

* wget

* whatis

* whereis

* which

* who

* whoami

* whois

* write

* xargs

* xrandr

* xz

* yacc

* yes

* zcat

* zdiff

* zdump

* zegrep

* zfgrep

* zgrep

* zip

Table of Contents

15. Internal Commands and Builtins

15.1. Job Control Commands

16. External Filters, Programs and Commands

16.1. Basic Commands

16.2. Complex Commands

16.3. Time / Date Commands

16.4. Text Processing Commands

16.5. File and Archiving Commands

16.6. Communications Commands

16.7. Terminal Control Commands

16.8. Math Commands

16.9. Miscellaneous Commands

17. System and Administrative Commands

17.1. Analyzing a System Script

________________________________________________________________

Chapter 15. Internal Commands and Builtins

A builtin is a command contained within the Bash tool set, literally

built in. This is either for performance reasons -- builtins execute

faster than external commands, which usually require forking off [58]

a separate process -- or because a particular builtin needs direct

access to the shell internals.

When a command or the shell itself initiates (or spawns) a new

subprocess to carry out a task, this is called forking. This new

process is the child, and the process that forked it off is the

parent. While the child process is doing its work, the parent process

is still executing.

Note that while a parent process gets the process ID of the child

process, and can thus pass arguments to it, the reverse is not true.

This can create problems that are subtle and hard to track down.

Example 15-1. A script that spawns multiple instances of itself

!/bin/bash

spawn.sh

PIDS=$(pidof sh $0) # Process IDs of the various instances of this script.

P_array=( $PIDS ) # Put them in an array (why?).

echo $PIDS # Show process IDs of parent and child processes.

let "instances = ${#P_array[*]} - 1" # Count elements, less 1.

# Why subtract 1?

echo "$instances instance(s) of this script running."

echo "[Hit Ctl-C to exit.]"; echo

sleep 1 # Wait.

sh $0 # Play it again, Sam.

exit 0 # Not necessary; script will never get to here.

# Why not?

After exiting with a Ctl-C,

+ do all the spawned instances of the script die?

If so, why?

Note:

----

Be careful not to run this script too long.

It will eventually eat up too many system resources.

Is having a script spawn multiple instances of itself

+ an advisable scripting technique.

Why or why not?

Generally, a Bash builtin does not fork a subprocess when it executes

within a script. An external system command or filter in a script

usually will fork a subprocess.

A builtin may be a synonym to a system command of the same name, but

Bash reimplements it internally. For example, the Bash echo command

is not the same as /bin/echo, although their behavior is almost

identical.

!/bin/bash

echo "This line uses the \"echo\" builtin."

/bin/echo "This line uses the /bin/echo system command."

A keyword is a reserved word, token or operator. Keywords have a

special meaning to the shell, and indeed are the building blocks of

the shell's syntax. As examples, for, while, do, and ! are keywords.

Similar to a builtin, a keyword is hard-coded into Bash, but unlike a

builtin, a keyword is not in itself a command, but a subunit of a

command construct. [59]

I/O

echo

prints (to stdout) an expression or variable (see Example

4-1).

echo Hello

echo $a

An echo requires the -e option to print escaped characters.

See Example 5-2.

Normally, each echo command prints a terminal newline, but the

-n option suppresses this.

Note

An echo can be used to feed a sequence of commands down a pipe.

if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]]

then

echo "$VAR contains the substring sequence \"txt\""

Note

An echo, in combination with command substitution can set a variable.

a=`echo "HELLO" | tr A-Z a-z`

See also Example 16-22, Example 16-3, Example 16-47, and Example

16-48.

Be aware that echo `command` deletes any linefeeds that the

output of command generates.

The $IFS (internal field separator) variable normally contains

\n (linefeed) as one of its set of whitespace characters. Bash

therefore splits the output of command at linefeeds into

arguments to echo. Then echo outputs these arguments,

separated by spaces.

bash$ ls -l /usr/share/apps/kjezz/sounds

-rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au

-rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au

bash$ echo `ls -l /usr/share/apps/kjezz/sounds`

total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root ro

ot ...

So, how can we embed a linefeed within an echoed character

string?

Embedding a linefeed?

echo "Why doesn't this string \n split on two lines?"

Doesn't split.

Let's try something else.

echo

echo $"A line of text containing

a linefeed."

Prints as two distinct lines (embedded linefeed).

But, is the "$" variable prefix really necessary?

echo

echo "This string splits

on two lines."

No, the "$" is not needed.

echo

echo "---------------"

echo

echo -n $"Another line of text containing

a linefeed."

Prints as two distinct lines (embedded linefeed).

Even the -n option fails to suppress the linefeed here.

echo

echo "---------------"

echo

However, the following doesn't work as expected.

Why not? Hint: Assignment to a variable.

string1=$"Yet another line of text containing

a linefeed (maybe)."

echo $string1

Yet another line of text containing a linefeed (maybe).

^

Linefeed becomes a space.

Thanks, Steve Parker, for pointing this out.

Note

This command is a shell builtin, and not the same as /bin/echo,

although its behavior is similar.

bash$ type -a echo

echo is a shell builtin

echo is /bin/echo

printf

The printf, formatted print, command is an enhanced echo. It

is a limited variant of the C language printf() library

function, and its syntax is somewhat different.

printf format-string... parameter...

This is the Bash builtin version of the /bin/printf or

/usr/bin/printf command. See the printf manpage (of the system

command) for in-depth coverage.

Caution

Older versions of Bash may not support printf.

Example 15-2. printf in action

!/bin/bash

printf demo

declare -r PI=3.14159265358979 # Read-only variable, i.e., a constant.

declare -r DecimalConstant=31373

Message1="Greetings,"

Message2="Earthling."

echo

printf "Pi to 2 decimal places = %1.2f" $PI

echo

printf "Pi to 9 decimal places = %1.9f" $PI # It even rounds off correctly.

printf "\n" # Prints a line feed,

# Equivalent to 'echo' . . .

printf "Constant = \t%d\n" $DecimalConstant # Inserts tab (\t).

printf "%s %s \n" $Message1 $Message2

echo

==========================================#

Simulation of C function, sprintf().

Loading a variable with a formatted string.

echo

Pi12=$(printf "%1.12f" $PI)

echo "Pi to 12 decimal places = $Pi12" # Roundoff error!

Msg=`printf "%s %s \n" $Message1 $Message2`

echo $Msg; echo $Msg

As it happens, the 'sprintf' function can now be accessed

+ as a loadable module to Bash,

+ but this is not portable.

exit 0

Formatting error messages is a useful application of printf

E_BADDIR=85

var=nonexistent_directory

error()

{

printf "$@" >&2

# Formats positional params passed, and sends them to stderr.

echo

exit $E_BADDIR

}

cd $var || error $"Can't cd to %s." "$var"

Thanks, S.C.

!/bin/bash

"Reading" variables.

echo -n "Enter the value of variable 'var1': "

The -n option to echo suppresses newline.

read var1

Note no ' in front of var1, since it is being set.

echo "var1 = $var1"

echo

A single 'read' statement can set multiple variables.

echo -n "Enter the values of variables 'var2' and 'var3' "

echo =n "(separated by a space or tab): "

read var2 var3

echo "var2 = $var2 var3 = $var3"

If you input only one value,

+ the other variable(s) will remain unset (null).

exit 0

A read without an associated variable assigns its input to the

dedicated variable $REPLY.

Example 15-4. What happens when read has no variable

!/bin/bash

read-novar.sh

echo

-------------------------- #

echo -n "Enter a value: "

read var

echo "\"var\" = "$var""

Everything as expected here.

-------------------------- #

echo

------------------------------------------------------------------- #

echo -n "Enter another value: "

read # No variable supplied for 'read', therefore...

#+ Input to 'read' assigned to default variable, $REPLY.

var="$REPLY"

echo "\"var\" = "$var""

This is equivalent to the first code block.

------------------------------------------------------------------- #

echo

echo "========================="

echo

This example is similar to the "reply.sh" script.

However, this one shows that $REPLY is available

+ even after a 'read' to a variable in the conventional way.

================================================================= #

In some instances, you might wish to discard the first value read.

In such cases, simply ignore the $REPLY variable.

{ # Code block.

read # Line 1, to be discarded.

read line2 # Line 2, saved in variable.

} <$0

echo "Line 2 of this script is:"

echo "$line2" # # read-novar.sh

echo # #!/bin/bash line discarded.

!/bin/bash

echo

echo "Enter a string terminated by a \\, then press <ENTER>."

echo "Then, enter a second string (no \\ this time), and again press <ENTER>."

read var1 # The "\" suppresses the newline, when reading $var1.

# first line \

# second line

echo "var1 = $var1"

var1 = first line second line

For each line terminated by a "\"

+ you get a prompt on the next line to continue feeding characters into var1.

echo; echo

echo "Enter another string terminated by a \\ , then press <ENTER>."

read -r var2 # The -r option causes the "\" to be read literally.

# first line \

echo "var2 = $var2"

var2 = first line \

Data entry terminates with the first <ENTER>.

echo

exit 0

The read command has some interesting options that permit

echoing a prompt and even reading keystrokes without hitting

ENTER.

Read a keypress without hitting ENTER.

read -s -n1 -p "Hit a key " keypress

echo; echo "Keypress was "\"$keypress\""."

-s option means do not echo input.

-n N option means accept only N characters of input.

-p option means echo the following prompt before reading input.

Using these options is tricky, since they need to be in the correct order.

The -n option to read also allows detection of the arrow keys

and certain of the other unusual keys.

Example 15-6. Detecting the arrow keys

!/bin/bash

arrow-detect.sh: Detects the arrow keys, and a few more.

Thank you, Sandro Magi, for showing me how.

--------------------------------------------

Character codes generated by the keypresses.

arrowup='\[A'

arrowdown='\[B'

arrowrt='\[C'

arrowleft='\[D'

insert='\[2'

delete='\[3'

--------------------------------------------

SUCCESS=0

OTHER=65

echo -n "Press a key... "

May need to also press ENTER if a key not listed above pressed.

read -n3 key # Read 3 characters.

echo -n "$key" | grep "$arrowup" #Check if character code detected.

if [ "$?" -eq $SUCCESS ]

then

echo "Up-arrow key pressed."

exit $SUCCESS

echo -n "$key" | grep "$arrowdown"

if [ "$?" -eq $SUCCESS ]

then

echo "Down-arrow key pressed."

exit $SUCCESS

echo -n "$key" | grep "$arrowrt"

if [ "$?" -eq $SUCCESS ]

then

echo "Right-arrow key pressed."

exit $SUCCESS

echo -n "$key" | grep "$arrowleft"

if [ "$?" -eq $SUCCESS ]

then

echo "Left-arrow key pressed."

exit $SUCCESS

echo -n "$key" | grep "$insert"

if [ "$?" -eq $SUCCESS ]

then

echo "\"Insert\" key pressed."

exit $SUCCESS

echo -n "$key" | grep "$delete"

if [ "$?" -eq $SUCCESS ]

then

echo "\"Delete\" key pressed."

exit $SUCCESS

echo " Some other key pressed."

exit $OTHER

========================================= #

Mark Alexander came up with a simplified

+ version of the above script (Thank you!).

It eliminates the need for grep.

!/bin/bash

uparrow= \x1b[A'

downarrow= \x1b[B'

leftarrow= \x1b[D'

rightarrow= \x1b[C'

read -s -n3 -p "Hit an arrow key: " x

case "$x" in

$uparrow)

echo "You pressed up-arrow"

;;

$downarrow)

echo "You pressed down-arrow"

;;

$leftarrow)

echo "You pressed left-arrow"

;;

$rightarrow)

echo "You pressed right-arrow"

;;

esac

exit $?

========================================= #

Antonio Macchi has a simpler alternative.

!/bin/bash

while true

read -sn1 a

test "$a" == `echo -en "\e"` || continue

read -sn1 a

test "$a" == "[" || continue

read -sn1 a

case "$a" in

A) echo "up";;

B) echo "down";;

C) echo "right";;

D) echo "left";;

esac

done

========================================= #

Exercise:

--------

1) Add detection of the "Home," "End," "PgUp," and "PgDn" keys.

Note

The -n option to read will not detect the ENTER (newline) key.

The -t option to read permits timed input (see Example 9-4 and

Example A-41).

The -u option takes the file descriptor of the target file.

The read command may also "read" its variable value from a

file redirected to stdin. If the file contains more than one

line, only the first line is assigned to the variable. If read

has more than one parameter, then each of these variables gets

assigned a successive whitespace-delineated string. Caution!

Example 15-7. Using read with file redirection

!/bin/bash

read var1 <data-file

echo "var1 = $var1"

var1 set to the entire first line of the input file "data-file"

read var2 var3 <data-file

echo "var2 = $var2 var3 = $var3"

Note non-intuitive behavior of "read" here.

1) Rewinds back to the beginning of input file.

2) Each variable is now set to a corresponding string,

separated by whitespace, rather than to an entire line of text.

3) The final variable gets the remainder of the line.

4) If there are more variables to be set than whitespace-terminated strings

on the first line of the file, then the excess variables remain empty.

echo "------------------------------------------------"

How to resolve the above problem with a loop:

while read line

echo "$line"

done <data-file

Thanks, Heiner Steven for pointing this out.

echo "------------------------------------------------"

Use $IFS (Internal Field Separator variable) to split a line of input to

"read", if you do not want the default to be whitespace.

echo "List of all users:"

OIFS=$IFS; IFS=: # /etc/passwd uses ":" for field separator.

while read name passwd uid gid fullname ignore

echo "$name ($fullname)"

done </etc/passwd # I/O redirection.

IFS=$OIFS # Restore original $IFS.

This code snippet also by Heiner Steven.

Setting the $IFS variable within the loop itself

+ eliminates the need for storing the original $IFS

+ in a temporary variable.

Thanks, Dim Segebart, for pointing this out.

echo "------------------------------------------------"

echo "List of all users:"

while IFS=: read name passwd uid gid fullname ignore

echo "$name ($fullname)"

done </etc/passwd # I/O redirection.

echo

echo "\$IFS still $IFS"

exit 0

Note

Piping output to a read, using echo to set variables will fail.

Yet, piping the output of cat seems to work.

cat file1 file2 |

while read line

echo $line

done

However, as Bjön Eriksson shows:

Example 15-8. Problems reading from a pipe

!/bin/sh

readpipe.sh

This example contributed by Bjon Eriksson.

shopt -s lastpipe

last="(null)"

cat $0 |

while read line

echo "{$line}"

last=$line

done

echo

echo "++++++++++++++++++++++"

printf "\nAll done, last: $last\n" # The output of this line

#+ changes if you uncomment line 5.

# (Bash, version -ge 4.2 required.)

exit 0 # End of code.

# (Partial) output of script follows.

# The 'echo' supplies extra brackets.

./readpipe.sh

{#!/bin/sh}

{last="(null)"}

{cat $0 |}

{while read line}

{do}

{echo "{$line}"}

{last=$line}

{done}

{printf "nAll done, last: $lastn"}

All done, last: (null)

The variable (last) is set within the loop/subshell

but its value does not persist outside the loop.

The gendiff script, usually found in /usr/bin on many Linux distros,

pipes the output of find to a while read construct.

find $1 $ -name "*$2" -o -name ".*$2" $ -print |

while read f; do

. . .

Tip

It is possible to paste text into the input field of a read (but not

multiple lines!). See Example A-38.

Filesystem

The familiar cd change directory command finds use in scripts

where execution of a command requires being in a specified

directory.

(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)

[from the previously cited example by Alan Cox]

The -P (physical) option to cd causes it to ignore symbolic

links.

cd - changes to $OLDPWD, the previous working directory.

Caution

The cd command does not function as expected when presented with two

forward slashes.

bash$ cd //

bash$ pwd

The output should, of course, be /. This is a problem both from the

command-line and in a script.

pwd

Print Working Directory. This gives the user's (or script's)

current directory (see Example 15-9). The effect is identical

to reading the value of the builtin variable $PWD.

pushd, popd, dirs

This command set is a mechanism for bookmarking working

directories, a means of moving back and forth through

directories in an orderly manner. A pushdown stack is used to

keep track of directory names. Options allow various

manipulations of the directory stack.

pushd dir-name pushes the path dir-name onto the directory

stack (to the top of the stack) and simultaneously changes the

current working directory to dir-name

popd removes (pops) the top directory path name off the

directory stack and simultaneously changes the current working

directory to the directory now at the top of the stack.

dirs lists the contents of the directory stack (compare this

with the $DIRSTACK variable). A successful pushd or popd will

automatically invoke dirs.

Scripts that require various changes to the current working

directory without hard-coding the directory name changes can

make good use of these commands. Note that the implicit

$DIRSTACK array variable, accessible from within a script,

holds the contents of the directory stack.

Example 15-9. Changing the current working directory

!/bin/bash

dir1=/usr/local

dir2=/var/spool

pushd $dir1

Will do an automatic 'dirs' (list directory stack to stdout).

echo "Now in directory `pwd`." # Uses back-quoted 'pwd'.

Now, do some stuff in directory 'dir1'.

pushd $dir2

echo "Now in directory `pwd`."

Now, do some stuff in directory 'dir2'.

echo "The top entry in the DIRSTACK array is $DIRSTACK."

popd

echo "Now back in directory `pwd`."

Now, do some more stuff in directory 'dir1'.

popd

echo "Now back in original working directory `pwd`."

exit 0

What happens if you don't 'popd' -- then exit the script?

Which directory do you end up in? Why?

Variables

let

The let command carries out arithmetic operations on

variables. [60] In many cases, it functions as a less complex

version of expr.

Example 15-10. Letting let do arithmetic.

!/bin/bash

echo

let a=11 # Same as 'a=11'

let a=a+5 # Equivalent to let "a = a + 5"

# (Double quotes and spaces make it more readable.)

echo "11 + 5 = $a" # 16

let "a <<= 3" # Equivalent to let "a = a << 3"

echo "\"\$a\" (=16) left-shifted 3 places = $a"

# 128

let "a /= 4" # Equivalent to let "a = a / 4"

echo "128 / 4 = $a" # 32

let "a -= 5" # Equivalent to let "a = a - 5"

echo "32 - 5 = $a" # 27

let "a *= 10" # Equivalent to let "a = a * 10"

echo "27 * 10 = $a" # 270

let "a %= 8" # Equivalent to let "a = a % 8"

echo "270 modulo 8 = $a (270 / 8 = 33, remainder $a)"

# 6

Does "let" permit C-style operators?

Yes, just as the (( ... )) double-parentheses construct does.

let a++ # C-style (post) increment.

echo "6++ = $a" # 6++ = 7

let a-- # C-style decrement.

echo "7-- = $a" # 7-- = 6

Of course, ++a, etc., also allowed . . .

echo

Trinary operator.

Note that $a is 6, see above.

let "t = a<7?7:11" # True

echo $t # 7

let a++

let "t = a<7?7:11" # False

echo $t # 11

exit

Caution

The let command can, in certain contexts, return a surprising exit

status.

Evgeniy Ivanov points out:

var=0

echo $? # 0

# As expected.

let var++

echo $? # 1

# The command was successful, so why isn't $?=0 ???

# Anomaly!

let var++

echo $? # 0

# As expected.

Likewise . . .

let var=0

echo $? # 1

# The command was successful, so why isn't $?=0 ???

However, as Jeff Gorak points out,

+ this is part of the design spec for 'let' . . .

"If the last ARG evaluates to 0, let returns 1;

let returns 0 otherwise." ['help let']

eval

eval arg1 [arg2] ... [argN]

Combines the arguments in an expression or list of expressions

and evaluates them. Any variables within the expression are

expanded. The net result is to convert a string into a

command.

Tip

The eval command can be used for code generation from the

command-line or within a script.

bash$ command_string="ps ax"

bash$ process="ps ax"

bash$ eval "$command_string" | grep "$process"

26973 pts/3 R+ 0:00 grep --color ps ax

26974 pts/3 R+ 0:00 ps ax

Each invocation of eval forces a re-evaluation of its

arguments.

a='$b'

b='$c'

c=d

echo $a # $b

# First level.

eval echo $a # $c

# Second level.

eval eval echo $a # d

# Third level.

Thank you, E. Choroba.

Example 15-11. Showing the effect of eval

!/bin/bash

Exercising "eval" ...

y=`eval ls -l` # Similar to y=`ls -l`

echo $y #+ but linefeeds removed because "echoed" variable is unquoted

echo

echo "$y" # Linefeeds preserved when variable is quoted.

echo; echo

y=`eval df` # Similar to y=`df`

echo $y #+ but linefeeds removed.

When LF's not preserved, it may make it easier to parse output,

+ using utilities such as "awk".

echo

echo "==========================================================="

echo

eval "`seq 3 | sed -e 's/.*/echo var&=ABCDEFGHIJ/'`"

var1=ABCDEFGHIJ

var2=ABCDEFGHIJ

var3=ABCDEFGHIJ

echo

echo "==========================================================="

echo

Now, showing how to do something useful with "eval" . . .

(Thank you, E. Choroba!)

version=3.4 # Can we split the version into major and minor

#+ part in one command?

echo "version = $version"

eval major=${version/./;minor=} # Replaces '.' in version by ';minor='

# The substitution yields '3; minor=4'

#+ so eval does minor=4, major=3

echo Major: $major, minor: $minor # Major: 3, minor: 4

Example 15-12. Using eval to select among variables

!/bin/bash

arr-choice.sh

Passing arguments to a function to select

+ one particular variable out of a group.

arr0=( 10 11 12 13 14 15 )

arr1=( 20 21 22 23 24 25 )

arr2=( 30 31 32 33 34 35 )

0 1 2 3 4 5 Element number (zero-indexed)

choose_array ()

{

eval array_member=\${arr${array_number}[element_number]}

# ^ ^^^^^^^^^^^^

# Using eval to construct the name of a variable,

#+ in this particular case, an array name.

echo "Element $element_number of array $array_number is $array_member"

} # Function can be rewritten to take parameters.

array_number=0 # First array.

element_number=3

choose_array # 13

array_number=2 # Third array.

element_number=4

choose_array # 34

array_number=3 # Null array (arr3 not allocated).

element_number=4

choose_array # (null)

Thank you, Antonio Macchi, for pointing this out.

Example 15-13. Echoing the command-line parameters

!/bin/bash

echo-params.sh

Call this script with a few command-line parameters.

For example:

sh echo-params.sh first second third fourth fifth

params=$# # Number of command-line parameters.

param=1 # Start at first command-line param.

while [ "$param" -le "$params" ]

echo -n "Command-line parameter "

echo -n \$param # Gives only the *name* of variable.

^^^ # $1, $2, $3, etc.

# Why?

# \$ escapes the first "$"

#+ so it echoes literally,

#+ and $param dereferences "$param" . . .

#+ . . . as expected.

echo -n " = "

eval echo \$param # Gives the *value* of variable.

^^^^ ^^^ # The "eval" forces the evaluation

#+ of \$

#+ as an indirect variable reference.

(( param ++ )) # On to the next.

done

exit $?

=================================================

$ sh echo-params.sh first second third fourth fifth

Command-line parameter $1 = first

Command-line parameter $2 = second

Command-line parameter $3 = third

Command-line parameter $4 = fourth

Command-line parameter $5 = fifth

Example 15-14. Forcing a log-off

!/bin/bash

Killing ppp to force a log-off.

For dialup connection, of course.

Script should be run as root user.

SERPORT=ttyS3

Depending on the hardware and even the kernel version,

+ the modem port on your machine may be different --

+ /dev/ttyS1 or /dev/ttyS2.

killppp="eval kill -9 `ps ax | awk '/ppp/ { print $1 }'`"

-------- process ID of ppp -------

$killppp # This variable is now a command.

The following operations must be done as root user.

chmod 666 /dev/$SERPORT # Restore r+w permissions, or else what?

Since doing a SIGKILL on ppp changed the permissions on the serial port,

+ we restore permissions to previous state.

rm /var/lock/LCK..$SERPORT # Remove the serial port lock file. Why?

exit $?

Exercises:

---------

1) Have script check whether root user is invoking it.

2) Do a check on whether the process to be killed

+ is actually running before attempting to kill it.

3) Write an alternate version of this script based on 'fuser':

+ if [ fuser -s /dev/modem ]; then . . .

Example 15-15. A version of rot13

!/bin/bash

A version of "rot13" using 'eval'.

Compare to "rot13.sh" example.

setvar_rot_13() # "rot13" scrambling

{

local varname=$1 varvalue=$2

eval $varname='$(echo "$varvalue" | tr a-z n-za-m)'

}

setvar_rot_13 var "foobar" # Run "foobar" through rot13.

echo $var # sbbone

setvar_rot_13 var "$var" # Run "sbbone" through rot13.

# Back to original variable.

echo $var # foobar

This example by Stephane Chazelas.

Modified by document author.

exit 0

Here is another example of using eval to evaluate a complex

expression, this one from an earlier version of YongYe's

Tetris game script.

eval ${1}+=\"${x} ${y} \"

Example A-53 uses eval to convert array elements into a

command list.

The eval command occurs in the older version of indirect

referencing.

eval var=\$var

Tip

The eval command can be used to parameterize brace expansion.

Caution

The eval command can be risky, and normally should be avoided when

there exists a reasonable alternative. An eval $COMMANDS executes the

contents of COMMANDS, which may contain such unpleasant surprises as

rm -rf *. Running an eval on unfamiliar code written by persons

unknown is living dangerously.

set

The set command changes the value of internal script

variables/options. One use for this is to toggle option flags

which help determine the behavior of the script. Another

application for it is to reset the positional parameters that

a script sees as the result of a command (set `command`). The

script can then parse the fields of the command output.

Example 15-16. Using set with positional parameters

!/bin/bash

ex34.sh

Script "set-test"

Invoke this script with three command-line parameters,

for example, "sh ex34.sh one two three".

echo

echo "Positional parameters before set \`uname -a\` :"

echo "Command-line argument #1 = $1"

echo "Command-line argument #2 = $2"

echo "Command-line argument #3 = $3"

set `uname -a` # Sets the positional parameters to the output

# of the command `uname -a`

echo

echo +++++

echo $_ # +++++

Flags set in script.

echo $- # hB

Anomalous behavior?

echo

echo "Positional parameters after set \`uname -a\` :"

$1, $2, $3, etc. reinitialized to result of `uname -a`

echo "Field #1 of 'uname -a' = $1"

echo "Field #2 of 'uname -a' = $2"

echo "Field #3 of 'uname -a' = $3"

echo \#\#\#

echo $_ # ###

echo

exit 0

More fun with positional parameters.

Example 15-17. Reversing the positional parameters

!/bin/bash

revposparams.sh: Reverse positional parameters.

Script by Dan Jacobson, with stylistic revisions by document author.

set a\ b c d\ e;

^ ^ Spaces escaped

^ ^ Spaces not escaped

OIFS=$IFS; IFS=:;

^ Saving old IFS and setting new one.

echo

until [ $# -eq 0 ]

do # Step through positional parameters.

echo "### k0 = "$k"" # Before

k=$1:$k; # Append each pos param to loop variable.

^

echo "### k = "$k"" # After

echo

shift;

done

set $k # Set new positional parameters.

echo -

echo $# # Count of positional parameters.

echo -

echo

for i # Omitting the "in list" sets the variable -- i --

#+ to the positional parameters.

echo $i # Display new positional parameters.

done

IFS=$OIFS # Restore IFS.

Question:

Is it necessary to set an new IFS, internal field separator,

+ in order for this script to work properly?

What happens if you don't? Try it.

And, why use the new IFS -- a colon -- in line 17,

+ to append to the loop variable?

What is the purpose of this?

exit 0

$ ./revposparams.sh

k0 =

k = a b

k0 = a b

k = c a b

k0 = c a b

k = d e c a b

d e

a b

Invoking set without any options or arguments simply lists all

the environmental and other variables that have been

initialized.

bash$ set

AUTHORCOPY=/home/bozo/posts

BASH=/bin/bash

BASH_VERSION= 2.05.8(1)-release'

...

XAUTHORITY=/home/bozo/.Xauthority

_=/etc/bashrc

variable22=abc

variable23=xzy

Using set with the -- option explicitly assigns the contents

of a variable to the positional parameters. If no variable

follows the -- it unsets the positional parameters.

Example 15-18. Reassigning the positional parameters

!/bin/bash

variable="one two three four five"

set -- $variable

Sets positional parameters to the contents of "$variable".

first_param=$1

second_param=$2

shift; shift # Shift past first two positional params.

shift 2 also works.

remaining_params="$*"

echo

echo "first parameter = $first_param" # one

echo "second parameter = $second_param" # two

echo "remaining parameters = $remaining_params" # three four five

echo; echo

Again.

set -- $variable

first_param=$1

second_param=$2

echo "first parameter = $first_param" # one

echo "second parameter = $second_param" # two

======================================================

set --

Unsets positional parameters if no variable specified.

first_param=$1

second_param=$2

echo "first parameter = $first_param" # (null value)

echo "second parameter = $second_param" # (null value)

exit 0

See also Example 11-2 and Example 16-56.

unset

The unset command deletes a shell variable, effectively

setting it to null. Note that this command does not affect

positional parameters.

bash$ unset PATH

bash$ echo $PATH

bash$

Example 15-19. "Unsetting" a variable

!/bin/bash

unset.sh: Unsetting a variable.

variable=hello # Initialized.

echo "variable = $variable"

unset variable # Unset.

# In this particular context,

#+ same effect as: variable=

echo "(unset) variable = $variable" # $variable is null.

if [ -z "$variable" ] # Try a string-length test.

then

echo "\$variable has zero length."

exit 0

Note

In most contexts, an undeclared variable and one that has been unset

are equivalent. However, the ${parameter:-default} parameter

substitution construct can distinguish between the two.

export

The export [61] command makes available variables to all child

processes of the running script or shell. One important use of

the export command is in startup files, to initialize and make

accessible environmental variables to subsequent user

processes.

Caution

Unfortunately, there is no way to export variables back to the parent

process, to the process that called or invoked the script or shell.

Example 15-20. Using export to pass a variable to an embedded

awk script

!/bin/bash

Yet another version of the "column totaler" script (col-totaler.sh)

+ that adds up a specified column (of numbers) in the target file.

This uses the environment to pass a script variable to 'awk' . . .

+ and places the awk script in a variable.

ARGS=2

E_WRONGARGS=85

if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.

then

echo "Usage: `basename $0` filename column-number"

exit $E_WRONGARGS

filename=$1

column_number=$2

===== Same as original script, up to this point =====#

export column_number

Export column number to environment, so it's available for retrieval.

-----------------------------------------------

awkscript='{ total += $ENVIRON["column_number"] }

END { print total }'

Yes, a variable can hold an awk script.

-----------------------------------------------

Now, run the awk script.

awk "$awkscript" "$filename"

Thanks, Stephane Chazelas.

exit 0

Tip

It is possible to initialize and export variables in the same

operation, as in export var1=xxx.

However, as Greg Keraunen points out, in certain situations this may

have a different effect than setting a variable, then exporting it.

bash$ export var=(a b); echo ${var[0]}

(a b)

bash$ var=(a b); export var; echo ${var[0]}

Note

A variable to be exported may require special treatment. See Example

M-2.

declare, typeset

The declare and typeset commands specify and/or restrict

properties of variables.

readonly

Same as declare -r, sets a variable as read-only, or, in

effect, as a constant. Attempts to change the variable fail

with an error message. This is the shell analog of the C

language const type qualifier.

getopts

This powerful tool parses command-line arguments passed to the

script. This is the Bash analog of the getopt external command

and the getopt library function familiar to C programmers. It

permits passing and concatenating multiple options [62] and

associated arguments to a script (for example scriptname -abc

-e /usr/local).

The getopts construct uses two implicit variables. $OPTIND is

the argument pointer (OPTion INDex) and $OPTARG (OPTion

ARGument) the (optional) argument attached to an option. A

colon following the option name in the declaration tags that

option as having an associated argument.

A getopts construct usually comes packaged in a while loop,

which processes the options and arguments one at a time, then

increments the implicit $OPTIND variable to point to the next.

Note

1. The arguments passed from the command-line to the script

must be preceded by a dash (-). It is the prefixed - that

lets getopts recognize command-line arguments as options. In

fact, getopts will not process arguments without the

prefixed -, and will terminate option processing at the

first argument encountered lacking them.

2. The getopts template differs slightly from the standard

while loop, in that it lacks condition brackets.

3. The getopts construct is a highly functional replacement for

the traditional getopt external command.

while getopts ":abcde:fg" Option

Initial declaration.

a, b, c, d, e, f, and g are the options (flags) expected.

The : after option 'e' shows it will have an argument passed with it.

case $Option in

a ) # Do something with variable 'a'.

b ) # Do something with variable 'b'.

...

e) # Do something with 'e', and also with $OPTARG,

# which is the associated argument passed with option 'e'.

...

g ) # Do something with variable 'g'.

esac

done

shift $(($OPTIND - 1))

Move argument pointer to next.

All this is not nearly as complicated as it looks <grin>.

Example 15-21. Using getopts to read the options/arguments

passed to a script

!/bin/bash

ex33.sh: Exercising getopts and OPTIND

Script modified 10/09/03 at the suggestion of Bill Gradwohl.

Here we observe how 'getopts' processes command-line arguments to script.

The arguments are parsed as "options" (flags) and associated arguments.

Try invoking this script with:

'scriptname -mn'

'scriptname -oq qOption' (qOption can be some arbitrary string.)

'scriptname -qXXX -r'

'scriptname -qr'

+ - Unexpected result, takes "r" as the argument to option "q"

'scriptname -q -r'

+ - Unexpected result, same as above

'scriptname -mnop -mnop' - Unexpected result

(OPTIND is unreliable at stating where an option came from.)

If an option expects an argument ("flag:"), then it will grab

+ whatever is next on the command-line.

NO_ARGS=0

E_OPTERROR=85

if [ $# -eq "$NO_ARGS" ] # Script invoked with no command-line args?

then

echo "Usage: `basename $0` options (-mnopqrs)"

exit $E_OPTERROR # Exit and explain usage.

# Usage: scriptname -options

# Note: dash (-) necessary

while getopts ":mnopq:rs" Option

case $Option in

m ) echo "Scenario #1: option -m- [OPTIND=${OPTIND}]";;

n | o ) echo "Scenario #2: option -$Option- [OPTIND=${OPTIND}]";;

p ) echo "Scenario #3: option -p- [OPTIND=${OPTIND}]";;

q ) echo "Scenario #4: option -q-\

with argument \"$OPTARG\" [OPTIND=${OPTIND}]";;

# Note that option 'q' must have an associated argument,

#+ otherwise it falls through to the default.

r | s ) echo "Scenario #5: option -$Option-";;

* ) echo "Unimplemented option chosen.";; # Default.

esac

done

shift $(($OPTIND - 1))

Decrements the argument pointer so it points to next argument.

$1 now references the first non-option item supplied on the command-line

+ if one exists.

exit $?

As Bill Gradwohl states,

"The getopts mechanism allows one to specify: scriptname -mnop -mnop

+ but there is no reliable way to differentiate what came

+ from where by using OPTIND."

There are, however, workarounds.

Script Behavior

source, . (dot command)

This command, when invoked from the command-line, executes a

script. Within a script, a source file-name loads the file

file-name. Sourcing a file (dot-command) imports code into the

script, appending to the script (same effect as the #include

directive in a C program). The net result is the same as if

the "sourced" lines of code were physically present in the

body of the script. This is useful in situations when multiple

scripts use a common data file or function library.

Example 15-22. "Including" a data file

!/bin/bash

Note that this example must be invoked with bash, i.e., bash ex38.sh

+ not sh ex38.sh !

. data-file # Load a data file.

Same effect as "source data-file", but more portable.

The file "data-file" must be present in current working directory,

+ since it is referred to by its basename.

Now, let's reference some data from that file.

echo "variable1 (from data-file) = $variable1"

echo "variable3 (from data-file) = $variable3"

let "sum = $variable2 + $variable4"

echo "Sum of variable2 + variable4 (from data-file) = $sum"

echo "message1 (from data-file) is \"$message1\""

Escaped quotes

echo "message2 (from data-file) is \"$message2\""

print_message This is the message-print function in the data-file.

exit $?

File data-file for Example 15-22, above. Must be present in

same directory.

This is a data file loaded by a script.

Files of this type may contain variables, functions, etc.

It loads with a 'source' or '.' command from a shell script.

Let's initialize some variables.

variable1=23

variable2=474

variable3=5

variable4=97

message1="Greetings from *** line $LINENO *** of the data file!"

message2="Enough for now. Goodbye."

print_message ()

{ # Echoes any message passed to it.

if [ -z "$1" ]

then

return 1 # Error, if argument missing.

echo

until [ -z "$1" ]

do # Step through arguments passed to function.

echo -n "$1" # Echo args one at a time, suppressing line feeds.

echo -n " " # Insert spaces between words.

shift # Next one.

done

echo

return 0

}

If the sourced file is itself an executable script, then it

will run, then return control to the script that called it. A

sourced executable script may use a return for this purpose.

Arguments may be (optionally) passed to the sourced file as

positional parameters.

source $filename $arg1 arg2

It is even possible for a script to source itself, though this

does not seem to have any practical applications.

Example 15-23. A (useless) script that sources itself

!/bin/bash

self-source.sh: a script sourcing itself "recursively."

From "Stupid Script Tricks," Volume II.

MAXPASSCNT=100 # Maximum number of execution passes.

echo -n "$pass_count "

At first execution pass, this just echoes two blank spaces,

+ since $pass_count still uninitialized.

let "pass_count += 1"

Assumes the uninitialized variable $pass_count

+ can be incremented the first time around.

This works with Bash and pdksh, but

+ it relies on non-portable (and possibly dangerous) behavior.

Better would be to initialize $pass_count to 0 before incrementing.

while [ "$pass_count" -le $MAXPASSCNT ]

. $0 # Script "sources" itself, rather than calling itself.

# ./$0 (which would be true recursion) doesn't work here. Why?

done

What occurs here is not actually recursion,

+ since the script effectively "expands" itself, i.e.,

+ generates a new section of code

+ with each pass through the 'while' loop',

with each 'source' in line 20.

Of course, the script interprets each newly 'sourced' "#!" line

+ as a comment, and not as the start of a new script.

echo

exit 0 # The net effect is counting from 1 to 100.

# Very impressive.

Exercise:

--------

Write a script that uses this trick to actually do something useful.

exit

Unconditionally terminates a script. [63] The exit command may

optionally take an integer argument, which is returned to the

shell as the exit status of the script. It is good practice to

end all but the simplest scripts with an exit 0, indicating a

successful run.

Note

If a script terminates with an exit lacking an argument, the exit

status of the script is the exit status of the last command executed

in the script, not counting the exit. This is equivalent to an exit

$?.

Note

An exit command may also be used to terminate a subshell.

exec

This shell builtin replaces the current process with a

specified command. Normally, when the shell encounters a

command, it forks off a child process to actually execute the

command. Using the exec builtin, the shell does not fork, and

the command exec'ed replaces the shell. When used in a script,

therefore, it forces an exit from the script when the exec'ed

command terminates. [64]

Example 15-24. Effects of exec

!/bin/bash

exec echo "Exiting \"$0\" at line $LINENO." # Exit from script here.

$LINENO is an internal Bash variable set to the line number it's on.

----------------------------------

The following lines never execute.

echo "This echo fails to echo."

exit 99 # This script will not exit here.

# Check exit value after script terminates

#+ with an 'echo $?'.

# It will *not* be 99.

Example 15-25. A script that exec's itself

!/bin/bash

self-exec.sh

Note: Set permissions on this script to 555 or 755,

then call it with ./self-exec.sh or sh ./self-exec.sh.

echo

echo "This line appears ONCE in the script, yet it keeps echoing."

echo "The PID of this instance of the script is still $."

Demonstrates that a subshell is not forked off.

echo "==================== Hit Ctl-C to exit ===================="

sleep 1

exec $0 # Spawns another instance of this same script

#+ that replaces the previous one.

echo "This line will never echo!" # Why not?

exit 99 # Will not exit here!

# Exit code will not be 99!

An exec also serves to reassign file descriptors. For example,

exec <zzz-file replaces stdin with the file zzz-file.

Note

The -exec option to find is not the same as the exec shell builtin.

shopt

This command permits changing shell options on the fly (see

Example 25-1 and Example 25-2). It often appears in the Bash

startup files, but also has its uses in scripts. Needs version

2 or later of Bash.

shopt -s cdspell

Allows minor misspelling of directory names with 'cd'

Option -s sets, -u unsets.

cd /hpme # Oops! Mistyped '/home'.

pwd # /home

# The shell corrected the misspelling.

caller

Putting a caller command inside a function echoes to stdout

information about the caller of that function.

!/bin/bash

function1 ()

{

# Inside function1 ().

caller 0 # Tell me about it.

}

function1 # Line 9 of script.

9 main test.sh

^ Line number that the function was called from.

^^^^ Invoked from "main" part of script.

^^^^^^^ Name of calling script.

caller 0 # Has no effect because it's not inside a function.

A caller command can also return caller information from a

script sourced within another script. Analogous to a function,

this is a "subroutine call."

You may find this command useful in debugging.

Commands

true

A command that returns a successful (zero) exit status, but

does nothing else.

bash$ true

bash$ echo $?

Endless loop

while true # alias for ":"

operation-1

operation-2

...

operation-n

# Need a way to break out of loop or script will hang.

done

false

A command that returns an unsuccessful exit status, but does

nothing else.

bash$ false

bash$ echo $?

Testing "false"

if false

then

echo "false evaluates \"true\""

else

echo "false evaluates \"false\""

false evaluates "false"

Looping while "false" (null loop)

while false

# The following code will not execute.

operation-1

operation-2

...

operation-n

# Nothing happens!

done

type [cmd]

Similar to the which external command, type cmd identifies

"cmd." Unlike which, type is a Bash builtin. The useful -a

option to type identifies keywords and builtins, and also

locates system commands with identical names.

bash$ type '['

[ is a shell builtin

bash$ type -a '['

[ is a shell builtin

[ is /usr/bin/[

bash$ type type

type is a shell builtin

The type command can be useful for testing whether a certain

command exists.

hash [cmds]

Records the path name of specified commands -- in the shell

hash table [65] -- so the shell or script will not need to

search the $PATH on subsequent calls to those commands. When

hash is called with no arguments, it simply lists the commands

that have been hashed. The -r option resets the hash table.

bind

The bind builtin displays or modifies readline [66] key

bindings.

help

Gets a short usage summary of a shell builtin. This is the

counterpart to whatis, but for builtins. The display of help

information got a much-needed update in the version 4 release

of Bash.

bash$ help exit

exit: exit [n]

Exit the shell with a status of N. If N is omitted, the exit status

is that of the last command executed.

________________________________________________________________

15.1. Job Control Commands

Certain of the following job control commands take a job identifier

as an argument. See the table at end of the chapter.

jobs

Lists the jobs running in the background, giving the job

number. Not as useful as ps.

Note

It is all too easy to confuse jobs and processes. Certain builtins,

such as kill, disown, and wait accept either a job number or a

process number as an argument. The fg, bg and jobs commands accept

only a job number.

bash$ sleep 100 &

[1] 1384

bash $ jobs

[1]+ Running sleep 100 &

"1" is the job number (jobs are maintained by the current shell).

"1384" is the PID or process ID number (processes are maintained by

the system). To kill this job/process, either a kill %1 or a kill

1384 works.

Thanks, S.C.

disown

Remove job(s) from the shell's table of active jobs.

fg, bg

The fg command switches a job running in the background into

the foreground. The bg command restarts a suspended job, and

runs it in the background. If no job number is specified, then

the fg or bg command acts upon the currently running job.

wait

Suspend script execution until all jobs running in background

have terminated, or until the job number or process ID

specified as an option terminates. Returns the exit status of

waited-for command.

You may use the wait command to prevent a script from exiting

before a background job finishes executing (this would create

a dreaded orphan process).

Example 15-26. Waiting for a process to finish before

proceeding

!/bin/bash

ROOT_UID=0 # Only users with $UID 0 have root privileges.

E_NOTROOT=65

E_NOPARAMS=66

if [ "$UID" -ne "$ROOT_UID" ]

then

echo "Must be root to run this script."

# "Run along kid, it's past your bedtime."

exit $E_NOTROOT

if [ -z "$1" ]

then

echo "Usage: `basename $0` find-string"

exit $E_NOPARAMS

echo "Updating 'locate' database..."

echo "This may take a while."

updatedb /usr & # Must be run as root.

wait

Don't run the rest of the script until 'updatedb' finished.

You want the the database updated before looking up the file name.

locate $1

Without the 'wait' command, in the worse case scenario,

+ the script would exit while 'updatedb' was still running,

+ leaving it as an orphan process.

exit 0

Optionally, wait can take a job identifier as an argument, for

example, wait%1 or wait $PPID. [67] See the job id table.

Tip

Within a script, running a command in the background with an

ampersand (&) may cause the script to hang until ENTER is hit. This

seems to occur with commands that write to stdout. It can be a major

annoyance.

!/bin/bash

test.sh

ls -l &

echo "Done."

bash$ ./test.sh

Done.

[bozo@localhost test-scripts]$ total 1

-rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh

As Walter Brameld IV explains it:

As far as I can tell, such scripts don't actually hang. It jus

seems that they do because the background command writes text

the console after the prompt. The user gets the impression tha

the prompt was never displayed. Here's the sequence of events:

1. Script launches background command.

2. Script exits.

3. Shell displays the prompt.

4. Background command continues running and writing text to th

console.

5. Background command finishes.

6. User doesn't see a prompt at the bottom of the output, thin

ks script

is hanging.

Placing a wait after the background command seems to remedy this.

!/bin/bash

test.sh

ls -l &

echo "Done."

wait

bash$ ./test.sh

Done.

[bozo@localhost test-scripts]$ total 1

-rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh

Redirecting the output of the command to a file or even to

/dev/null also takes care of this problem.

suspend

This has a similar effect to Control-Z, but it suspends the

shell (the shell's parent process should resume it at an

appropriate time).

logout

Exit a login shell, optionally specifying an exit status.

times

Gives statistics on the system time elapsed when executing

commands, in the following form:

0m0.020s 0m0.020s

This capability is of relatively limited value, since it is

not common to profile and benchmark shell scripts.

kill

Forcibly terminate a process by sending it an appropriate

terminate signal (see Example 17-6).

Example 15-27. A script that kills itself

!/bin/bash

self-destruct.sh

kill $ # Script kills its own process here.

# Recall that "$" is the script's PID.

echo "This line will not echo."

Instead, the shell sends a "Terminated" message to stdout.

exit 0 # Normal exit? No!

After this script terminates prematurely,

+ what exit status does it return?

sh self-destruct.sh

echo $?

143 143 = 128 + 15

TERM signal

Note

kill -l lists all the signals (as does the file

/usr/include/asm/signal.h). A kill -9 is a sure kill, which will

usually terminate a process that stubbornly refuses to die with a

plain kill. Sometimes, a kill -15 works. A zombie process, that is, a

child process that has terminated, but that the parent process has

not (yet) killed, cannot be killed by a logged-on user -- you can't

kill something that is already dead -- but init will generally clean

it up sooner or later.

killall

The killall command kills a running process by name, rather

than by process ID. If there are multiple instances of a

particular command running, then doing a killall on that

command will terminate them all.

Note

This refers to the killall command in /usr/bin, not the killall

script in /etc/rc.d/init.d.

command

The command directive disables aliases and functions for the

command immediately following it.

bash$ command ls

Note

This is one of three shell directives that effect script command

processing. The others are builtin and enable.

builtin

Invoking builtin BUILTIN_COMMAND runs the command

BUILTIN_COMMAND as a shell builtin, temporarily disabling both

functions and external system commands with the same name.

enable

This either enables or disables a shell builtin command. As an

example, enable -n kill disables the shell builtin kill, so

that when Bash subsequently encounters kill, it invokes the

external command /bin/kill.

The -a option to enable lists all the shell builtins,

indicating whether or not they are enabled. The -f filename

option lets enable load a builtin as a shared library (DLL)

module from a properly compiled object file. [68].

autoload

This is a port to Bash of the ksh autoloader. With autoload in

place, a function with an autoload declaration will load from

an external file at its first invocation. [69] This saves

system resources.

Note that autoload is not a part of the core Bash

installation. It needs to be loaded in with enable -f (see

above).

Table 15-1. Job identifiers

Notation Meaning

%N Job number [N]

%S Invocation (command-line) of job begins with string S

%?S Invocation (command-line) of job contains within it string S

%% "current" job (last job stopped in foreground or started in

background)

%+ "current" job (last job stopped in foreground or started in

background)

%- Last job

$! Last background process

________________________________________________________________

Chapter 16. External Filters, Programs and Commands

Standard UNIX commands make shell scripts more versatile. The power

of scripts comes from coupling system commands and shell directives

with simple programming constructs.

________________________________________________________________

16.1. Basic Commands

The first commands a novice learns

The basic file "list" command. It is all too easy to

underestimate the power of this humble command. For example,

using the -R, recursive option, ls provides a tree-like

listing of a directory structure. Other useful options are -S,

sort listing by file size, -t, sort by file modification time,

-v, sort by (numerical) version numbers embedded in the

filenames, [70] -b, show escape characters, and -i, show file

inodes (see Example 16-4).

bash$ ls -l

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt

bash$ ls -lv

total 0

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt

-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt

Tip

The ls command returns a non-zero exit status when attempting to list

a non-existent file.

bash$ ls abc

ls: abc: No such file or directory

bash$ echo $?

Example 16-1. Using ls to create a table of contents for

burning a CDR disk

!/bin/bash

ex40.sh (burn-cd.sh)

Script to automate burning a CDR.

SPEED=10 # May use higher speed if your hardware supports it.

IMAGEFILE=cdimage.iso

CONTENTSFILE=contents

DEVICE=/dev/cdrom For older versions of cdrecord

DEVICE="1,0,0"

DEFAULTDIR=/opt # This is the directory containing the data to be burned.

# Make sure it exists.

# Exercise: Add a test for this.

Uses Joerg Schilling's "cdrecord" package:

http://www.fokus.fhg.de/usr/schilling/cdrecord.html

If this script invoked as an ordinary user, may need to suid cdrecord

+ chmod u+s /usr/bin/cdrecord, as root.

Of course, this creates a security hole, though a relatively minor one.

if [ -z "$1" ]

then

IMAGE_DIRECTORY=$DEFAULTDIR

# Default directory, if not specified on command-line.

else

IMAGE_DIRECTORY=$1

Create a "table of contents" file.

ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$CONTENTSFILE

The "l" option gives a "long" file listing.

The "R" option makes the listing recursive.

The "F" option marks the file types (directories get a trailing /).

echo "Creating table of contents."

Create an image file preparatory to burning it onto the CDR.

mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY

echo "Creating ISO9660 file system image ($IMAGEFILE)."

Burn the CDR.

echo "Burning the disk."

echo "Please be patient, this will take a while."

wodim -v -isosize dev=$DEVICE $IMAGEFILE

In newer Linux distros, the "wodim" utility assumes the

+ functionality of "cdrecord."

exitcode=$?

echo "Exit code = $exitcode"

exit $exitcode

cat, tac

cat, an acronym for concatenate, lists a file to stdout. When

combined with redirection (> or >>), it is commonly used to

concatenate files.

Uses of 'cat'

cat filename # Lists the file.

cat file.1 file.2 file.3 > file.123 # Combines three files into one.

The -n option to cat inserts consecutive numbers before all

lines of the target file(s). The -b option numbers only the

non-blank lines. The -v option echoes nonprintable characters,

using ^ notation. The -s option squeezes multiple consecutive

blank lines into a single blank line.

See also Example 16-28 and Example 16-24.

Note

In a pipe, it may be more efficient to redirect the stdin to a file,

rather than to cat the file.

cat filename | tr a-z A-Z

tr a-z A-Z < filename # Same effect, but starts one less process,

#+ and also dispenses with the pipe.

tac, is the inverse of cat, listing a file backwards from its

end.

rev

reverses each line of a file, and outputs to stdout. This does

not have the same effect as tac, as it preserves the order of

the lines, but flips each one around (mirror image).

bash$ cat file1.txt

This is line 1.

This is line 2.

bash$ tac file1.txt

This is line 2.

This is line 1.

bash$ rev file1.txt

.1 enil si sihT

.2 enil si sihT

This is the file copy command. cp file1 file2 copies file1 to

file2, overwriting file2 if it already exists (see Example

16-6).

Tip

Particularly useful are the -a archive flag (for copying an entire

directory tree), the -u update flag (which prevents overwriting

identically-named newer files), and the -r and -R recursive flags.

cp -u source_dir/* dest_dir

"Synchronize" dest_dir to source_dir

+ by copying over all newer and not previously existing files.

This is the file move command. It is equivalent to a

combination of cp and rm. It may be used to move multiple

files to a directory, or even to rename a directory. For some

examples of using mv in a script, see Example 10-11 and

Example A-2.

Note

When used in a non-interactive script, mv takes the -f (force) option

to bypass user input.

When a directory is moved to a preexisting directory, it becomes a

subdirectory of the destination directory.

bash$ mv source_directory target_directory

bash$ ls -lF target_directory

total 1

drwxrwxr-x 2 bozo bozo 1024 May 28 19:20 source_directory/

Delete (remove) a file or files. The -f option forces removal

of even readonly files, and is useful for bypassing user input

in a script.

Note

The rm command will, by itself, fail to remove filenames beginning

with a dash. Why? Because rm sees a dash-prefixed filename as an

option.

bash$ rm -badname

rm: invalid option -- b

Try `rm --help' for more information.

One clever workaround is to precede the filename with a " -- " (the

end-of-options flag).

bash$ rm -- -badname

Another method to is to preface the filename to be removed

with a dot-slash .

bash$ rm ./-badname

Warning

When used with the recursive flag -r, this command removes files all

the way down the directory tree from the current directory. A

careless rm -rf * can wipe out a big chunk of a directory structure.

rmdir

Remove directory. The directory must be empty of all files --

including "invisible" dotfiles [71] -- for this command to

succeed.

mkdir

Make directory, creates a new directory. For example, mkdir -p

project/programs/December creates the named directory. The -p

option automatically creates any necessary parent directories.

chmod

Changes the attributes of an existing file or directory (see

Example 15-14).

chmod +x filename

Makes "filename" executable for all users.

chmod u+s filename

Sets "suid" bit on "filename" permissions.

An ordinary user may execute "filename" with same privileges as the file's o

wner.

(This does not apply to shell scripts.)

chmod 644 filename

Makes "filename" readable/writable to owner, readable to others

+ (octal mode).

chmod 444 filename

Makes "filename" read-only for all.

Modifying the file (for example, with a text editor)

+ not allowed for a user who does not own the file (except for root),

+ and even the file owner must force a file-save

+ if she modifies the file.

Same restrictions apply for deleting the file.

chmod 1777 directory-name

Gives everyone read, write, and execute permission in directory,

+ however also sets the "sticky bit".

This means that only the owner of the directory,

+ owner of the file, and, of course, root

+ can delete any particular file in that directory.

chmod 111 directory-name

Gives everyone execute-only permission in a directory.

This means that you can execute and READ the files in that directory

+ (execute permission necessarily includes read permission

+ because you can't execute a file without being able to read it).

But you can't list the files or search for them with the "find" command.

These restrictions do not apply to root.

chmod 000 directory-name

No permissions at all for that directory.

Can't read, write, or execute files in it.

Can't even list files in it or "cd" to it.

But, you can rename (mv) the directory

+ or delete it (rmdir) if it is empty.

You can even symlink to files in the directory,

+ but you can't read, write, or execute the symlinks.

These restrictions do not apply to root.

chattr

Change file attributes. This is analogous to chmod above, but

with different options and a different invocation syntax, and

it works only on ext2/ext3 filesystems.

One particularly interesting chattr option is i. A chattr +i

filename marks the file as immutable. The file cannot be

modified, linked to, or deleted, not even by root. This file

attribute can be set or removed only by root. In a similar

fashion, the a option marks the file as append only.

root# chattr +i file1.txt

root# rm file1.txt

rm: remove write-protected regular file `file1.txt'? y

rm: cannot remove `file1.txt': Operation not permitted

If a file has the s (secure) attribute set, then when it is

deleted its block is overwritten with binary zeroes. [72]

If a file has the u (undelete) attribute set, then when it is

deleted, its contents can still be retrieved (undeleted).

If a file has the c (compress) attribute set, then it will

automatically be compressed on writes to disk, and

uncompressed on reads.

Note

The file attributes set with chattr do not show in a file listing (ls

-l).

Creates links to pre-existings files. A "link" is a reference

to a file, an alternate name for it. The ln command permits

referencing the linked file by more than one name and is a

superior alternative to aliasing (see Example 4-6).

The ln creates only a reference, a pointer to the file only a

few bytes in size.

The ln command is most often used with the -s, symbolic or

"soft" link flag. Advantages of using the -s flag are that it

permits linking across file systems or to directories.

The syntax of the command is a bit tricky. For example: ln -s

oldfile newfile links the previously existing oldfile to the

newly created link, newfile.

Caution

If a file named newfile has previously existed, an error message will

result.

Which type of link to use?

As John Macdonald explains it:

Both of these [types of links] provide a certain measure of dual

reference -- if you edit the contents of the file using any name,

your changes will affect both the original name and either a hard or

soft new name. The differences between them occurs when you work at a

higher level. The advantage of a hard link is that the new name is

totally independent of the old name -- if you remove or rename the

old name, that does not affect the hard link, which continues to

point to the data while it would leave a soft link hanging pointing

to the old name which is no longer there. The advantage of a soft

link is that it can refer to a different file system (since it is

just a reference to a file name, not to actual data). And, unlike a

hard link, a symbolic link can refer to a directory.

Links give the ability to invoke a script (or any other type

of executable) with multiple names, and having that script

behave according to how it was invoked.

Example 16-2. Hello or Good-bye

!/bin/bash

hello.sh: Saying "hello" or "goodbye"

+ depending on how script is invoked.

Make a link in current working directory ($PWD) to this script:

ln -s hello.sh goodbye

Now, try invoking this script both ways:

./hello.sh

./goodbye

HELLO_CALL=65

GOODBYE_CALL=66

if [ $0 = "./goodbye" ]

then

echo "Good-bye!"

# Some other goodbye-type commands, as appropriate.

exit $GOODBYE_CALL

echo "Hello!"

Some other hello-type commands, as appropriate.

exit $HELLO_CALL

man, info

These commands access the manual and information pages on

system commands and installed utilities. When available, the

info pages usually contain more detailed descriptions than do

the man pages.

There have been various attempts at "automating" the writing

of man pages. For a script that makes a tentative first step

in that direction, see Example A-39.

________________________________________________________________

16.2. Complex Commands

Commands for more advanced users

find

-exec COMMAND \;

Carries out COMMAND on each file that find matches. The

command sequence terminates with ; (the ";" is escaped to make

certain the shell passes it to find literally, without

interpreting it as a special character).

bash$ find ~/ -name '*.txt'

/home/bozo/.kde/share/apps/karm/karmdata.txt

/home/bozo/misc/irmeyc.txt

/home/bozo/test-scripts/1.txt

If COMMAND contains {}, then find substitutes the full path

name of the selected file for "{}".

find ~/ -name 'core*' -exec rm {} \;

Removes all core dump files from user's home directory.

find /home/bozo/projects -mtime -1

^ Note minus sign!

Lists all files in /home/bozo/projects directory tree

+ that were modified within the last day (current_day - 1).

find /home/bozo/projects -mtime 1

Same as above, but modified exactly one day ago.

mtime = last modification time of the target file

ctime = last status change time (via 'chmod' or otherwise)

atime = last access time

DIR=/home/bozo/junk_files

find "$DIR" -type f -atime +5 -exec rm {} \;

^ ^^

Curly brackets are placeholder for the path name output by "find."

Deletes all files in "/home/bozo/junk_files"

+ that have not been accessed in at least 5 days (plus sign ... +5).

"-type filetype", where

f = regular file

d = directory

l = symbolic link, etc.

(The 'find' manpage and info page have complete option listings.)

find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {

} \;

Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.

There a few extraneous hits. Can they be filtered out?

Possibly by:

find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \

| grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*

[:digit:] is one of the character classes

+ introduced with the POSIX 1003.2 standard.

Thanks, Stéphane Chazelas.

Note

The -exec option to find should not be confused with the exec shell

builtin.

Example 16-3. Badname, eliminate file names in current

directory containing bad characters and whitespace.

!/bin/bash

badname.sh

Delete filenames in current directory containing bad characters.

for filename in *

badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\<\>\&\*\|\$]/p`

badname=`echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p'` also works.

Deletes files containing these nasties: + { ; " \ = ? ~ ( ) < > & * | $

rm $badname 2>/dev/null

^^^^^^^^^^^ Error messages deep-sixed.

done

Now, take care of files containing all manner of whitespace.

find . -name "* *" -exec rm -f {} \;

The path name of the file that _find_ finds replaces the "{}".

The '\' ensures that the ';' is interpreted literally, as end of command.

exit 0

---------------------------------------------------------------------

Commands below this line will not execute because of _exit_ command.

An alternative to the above script:

find . -name '*[+{;"\\=?~()<>&*|$ ]*' -maxdepth 0 \

-exec rm -f '{}' \;

The "-maxdepth 0" option ensures that _find_ will not search

+ subdirectories below $PWD.

(Thanks, S.C.)

Example 16-4. Deleting a file by its inode number

!/bin/bash

idelete.sh: Deleting a file by its inode number.

This is useful when a filename starts with an illegal character,

+ such as ? or -.

ARGCOUNT=1 # Filename arg must be passed to script.

E_WRONGARGS=70

E_FILE_NOT_EXIST=71

E_CHANGED_MIND=72

if [ $# -ne "$ARGCOUNT" ]

then

echo "Usage: `basename $0` filename"

exit $E_WRONGARGS

if [ ! -e "$1" ]

then

echo "File \""$1"\" does not exist."

exit $E_FILE_NOT_EXIST

inum=`ls -i | grep "$1" | awk '{print $1}'`

inum = inode (index node) number of file

-----------------------------------------------------------------------

Every file has an inode, a record that holds its physical address info.

-----------------------------------------------------------------------

echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? "

The '-v' option to 'rm' also asks this.

read answer

case "$answer" in

[nN]) echo "Changed your mind, huh?"

exit $E_CHANGED_MIND

;;

) echo "Deleting file \"$1\".";;

esac

find . -inum $inum -exec rm {} \;

^^

Curly brackets are placeholder

+ for text output by "find."

echo "File "\"$1"\" deleted!"

exit 0

The find command also works without the -exec option.

!/bin/bash

Find suid root files.

A strange suid file might indicate a security hole,

+ or even a system intrusion.

directory="/usr/sbin"

Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc.

permissions="+4000" # suid root (dangerous!)

for file in $( find "$directory" -perm "$permissions" )

ls -ltF --author "$file"

done

See Example 16-30, Example 3-4, and Example 11-10 for scripts

using find. Its manpage provides more detail on this complex

and powerful command.

xargs

A filter for feeding arguments to a command, and also a tool

for assembling the commands themselves. It breaks a data

stream into small enough chunks for filters and commands to

process. Consider it as a powerful replacement for backquotes.

In situations where command substitution fails with a too many

arguments error, substituting xargs often works. [73]

Normally, xargs reads from stdin or from a pipe, but it can

also be given the output of a file.

The default command for xargs is echo. This means that input

piped to xargs may have linefeeds and other whitespace

characters stripped out.

bash$ ls -l

total 0

-rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1

-rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2

bash$ ls -l | xargs

total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 J

an...

bash$ find ~/mail -type f | xargs grep "Linux"

./misc:User-Agent: slrn/0.9.8.1 (Linux)

./sent-mail-jul-2005: hosted by the Linux Documentation Project.

./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)

./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article

./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem

. . .

ls | xargs -p -l gzip gzips every file in current directory,

one at a time, prompting before each operation.

Note

Note that xargs processes the arguments passed to it sequentially,

one at a time.

bash$ find /usr/bin | xargs file

/usr/bin: directory

/usr/bin/foomatic-ppd-options: perl script text executable

. . .

Tip

An interesting xargs option is -n NN, which limits to NN the number

of arguments passed.

ls | xargs -n 8 echo lists the files in the current directory in 8

columns.

Tip

Another useful option is -0, in combination with find -print0 or grep

-lZ. This allows handling arguments containing whitespace or quotes.

find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f

grep -rliwZ GUI / | xargs -0 rm -f

Either of the above will remove any file containing "GUI". (Thanks,

S.C.)

Or:

cat /proc/"$pid"/"$OPTION" | xargs -0 echo

Formats output: ^^^^^^^^^^^^^^^

From Han Holl's fixup of "get-commandline.sh"

+ script in "/dev and /proc" chapter.

Tip

The -P option to xargs permits running processes in parallel. This

speeds up execution in a machine with a multicore CPU.

!/bin/bash

ls *gif | xargs -t -n1 -P2 gif2png

Converts all the gif images in current directory to png.

Options:

=======

-t Print command to stderr.

-n1 At most 1 argument per command line.

-P2 Run up to 2 processes simultaneously.

Thank you, Roberto Polli, for the inspiration.

Example 16-5. Logfile: Using xargs to monitor system log

!/bin/bash

Generates a log file in current directory

from the tail end of /var/log/messages.

Note: /var/log/messages must be world readable

if this script invoked by an ordinary user.

#root chmod 644 /var/log/messages

LINES=5

( date; uname -a ) >>logfile

Time and machine name

echo ---------------------------------------------------------- >>logfile

tail -n $LINES /var/log/messages | xargs | fmt -s >>logfile

echo >>logfile

exit 0

Note:

----

As Frank Wang points out,

+ unmatched quotes (either single or double quotes) in the source file

+ may give xargs indigestion.

He suggests the following substitution for line 15:

tail -n $LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s >>logfile

Exercise:

--------

Modify this script to track changes in /var/log/messages at intervals

+ of 20 minutes.

Hint: Use the "watch" command.

As in find, a curly bracket pair serves as a placeholder for

replacement text.

Example 16-6. Copying files in current directory to another

!/bin/bash

copydir.sh

Copy (verbose) all files in current directory ($PWD)

+ to directory specified on command-line.

E_NOARGS=85

if [ -z "$1" ] # Exit if no argument given.

then

echo "Usage: `basename $0` directory-to-copy-to"

exit $E_NOARGS

ls . | xargs -i -t cp ./{} $1

^^ ^^ ^^

-t is "verbose" (output command-line to stderr) option.

-i is "replace strings" option.

{} is a placeholder for output text.

This is similar to the use of a curly-bracket pair in "find."

List the files in current directory (ls .),

+ pass the output of "ls" as arguments to "xargs" (-i -t options),

+ then copy (cp) these arguments ({}) to new directory ($1).

The net result is the exact equivalent of

+ cp * $1

+ unless any of the filenames has embedded "whitespace" characters.

exit 0

Example 16-7. Killing processes by name

!/bin/bash

kill-byname.sh: Killing processes by name.

Compare this script with kill-process.sh.

For instance,

+ try "./kill-byname.sh xterm" --

+ and watch all the xterms on your desktop disappear.

Warning:

-------

This is a fairly dangerous script.

Running it carelessly (especially as root)

+ can cause data loss and other undesirable effects.

E_BADARGS=66

if test -z "$1" # No command-line arg supplied?

then

echo "Usage: `basename $0` Process(es)_to_kill"

exit $E_BADARGS

PROCESS_NAME="$1"

ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/nul

^^ ^^

---------------------------------------------------------------

Notes:

-i is the "replace strings" option to xargs.

The curly brackets are the placeholder for the replacement.

2&>/dev/null suppresses unwanted error messages.

Can grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"?

---------------------------------------------------------------

exit $?

The "killall" command has the same effect as this script,

+ but using it is not quite as educational.

Example 16-8. Word frequency analysis using xargs

!/bin/bash

wf2.sh: Crude word frequency analysis on a text file.

Uses 'xargs' to decompose lines of text into single words.

Compare this example to the "wf.sh" script later on.

Check for input file on command-line.

ARGS=1

E_BADARGS=85

E_NOFILE=86

if [ $# -ne "$ARGS" ]

Correct number of arguments passed to script?

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

if [ ! -f "$1" ] # Does file exist?

then

echo "File \"$1\" does not exist."

exit $E_NOFILE

cat "$1" | xargs -n1 | \

List the file, one word per line.

tr A-Z a-z | \

Shift characters to lowercase.

sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\

/g' | \

Filter out periods and commas, and

+ change space between words to linefeed,

sort | uniq -c | sort -nr

Finally remove duplicates, prefix occurrence count

+ and sort numerically.

This does the same job as the "wf.sh" example,

+ but a bit more ponderously, and it runs more slowly (why?).

exit $?

expr

All-purpose expression evaluator: Concatenates and evaluates

the arguments according to the operation given (arguments must

be separated by spaces). Operations may be arithmetic,

comparison, string, or logical.

expr 3 + 5

returns 8

expr 5 % 3

returns 2

expr 1 / 0

returns the error message, expr: division by zero

Illegal arithmetic operations not allowed.

expr 5 \* 3

returns 15

The multiplication operator must be escaped when used in

an arithmetic expression with expr.

y=`expr $y + 1`

Increment a variable, with the same effect as let y=y+1

and y=$(($y+1)). This is an example of arithmetic

expansion.

z=`expr substr $string $position $length`

Extract substring of $length characters, starting at

$position.

Example 16-9. Using expr

!/bin/bash

Demonstrating some of the uses of 'expr'

=======================================

echo

Arithmetic Operators

---------- ---------

echo "Arithmetic Operators"

echo

a=`expr 5 + 3`

echo "5 + 3 = $a"

a=`expr $a + 1`

echo

echo "a + 1 = $a"

echo "(incrementing a variable)"

a=`expr 5 % 3`

modulo

echo

echo "5 mod 3 = $a"

echo

Logical Operators

------- ---------

Returns 1 if true, 0 if false,

+ opposite of normal Bash convention.

echo "Logical Operators"

echo

x=24

y=25

b=`expr $x = $y` # Test equality.

echo "b = $b" # 0 ( $x -ne $y )

echo

a=3

b=`expr $a \> 10`

echo 'b=`expr $a \> 10`, therefore...'

echo "If a > 10, b = 0 (false)"

echo "b = $b" # 0 ( 3 ! -gt 10 )

echo

b=`expr $a \< 10`

echo "If a < 10, b = 1 (true)"

echo "b = $b" # 1 ( 3 -lt 10 )

echo

Note escaping of operators.

b=`expr $a \<= 3`

echo "If a <= 3, b = 1 (true)"

echo "b = $b" # 1 ( 3 -le 3 )

There is also a "\>=" operator (greater than or equal to).

echo

String Operators

------ ---------

echo "String Operators"

echo

a=1234zipper43231

echo "The string being operated upon is \"$a\"."

length: length of string

b=`expr length $a`

echo "Length of \"$a\" is $b."

index: position of first character in substring

that matches a character in string

b=`expr index $a 23`

echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."

substr: extract substring, starting position & length specified

b=`expr substr $a 2 6`

echo "Substring of \"$a\", starting at position 2,\

and 6 chars long is \"$b\"."

The default behavior of the 'match' operations is to

+ search for the specified match at the BEGINNING of the string.

Using Regular Expressions ...

b=`expr match "$a" '[0-9]*'` # Numerical count.

echo Number of digits at the beginning of \"$a\" is $b.

b=`expr match "$a" '$[0-9]*$'` # Note that escaped parentheses

== == #+ trigger substring match.

echo "The digits at the beginning of \"$a\" are \"$b\"."

echo

exit 0

Important

The : (null) operator can substitute for match. For example, b=`expr

$a : [0-9]*` is the exact equivalent of b=`expr match $a [0-9]*` in

the above listing.

!/bin/bash

echo

echo "String operations using \"expr \$string : \" construct"

echo "==================================================="

echo

a=1234zipper5FLIPPER43231

echo "The string being operated upon is \"`expr "$a" : '$.*$'`\"."

Escaped parentheses grouping operator. == ==

***************************

+ Escaped parentheses

+ match a substring

***************************

If no escaped parentheses ...

+ then 'expr' converts the string operand to an integer.

echo "Length of \"$a\" is `expr "$a" : '.*'`." # Length of string

echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."

------------------------------------------------------------------------- #

echo

echo "The digits at the beginning of \"$a\" are `expr "$a" : '$[0-9]*$'`."

== ==

echo "The first 7 characters of \"$a\" are `expr "$a" : '$.......$'`."

===== == ==

Again, escaped parentheses force a substring match.

echo "The last 7 characters of \"$a\" are `expr "$a" : '.*$.......$'`."

==== end of string operator ^^

(In fact, means skip over one or more of any characters until specified

+ substring found.)

echo

exit 0

The above script illustrates how expr uses the escaped parentheses --

$ ... $ -- grouping operator in tandem with regular expression

parsing to match a substring. Here is a another example, this time

from "real life."

Strip the whitespace from the beginning and end.

LRFDATE=`expr "$LRFDATE" : '[[:space:]]*$.*$[[:space:]]* `

From Peter Knowles' "booklistgen.sh" script

+ for converting files to Sony Librie/PRS-50X format.

(http://booklistgensh.peterknowles.com)

Perl, sed, and awk have far superior string parsing facilities. A

short sed or awk "subroutine" within a script (see Section 36.2) is

an attractive alternative to expr.

See Section 10.1 for more on using expr in string operations.

________________________________________________________________

16.3. Time / Date Commands

Time/date and timing

date

Simply invoked, date prints the date and time to stdout. Where

this command gets interesting is in its formatting and parsing

options.

Example 16-10. Using date

!/bin/bash

Exercising the 'date' command

echo "The number of days since the year's beginning is `date +%j`."

Needs a leading '+' to invoke formatting.

%j gives day of year.

echo "The number of seconds elapsed since 01/01/1970 is `date +%s`."

%s yields number of seconds since "UNIX epoch" began,

+ but how is this useful?

prefix=temp

suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific.

filename=$prefix.$suffix

echo "Temporary filename = $filename"

It's great for creating "unique and random" temp filenames,

+ even better than using $.

Read the 'date' man page for more formatting options.

exit 0

The -u option gives the UTC (Universal Coordinated Time).

bash$ date

Fri Mar 29 21:07:39 MST 2002

bash$ date -u

Sat Mar 30 04:07:42 UTC 2002

This option facilitates calculating the time between different

dates.

Example 16-11. Date calculations

!/bin/bash

date-calc.sh

Author: Nathan Coulter

Used in ABS Guide with permission (thanks!).

MPHR=60 # Minutes per hour.

HPD=24 # Hours per day.

diff () {

printf '%s' $(( $(date -u -d"$TARGET" +%s) -

$(date -u -d"$CURRENT" +%s)))

%d = day of month.

}

CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z')

TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z')

%F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone.

printf '\nIn 2007, %s ' \

"$(date -d"$CURRENT +

$(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')"

%B = name of month ^ halfway

printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')"

printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')"

printf '\nOn %s at %s, there were\n' \

$(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T)

DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD ))

CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z')

HOURS=$(( $(diff) / $MPHR / $MPHR ))

CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z')

MINUTES=$(( $(diff) / $MPHR ))

CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z')

printf '%s days, %s hours, ' "$DAYS" "$HOURS"

printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)"

printf 'until Christmas Dinner!\n\n'

Exercise:

--------

Rewrite the diff () function to accept passed parameters,

+ rather than using global variables.

The date command has quite a number of output options. For

example %N gives the nanosecond portion of the current time.

One interesting use for this is to generate random integers.

date +%N | sed -e 's/000$//' -e 's/^0//'

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Strip off leading and trailing zeroes, if present.

Length of generated integer depends on

+ how many zeroes stripped off.

115281032

63408725

394504284

There are many more options (try man date).

date +%j

Echoes day of the year (days elapsed since January 1).

date +%k%M

Echoes hour and minute in 24-hour format, as a single digit string.

The 'TZ' parameter permits overriding the default time zone.

date # Mon Mar 28 21:42:16 MST 2005

TZ=EST date # Mon Mar 28 23:42:16 EST 2005

Thanks, Frank Kannemann and Pete Sjoberg, for the tip.

SixDaysAgo=$(date --date='6 days ago')

OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month!)

OneYearAgo=$(date --date='1 year ago')

Concatenates the list files,

sorts them,

removes duplicate lines,

and finally writes the result to an output file.

The useful -c option prefixes each line of the input file with

its number of occurrences.

bash$ cat testfile

This line occurs only once.

This line occurs twice.

This line occurs three times.

bash$ uniq -c testfile

1 This line occurs only once.

2 This line occurs twice.

3 This line occurs three times.

bash$ sort testfile | uniq -c | sort -nr

3 This line occurs three times.

2 This line occurs twice.

1 This line occurs only once.

The sort INPUTFILE | uniq -c | sort -nr command string

produces a frequency of occurrence listing on the INPUTFILE

file (the -nr options to sort cause a reverse numerical sort).

This template finds use in analysis of log files and

dictionary lists, and wherever the lexical structure of a

document needs to be examined.

Example 16-12. Word Frequency Analysis

!/bin/bash

wf.sh: Crude word frequency analysis on a text file.

This is a more efficient version of the "wf2.sh" script.

Check for input file on command-line.

ARGS=1

E_BADARGS=85

E_NOFILE=86

if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script?

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

if [ ! -f "$1" ] # Check if file exists.

then

echo "File \"$1\" does not exist."

exit $E_NOFILE

main ()

sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\

/g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr

=========================

Frequency of occurrence

Filter out periods and commas, and

+ change space between words to linefeed,

+ then shift characters to lowercase, and

+ finally prefix occurrence count and sort numerically.

Arun Giridhar suggests modifying the above to:

. . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr

This adds a secondary sort key, so instances of

+ equal occurrence are sorted alphabetically.

As he explains it:

"This is effectively a radix sort, first on the

+ least significant column

+ (word or string, optionally case-insensitive)

+ and last on the most significant column (frequency)."

As Frank Wang explains, the above is equivalent to

+ . . . | sort | uniq -c | sort +0 -nr

+ and the following also works:

+ . . . | sort | uniq -c | sort -k1nr -k

exit 0

Exercises:

---------

1) Add 'sed' commands to filter out other punctuation,

+ such as semicolons.

2) Modify the script to also filter out multiple spaces and

+ other whitespace.

bash$ cat testfile

This line occurs only once.

This line occurs twice.

This line occurs three times.

bash$ ./wf.sh testfile

6 this

6 occurs

6 line

3 times

3 three

2 twice

1 only

1 once

expand, unexpand

The expand filter converts tabs to spaces. It is often used in

a pipe.

The unexpand filter converts spaces to tabs. This reverses the

effect of expand.

cut

A tool for extracting fields from files. It is similar to the

print $N command set in awk, but more limited. It may be

simpler to use cut in a script than awk. Particularly

important are the -d (delimiter) and -f (field specifier)

options.

Using cut to obtain a listing of the mounted filesystems:

cut -d ' ' -f1,2 /etc/mtab

Using cut to list the OS and kernel version:

uname -a | cut -d" " -f1,3,11,12

Using cut to extract message headers from an e-mail folder:

bash$ grep '^Subject:' read-messages | cut -c10-80

Re: Linux suitable for mission-critical apps?

MAKE MILLIONS WORKING AT HOME!!!

Spam complaint

Re: Spam complaint

Using cut to parse a file:

List all the users in /etc/passwd.

FILENAME=/etc/passwd

for user in $(cut -d: -f1 $FILENAME)

echo $user

done

Thanks, Oleg Philon for suggesting this.

cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{

print $2, $3 }' filename

Note

It is even possible to specify a linefeed as a delimiter. The trick

is to actually embed a linefeed (RETURN) in the command sequence.

bash$ cut -d'

' -f3,7,19 testfile

This is line 3 of testfile.

This is line 7 of testfile.

This is line 19 of testfile.

Thank you, Jaka Kranjc, for pointing this out.

!/bin/bash

script-detector.sh: Detects scripts within a directory.

TESTCHARS=2 # Test first 2 characters.

SHABANG='#!' # Scripts begin with a "sha-bang."

for file in * # Traverse all the files in current directory.

if [[ `head -c$TESTCHARS "$file"` = "$SHABANG" ]]

# head -c2 #!

# The '-c' option to "head" outputs a specified

#+ number of characters, rather than lines (the default).

then

echo "File \"$file\" is a script."

else

echo "File \"$file\" is *not* a script."

done

exit 0

Exercises:

---------

1) Modify this script to take as an optional argument

+ the directory to scan for scripts

+ (rather than just the current working directory).

2) As it stands, this script gives "false positives" for

+ Perl, awk, and other scripting language scripts.

Correct this.

Example 16-14. Generating 10-digit random numbers

!/bin/bash

rnd.sh: Outputs a 10-digit random number

Script by Stephane Chazelas.

head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'

=================================================================== #

Analysis

--------

head:

-c4 option takes first 4 bytes.

od:

-N4 option limits output to 4 bytes.

-tu4 option selects unsigned decimal format for output.

sed:

-n option, in combination with "p" flag to the "s" command,

outputs only matched lines.

The author of this script explains the action of 'sed', as follows.

head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'

----------------------------------> |

Assume output up to "sed" --------> |

is 0000000 1198195154\n

sed begins reading characters: 0000000 1198195154\n.

Here it finds a newline character,

+ so it is ready to process the first line (0000000 1198195154).

It looks at its <range><action>s. The first and only one is

range action

1 s/.* //p

The line number is in the range, so it executes the action:

+ tries to substitute the longest string ending with a space in the line

("0000000 ") with nothing (//), and if it succeeds, prints the result

("p" is a flag to the "s" command here, this is different

+ from the "p" command).

sed is now ready to continue reading its input. (Note that before

+ continuing, if -n option had not been passed, sed would have printed

+ the line once again).

Now, sed reads the remainder of the characters, and finds the

+ end of the file.

It is now ready to process its 2nd line (which is also numbered ' as

+ it's the last one).

It sees it is not matched by any <range>, so its job is done.

In few word this sed commmand means:

"On the first line only, remove any character up to the right-most space,

+ then print it."

A better way to do this would have been:

sed -e 's/.* //;q'

Here, two <range><action>s (could have been written

sed -e 's/.* //' -e q):

range action

nothing (matches line) s/.* //

nothing (matches line) q (quit)

Here, sed only reads its first line of input.

It performs both actions, and prints the line (substituted) before

+ quitting (because of the "q" action) since the "-n" option is not passed.

=================================================================== #

An even simpler altenative to the above one-line script would be:

head -c4 /dev/urandom| od -An -tu4

exit

!/bin/bash

filename=sys.log

cat /dev/null > $filename; echo "Creating / cleaning out file."

Creates the file if it does not already exist,

+ and truncates it to zero length if it does.

: > filename and > filename also work.

tail /var/log/messages > $filename

/var/log/messages must have world read permission for this to work.

echo "$filename contains tail end of system log."

exit 0

Tip

To list a specific line of a text file, pipe the output of head to

tail -n 1. For example head -n 8 database.txt | tail -n 1 lists the

8th line of the file database.txt.

To set a variable to a given block of a text file:

var=$(head -n $m $filename | tail -n $n)

filename = name of file

m = from beginning of file, number of lines to end of block

n = number of lines to set variable to (trim from end of block)

Note

Newer implementations of tail deprecate the older tail -$LINES

filename usage. The standard tail -n $LINES filename is correct.

See also Example 16-5, Example 16-39 and Example 32-6.

grep

A multi-purpose file search tool that uses Regular

Expressions. It was originally a command/filter in the

venerable ed line editor: g/re/p -- global - regular

expression - print.

grep pattern [file...]

Search the target file(s) for occurrences of pattern, where

pattern may be literal text or a Regular Expression.

bash$ grep '[rst]ystem. osinfo.txt

The GPL governs the distribution of the Linux operating system.

If no target file(s) specified, grep works as a filter on

stdout, as in a pipe.

bash$ ps ax | grep clock

765 tty1 S 0:00 xclock

901 pts/1 S 0:00 grep clock

The -i option causes a case-insensitive search.

The -w option matches only whole words.

The -l option lists only the files in which matches were

found, but not the matching lines.

The -r (recursive) option searches files in the current

working directory and all subdirectories below it.

The -n option lists the matching lines, together with line

numbers.

bash$ grep -n Linux osinfo.txt

2:This is a file containing information about Linux.

6:The GPL governs the distribution of the Linux operating system.

The -v (or --invert-match) option filters out matches.

grep pattern1 *.txt | grep -v pattern2

Matches all lines in "*.txt" files containing "pattern1",

but not "pattern2".

The -c (--count) option gives a numerical count of matches,

rather than actually listing the matches.

grep -c txt *.sgml # (number of occurrences of "txt" in "*.sgml" files)

grep -cz .

^ dot

means count (-c) zero-separated (-z) items matching "."

that is, non-empty ones (containing at least 1 character).

printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz . # 3

printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz ' # 5

printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^' # 5

printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c ' # 9

By default, newline chars (\n) separate items to match.

Note that the -z option is GNU "grep" specific.

Thanks, S.C.

The --color (or --colour) option marks the matching string in

color (on the console or in an xterm window). Since grep

prints out each entire line containing the matching pattern,

this lets you see exactly what is being matched. See also the

-o option, which shows only the matching portion of the

line(s).

Example 16-16. Printing out the From lines in stored e-mail

messages

!/bin/bash

from.sh

Emulates the useful 'from' utility in Solaris, BSD, etc.

Echoes the "From" header line in all messages

+ in your e-mail directory.

MAILDIR=~/mail/* # No quoting of variable. Why?

Maybe check if-exists $MAILDIR: if [ -d $MAILDIR ] . . .

GREP_OPTS="-H -A 5 --color" # Show file, plus extra context lines

#+ and display "From" in color.

TARGETSTR="^From" # "From" at beginning of line.

for file in $MAILDIR # No quoting of variable.

grep $GREP_OPTS "$TARGETSTR" "$file"

# ^^^^^^^^^^ # Again, do not quote this variable.

echo

done

exit $?

You might wish to pipe the output of this script to 'more'

+ or redirect it to a file . . .

When invoked with more than one target file given, grep

specifies which file contains matches.

bash$ grep Linux osinfo.txt misc.txt

osinfo.txt:This is a file containing information about Linux.

osinfo.txt:The GPL governs the distribution of the Linux operating system.

misc.txt:The Linux operating system is steadily gaining in popularity.

Tip

To force grep to show the filename when searching only one target

file, simply give /dev/null as the second file.

bash$ grep Linux osinfo.txt /dev/null

osinfo.txt:This is a file containing information about Linux.

osinfo.txt:The GPL governs the distribution of the Linux operating system.

If there is a successful match, grep returns an exit status of

0, which makes it useful in a condition test in a script,

especially in combination with the -q option to suppress

output.

SUCCESS=0 # if grep lookup succeeds

word=Linux

filename=data.file

grep -q "$word" "$filename" # The "-q" option

#+ causes nothing to echo to stdout.

if [ $? -eq $SUCCESS ]

if grep -q "$word" "$filename" can replace lines 5 - 7.

then

echo "$word found in $filename"

else

echo "$word not found in $filename"

Example 32-6 demonstrates how to use grep to search for a word

pattern in a system logfile.

Example 16-17. Emulating grep in a script

!/bin/bash

grp.sh: Rudimentary reimplementation of grep.

E_BADARGS=85

if [ -z "$1" ] # Check for argument to script.

then

echo "Usage: `basename $0` pattern"

exit $E_BADARGS

echo

for file in * # Traverse all files in $PWD.

output=$(sed -n /"$1"/p $file) # Command substitution.

if [ ! -z "$output" ] # What happens if "$output" is not quoted?

then

echo -n "$file: "

echo "$output"

fi # sed -ne "/$1/s|^|${file}: |p" is equivalent to above.

echo

done

echo

exit 0

Exercises:

---------

1) Add newlines to output, if more than one match in any given file.

2) Add features.

How can grep search for two (or more) separate patterns? What

if you want grep to display all lines in a file or files that

contain both "pattern1" and "pattern2"?

One method is to pipe the result of grep pattern1 to grep

pattern2.

For example, given the following file:

Filename: tstfile

This is a sample file.

This is an ordinary text file.

This file does not contain any unusual text.

This file is not unusual.

Here is some text.

Now, let's search this file for lines containing both "file"

and "text" . . .

bash$ grep file tstfile

Filename: tstfile

This is a sample file.

This is an ordinary text file.

This file does not contain any unusual text.

This file is not unusual.

bash$ grep file tstfile | grep text

This is an ordinary text file.

This file does not contain any unusual text.

Now, for an interesting recreational use of grep . . .

Example 16-18. Crossword puzzle solver

!/bin/bash

cw-solver.sh

This is actually a wrapper around a one-liner (line 46).

Crossword puzzle and anagramming word game solver.

You know some of the letters in the word you're looking for,

+ so you need a list of all valid words

+ with the known letters in given positions.

For example: w...i....n

1???5????10

w in position 1, 3 unknowns, i in the 5th, 4 unknowns, n at the end.

(See comments at end of script.)

E_NOPATT=71

DICT=/usr/share/dict/word.lst

^^^^^^^^ Looks for word list here.

ASCII word list, one word per line.

If you happen to need an appropriate list,

+ download the author's "yawl" word list package.

http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz

or

http://bash.deta.in/yawl-0.3.2.tar.gz

if [ -z "$1" ] # If no word pattern specified

then #+ as a command-line argument . . .

echo #+ . . . then . . .

echo "Usage:" #+ Usage message.

echo

echo ""$0" \"pattern,\""

echo "where \"pattern\" is in the form"

echo "xxx..x.x..."

echo

echo "The x's represent known letters,"

echo "and the periods are unknown letters (blanks)."

echo "Letters and periods can be in any position."

echo "For example, try: sh cw-solver.sh w...i....n"

echo

exit $E_NOPATT

echo

===============================================

This is where all the work gets done.

grep ^"$1"$ "$DICT" # Yes, only one line!

| |

^ is start-of-word regex anchor.

$ is end-of-word regex anchor.

From _Stupid Grep Tricks_, vol. 1,

+ a book the ABS Guide author may yet get around

+ to writing . . . one of these days . . .

===============================================

echo

exit $? # Script terminates here.

If there are too many words generated,

+ redirect the output to a file.

$ sh cw-solver.sh w...i....n

wellington

workingman

workingmen

egrep -- extended grep -- is the same as grep -E. This uses a

somewhat different, extended set of Regular Expressions, which

can make the search a bit more flexible. It also allows the

boolean | (or) operator.

bash $ egrep 'matches|Matches' file.txt

Line 1 matches.

Line 3 Matches.

Line 4 contains matches, but also Matches

fgrep -- fast grep -- is the same as grep -F. It does a

literal string search (no Regular Expressions), which

generally speeds things up a bit.

Note

On some Linux distros, egrep and fgrep are symbolic links to, or

aliases for grep, but invoked with the -E and -F options,

respectively.

Example 16-19. Looking up definitions in Webster's 1913

Dictionary

!/bin/bash

dict-lookup.sh

This script looks up definitions in the 1913 Webster's Dictionary.

This Public Domain dictionary is available for download

+ from various sites, including

+ Project Gutenberg (http://www.gutenberg.org/etext/247).

Convert it from DOS to UNIX format (with only LF at end of line)

+ before using it with this script.

Store the file in plain, uncompressed ASCII text.

Set DEFAULT_DICTFILE variable below to path/filename.

E_BADARGS=85

MAXCONTEXTLINES=50 # Maximum number of lines to show.

DEFAULT_DICTFILE="/usr/share/dict/webster1913-dict.txt"

# Default dictionary file pathname.

# Change this as necessary.

Note:

----

This particular edition of the 1913 Webster's

+ begins each entry with an uppercase letter

+ (lowercase for the remaining characters).

Only the very first line of an entry begins this way,

+ and that's why the search algorithm below works.

if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]]

Must at least specify word to look up, and

+ it must start with an uppercase letter.

then

echo "Usage: `basename $0` Word-to-define [dictionary-file]"

echo

echo "Note: Word to look up must start with capital letter,"

echo "with the rest of the word in lowercase."

echo "--------------------------------------------"

echo "Examples: Abandon, Dictionary, Marking, etc."

exit $E_BADARGS

if [ -z "$2" ] # May specify different dictionary

#+ as an argument to this script.

then

dictfile=$DEFAULT_DICTFILE

else

dictfile="$2"

---------------------------------------------------------

Definition=$(fgrep -A $MAXCONTEXTLINES "$1 \\" "$dictfile")

Definitions in form "Word \..."

And, yes, "fgrep" is fast enough

+ to search even a very large text file.

Now, snip out just the definition block.

echo "$Definition" |

sed -n '1,/^[A-Z]/p' |

Print from first line of output

+ to the first line of the next entry.

sed '$d' | sed '$d'

Delete last two lines of output

+ (blank line and first line of next entry).

---------------------------------------------------------

exit $?

Exercises:

---------

1) Modify the script to accept any type of alphabetic input

+ (uppercase, lowercase, mixed case), and convert it

+ to an acceptable format for processing.

2) Convert the script to a GUI application,

+ using something like 'gdialog' or 'zenity' . . .

The script will then no longer take its argument(s)

+ from the command-line.

3) Modify the script to parse one of the other available

+ Public Domain Dictionaries, such as the U.S. Census Bureau Gazetteer.

Note

See also Example A-41 for an example of speedy fgrep lookup on a

large text file.

agrep (approximate grep) extends the capabilities of grep to

approximate matching. The search string may differ by a

specified number of characters from the resulting matches.

This utility is not part of the core Linux distribution.

Tip

To search compressed files, use zgrep, zegrep, or zfgrep. These also

work on non-compressed files, though slower than plain grep, egrep,

fgrep. They are handy for searching through a mixed set of files,

some compressed, some not.

To search bzipped files, use bzgrep.

look

The command look works like grep, but does a lookup on a

"dictionary," a sorted word list. By default, look searches

for a match in /usr/dict/words, but a different dictionary

file may be specified.

Example 16-20. Checking words in a list for validity

!/bin/bash

lookup: Does a dictionary lookup on each word in a data file.

file=words.data # Data file from which to read words to test.

echo

echo "Testing file $file"

echo

while [ "$word" != end ] # Last word in data file.

do # ^^^

read word # From data file, because of redirection at end of loop.

look $word > /dev/null # Don't want to display lines in dictionary file.

# Searches for words in the file /usr/share/dict/words

#+ (usually a link to linux.words).

lookup=$? # Exit status of 'look' command.

if [ "$lookup" -eq 0 ]

then

echo "\"$word\" is valid."

else

echo "\"$word\" is invalid."

done <"$file" # Redirects stdin to $file, so "reads" come from there.

echo

exit 0

----------------------------------------------------------------

Code below line will not execute because of "exit" command above.

Stephane Chazelas proposes the following, more concise alternative:

while read word && [[ $word != end ]]

do if look "$word" > /dev/null

then echo "\"$word\" is valid."

else echo "\"$word\" is invalid."

done <"$file"

exit 0

sed, awk

Scripting languages especially suited for parsing text files

and command output. May be embedded singly or in combination

in pipes and shell scripts.

sed

Non-interactive "stream editor", permits using many ex

commands in batch mode. It finds many uses in shell scripts.

awk

Programmable file extractor and formatter, good for

manipulating and/or extracting fields (columns) in structured

text files. Its syntax is similar to C.

wc gives a "word count" on a file or I/O stream:

bash $ wc /usr/share/doc/sed-4.1.2/README

13 70 447 README

[13 lines 70 words 447 characters]

wc -w gives only the word count.

wc -l gives only the line count.

wc -c gives only the byte count.

wc -m gives only the character count.

wc -L gives only the length of the longest line.

Using wc to count how many .txt files are in current working

directory:

$ ls *.txt | wc -l

Will work as long as none of the "*.txt" files

+ have a linefeed embedded in their name.

Alternative ways of doing this are:

find . -maxdepth 1 -name \*.txt -print0 | grep -cz .

(shopt -s nullglob; set -- *.txt; echo $#)

Thanks, S.C.

Using wc to total up the size of all the files whose names

begin with letters in the range d - h

bash$ wc [d-h]* | grep total | awk '{print $3}'

71832

Using wc to count the instances of the word "Linux" in the

main source file for this book.

bash$ grep Linux abs-book.sgml | wc -l

138

See also Example 16-39 and Example 20-8.

Certain commands include some of the functionality of wc as

options.

... | grep foo | wc -l

This frequently used construct can be more concisely rendered.

... | grep -c foo

Just use the "-c" (or "--count") option of grep.

Thanks, S.C.

character translation filter.

Caution

Must use quoting and/or brackets, as appropriate. Quotes prevent the

shell from reinterpreting the special characters in tr command

sequences. Brackets should be quoted to prevent expansion by the

shell.

Either tr "A-Z" "*" <filename or tr A-Z \* <filename changes

all the uppercase letters in filename to asterisks (writes to

stdout). On some systems this may not work, but tr A-Z '[**]'

will.

The -d option deletes a range of characters.

echo "abcdef" # abcdef

echo "abcdef" | tr -d b-d # aef

tr -d 0-9 <filename

Deletes all digits from the file "filename".

The --squeeze-repeats (or -s) option deletes all but the first

instance of a string of consecutive characters. This option is

useful for removing excess whitespace.

bash$ echo "XXXXX" | tr --squeeze-repeats 'X'

The -c "complement" option inverts the character set to match.

With this option, tr acts only upon those characters not

matching the specified set.

bash$ echo "acfdeb123" | tr -c b-d +

+c+d+b++++

Note that tr recognizes POSIX character classes. [74]

bash$ echo "abcd2ef1" | tr '[:alpha:]' -

----2--1

Example 16-21. toupper: Transforms a file to all uppercase.

!/bin/bash

Changes a file to all uppercase.

E_BADARGS=85

if [ -z "$1" ] # Standard check for command-line arg.

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

tr a-z A-Z <"$1"

Same effect as above, but using POSIX character set notation:

tr '[:lower:]' '[:upper:]' <"$1"

Thanks, S.C.

Or even . . .

cat "$1" | tr a-z A-Z

Or dozens of other ways . . .

exit 0

Exercise:

Rewrite this script to give the option of changing a file

+ to either upper or lowercase.

Hint: Use either the "case" or "select" command.

Example 16-22. lowercase: Changes all filenames in working

directory to lowercase.

!/bin/bash

Changes every filename in working directory to all lowercase.

Inspired by a script of John Dubois,

+ which was translated into Bash by Chet Ramey,

+ and considerably simplified by the author of the ABS Guide.

for filename in * # Traverse all files in directory.

fname=`basename $filename`

n=`echo $fname | tr A-Z a-z` # Change name to lowercase.

if [ "$fname" != "$n" ] # Rename only files not already lowercase.

then

mv $fname $n

done

exit $?

Code below this line will not execute because of "exit".

--------------------------------------------------------#

To run it, delete script above line.

The above script will not work on filenames containing blanks or newlines.

Stephane Chazelas therefore suggests the following alternative:

for filename in * # Not necessary to use basename,

# since "*" won't return any file containing "/".

do n=`echo "$filename/" | tr '[:upper:]' '[:lower:]'`

POSIX char set notation.

Slash added so that trailing newlines are not

removed by command substitution.

# Variable substitution:

n=${n%/} # Removes trailing slash, added above, from filename.

[[ $filename == $n ]] || mv "$filename" "$n"

# Checks if filename already lowercase.

done

exit $?

Example 16-23. du: DOS to UNIX text file conversion.

!/bin/bash

Du.sh: DOS to UNIX text file converter.

E_WRONGARGS=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` filename-to-convert"

exit $E_WRONGARGS

NEWFILENAME=$1.unx

CR='\015' # Carriage return.

# 015 is octal ASCII code for CR.

# Lines in a DOS text file end in CR-LF.

# Lines in a UNIX text file end in LF only.

tr -d $CR < $1 > $NEWFILENAME

Delete CR's and write to new file.

echo "Original DOS text file is \"$1\"."

echo "Converted UNIX text file is \"$NEWFILENAME\"."

exit 0

Exercise:

--------

Change the above script to convert from UNIX to DOS.

Example 16-24. rot13: ultra-weak encryption.

!/bin/bash

rot13.sh: Classic rot13 algorithm,

encryption that might fool a 3-year old

for about 10 minutes.

Usage: ./rot13.sh filename

or ./rot13.sh <filename

or ./rot13.sh and supply keyboard input (stdin)

cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' # "a" goes to "n", "b" to "o" ...

The cat "$@" construct

+ permits input either from stdin or from files.

exit 0

Example 16-25. Generating "Crypto-Quote" Puzzles

!/bin/bash

crypto-quote.sh: Encrypt quotes

Will encrypt famous quotes in a simple monoalphabetic substitution.

The result is similar to the "Crypto Quote" puzzles

+ seen in the Op Ed pages of the Sunday paper.

key=ETAOINSHRDLUBCFGJMQPVWZYXK

The "key" is nothing more than a scrambled alphabet.

Changing the "key" changes the encryption.

The 'cat "$@"' construction gets input either from stdin or from files.

If using stdin, terminate input with a Control-D.

Otherwise, specify filename as command-line parameter.

cat "$@" | tr "a-z" "A-Z" | tr "A-Z" "$key"

| to uppercase | encrypt

Will work on lowercase, uppercase, or mixed-case quotes.

Passes non-alphabetic characters through unchanged.

Try this script with something like:

"Nothing so needs reforming as other people's habits."

--Mark Twain

Output is:

"CFPHRCS QF CIIOQ MINFMBRCS EQ FPHIM GIFGUI'Q HETRPQ."

--BEML PZERC

To reverse the encryption:

cat "$@" | tr "$key" "A-Z"

This simple-minded cipher can be broken by an average 12-year old

+ using only pencil and paper.

exit 0

Exercise:

--------

Modify the script so that it will either encrypt or decrypt,

+ depending on command-line argument(s).

Of course, tr lends itself to code obfuscation.

!/bin/bash

jabh.sh

x="wftedskaebjgdBstbdbsmnjgz"

echo $x | tr "a-z" 'oh, turtleneck Phrase Jar!'

Based on the Wikipedia "Just another Perl hacker" article.

tr variants

The tr utility has two historic variants. The BSD version does not

use brackets (tr a-z A-Z), but the SysV one does (tr '[a-z]'

'[A-Z]'). The GNU version of tr resembles the BSD one.

fold

A filter that wraps lines of input to a specified width. This

is especially useful with the -s option, which breaks lines at

word spaces (see Example 16-26 and Example A-1).

fmt

Simple-minded file formatter, used as a filter in a pipe to

"wrap" long lines of text output.

Example 16-26. Formatted file listing.

!/bin/bash

WIDTH=40 # 40 columns wide.

b=`ls /usr/local/bin` # Get a file listing...

echo $b | fmt -w $WIDTH

Could also have been done by

echo $b | fold - -s -w $WIDTH

exit 0

!/bin/bash

colms.sh

A minor modification of the example file in the "column" man page.

(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \

; ls -l | sed 1d) | column -t

^^^^^^ ^^

The "sed 1d" in the pipe deletes the first line of output,

+ which would be "total N",

+ where "N" is the total number of files found by "ls -l".

The -t option to "column" pretty-prints a table.

exit 0

colrm

Column removal filter. This removes columns (characters) from

a file and writes the file, lacking the range of specified

columns, back to stdout. colrm 2 4 <filename removes the

second through fourth characters from each line of the text

file filename.

Caution

If the file contains tabs or nonprintable characters, this may cause

unpredictable behavior. In such cases, consider using expand and

unexpand in a pipe preceding colrm.

Line numbering filter: nl filename lists filename to stdout,

but inserts consecutive numbers at the beginning of each

non-blank line. If filename omitted, operates on stdin.

The output of nl is very similar to cat -b, since, by default

nl does not list blank lines.

Example 16-28. nl: A self-numbering script.

!/bin/bash

line-number.sh

This script echoes itself twice to stdout with its lines numbered.

echo " line number = $LINENO" # 'nl' sees this as line 4

(nl does not number blank lines).

'cat -n' sees it correctly as line #6.

nl `basename $0`

echo; echo # Now, let's try it with 'cat -n'

cat -n `basename $0`

The difference is that 'cat -n' numbers the blank lines.

Note that 'nl -ba' will also do so.

exit 0

-----------------------------------------------------------------

Print formatting filter. This will paginate files (or stdout)

into sections suitable for hard copy printing or viewing on

screen. Various options permit row and column manipulation,

joining lines, setting margins, numbering lines, adding page

headers, and merging files, among other things. The pr command

combines much of the functionality of nl, paste, fold, column,

and expand.

pr -o 5 --width=65 fileZZZ | more gives a nice paginated

listing to screen of fileZZZ with margins set at 5 and 65.

A particularly useful option is -d, forcing double-spacing

(same effect as sed -G).

gettext

The GNU gettext package is a set of utilities for localizing

and translating the text output of programs into foreign

languages. While originally intended for C programs, it now

supports quite a number of programming and scripting

languages.

The gettext program works on shell scripts. See the info page.

msgfmt

A program for generating binary message catalogs. It is used

for localization.

iconv

A utility for converting file(s) to a different encoding

(character set). Its chief use is for localization.

Convert a string from UTF-8 to UTF-16 and print to the BookList

function write_utf8_string {

STRING=$1

BOOKLIST=$2

echo -n "$STRING" | iconv -f UTF8 -t UTF16 | \

cut -b 3- | tr -d \\n >> "$BOOKLIST"

}

From Peter Knowles' "booklistgen.sh" script

+ for converting files to Sony Librie/PRS-50X format.

(http://booklistgensh.peterknowles.com)

recode

Consider this a fancier version of iconv, above. This very

versatile utility for converting a file to a different

encoding scheme. Note that recode is not part of the standard

Linux installation.

TeX, gs

TeX and Postscript are text markup languages used for

preparing copy for printing or formatted video display.

TeX is Donald Knuth's elaborate typsetting system. It is often

convenient to write a shell script encapsulating all the

options and arguments passed to one of these markup languages.

Ghostscript (gs) is a GPL-ed Postscript interpreter.

texexec

Utility for processing TeX and pdf files. Found in /usr/bin on

many Linux distros, it is actually a shell wrapper that calls

Perl to invoke Tex.

texexec --pdfarrange --result=Concatenated.pdf *pdf

Concatenates all the pdf files in the current working directory

+ into the merged file, Concatenated.pdf . . .

(The --pdfarrange option repaginates a pdf file. See also --pdfcombine.)

The above command-line could be parameterized and put into a shell script.

enscript

Utility for converting plain text file to PostScript

For example, enscript filename.txt -p filename.ps produces the

PostScript output file filename.ps.

groff, tbl, eqn

Yet another text markup and display formatting language is

groff. This is the enhanced GNU version of the venerable UNIX

roff/troff display and typesetting package. Manpages use

groff.

The tbl table processing utility is considered part of groff,

as its function is to convert table markup into groff

commands.

The eqn equation processing utility is likewise part of groff,

and its function is to convert equation markup into groff

commands.

Example 16-29. manview: Viewing formatted manpages

!/bin/bash

manview.sh: Formats the source of a man page for viewing.

This script is useful when writing man page source.

It lets you look at the intermediate results on the fly

+ while working on it.

E_WRONGARGS=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` filename"

exit $E_WRONGARGS

---------------------------

groff -Tascii -man $1 | less

From the man page for groff.

---------------------------

If the man page includes tables and/or equations,

+ then the above code will barf.

The following line can handle such cases.

gtbl < "$1" | geqn -Tlatin1 | groff -Tlatin1 -mtty-char -man

Thanks, S.C.

exit $? # See also the "maned.sh" script.

!/bin/bash

Copying a directory tree using cpio.

Advantages of using 'cpio':

Speed of copying. It's faster than 'tar' with pipes.

Well suited for copying special files (named pipes, etc.)

+ that 'cp' may choke on.

ARGS=2

E_BADARGS=65

if [ $# -ne "$ARGS" ]

then

echo "Usage: `basename $0` source destination"

exit $E_BADARGS

source="$1"

destination="$2"

find "$source" -depth | cpio -admvp "$destination"

^^^^^ ^^^^^

Read the 'find' and 'cpio' info pages to decipher these options.

The above works only relative to $PWD (current directory) . . .

+ full pathnames are specified.

Exercise:

--------

Add code to check the exit status ($?) of the 'find | cpio' pipe

+ and output appropriate error messages if anything went wrong.

exit $?

rpm2cpio

This command extracts a cpio archive from an rpm one.

Example 16-31. Unpacking an rpm archive

!/bin/bash

de-rpm.sh: Unpack an 'rpm' archive

: ${1?"Usage: `basename $0` target-file"}

Must specify 'rpm' archive name as an argument.

TEMPFILE=$.cpio # Tempfile with "unique" name.

# $ is process ID of script.

rpm2cpio < $1 > $TEMPFILE # Converts rpm archive into

#+ cpio archive.

cpio --make-directories -F $TEMPFILE -i # Unpacks cpio archive.

rm -f $TEMPFILE # Deletes cpio archive.

exit 0

Exercise:

Add check for whether 1) "target-file" exists and

+ 2) it is an rpm archive.

Hint: Parse output of 'file' command.

pax

The pax portable archive exchange toolkit facilitates periodic

file backups and is designed to be cross-compatible between

various flavors of UNIX. It was designed to replace tar and

cpio.

pax -wf daily_backup.pax ~/linux-server/files

Creates a tar archive of all files in the target directory.

Note that the options to pax must be in the correct order --

+ pax -fw has an entirely different effect.

pax -f daily_backup.pax

Lists the files in the archive.

pax -rf daily_backup.pax ~/bsd-server/files

Restores the backed-up files from the Linux machine

+ onto a BSD one.

Note that pax handles many of the standard archiving and

compression commands.

Compression

gzip

The standard GNU/UNIX compression utility, replacing the

inferior and proprietary compress. The corresponding

decompression command is gunzip, which is the equivalent of

gzip -d.

Note

The -c option sends the output of gzip to stdout. This is useful when

piping to other commands.

The zcat filter decompresses a gzipped file to stdout, as

possible input to a pipe or redirection. This is, in effect, a

cat command that works on compressed files (including files

processed with the older compress utility). The zcat command

is equivalent to gzip -dc.

Caution

On some commercial UNIX systems, zcat is a synonym for uncompress -c,

and will not work on gzipped files.

Find sh and Bash scripts in a given directory:

DIRECTORY=/usr/local/bin

KEYWORD=Bourne

Bourne and Bourne-Again shell scripts

file $DIRECTORY/* | fgrep $KEYWORD

Output:

/usr/local/bin/burn-cd: Bourne-Again shell script text executable

/usr/local/bin/burnit: Bourne-Again shell script text executable

/usr/local/bin/cassette.sh: Bourne shell script text executable

/usr/local/bin/copy-cd: Bourne-Again shell script text executable

. . .

Example 16-32. Stripping comments from C program files

!/bin/bash

strip-comment.sh: Strips out the comments (/* COMMENT */) in a C program.

E_NOARGS=0

E_ARGERROR=66

E_WRONG_FILE_TYPE=67

if [ $# -eq "$E_NOARGS" ]

then

echo "Usage: `basename $0` C-program-file" >&2 # Error message to stderr.

exit $E_ARGERROR

Test for correct file type.

type=`file $1 | awk '{ print $2, $3, $4, $5 }'`

"file $1" echoes file type . . .

Then awk removes the first field, the filename . . .

Then the result is fed into the variable "type."

correct_type="ASCII C program text"

if [ "$type" != "$correct_type" ]

then

echo

echo "This script works on C program files only."

echo

exit $E_WRONG_FILE_TYPE

Rather cryptic sed script:

--------

sed '

/^\/\*/d

/.*\*\//d

' $1

--------

Easy to understand if you take several hours to learn sed fundamentals.

Need to add one more line to the sed script to deal with

+ case where line of code has a comment following it on same line.

This is left as a non-trivial exercise.

Also, the above code deletes non-comment lines with a "*/" . . .

+ not a desirable result.

exit 0

----------------------------------------------------------------

Code below this line will not execute because of 'exit 0' above.

Stephane Chazelas suggests the following alternative:

usage() {

echo "Usage: `basename $0` C-program-file" >&2

exit 1

}

WEIRD=`echo -n -e '\377'` # or WEIRD= \377'

[[ $# -eq 1 ]] || usage

case `file "$1"` in

*"C program text"*) sed -e "s%/\*%${WEIRD}%g;s%\*/%${WEIRD}%g" "$1" \

| tr '\377\n' '\n\377' \

| sed -ne 'p;n' \

| tr -d '\n' | tr '\377' '\n';;

*) usage;;

esac

This is still fooled by things like:

printf("/*");

or

/* /* buggy embedded comment */

To handle all special cases (comments in strings, comments in string

+ where there is a \", \\" ...),

+ the only way is to write a C parser (using lex or yacc perhaps?).

exit 0

which

which command gives the full path to "command." This is useful

for finding out whether a particular command or utility is

installed on the system.

$bash which rm

/usr/bin/rm

For an interesting use of this command, see Example 36-16.

whereis

Similar to which, above, whereis command gives the full path

to "command," but also to its manpage.

$bash whereis rm

rm: /bin/rm /usr/share/man/man1/rm.1.bz2

whatis

whatis command looks up "command" in the whatis database. This

is useful for identifying system commands and important

configuration files. Consider it a simplified man command.

$bash whatis whatis

whatis (1) - search the whatis database for complete words

Example 16-33. Exploring /usr/X11R6/bin

!/bin/bash

What are all those mysterious binaries in /usr/X11R6/bin?

DIRECTORY="/usr/X11R6/bin"

Try also "/bin", "/usr/bin", "/usr/local/bin", etc.

for file in $DIRECTORY/*

whatis `basename $file` # Echoes info about the binary.

done

exit 0

Note: For this to work, you must create a "whatis" database

+ with /usr/sbin/makewhatis.

You may wish to redirect output of this script, like so:

./what.sh >>whatis.db

or view it a page at a time on stdout,

./what.sh | less

file: test1.txt

# owner: bozo

# group: bozgrp

user::rw-

group::rw-

other::r--

# file: test2.txt

# owner: bozo

# group: bozgrp

user::rw-

group::rw-

other::r--

bash$ setfacl -m u:bozo:rw yearly_budget.csv

bash$ getfacl yearly_budget.csv

file: yearly_budget.csv

# owner: accountant

# group: budgetgrp

user::rw-

user:bozo:rw-

user:accountant:rw-

group::rw-

mask::rw-

other::r--

readlink

Disclose the file that a symbolic link points to.

bash$ readlink /usr/bin/awk

../../bin/gawk

strings

Use the strings command to find printable strings in a binary

or data file. It will list sequences of printable characters

found in the target file. This might be handy for a quick 'n

dirty examination of a core dump or for looking at an unknown

graphic image file (strings image-file | more might show

something like JFIF, which would identify the file as a jpeg

graphic). In a script, you would probably parse the output of

strings with grep or sed. See Example 11-8 and Example 11-10.

Example 16-34. An "improved" strings command

!/bin/bash

wstrings.sh: "word-strings" (enhanced "strings" command)

This script filters the output of "strings" by checking it

+ against a standard word list file.

This effectively eliminates gibberish and noise,

+ and outputs only recognized words.

===========================================================

Standard Check for Script Argument(s)

ARGS=1

E_BADARGS=85

E_NOFILE=86

if [ $# -ne $ARGS ]

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

if [ ! -f "$1" ] # Check if file exists.

then

echo "File \"$1\" does not exist."

exit $E_NOFILE

===========================================================

MINSTRLEN=3 # Minimum string length.

WORDFILE=/usr/share/dict/linux.words # Dictionary file.

May specify a different word list file

+ of one-word-per-line format.

For example, the "yawl" word-list package,

http://bash.deta.in/yawl-0.3.2.tar.gz

wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \

tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

Translate output of 'strings' command with multiple passes of 'tr'.

"tr A-Z a-z" converts to lowercase.

"tr '[:space:]'" converts whitespace characters to Z's.

"tr -cs '[:alpha:]' Z" converts non-alphabetic characters to Z's,

+ and squeezes multiple consecutive Z's.

"tr -s '\173-\377' Z" converts all characters past 'z' to Z's

+ and squeezes multiple consecutive Z's,

+ which gets rid of all the weird characters that the previous

+ translation failed to deal with.

Finally, "tr Z ' '" converts all those Z's to whitespace,

+ which will be seen as word separators in the loop below.

***********************************************************************

Note the technique of feeding/piping the output of 'tr' back to itself,

+ but with different arguments and/or options on each successive pass.

***********************************************************************

for word in $wlist # Important:

# $wlist must not be quoted here.

# "$wlist" does not work.

# Why not?

strlen=${#word} # String length.

if [ "$strlen" -lt "$MINSTRLEN" ] # Skip over short strings.

then

continue

grep -Fw $word "$WORDFILE" # Match whole words only.

^^^ # "Fixed strings" and

#+ "whole words" options.

done

exit $?

Comparison

diff, patch

diff: flexible file comparison utility. It compares the target

files line-by-line sequentially. In some applications, such as

comparing word dictionaries, it may be helpful to filter the

files through sort and uniq before piping them to diff. diff

file-1 file-2 outputs the lines in the files that differ, with

carets showing which file each particular line belongs to.

The --side-by-side option to diff outputs each compared file,

line by line, in separate columns, with non-matching lines

marked. The -c and -u options likewise make the output of the

command easier to interpret.

There are available various fancy frontends for diff, such as

sdiff, wdiff, xdiff, and mgdiff.

Tip

The diff command returns an exit status of 0 if the compared files

are identical, and 1 if they differ (or 2 when binary files are being

compared). This permits use of diff in a test construct within a

shell script (see below).

A common use for diff is generating difference files to be

used with patch The -e option outputs files suitable for ed or

ex scripts.

patch: flexible versioning utility. Given a difference file

generated by diff, patch can upgrade a previous version of a

package to a newer version. It is much more convenient to

distribute a relatively small "diff" file than the entire body

of a newly revised package. Kernel "patches" have become the

preferred method of distributing the frequent releases of the

Linux kernel.

patch -p1 <patch-file

Takes all the changes listed in 'patch-file'

and applies them to the files referenced therein.

This upgrades to a newer version of the package.

Patching the kernel:

cd /usr/src

gzip -cd patchXX.gz | patch -p0

Upgrading kernel source using 'patch'.

From the Linux kernel docs "README",

by anonymous author (Alan Cox?).

Note

The diff command can also recursively compare directories (for the

filenames present).

bash$ diff -r ~/notes1 ~/notes2

Only in /home/bozo/notes1: file02

Only in /home/bozo/notes1: file03

Only in /home/bozo/notes2: file04

Tip

Use zdiff to compare gzipped files.

Tip

Use diffstat to create a histogram (point-distribution graph) of

output from diff.

diff3, merge

An extended version of diff that compares three files at a

time. This command returns an exit value of 0 upon successful

execution, but unfortunately this gives no information about

the results of the comparison.

bash$ diff3 file-1 file-2 file-3

====

1:1c

This is line 1 of "file-1".

2:1c

This is line 1 of "file-2".

3:1c

This is line 1 of "file-3"

The merge (3-way file merge) command is an interesting adjunct

to diff3. Its syntax is merge Mergefile file1 file2. The

result is to output to Mergefile the changes that lead from

file1 to file2. Consider this command a stripped-down version

of patch.

sdiff

Compare and/or edit two files in order to merge them into an

output file. Because of its interactive nature, this command

would find little use in a script.

cmp

The cmp command is a simpler version of diff, above. Whereas

diff reports the differences between two files, cmp merely

shows at what point they differ.

Note

Like diff, cmp returns an exit status of 0 if the compared files are

identical, and 1 if they differ. This permits use in a test construct

within a shell script.

Example 16-35. Using cmp to compare two files within a script.

!/bin/bash

file-comparison.sh

ARGS=2 # Two args to script expected.

E_BADARGS=85

E_UNREADABLE=86

if [ $# -ne "$ARGS" ]

then

echo "Usage: `basename $0` file1 file2"

exit $E_BADARGS

if [[ ! -r "$1" || ! -r "$2" ]]

then

echo "Both files to be compared must exist and be readable."

exit $E_UNREADABLE

cmp $1 $2 &> /dev/null

Redirection to /dev/null buries the output of the "cmp" command.

cmp -s $1 $2 has same result ("-s" silent flag to "cmp")

Thank you Anders Gustavsson for pointing this out.

Also works with 'diff', i.e.,

+ diff $1 $2 &> /dev/null

if [ $? -eq 0 ] # Test exit status of "cmp" command.

then

echo "File \"$1\" is identical to file \"$2\"."

else

echo "File \"$1\" differs from file \"$2\"."

exit 0

Tip

Use zcmp on gzipped files.

comm

Versatile file comparison utility. The files must be sorted

for this to be useful.

comm -options first-file second-file

comm file-1 file-2 outputs three columns:

+ column 1 = lines unique to file-1

+ column 2 = lines unique to file-2

+ column 3 = lines common to both.

The options allow suppressing output of one or more columns.

+ -1 suppresses column 1

+ -2 suppresses column 2

+ -3 suppresses column 3

+ -12 suppresses both columns 1 and 2, etc.

This command is useful for comparing "dictionaries" or word

lists -- sorted text files with one word per line.

Utilities

basename

Strips the path information from a file name, printing only

the file name. The construction basename $0 lets the script

know its name, that is, the name it was invoked by. This can

be used for "usage" messages if, for example a script is

called with missing arguments:

echo "Usage: `basename $0` arg1 arg2 ... argn"

dirname

Strips the basename from a filename, printing only the path

information.

Note

basename and dirname can operate on any arbitrary string. The

argument does not need to refer to an existing file, or even be a

filename for that matter (see Example A-7).

Example 16-36. basename and dirname

!/bin/bash

address=/home/bozo/daily-journal.txt

echo "Basename of /home/bozo/daily-journal.txt = `basename $address`"

echo "Dirname of /home/bozo/daily-journal.txt = `dirname $address`"

echo

echo "My own home is `basename ~/`." # `basename ~` also works.

echo "The home of my home is `dirname ~/`." # `dirname ~` also works.

exit 0

split, csplit

These are utilities for splitting a file into smaller chunks.

Their usual use is for splitting up large files in order to

back them up on floppies or preparatory to e-mailing or

uploading them.

The csplit command splits a file according to context, the

split occuring where patterns are matched.

Example 16-37. A script that copies itself in sections

!/bin/bash

splitcopy.sh

A script that splits itself into chunks,

+ then reassembles the chunks into an exact copy

+ of the original script.

CHUNKSIZE=4 # Size of first chunk of split files.

OUTPREFIX=xx # csplit prefixes, by default,

#+ files with "xx" ...

csplit "$0" "$CHUNKSIZE"

Some comment lines for padding . . .

Line 15

Line 16

Line 17

Line 18

Line 19

Line 20

cat "$OUTPREFIX"* > "$0.copy" # Concatenate the chunks.

rm "$OUTPREFIX"* # Get rid of the chunks.

exit $?

Encoding and Encryption

sum, cksum, md5sum, sha1sum

These are utilities for generating checksums. A checksum is a

number [77] mathematically calculated from the contents of a

file, for the purpose of checking its integrity. A script

might refer to a list of checksums for security purposes, such

as ensuring that the contents of key system files have not

been altered or corrupted. For security applications, use the

md5sum (message digest 5 checksum) command, or better yet, the

newer sha1sum (Secure Hash Algorithm). [78]

bash$ cksum /boot/vmlinuz

1670054224 804083 /boot/vmlinuz

bash$ echo -n "Top Secret" | cksum

3391003827 10

bash$ md5sum /boot/vmlinuz

0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz

bash$ echo -n "Top Secret" | md5sum

8babc97a6f62a4649716f4df8d61728f -

Note

The cksum command shows the size, in bytes, of its target, whether

file or stdout.

The md5sum and sha1sum commands display a dash when they receive

their input from stdout.

Example 16-38. Checking file integrity

!/bin/bash

file-integrity.sh: Checking whether files in a given directory

have been tampered with.

E_DIR_NOMATCH=80

E_BAD_DBFILE=81

dbfile=File_record.md5

Filename for storing records (database file).

set_up_database ()

{

echo ""$directory"" > "$dbfile"

# Write directory name to first line of file.

md5sum "$directory"/* >> "$dbfile"

# Append md5 checksums and filenames.

}

check_database ()

{

local n=0

local filename

local checksum

# ------------------------------------------- #

# This file check should be unnecessary,

#+ but better safe than sorry.

if [ ! -r "$dbfile" ]

then

echo "Unable to read checksum database file!"

exit $E_BAD_DBFILE

# ------------------------------------------- #

while read record[n]

directory_checked="${record[0]}"

if [ "$directory_checked" != "$directory" ]

then

echo "Directories do not match up!"

# Tried to use file for a different directory.

exit $E_DIR_NOMATCH

if [ "$n" -gt 0 ] # Not directory name.

then

filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' )

# md5sum writes records backwards,

#+ checksum first, then filename.

checksum[n]=$( md5sum "${filename[n]}" )

if [ "${record[n]}" = "${checksum[n]}" ]

then

echo "${filename[n]} unchanged."

elif [ "`basename ${filename[n]}`" != "$dbfile" ]

# Skip over checksum database file,

#+ as it will change with each invocation of script.

# ---

# This unfortunately means that when running

#+ this script on $PWD, tampering with the

#+ checksum database file will not be detected.

# Exercise: Fix this.

then

echo "${filename[n]} : CHECKSUM ERROR!"

# File has been changed since last checked.

let "n+=1"

done <"$dbfile" # Read from checksum database file.

}

=================================================== #

main ()

if [ -z "$1" ]

then

directory="$PWD" # If not specified,

else #+ use current working directory.

directory="$1"

clear # Clear screen.

echo " Running file integrity check on $directory"

echo

------------------------------------------------------------------ #

if [ ! -r "$dbfile" ] # Need to create database file?

then

echo "Setting up database file, \""$directory"/"$dbfile"\"."; echo

set_up_database

------------------------------------------------------------------ #

check_database # Do the actual work.

echo

You may wish to redirect the stdout of this script to a file,

+ especially if the directory checked has many files in it.

exit 0

For a much more thorough file integrity check,

+ consider the "Tripwire" package,

+ http://sourceforge.net/projects/tripwire/.

Also see Example A-19, Example 36-16, and Example 10-2 for

creative uses of the md5sum command.

Note

There have been reports that the 128-bit md5sum can be cracked, so

the more secure 160-bit sha1sum is a welcome new addition to the

checksum toolkit.

bash$ md5sum testfile

e181e2c8720c60522c4c4c981108e367 testfile

bash$ sha1sum testfile

5d7425a9c08a66c3177f1e31286fa40986ffc996 testfile

Security consultants have demonstrated that even sha1sum can

be compromised. Fortunately, newer Linux distros include

longer bit-length sha224sum, sha256sum, sha384sum, and

sha512sum commands.

uuencode

This utility encodes binary files (images, sound files,

compressed files, etc.) into ASCII characters, making them

suitable for transmission in the body of an e-mail message or

in a newsgroup posting. This is especially useful where MIME

(multimedia) encoding is not available.

uudecode

This reverses the encoding, decoding uuencoded files back into

the original binaries.

Example 16-39. Uudecoding encoded files

!/bin/bash

Uudecodes all uuencoded files in current working directory.

lines=35 # Allow 35 lines for the header (very generous).

for File in * # Test all the files in $PWD.

search1=`head -n $lines $File | grep begin | wc -w`

search2=`tail -n $lines $File | grep end | wc -w`

# Uuencoded files have a "begin" near the beginning,

#+ and an "end" near the end.

if [ "$search1" -gt 0 ]

then

if [ "$search2" -gt 0 ]

then

echo "uudecoding - $File -"

uudecode $File

done

Note that running this script upon itself fools it

+ into thinking it is a uuencoded file,

+ because it contains both "begin" and "end".

Exercise:

--------

Modify this script to check each file for a newsgroup header,

+ and skip to next if not found.

exit 0

Tip

The fold -s command may be useful (possibly in a pipe) to process

long uudecoded text messages downloaded from Usenet newsgroups.

mimencode, mmencode

The mimencode and mmencode commands process multimedia-encoded

e-mail attachments. Although mail user agents (such as pine or

kmail) normally handle this automatically, these particular

utilities permit manipulating such attachments manually from

the command-line or in batch processing mode by means of a

shell script.

crypt

At one time, this was the standard UNIX file encryption

utility. [79] Politically-motivated government regulations

prohibiting the export of encryption software resulted in the

disappearance of crypt from much of the UNIX world, and it is

still missing from most Linux distributions. Fortunately,

programmers have come up with a number of decent alternatives

to it, among them the author's very own

[ftp://metalab.unc.edu/pub/Linux/utils/file/cruft-0.2.tar.gz]

cruft (see Example A-4).

openssl

This is an Open Source implementation of Secure Sockets Layer

encryption.

To encrypt a file:

openssl aes-128-ecb -salt -in file.txt -out file.encrypted \

-pass pass:my_password

^^^^^^^^^^^ User-selected password.

aes-128-ecb is the encryption method chosen.

To decrypt an openssl-encrypted file:

openssl aes-128-ecb -d -salt -in file.encrypted -out file.txt \

-pass pass:my_password

^^^^^^^^^^^ User-selected password.

Piping openssl to/from tar makes it possible to encrypt an

entire directory tree.

To encrypt a directory:

sourcedir="/home/bozo/testfiles"

encrfile="encr-dir.tar.gz"

password=my_secret_password

tar czvf - "$sourcedir" |

openssl des3 -salt -out "$encrfile" -pass pass:"$password"

^^^^ Uses des3 encryption.

Writes encrypted file "encr-dir.tar.gz" in current working directory.

To decrypt the resulting tarball:

openssl des3 -d -salt -in "$encrfile" -pass pass:"$password" |

tar -xzv

Decrypts and unpacks into current working directory.

Of course, openssl has many other uses, such as obtaining

signed certificates for Web sites. See the info page.

shred

Securely erase a file by overwriting it multiple times with

random bit patterns before deleting it. This command has the

same effect as Example 16-61, but does it in a more thorough

and elegant manner.

This is one of the GNU fileutils.

Caution

Advanced forensic technology may still be able to recover the

contents of a file, even after application of shred.

Miscellaneous

mktemp

Create a temporary file [80] with a "unique" filename. When

invoked from the command-line without additional arguments, it

creates a zero-length file in the /tmp directory.

bash$ mktemp

/tmp/tmp.zzsvql3154

PREFIX=filename

tempfile=`mktemp $PREFIX.XXXXXX`

^^^^^^ Need at least 6 placeholders

+ in the filename template.

If no filename template supplied,

+ "tmp.XXXXXXXXXX" is the default.

echo "tempfile name = $tempfile"

tempfile name = filename.QA2ZpY

or something similar...

Creates a file of that name in the current working directory

+ with 600 file permissions.

A "umask 177" is therefore unnecessary,

+ but it's good programming practice nevertheless.

make

Utility for building and compiling binary packages. This can

also be used for any set of operations triggered by

incremental changes in source files.

The make command checks a Makefile, a list of file

dependencies and operations to be carried out.

The make utility is, in effect, a powerful scripting language

similar in many ways to Bash, but with the capability of

recognizing dependencies. For in-depth coverage of this useful

tool set, see the GNU software documentation site.

install

Special purpose file copying command, similar to cp, but

capable of setting permissions and attributes of the copied

files. This command seems tailormade for installing software

packages, and as such it shows up frequently in Makefiles (in

the make install : section). It could likewise prove useful in

installation scripts.

dos2unix

This utility, written by Benjamin Lin and collaborators,

converts DOS-formatted text files (lines terminated by CR-LF)

to UNIX format (lines terminated by LF only), and vice-versa.

ptx

The ptx [targetfile] command outputs a permuted index

(cross-reference list) of the targetfile. This may be further

filtered and formatted in a pipe, if necessary.

more, less

Pagers that display a text file or stream to stdout, one

screenful at a time. These may be used to filter the output of

stdout . . . or of a script.

An interesting application of more is to "test drive" a

command sequence, to forestall potentially unpleasant

consequences.

ls /home/bozo | awk '{print "rm -rf " $1}' | more

^^^^

Testing the effect of the following (disastrous) command-line:

ls /home/bozo | awk '{print "rm -rf " $1}' | sh

Hand off to the shell to execute . . . ^^

The less pager has the interesting property of doing a

formatted display of man page source. See Example A-39.

________________________________________________________________

16.6. Communications Commands

Certain of the following commands find use in network data transfer

and analysis, as well as in chasing spammers.

Information and Statistics

host

Searches for information about an Internet host by name or IP

address, using DNS.

bash$ host surfacemail.com

surfacemail.com. has address 202.92.42.236

ipcalc

Displays IP information for a host. With the -h option, ipcalc

does a reverse DNS lookup, finding the name of the host

(server) from the IP address.

bash$ ipcalc -h 202.92.42.236

HOSTNAME=surfacemail.com

nslookup

Do an Internet "name server lookup" on a host by IP address.

This is essentially equivalent to ipcalc -h or dig -x . The

command may be run either interactively or noninteractively,

i.e., from within a script.

The nslookup command has allegedly been "deprecated," but it

is still useful.

bash$ nslookup -sil 66.97.104.180

nslookup kuhleersparnis.ch

Server: 135.116.137.2

Address: 135.116.137.2#53

Non-authoritative answer:

Name: kuhleersparnis.ch

dig

Domain Information Groper. Similar to nslookup, dig does an

Internet name server lookup on a host. May be run from the

command-line or from within a script.

Some interesting options to dig are +time=N for setting a

query timeout to N seconds, +nofail for continuing to query

servers until a reply is received, and -x for doing a reverse

address lookup.

Compare the output of dig -x with ipcalc -h and nslookup.

bash$ dig -x 81.9.6.2

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649

;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:

;2.6.9.81.in-addr.arpa. IN PTR

;; AUTHORITY SECTION:

6.9.81.in-addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net.

2002031705 900 600 86400 3600

;; Query time: 537 msec

;; SERVER: 135.116.137.2#53(135.116.137.2)

;; WHEN: Wed Jun 26 08:35:24 2002

;; MSG SIZE rcvd: 91

Example 16-40. Finding out where to report a spammer

!/bin/bash

spam-lookup.sh: Look up abuse contact to report a spammer.

Thanks, Michael Zick.

Check for command-line arg.

ARGCOUNT=1

E_WRONGARGS=85

if [ $# -ne "$ARGCOUNT" ]

then

echo "Usage: `basename $0` domain-name"

exit $E_WRONGARGS

dig +short $1.contacts.abuse.net -c in -t txt

Also try:

dig +nssearch $1

Tries to find "authoritative name servers" and display SOA records.

The following also works:

whois -h whois.abuse.net $1

^^ ^^^^^^^^^^^^^^^ Specify host.

Can even lookup multiple spammers with this, i.e."

whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .

Exercise:

--------

Expand the functionality of this script

+ so that it automatically e-mails a notification

+ to the responsible ISP's contact address(es).

Hint: use the "mail" command.

exit $?

spam-lookup.sh chinatietong.com

A known spam domain.

"crnet_mgr@chinatietong.com"

"crnet_tec@chinatietong.com"

"postmaster@chinatietong.com"

For a more elaborate version of this script,

+ see the SpamViz home page, http://www.spamviz.net/index.html.

Example 16-41. Analyzing a spam domain

! /bin/bash

is-spammer.sh: Identifying spam domains

$Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $

Above line is RCS ID info.

This is a simplified version of the "is_spammer.bash

+ script in the Contributed Scripts appendix.

is-spammer <domain.name>

Uses an external program: 'dig'

Tested with version: 9.2.4rc5

Uses functions.

Uses IFS to parse strings by assignment into arrays.

And even does something useful: checks e-mail blacklists.

Use the domain.name(s) from the text body:

http://www.good_stuff.spammer.biz/just_ignore_everything_else

^^^^^^^^^^^

Or the domain.name(s) from any e-mail address:

Really_Good_Offer@spammer.biz

as the only argument to this script.

(PS: have your Inet connection running)

So, to invoke this script in the above two instances:

is-spammer.sh spammer.biz

Whitespace == :Space:Tab:Line Feed:Carriage Return:

WSP_IFS= \x20' \x09' \x0A' \x0D'

No Whitespace == Line Feed:Carriage Return

No_WSP= \x0A' \x0D'

Field separator for dotted decimal ip addresses

ADR_IFS=${No_WSP}'.'

Get the dns text resource record.

get_txt <error_code> <list_query>

get_txt() {

# Parse $1 by assignment at the dots.

local -a dns

IFS=$ADR_IFS

dns=( $1 )

IFS=$WSP_IFS

if [ "${dns[0]}" == '127' ]

then

# See if there is a reason.

echo $(dig +short $2 -t txt)

}

Get the dns address resource record.

chk_adr <rev_dns> <list_server>

chk_adr() {

local reply

local server

local reason

server=${1}${2}

reply=$( dig +short ${server} )

# If reply might be an error code . . .

if [ ${#reply} -gt 6 ]

then

reason=$(get_txt ${reply} ${server} )

reason=${reason:-${reply}}

echo ${reason:-' not blacklisted.'}

}

Need to get the IP address from the name.

echo 'Get address of: '$1

ip_adr=$(dig +short $1)

dns_reply=${ip_adr:-' no answer '}

echo ' Found address: '${dns_reply}

A valid reply is at least 4 digits plus 3 dots.

if [ ${#ip_adr} -gt 6 ]

then

echo

declare query

# Parse by assignment at the dots.

declare -a dns

IFS=$ADR_IFS

dns=( ${ip_adr} )

IFS=$WSP_IFS

# Reorder octets into dns query order.

rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'

See: http://www.spamhaus.org (Conservative, well maintained)

echo -n 'spamhaus.org says: '

echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')

See: http://ordb.org (Open mail relays)

echo -n ' ordb.org says: '

echo $(chk_adr ${rev_dns} 'relays.ordb.org')

See: http://www.spamcop.net/ (You can report spammers here)

echo -n ' spamcop.net says: '

echo $(chk_adr ${rev_dns} 'bl.spamcop.net')

# # other blacklist operations # # #

See: http://cbl.abuseat.org.

echo -n ' abuseat.org says: '

echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')

See: http://dsbl.org/usage (Various mail relays)

echo

echo 'Distributed Server Listings'

echo -n ' list.dsbl.org says: '

echo $(chk_adr ${rev_dns} 'list.dsbl.org')

echo -n ' multihop.dsbl.org says: '

echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')

echo -n 'unconfirmed.dsbl.org says: '

echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')

else

echo

echo 'Could not use that address.'

exit 0

Exercises:

--------

1) Check arguments to script,

and exit with appropriate error message if necessary.

2) Check if on-line at invocation of script,

and exit with appropriate error message if necessary.

3) Substitute generic variables for "hard-coded" BHL domains.

4) Set a time-out for the script using the "+time=" option

to the 'dig' command.

For a much more elaborate version of the above script, see

Example A-28.

traceroute

Trace the route taken by packets sent to a remote host. This

command works within a LAN, WAN, or over the Internet. The

remote host may be specified by an IP address. The output of

this command may be filtered by grep or sed in a pipe.

bash$ traceroute 81.9.6.2

traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets

1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms

2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms

3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms *

...

ping

Broadcast an ICMP ECHO_REQUEST packet to another machine,

either on a local or remote network. This is a diagnostic tool

for testing network connections, and it should be used with

caution.

bash$ ping localhost

PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.

64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709

usec

64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286

usec

--- localhost.localdomain ping statistics ---

2 packets transmitted, 2 packets received, 0% packet loss

round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms

A successful ping returns an exit status of 0. This can be

tested for in a script.

HNAME=news-15.net # Notorious spammer.

HNAME=$HOST # Debug: test for localhost.

count=2 # Send only two pings.

if [[ `ping -c $count "$HNAME"` ]]

then

echo ""$HNAME" still up and broadcasting spam your way."

else

echo ""$HNAME" seems to be down. Pity."

whois

Perform a DNS (Domain Name System) lookup. The -h option

permits specifying which particular whois server to query. See

Example 4-6 and Example 16-40.

finger

Retrieve information about users on a network. Optionally,

this command can display a user's ~/.plan, ~/.project, and

~/.forward files, if present.

bash$ finger

bozo Bozo Bozeman tty1 8 Jun 25 16:59 (:0)

bozo Bozo Bozeman ttyp0 Jun 25 16:59 (:0.0)

bozo Bozo Bozeman ttyp1 Jun 25 17:07 (:0.0)

bash$ finger bozo

Directory: /home/bozo Shell: /bin/bash

Office: 2355 Clown St., 543-1234

On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle

On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle

On since Fri Aug 31 20:13 (MST) on pts/1

On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle

Mail last read Tue Jul 3 10:08 2007 (MST)

No Plan.

Out of security considerations, many networks disable finger

and its associated daemon. [81]

chfn

Change information disclosed by the finger command.

vrfy

Verify an Internet e-mail address.

This command seems to be missing from newer Linux distros.

Remote Host Access

sx, rx

The sx and rx command set serves to transfer files to and from

a remote host using the xmodem protocol. These are generally

part of a communications package, such as minicom.

sz, rz

The sz and rz command set serves to transfer files to and from

a remote host using the zmodem protocol. Zmodem has certain

advantages over xmodem, such as faster transmission rate and

resumption of interrupted file transfers. Like sx and rx,

these are generally part of a communications package.

ftp

Utility and protocol for uploading / downloading files to or

from a remote host. An ftp session can be automated in a

script (see Example 19-6 and Example A-4).

uucp, uux, cu

uucp: UNIX to UNIX copy. This is a communications package for

transferring files between UNIX servers. A shell script is an

effective way to handle a uucp command sequence.

Since the advent of the Internet and e-mail, uucp seems to

have faded into obscurity, but it still exists and remains

perfectly workable in situations where an Internet connection

is not available or appropriate. The advantage of uucp is that

it is fault-tolerant, so even if there is a service

interruption the copy operation will resume where it left off

when the connection is restored.

---

uux: UNIX to UNIX execute. Execute a command on a remote

system. This command is part of the uucp package.

---

cu: Call Up a remote system and connect as a simple terminal.

It is a sort of dumbed-down version of telnet. This command is

part of the uucp package.

telnet

Utility and protocol for connecting to a remote host.

Caution

The telnet protocol contains security holes and should therefore

probably be avoided. Its use within a shell script is not

recommended.

wget

The wget utility noninteractively retrieves or downloads files

from a Web or ftp site. It works well in a script.

wget -p http://www.xyz23.com/file01.html

The -p or --page-requisite option causes wget to fetch all files

+ required to display the specified page.

wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE

The -r option recursively follows and retrieves all links

+ on the specified site.

wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2

The -c option lets wget resume an interrupted download.

This works with ftp servers and many HTTP sites.

Example 16-42. Getting a stock quote

!/bin/bash

quote-fetch.sh: Download a stock quote.

E_NOPARAMS=86

if [ -z "$1" ] # Must specify a stock (symbol) to fetch.

then echo "Usage: `basename $0` stock-symbol"

exit $E_NOPARAMS

stock_symbol=$1

file_suffix=.html

Fetches an HTML file, so name it appropriately.

URL='http://finance.yahoo.com/q?s='

Yahoo finance board, with stock query suffix.

-----------------------------------------------------------

wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"

-----------------------------------------------------------

To look up stuff on http://search.yahoo.com:

-----------------------------------------------------------

URL="http://search.yahoo.com/search?fr=ush-news&p=${query}"

wget -O "$savefilename" "${URL}"

-----------------------------------------------------------

Saves a list of relevant URLs.

exit $?

Exercises:

---------

1) Add a test to ensure the user running the script is on-line.

(Hint: parse the output of 'ps -ax' for "ppp" or "connect."

2) Modify this script to fetch the local weather report,

+ taking the user's zip code as an argument.

!/bin/bash

fc4upd.sh

Script author: Frank Wang.

Slight stylistic modifications by ABS Guide author.

Used in ABS Guide with permission.

Download Fedora Core 4 update from mirror site using rsync.

Should also work for newer Fedora Cores -- 5, 6, . . .

Only download latest package if multiple versions exist,

+ to save space.

URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/

URL=rsync://ftp.kddilabs.jp/fedora/core/updates/

URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/

DEST=${1:-/var/www/html/fedora/updates/}

LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt

PID_FILE=/var/run/${0##*/}.pid

E_RETURN=85 # Something unexpected happened.

General rsync options

-r: recursive download

-t: reserve time

-v: verbose

OPTS="-rtv --delete-excluded --delete-after --partial"

rsync include pattern

Leading slash causes absolute path name match.

INCLUDE=(

"/4/i386/kde-i18n-Chinese*"

^ ^

Quoting is necessary to prevent globbing.

)

rsync exclude pattern

Temporarily comment out unwanted pkgs using "#" . . .

EXCLUDE=(

/testing

/4/SRPMS

/4/ppc

/4/x86_64

/4/i386/debug

"/4/i386/kde-i18n-*"

"/4/i386/openoffice.org-langpack-*"

"/4/i386/*i586.rpm"

"/4/i386/GFS-*"

"/4/i386/cman-*"

"/4/i386/dlm-*"

"/4/i386/gnbd-*"

"/4/i386/kernel-smp*"

"/4/i386/kernel-xen*"

"/4/i386/xen-*"

)

init () {

# Let pipe command return possible rsync error, e.g., stalled network.

set -o pipefail # Newly introduced in Bash, version 3.

TMP=${TMPDIR:-/tmp}/${0##*/}.$ # Store refined download list.

trap "{

rm -f $TMP 2>/dev/null

}" EXIT # Clear temporary file on exit.

}

check_pid () {

Check if process exists.

if [ -s "$PID_FILE" ]; then

echo "PID file exists. Checking ..."

PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)

if /bin/ps --pid $PID &>/dev/null; then

echo "Process $PID found. ${0##*/} seems to be running!"

/usr/bin/logger -t ${0##*/} \

"Process $PID found. ${0##*/} seems to be running!"

exit $E_RETURN

echo "Process $PID not found. Start new process . . ."

}

Set overall file update range starting from root or $URL,

+ according to above patterns.

set_range () {

include=

exclude=

for p in "${INCLUDE[@]}"; do

include="$include --include \"$p\""

done

for p in "${EXCLUDE[@]}"; do

exclude="$exclude --exclude \"$p\""

done

}

Retrieve and refine rsync update list.

get_list () {

echo $ > $PID_FILE || {

echo "Can't write to pid file $PID_FILE"

exit $E_RETURN

}

echo -n "Retrieving and refining update list . . ."

# Retrieve list -- 'eval' is needed to run rsync as a single command.

# $3 and $4 is the date and time of file creation.

# $5 is the full package name.

previous=

pre_file=

pre_date=0

eval /bin/nice /usr/bin/rsync \

-r $include $exclude $URL | \

egrep '^dr.x|^-r' | \

awk '{print $3, $4, $5}' | \

sort -k3 | \

{ while read line; do

# Get seconds since epoch, to filter out obsolete pkgs.

cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)

# echo $cur_date

# Get file name.

cur_file=$(echo $line | awk '{print $3}')

# echo $cur_file

# Get rpm pkg name from file name, if possible.

if [[ $cur_file == *rpm ]]; then

pkg_name=$(echo $cur_file | sed -r -e \

's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/')

else

pkg_name=

# echo $pkg_name

if [ -z "$pkg_name" ]; then # If not a rpm file,

echo $cur_file >> $TMP #+ then append to download list.

elif [ "$pkg_name" != "$previous" ]; then # A new pkg found.

echo $pre_file >> $TMP # Output latest file.

previous=$pkg_name # Save current.

pre_date=$cur_date

pre_file=$cur_file

elif [ "$cur_date" -gt "$pre_date" ]; then

# If same pkg, but newer,

pre_date=$cur_date #+ then update latest pointer.

pre_file=$cur_file

done

echo $pre_file >> $TMP # TMP contains ALL

#+ of refined list now.

# echo "subshell=$BASH_SUBSHELL"

} # Bracket required here to let final "echo $pre_file >> $TMP"

# Remained in the same subshell ( 1 ) with the entire loop.

RET=$? # Get return code of the pipe command.

[ "$RET" -ne 0 ] && {

echo "List retrieving failed with code $RET"

exit $E_RETURN

}

echo "done"; echo

}

Real rsync download part.

get_file () {

echo "Downloading..."

/bin/nice /usr/bin/rsync \

$OPTS \

--filter "merge,+/ $TMP" \

--exclude '*' \

$URL $DEST \

| /usr/bin/tee $LOG

RET=$?

# --filter merge,+/ is crucial for the intention.

# + modifier means include and / means absolute path.

# Then sorted list in $TMP will contain ascending dir name and

#+ prevent the following --exclude '*' from "shortcutting the circuit."

echo "Done"

rm -f $PID_FILE 2>/dev/null

return $RET

}

-------

Main

init

check_pid

set_range

get_list

get_file

RET=$?

-------

if [ "$RET" -eq 0 ]; then

/usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."

else

/usr/bin/logger -t ${0##*/} \

"Fedora update mirrored with failure code: $RET"

exit $RET

!/bin/bash

remote.bash: Using ssh.

This example by Michael Zick.

Used with permission.

Presumptions:

------------

fd-2 isn't being captured ( '2>/dev/null' ).

ssh/sshd presumes stderr ('2') will display to user.

sshd is running on your machine.

For any 'standard' distribution, it probably is,

+ and without any funky ssh-keygen having been done.

Try ssh to your machine from the command-line:

$ ssh $HOSTNAME

Without extra set-up you'll be asked for your password.

enter password

when done, $ exit

Did that work? If so, you're ready for more fun.

Try ssh to your machine as 'root':

$ ssh -l root $HOSTNAME

When asked for password, enter root's, not yours.

Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain

Enter 'exit' when done.

The above gives you an interactive shell.

It is possible for sshd to be set up in a 'single command' mode,

+ but that is beyond the scope of this example.

The only thing to note is that the following will work in

+ 'single command' mode.

A basic, write stdout (local) command.

ls -l

Now the same basic command on a remote machine.

Pass a different 'USERNAME' 'HOSTNAME' if desired:

USER=${USERNAME:-$(whoami)}

HOST=${HOSTNAME:-$(hostname)}

Now excute the above command-line on the remote host,

+ with all transmissions encrypted.

ssh -l ${USER} ${HOST} " ls -l "

The expected result is a listing of your username's home

+ directory on the remote machine.

To see any difference, run this script from somewhere

+ other than your home directory.

In other words, the Bash command is passed as a quoted line

+ to the remote shell, which executes it on the remote machine.

In this case, sshd does ' bash -c "ls -l" ' on your behalf.

For information on topics such as not having to enter a

+ password/passphrase for every command-line, see

+ man ssh

+ man ssh-keygen

+ man sshd_config.

exit 0

Caution

Within a loop, ssh may cause unexpected behavior. According to a

[http://groups-beta.google.com/group/comp.unix.shell/msg/dcb446b5fff7

d230] Usenet post in the comp.unix shell archives, ssh inherits the

loop's stdin. To remedy this, pass ssh either the -n or -f option.

Thanks, Jason Bechtel, for pointing this out.

scp

Secure copy, similar in function to rcp, copies files between

two different networked machines, but does so using

authentication, and with a security level similar to ssh.

Local Network

write

This is a utility for terminal-to-terminal communication. It

allows sending lines from your terminal (console or xterm) to

that of another user. The mesg command may, of course, be used

to disable write access to a terminal

Since write is interactive, it would not normally find use in

a script.

netconfig

A command-line utility for configuring a network adapter

(using DHCP). This command is native to Red Hat centric Linux

distros.

Mail

mail

Send or read e-mail messages.

This stripped-down command-line mail client works fine as a

command embedded in a script.

Example 16-45. A script that mails itself

!/bin/sh

self-mailer.sh: Self-mailing script

adr=${1:-`whoami`} # Default to current user, if not specified.

Typing 'self-mailer.sh wiseguy@superdupergenius.com'

+ sends this script to that addressee.

Just 'self-mailer.sh' (no argument) sends the script

+ to the person invoking it, for example, bozo@localhost.localdomain.

For more on the ${parameter:-default} construct,

+ see the "Parameter Substitution" section

+ of the "Variables Revisited" chapter.

============================================================================

cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr"

============================================================================

--------------------------------------------

Greetings from the self-mailing script.

A mischievous person has run this script,

+ which has caused it to mail itself to you.

Apparently, some people have nothing better

+ to do with their time.

--------------------------------------------

echo "At `date`, script \"`basename $0`\" mailed to "$adr"."

exit 0

Note that the "mailx" command (in "send" mode) may be substituted

+ for "mail" ... but with somewhat different options.

mailto

Similar to the mail command, mailto sends e-mail messages from

the command-line or in a script. However, mailto also permits

sending MIME (multimedia) messages.

mailstats

Show mail statistics. This command may be invoked only by

root.

root# mailstats

Statistics from Tue Jan 1 20:32:08 2008

M msgsfr bytes_from msgsto bytes_to msgsrej msgsdis msgsqur Mailer

4 1682 24118K 0 0K 0 0 0 esmtp

9 212 640K 1894 25131K 0 0 0 local

=====================================================================

T 1894 24758K 1894 25131K 0 0 0

C 414 0

vacation

This utility automatically replies to e-mails that the

intended recipient is on vacation and temporarily unavailable.

It runs on a network, in conjunction with sendmail, and is not

applicable to a dial-up POPmail account.

________________________________________________________________

16.7. Terminal Control Commands

Command affecting the console or terminal

tput

Initialize terminal and/or fetch information about it from

terminfo data. Various options permit certain terminal

operations: tput clear is the equivalent of clear; tput reset

is the equivalent of reset.

bash$ tput longname

xterm terminal emulator (X Window System)

Issuing a tput cup X Y moves the cursor to the (X,Y)

coordinates in the current terminal. A clear to erase the

terminal screen would normally precede this.

Some interesting options to tput are:

+ bold, for high-intensity text

+ smul, to underline text in the terminal

+ smso, to render text in reverse

+ sgr0, to reset the terminal parameters (to normal), without

clearing the screen

Example scripts using tput:

1. Example 36-15

2. Example 36-13

3. Example A-44

4. Example A-42

5. Example 27-2

Note that stty offers a more powerful command set for controlling a

terminal.

infocmp

This command prints out extensive information about the current

terminal. It references the terminfo database.

bash$ infocmp

Reconstructed via infocmp from file:

/usr/share/terminfo/r/rxvt

rxvt|rxvt terminal emulator (X Window System),

am, bce, eo, km, mir, msgr, xenl, xon,

colors#8, cols#80, it#8, lines#24, pairs#64,

acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,

bel=^G, blink=\E[5m, bold=\E[1m,

civis=\E[?25l,

clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M,

...

reset

Reset terminal parameters and clear text screen. As with clear, the

cursor and prompt reappear in the upper lefthand corner of the

terminal.

clear

The clear command simply clears the text screen at the console or in

an xterm. The prompt and cursor reappear at the upper lefthand corner

of the screen or xterm window. This command may be used either at the

command line or in a script. See Example 11-26.

resize

Echoes commands necessary to set $TERM and $TERMCAP to duplicate the

size (dimensions) of the current terminal.

bash$ resize

set noglob;

setenv COLUMNS '80';

setenv LINES '24';

unset noglob;

script

This utility records (saves to a file) all the user keystrokes at the

command-line in a console or an xterm window. This, in effect,

creates a record of a session.

________________________________________________________________

16.8. Math Commands

"Doing the numbers"

factor

Decompose an integer into prime factors.

bash$ factor 27417

27417: 3 13 19 37

Example 16-46. Generating prime numbers

!/bin/bash

primes2.sh

Generating prime numbers the quick-and-easy way,

+ without resorting to fancy algorithms.

CEILING=10000 # 1 to 10000

PRIME=0

E_NOTPRIME=

is_prime ()

{

local factors

factors=( $(factor $1) ) # Load output of `factor` into array.

if [ -z "${factors[2]}" ]

Third element of "factors" array:

+ ${factors[2]} is 2nd factor of argument.

If it is blank, then there is no 2nd factor,

+ and the argument is therefore prime.

then

return $PRIME # 0

else

return $E_NOTPRIME # null

}

echo

for n in $(seq $CEILING)

if is_prime $n

then

printf %5d $n

fi # ^ Five positions per number suffices.

done # For a higher $CEILING, adjust upward, as necessary.

echo

exit

Bash can't handle floating point calculations, and it lacks

operators for certain important mathematical functions.

Fortunately, bc gallops to the rescue.

Not just a versatile, arbitrary precision calculation utility,

bc offers many of the facilities of a programming language. It

has a syntax vaguely resembling C.

Since it is a fairly well-behaved UNIX utility, and may

therefore be used in a pipe, bc comes in handy in scripts.

Here is a simple template for using bc to calculate a script

variable. This uses command substitution.

variable=$(echo "OPTIONS; OPERATIONS" | bc)

Example 16-47. Monthly Payment on a Mortgage

!/bin/bash

monthlypmt.sh: Calculates monthly payment on a mortgage.

This is a modification of code in the

+ "mcalc" (mortgage calculator) package,

+ by Jeff Schmidt

+ and

+ Mendel Cooper (yours truly, the ABS Guide author).

http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz

echo

echo "Given the principal, interest rate, and term of a mortgage,"

echo "calculate the monthly payment."

bottom=1.0

echo

echo -n "Enter principal (no commas) "

read principal

echo -n "Enter interest rate (percent) " # If 12%, enter "12", not ".12".

read interest_r

echo -n "Enter term (months) "

read term

interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.

# ^^^^^^^^^^^^^^^^^ Divide by 100.

# "scale" determines how many decimal places.

interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)

top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)

# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

# Standard formula for figuring interest.

echo; echo "Please be patient. This may take a while."

let "months = $term - 1"

====================================================================

for ((x=$months; x > 0; x--))

bot=$(echo "scale=9; $interest_rate^$x" | bc)

bottom=$(echo "scale=9; $bottom+$bot" | bc)

bottom = $(($bottom + $bot"))

done

====================================================================

--------------------------------------------------------------------

Rick Boivie pointed out a more efficient implementation

+ of the above loop, which decreases computation time by 2/3.

for ((x=1; x <= $months; x++))

do

bottom=$(echo "scale=9; $bottom * $interest_rate + 1" | bc)

done

And then he came up with an even more efficient alternative,

+ one that cuts down the run time by about 95%!

bottom=`{

echo "scale=9; bottom=$bottom; interest_rate=$interest_rate"

for ((x=1; x <= $months; x++))

do

echo 'bottom = bottom * interest_rate + 1'

done

echo 'bottom'

} | bc` # Embeds a 'for loop' within command substitution.

--------------------------------------------------------------------------

On the other hand, Frank Wang suggests:

bottom=$(echo "scale=9; ($interest_rate^$term-1)/($interest_rate-1)" | bc)

Because . . .

The algorithm behind the loop

+ is actually a sum of geometric proportion series.

The sum formula is e0(1-q^n)/(1-q),

+ where e0 is the first element and q=e(n+1)/e(n)

+ and n is the number of elements.

--------------------------------------------------------------------------

# let "payment = $top/$bottom"

payment=$(echo "scale=2; $top/$bottom" | bc)

# Use two decimal places for dollars and cents.

echo

echo "monthly payment = \$payment" # Echo a dollar sign in front of amount.

echo

exit 0

# Exercises:

# 1) Filter input to permit commas in principal amount.

# 2) Filter input to permit interest to be entered as percent or decimal.

# 3) If you are really ambitious,

#+ expand this script to print complete amortization tables.

Example 16-48. Base Conversion

!/bin/bash

Shellscript: base.sh - print number to different bases (Bourne Shell)

Author : Heiner Steven (heiner.steven@odn.de)

Date : 07-03-95

Category : Desktop

$Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $

==> Above line is RCS ID info.

Description

Changes

21-03-95 stv fixed error occuring with 0xb as input (0.2)

==> Used in ABS Guide with the script author's permission.

==> Comments added by ABS Guide author.

NOARGS=85

PN=`basename "$0"` # Program name

VER=`echo '$Revision: 1.2 | cut -d' ' -f2` # ==> VER=1.2

Usage () {

echo "$PN - print number to different bases, $VER (stv '95)

usage: $PN [number ...]

If no number is given, the numbers are read from standard input.

A number may be

binary (base 2) starting with 0b (i.e. 0b1100)

octal (base 8) starting with 0 (i.e. 014)

hexadecimal (base 16) starting with 0x (i.e. 0xc)

decimal otherwise (i.e. 12)" >&2

exit $NOARGS

} # ==> Prints usage message.

Msg () {

for i # ==> in [list] missing. Why?

do echo "$PN: $i" >&2

done

}

Fatal () { Msg "$@"; exit 66; }

PrintBases () {

# Determine base of the number

for i # ==> in [list] missing...

do # ==> so operates on command-line arg(s).

case "$i" in

0b*) ibase=2;; # binary

0x*|[a-f]*|[A-F]*) ibase=16;; # hexadecimal

0*) ibase=8;; # octal

[1-9]*) ibase=10;; # decimal

Msg "illegal number $i - ignored"

continue;;

esac

# Remove prefix, convert hex digits to uppercase (bc needs this).

number=`echo "$i" | sed -e 's:^0[bBxX]::' | tr '[a-f]' '[A-F]'`

# ==> Uses ":" as sed separator, rather than "/".

# Convert number to decimal

dec=`echo "ibase=$ibase; $number" | bc` # ==> 'bc' is calculator util

ity.

case "$dec" in

[0-9]*) ;; # number ok

*) continue;; # error: ignore

esac

# Print all conversions in one line.

# ==> 'here document' feeds command list to 'bc'.

echo `bc <<!

obase=16; "hex="; $dec

obase=10; "dec="; $dec

obase=8; "oct="; $dec

obase=2; "bin="; $dec

` | sed -e 's: : :g'

done

}

while [ $# -gt 0 ]

==> Is a "while loop" really necessary here,

==>+ since all the cases either break out of the loop

==>+ or terminate the script.

==> (Above comment by Paulo Marcel Coelho Aragao.)

case "$1" in

--) shift; break;;

-h) Usage;; # ==> Help message.

-*) Usage;;

*) break;; # First number

esac # ==> Error checking for illegal input might be appropriate.

shift

done

if [ $# -gt 0 ]

then

PrintBases "$@"

else # Read from stdin.

while read line

PrintBases $line

done

exit

An alternate method of invoking bc involves using a here

document embedded within a command substitution block. This is

especially appropriate when a script needs to pass a list of

options and commands to bc.

variable=`bc << LIMIT_STRING

options

statements

operations

LIMIT_STRING

...or...

variable=$(bc << LIMIT_STRING

options

statements

operations

LIMIT_STRING

)

Example 16-49. Invoking bc using a here document

!/bin/bash

Invoking 'bc' using command substitution

in combination with a 'here document'.

var1=`bc << EOF

18.33 * 19.78

EOF

echo $var1 # 362.56

$( ... ) notation also works.

v1=23.53

v2=17.881

v3=83.501

v4=171.63

var2=$(bc << EOF

scale = 4

a = ( $v1 + $v2 )

b = ( $v3 * $v4 )

a * b + 15.35

EOF

)

echo $var2 # 593487.8452

var3=$(bc -l << EOF

scale = 9

s ( 1.7 )

EOF

)

Returns the sine of 1.7 radians.

The "-l" option calls the 'bc' math library.

echo $var3 # .991664810

Now, try it in a function...

hypotenuse () # Calculate hypotenuse of a right triangle.

{ # c = sqrt( a^2 + b^2 )

hyp=$(bc -l << EOF

scale = 9

sqrt ( $1 * $1 + $2 * $2 )

EOF

)

Can't directly return floating point values from a Bash function.

But, can echo-and-capture:

echo "$hyp"

}

hyp=$(hypotenuse 3.68 7.31)

echo "hypotenuse = $hyp" # 8.184039344

exit 0

Example 16-50. Calculating PI

!/bin/bash

cannon.sh: Approximating PI by firing cannonballs.

Author: Mendel Cooper

License: Public Domain

Version 2.2, reldate 13oct08.

This is a very simple instance of a "Monte Carlo" simulation:

+ a mathematical model of a real-life event,

+ using pseudorandom numbers to emulate random chance.

Consider a perfectly square plot of land, 10000 units on a side.

This land has a perfectly circular lake in its center,

+ with a diameter of 10000 units.

The plot is actually mostly water, except for land in the four corners.

(Think of it as a square with an inscribed circle.)

We will fire iron cannonballs from an old-style cannon

+ at the square.

All the shots impact somewhere on the square,

+ either in the lake or on the dry corners.

Since the lake takes up most of the area,

+ most of the shots will SPLASH! into the water.

Just a few shots will THUD! into solid ground

+ in the four corners of the square.

If we take enough random, unaimed shots at the square,

+ Then the ratio of SPLASHES to total shots will approximate

+ the value of PI/4.

The simplified explanation is that the cannon is actually

+ shooting only at the upper right-hand quadrant of the square,

+ i.e., Quadrant I of the Cartesian coordinate plane.

Theoretically, the more shots taken, the better the fit.

However, a shell script, as opposed to a compiled language

+ with floating-point math built in, requires some compromises.

This decreases the accuracy of the simulation.

DIMENSION=10000 # Length of each side of the plot.

# Also sets ceiling for random integers generated.

MAXSHOTS=1000 # Fire this many shots.

# 10000 or more would be better, but would take too long.

PMULTIPLIER=4.0 # Scaling factor.

declare -r M_PI=3.141592654

# Actual 9-place value of PI, for comparison purposes.

get_random ()

{

SEED=$(head -n 1 /dev/urandom | od -N 1 | awk '{ print $2 }')

RANDOM=$SEED # From "seeding-random.sh"

#+ example script.

let "rnum = $RANDOM % $DIMENSION" # Range less than 10000.

echo $rnum

}

distance= # Declare global variable.

hypotenuse () # Calculate hypotenuse of a right triangle.

{ # From "alt-bc.sh" example.

distance=$(bc -l << EOF

scale = 0

sqrt ( $1 * $1 + $2 * $2 )

EOF

)

Setting "scale" to zero rounds down result to integer value,

+ a necessary compromise in this script.

It decreases the accuracy of this simulation.

}

==========================================================

main() {

"Main" code block, mimicking a C-language main() function.

Initialize variables.

shots=0

splashes=0

thuds=0

Pi=0

error=0

while [ "$shots" -lt "$MAXSHOTS" ] # Main loop.

xCoord=$(get_random) # Get random X and Y coords.

yCoord=$(get_random)

hypotenuse $xCoord $yCoord # Hypotenuse of

#+ right-triangle = distance.

((shots++))

printf "#%4d " $shots

printf "Xc = %4d " $xCoord

printf "Yc = %4d " $yCoord

printf "Distance = %5d " $distance # Distance from

#+ center of lake

#+ -- the "origin" --

#+ coordinate (0,0).

if [ "$distance" -le "$DIMENSION" ]

then

echo -n "SPLASH! "

((splashes++))

else

echo -n "THUD! "

((thuds++))

Pi=$(echo "scale=9; $PMULTIPLIER*$splashes/$shots" | bc)

# Multiply ratio by 4.0.

echo -n "PI ~ $Pi"

echo

done

echo

echo "After $shots shots, PI looks like approximately $Pi"

Tends to run a bit high,

+ possibly due to round-off error and imperfect randomness of $RANDOM.

But still usually within plus-or-minus 5% . . .

+ a pretty fair rough approximation.

error=$(echo "scale=9; $Pi - $M_PI" | bc)

pct_error=$(echo "scale=2; 100.0 * $error / $M_PI" | bc)

echo -n "Deviation from mathematical value of PI = $error"

echo " ($pct_error% error)"

echo

End of "main" code block.

}

==========================================================

exit 0

One might well wonder whether a shell script is appropriate for

+ an application as complex and computation-intensive as a simulation.

There are at least two justifications.

1) As a proof of concept: to show it can be done.

2) To prototype and test the algorithms before rewriting

+ it in a compiled high-level language.

The P command prints the string between the preceding brackets.

And now for some simple arithmetic.

echo "7 8 * p" | dc # 56

Pushes 7, then 8 onto the stack,

+ multiplies ("*" operator), then prints the result ("p" operator).

Most persons avoid dc, because of its non-intuitive input and

rather cryptic operators. Yet, it has its uses.

Example 16-51. Converting a decimal number to hexadecimal

!/bin/bash

hexconvert.sh: Convert a decimal number to hexadecimal.

E_NOARGS=85 # Command-line arg missing.

BASE=16 # Hexadecimal.

if [ -z "$1" ]

then # Need a command-line argument.

echo "Usage: $0 number"

exit $E_NOARGS

fi # Exercise: add argument validity checking.

hexcvt ()

{

if [ -z "$1" ]

then

echo 0

return # "Return" 0 if no arg passed to function.

echo ""$1" "$BASE" o p" | dc

o sets radix (numerical base) of output.

p prints the top of stack.

For other options: 'man dc' ...

return

}

hexcvt "$1"

exit

Studying the info page for dc is a painful path to

understanding its intricacies. There seems to be a small,

select group of dc wizards who delight in showing off their

mastery of this powerful, but arcane utility.

bash$ echo "16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D68736142snlbxq" | dc

Bash

dc <<< 10k5v1+2/p # 1.6180339887

^^^ Feed operations to dc using a Here String.

^^^ Pushes 10 and sets that as the precision (10k).

^^ Pushes 5 and takes its square root

(5v, v = square root).

^^ Pushes 1 and adds it to the running total (1+).

^^ Pushes 2 and divides the running total by that (2/).

^ Pops and prints the result (p)

The result is 1.6180339887 ...

... which happens to be the Pythagorean Golden Ratio, to 10 places.

Example 16-52. Factoring

!/bin/bash

factr.sh: Factor a number

MIN=2 # Will not work for number smaller than this.

E_NOARGS=85

E_TOOSMALL=86

if [ -z $1 ]

then

echo "Usage: $0 number"

exit $E_NOARGS

if [ "$1" -lt "$MIN" ]

then

echo "Number to factor must be $MIN or greater."

exit $E_TOOSMALL

Exercise: Add type checking (to reject non-integer arg).

echo "Factors of $1:"

-------------------------------------------------------

echo "$1[p]s2[lip/dli%0=1dvsr]s12sid2%0=13sidvsr[dli%0=\

1lrli2+dsi!>.]ds.xd1<2" | dc

-------------------------------------------------------

Above code written by Michel Charpentier <charpov@cs.unh.edu>

(as a one-liner, here broken into two lines for display purposes).

Used in ABS Guide with permission (thanks!).

exit

# $ sh factr.sh 270138

# 2

# 3

# 11

# 4093

awk

Yet another way of doing floating point math in a script is

using awk's built-in math functions in a shell wrapper.

Example 16-53. Calculating the hypotenuse of a triangle

!/bin/bash

hypotenuse.sh: Returns the "hypotenuse" of a right triangle.

(square root of sum of squares of the "legs")

ARGS=2 # Script needs sides of triangle passed.

E_BADARGS=85 # Wrong number of arguments.

if [ $# -ne "$ARGS" ] # Test number of arguments to script.

then

echo "Usage: `basename $0` side_1 side_2"

exit $E_BADARGS

AWKSCRIPT=' { printf( "%3.7f\n", sqrt($1*$1 + $2*$2) ) } '

command(s) / parameters passed to awk

Now, pipe the parameters to awk.

echo -n "Hypotenuse of $1 and $2 = "

echo $1 $2 | awk "$AWKSCRIPT"

^^^^^^^^^^^^

An echo-and-pipe is an easy way of passing shell parameters to awk.

exit

Exercise: Rewrite this script using 'bc' rather than awk.

Which method is more intuitive?

________________________________________________________________

16.9. Miscellaneous Commands

Command that fit in no special category

jot, seq

These utilities emit a sequence of integers, with a

user-selectable increment.

The default separator character between each integer is a

newline, but this can be changed with the -s option.

bash$ seq 5

bash$ seq -s : 5

1:2:3:4:5

Both jot and seq come in handy in a for loop.

Example 16-54. Using seq to generate loop arguments

!/bin/bash

Using "seq"

echo

for a in `seq 80` # or for a in $( seq 80 )

Same as for a in 1 2 3 4 5 ... 80 (saves much typing!).

May also use 'jot' (if present on system).

echo -n "$a "

done # 1 2 3 4 5 ... 80

Example of using the output of a command to generate

the [list] in a "for" loop.

echo; echo

COUNT=80 # Yes, 'seq' also accepts a replaceable parameter.

for a in `seq $COUNT` # or for a in $( seq $COUNT )

echo -n "$a "

done # 1 2 3 4 5 ... 80

echo; echo

BEGIN=75

END=80

for a in `seq $BEGIN $END`

Giving "seq" two arguments starts the count at the first one,

+ and continues until it reaches the second.

echo -n "$a "

done # 75 76 77 78 79 80

echo; echo

BEGIN=45

INTERVAL=5

END=80

for a in `seq $BEGIN $INTERVAL $END`

Giving "seq" three arguments starts the count at the first one,

+ uses the second for a step interval,

+ and continues until it reaches the third.

echo -n "$a "

done # 45 50 55 60 65 70 75 80

echo; echo

exit 0

A simpler example:

Create a set of 10 files,

+ named file.1, file.2 . . . file.10.

COUNT=10

PREFIX=file

for filename in `seq $COUNT`

touch $PREFIX.$filename

# Or, can do other operations,

#+ such as rm, grep, etc.

done

Example 16-55. Letter Count"

!/bin/bash

letter-count.sh: Counting letter occurrences in a text file.

Written by Stefano Palmeri.

Used in ABS Guide with permission.

Slightly modified by document author.

MINARGS=2 # Script requires at least two arguments.

E_BADARGS=65

FILE=$1

let LETTERS=$#-1 # How many letters specified (as command-line args).

# (Subtract 1 from number of command-line args.)

show_help(){

echo

echo Usage: `basename $0` file letters

echo Note: `basename $0` arguments are case sensitive.

echo Example: `basename $0` foobar.txt G n U L i N U x.

echo

}

Checks number of arguments.

if [ $# -lt $MINARGS ]; then

echo

echo "Not enough arguments."

echo

show_help

exit $E_BADARGS

Checks if file exists.

if [ ! -f $FILE ]; then

echo "File \"$FILE\" does not exist."

exit $E_BADARGS

Counts letter occurrences .

for n in `seq $LETTERS`; do

shift

if [[ `echo -n "$1" | wc -c` -eq 1 ]]; then # Checks arg.

echo "$1" -\> `cat $FILE | tr -cd "$1" | wc -c` # Counting.

else

echo "$1 is not a single char."

done

exit $?

This script has exactly the same functionality as letter-count2.sh,

+ but executes faster.

Why?

Note

Somewhat more capable than seq, jot is a classic UNIX utility that is

not normally included in a standard Linux distro. However, the source

rpm is available for download from the

[http://www.mit.edu/afs/athena/system/rhlinux/athena-9.0/free/SRPMS/a

thena-jot-9.0-3.src.rpm] MIT repository.

Unlike seq, jot can generate a sequence of random numbers, using the

-r option.

bash$ jot -r 3 999

1069

1272

1428

getopt

The getopt command parses command-line options preceded by a

dash. This external command corresponds to the getopts Bash

builtin. Using getopt permits handling long options by means

of the -l flag, and this also allows parameter reshuffling.

Example 16-56. Using getopt to parse command-line options

!/bin/bash

Using getopt

Try the following when invoking this script:

sh ex33a.sh -a

sh ex33a.sh -abc

sh ex33a.sh -a -b -c

sh ex33a.sh -d

sh ex33a.sh -dXYZ

sh ex33a.sh -d XYZ

sh ex33a.sh -abcd

sh ex33a.sh -abcdZ

sh ex33a.sh -z

sh ex33a.sh a

Explain the results of each of the above.

E_OPTERR=65

if [ "$#" -eq 0 ]

then # Script needs at least one command-line argument.

echo "Usage $0 -[options a,b,c]"

exit $E_OPTERR

set -- `getopt "abcd:" "$@"`

Sets positional parameters to command-line arguments.

What happens if you use "$*" instead of "$@"?

while [ ! -z "$1" ]

case "$1" in

-a) echo "Option \"a\"";;

-b) echo "Option \"b\"";;

-c) echo "Option \"c\"";;

-d) echo "Option \"d\" $2";;

*) break;;

esac

shift

done

It is usually better to use the 'getopts' builtin in a script.

See "ex33.sh."

exit 0

Note

As Peggy Russell points out:

It is often necessary to include an eval to correctly process

whitespace and quotes.

args=$(getopt -o a:bc:d -- "$@")

eval set -- "$args"

See Example 10-5 for a simplified emulation of getopt.

run-parts

The run-parts command [82] executes all the scripts in a

target directory, sequentially in ASCII-sorted filename order.

Of course, the scripts need to have execute permission.

The cron daemon invokes run-parts to run the scripts in the

/etc/cron.* directories.

yes

In its default behavior the yes command feeds a continuous

string of the character y followed by a line feed to stdout. A

control-C terminates the run. A different output string may be

specified, as in yes different string, which would continually

output different string to stdout.

One might well ask the purpose of this. From the command-line

or in a script, the output of yes can be redirected or piped

into a program expecting user input. In effect, this becomes a

sort of poor man's version of expect.

yes | fsck /dev/hda1 runs fsck non-interactively (careful!).

yes | rm -r dirname has same effect as rm -rf dirname

(careful!).

Warning

Caution advised when piping yes to a potentially dangerous system

command, such as fsck or fdisk. It might have unintended

consequences.

Note

The yes command parses variables, or more accurately, it echoes

parsed variables. For example:

bash$ yes $BASH_VERSION

3.1.17(1)-release

. . .

This particular "feature" may be used to create a very large ASCII

file on the fly:

bash$ yes $PATH > huge_file.txt

Ctl-C

Hit Ctl-C very quickly, or you just might get more than you

bargained for. . . .

The yes command may be emulated in a very simple script

function.

yes ()

{ # Trivial emulation of "yes" ...

local DEFAULT_TEXT="y"

while [ true ] # Endless loop.

if [ -z "$1" ]

then

echo "$DEFAULT_TEXT"

else # If argument ...

echo "$1" # ... expand and echo it.

done # The only things missing are the

} #+ --help and --version options.

banner

Prints arguments as a large vertical banner to stdout, using

an ASCII character (default '#'). This may be redirected to a

printer for hardcopy.

Note that banner has been dropped from many Linux distros,

presumably because it is no longer considered useful.

printenv

Show all the environmental variables set for a particular

user.

bash$ printenv | grep HOME

HOME=/home/bozo

The lp and lpr commands send file(s) to the print queue, to be

printed as hard copy. [83] These commands trace the origin of

their names to the line printers of another era. [84]

bash$ lp file1.txt or bash lp <file1.txt

It is often useful to pipe the formatted output from pr to lp.

bash$ pr -options file1.txt | lp

Formatting packages, such as groff and Ghostscript may send

their output directly to lp.

bash$ groff -Tascii file.tr | lp

bash$ gs -options | lp file.ps

Related commands are lpq, for viewing the print queue, and

lprm, for removing jobs from the print queue.

tee

[UNIX borrows an idea from the plumbing trade.]

This is a redirection operator, but with a difference. Like

the plumber's tee, it permits "siphoning off" to a file the

output of a command or commands within a pipe, but without

affecting the result. This is useful for printing an ongoing

process to a file or paper, perhaps to keep track of it for

debugging purposes.

(redirection)

|----> to file

==========================|====================

command ---> command ---> |tee ---> command ---> ---> output of pipe

===============================================

cat listfile* | sort | tee check.file | uniq > result.file

^^^^^^^^^^^^^^ ^^^^

The file "check.file" contains the concatenated sorted "listfiles,"

+ before the duplicate lines are removed by 'uniq.'

mkfifo

This obscure command creates a named pipe, a temporary

first-in-first-out buffer for transferring data between

processes. [85] Typically, one process writes to the FIFO, and

the other reads from it. See Example A-14.

!/bin/bash

This short script by Omair Eshkenazi.

Used in ABS Guide with permission (thanks!).

mkfifo pipe1 # Yes, pipes can be given names.

mkfifo pipe2 # Hence the designation "named pipe."

(cut -d' ' -f1 | tr "a-z" "A-Z") >pipe2 <pipe1 &

ls -l | tr -s ' ' | cut -d' ' -f3,9- | tee pipe1 |

cut -d' ' -f2 | paste - pipe2

rm -f pipe1

rm -f pipe2

No need to kill background processes when script terminates (why not?).

exit $?

Now, invoke the script and explain the output:

sh mkfifo-example.sh

4830.tar.gz BOZO

pipe1 BOZO

pipe2 BOZO

mkfifo-example.sh BOZO

Mixed.msg BOZO

pathchk

This command checks the validity of a filename. If the

filename exceeds the maximum allowable length (255 characters)

or one or more of the directories in its path is not

searchable, then an error message results.

Unfortunately, pathchk does not return a recognizable error

code, and it is therefore pretty much useless in a script.

Consider instead the file test operators.

Though this somewhat obscure and much feared data duplicator

command originated as a utility for exchanging data on

magnetic tapes between UNIX minicomputers and IBM mainframes,

it still has its uses. The dd command simply copies a file (or

stdin/stdout), but with conversions. Possible conversions

include ASCII/EBCDIC, [86] upper/lower case, swapping of byte

pairs between input and output, and skipping and/or truncating

the head or tail of the input file.

Converting a file to all uppercase:

dd if=$filename conv=ucase > $filename.uppercase

lcase # For lower case conversion

Some basic options to dd are:

+ if=INFILE

INFILE is the source file.

+ of=OUTFILE

OUTFILE is the target file, the file that will have the data

written to it.

+ bs=BLOCKSIZE

This is the size of each block of data being read and

written, usually a power of 2.

+ skip=BLOCKS

How many blocks of data to skip in INFILE before starting to

copy. This is useful when the INFILE has "garbage" or

garbled data in its header or when it is desirable to copy

only a portion of the INFILE.

+ seek=BLOCKS

How many blocks of data to skip in OUTFILE before starting

to copy, leaving blank data at beginning of OUTFILE.

+ count=BLOCKS

Copy only this many blocks of data, rather than the entire

INFILE.

+ conv=CONVERSION

Type of conversion to be applied to INFILE data before

copying operation.

A dd --help lists all the options this powerful utility takes.

Example 16-57. A script that copies itself

!/bin/bash

self-copy.sh

This script copies itself.

file_subscript=copy

dd if=$0 of=$0.$file_subscript 2>/dev/null

Suppress messages from dd: ^^^^^^^^^^^

exit $?

A program whose only output is its own source code

+ is called a "quine" per Willard Quine.

Does this script qualify as a quine?

Example 16-58. Exercising dd

!/bin/bash

exercising-dd.sh

Script by Stephane Chazelas.

Somewhat modified by ABS Guide author.

infile=$0 # This script.

outfile=log.txt # Output file left behind.

n=8

p=11

dd if=$infile of=$outfile bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null

Extracts characters n to p (8 to 11) from this script ("bash").

----------------------------------------------------------------

echo -n "hello vertical world" | dd cbs=1 conv=unblock 2> /dev/null

Echoes "hello vertical world" vertically downward.

Why? A newline follows each character dd emits.

exit $?

To demonstrate just how versatile dd is, let's use it to

capture keystrokes.

Example 16-59. Capturing Keystrokes

!/bin/bash

dd-keypress.sh: Capture keystrokes without needing to press ENTER.

keypresses=4 # Number of keypresses to capture.

old_tty_setting=$(stty -g) # Save old terminal settings.

echo "Press $keypresses keys."

stty -icanon -echo # Disable canonical mode.

# Disable local echo.

keys=$(dd bs=1 count=$keypresses 2> /dev/null)

'dd' uses stdin, if "if" (input file) not specified.

stty "$old_tty_setting" # Restore old terminal settings.

echo "You pressed the \"$keys\" keys."

Thanks, Stephane Chazelas, for showing the way.

exit 0

The dd command can do random access on a data stream.

echo -n . | dd bs=1 seek=4 of=file conv=notrunc

The "conv=notrunc" option means that the output file

+ will not be truncated.

Thanks, S.C.

The dd command can copy raw data and disk images to and from

devices, such as floppies and tape drives (Example A-5). A

common use is creating boot floppies.

dd if=kernel-image of=/dev/fd0H1440

Similarly, dd can copy the entire contents of a floppy, even

one formatted with a "foreign" OS, to the hard drive as an

image file.

dd if=/dev/fd0 of=/home/bozo/projects/floppy.img

Likewise, dd can create bootable flash drives and SD cards.

dd if=image.iso of=/dev/sdb

Example 16-60. Preparing a bootable SD card for the Raspberry

!/bin/bash

rp.sdcard.sh

Preparing an SD card with a bootable image for the Raspberry Pi.

$1 = imagefile name

$2 = sdcard (device file)

Otherwise defaults to the defaults, see below.

DEFAULTbs=4M # Block size, 4 mb default.

DEFAULTif="2013-07-26-wheezy-raspbian.img" # Commonly used distro.

DEFAULTsdcard="/dev/mmcblk0" # May be different. Check!

ROOTUSER_NAME=root # Must run as root!

E_NOTROOT=81

E_NOIMAGE=82

username=$(id -nu) # Who is running this script?

if [ "$username" != "$ROOTUSER_NAME" ]

then

echo "This script must run as root or with root privileges."

exit $E_NOTROOT

if [ -n "$1" ]

then

imagefile="$1"

else

imagefile="$DEFAULTif"

if [ -n "$2" ]

then

sdcard="$2"

else

sdcard="$DEFAULTsdcard"

if [ ! -e $imagefile ]

then

echo "Image file \"$imagefile\" not found!"

exit $E_NOIMAGE

echo "Last chance to change your mind!"; echo

read -s -n1 -p "Hit a key to write $imagefile to $sdcard [Ctl-c to exit]."

echo; echo

echo "Writing $imagefile to $sdcard ..."

dd bs=$DEFAULTbs if=$imagefile of=$sdcard

exit $?

Exercises:

---------

1) Provide additional error checking.

2) Have script autodetect device file for SD card (difficult!).

3) Have script sutodetect image file (*img) in $PWD.

Other applications of dd include initializing temporary swap

files (Example 31-2) and ramdisks (Example 31-3). It can even

do a low-level copy of an entire hard drive partition,

although this is not necessarily recommended.

People (with presumably nothing better to do with their time)

are constantly thinking of interesting applications of dd.

Example 16-61. Securely deleting a file

!/bin/bash

blot-out.sh: Erase "all" traces of a file.

This script overwrites a target file alternately

+ with random bytes, then zeros before finally deleting it.

After that, even examining the raw disk sectors by conventional methods

+ will not reveal the original file data.

PASSES=7 # Number of file-shredding passes.

# Increasing this slows script execution,

#+ especially on large target files.

BLOCKSIZE=1 # I/O with /dev/urandom requires unit block size,

#+ otherwise you get weird results.

E_BADARGS=70 # Various error exit codes.

E_NOT_FOUND=71

E_CHANGED_MIND=72

if [ -z "$1" ] # No filename specified.

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

file=$1

if [ ! -e "$file" ]

then

echo "File \"$file\" not found."

exit $E_NOT_FOUND

echo; echo -n "Are you absolutely sure you want to blot out \"$file\" (y/n)? "

read answer

case "$answer" in

[nN]) echo "Changed your mind, huh?"

exit $E_CHANGED_MIND

;;

) echo "Blotting out file \"$file\".";;

esac

flength=$(ls -l "$file" | awk '{print $5}') # Field 5 is file length.

pass_count=1

chmod u+w "$file" # Allow overwriting/deleting the file.

echo

while [ "$pass_count" -le "$PASSES" ]

echo "Pass #$pass_count"

sync # Flush buffers.

dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength

# Fill with random bytes.

sync # Flush buffers again.

dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength

# Fill with zeros.

sync # Flush buffers yet again.

let "pass_count += 1"

echo

done

rm -f $file # Finally, delete scrambled and shredded file.

sync # Flush buffers a final time.

echo "File \"$file\" blotted out and deleted."; echo

exit 0

This is a fairly secure, if inefficient and slow method

+ of thoroughly "shredding" a file.

The "shred" command, part of the GNU "fileutils" package,

+ does the same thing, although more efficiently.

The file cannot not be "undeleted" or retrieved by normal methods.

However . . .

+ this simple method would not likely withstand

+ sophisticated forensic analysis.

This script may not play well with a journaled file system.

Exercise (difficult): Fix it so it does.

Tom Vier's "wipe" file-deletion package does a much more thorough job

+ of file shredding than this simple script.

http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2

For an in-depth analysis on the topic of file deletion and security,

+ see Peter Gutmann's paper,

+ "Secure Deletion of Data From Magnetic and Solid-State Memory".

http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html

See also the dd thread entry in the bibliography.

The od, or octal dump filter converts input (or files) to

octal (base-8) or other bases. This is useful for viewing or

processing binary data files or otherwise unreadable system

device files, such as /dev/urandom, and as a filter for binary

data.

head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'

Sample output: 1324725719, 3918166450, 2989231420, etc.

From rnd.sh example script, by Stéphane Chazelas

The -C option nicely formats the output in tabular form.

objdump

Displays information about an object file or binary executable

in either hexadecimal form or as a disassembled listing (with

the -d option).

bash$ objdump -d /bin/ls

/bin/ls: file format elf32-i386

Disassembly of section .init:

080490bc <.init>:

80490bc: 55 push %ebp

80490bd: 89 e5 mov %esp,%ebp

. . .

mcookie

This command generates a "magic cookie," a 128-bit

(32-character) pseudorandom hexadecimal number, normally used

as an authorization "signature" by the X server. This also

available for use in a script as a "quick 'n dirty" random

number.

random000=$(mcookie)

Of course, a script could use md5sum for the same purpose.

Generate md5 checksum on the script itself.

random001=`md5sum $0 | awk '{print $1}'`

Uses 'awk' to strip off the filename.

The mcookie command gives yet another way to generate a

"unique" filename.

Example 16-62. Filename generator

!/bin/bash

tempfile-name.sh: temp filename generator

BASE_STR=`mcookie` # 32-character magic cookie.

POS=11 # Arbitrary position in magic cookie string.

LEN=5 # Get $LEN consecutive characters.

prefix=temp # This is, after all, a "temp" file.

# For more "uniqueness," generate the

#+ filename prefix using the same method

#+ as the suffix, below.

suffix=${BASE_STR:POS:LEN}

# Extract a 5-character string,

#+ starting at position 11.

temp_filename=$prefix.$suffix

# Construct the filename.

echo "Temp filename = "$temp_filename""

sh tempfile-name.sh

Temp filename = temp.e19ea

Compare this method of generating "unique" filenames

+ with the 'date' method in ex51.sh.

exit 0

units

This utility converts between different units of measure.

While normally invoked in interactive mode, units may find use

in a script.

Example 16-63. Converting meters to miles

!/bin/bash

unit-conversion.sh

Must have 'units' utility installed.

convert_units () # Takes as arguments the units to convert.

{

cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}')

# Strip off everything except the actual conversion factor.

echo "$cf"

}

Unit1=miles

Unit2=meters

cfactor=`convert_units $Unit1 $Unit2`

quantity=3.73

result=$(echo $quantity*$cfactor | bc)

echo "There are $result $Unit2 in $quantity $Unit1."

What happens if you pass incompatible units,

+ such as "acres" and "miles" to the function?

exit 0

Exercise: Edit this script to accept command-line parameters,

with appropriate error checking, of course.

A hidden treasure, m4 is a powerful macro [87] processing

filter, virtually a complete language. Although originally

written as a pre-processor for RatFor, m4 turned out to be

useful as a stand-alone utility. In fact, m4 combines some of

the functionality of eval, tr, and awk, in addition to its

extensive macro expansion facilities.

The April, 2002 issue of Linux Journal has a very nice article

on m4 and its uses.

Example 16-64. Using m4

!/bin/bash

m4.sh: Using the m4 macro processor

Strings

string=abcdA01

echo "len($string)" | m4 # 7

echo "substr($string,4)" | m4 # A01

echo "regexp($string,[0-1][0-1],\&Z)" | m4 # 01Z

Arithmetic

var=99

echo "incr($var)" | m4 # 100

echo "eval($var / 3)" | m4 # 33

exit

xmessage

This X-based variant of echo pops up a message/query window on

the desktop.

xmessage Left click to continue -button okay

zenity

The [http://freshmeat.net/projects/zenity] zenity utility is

adept at displaying GTK+ dialog widgets and very suitable for

scripting purposes.

doexec

The doexec command enables passing an arbitrary list of

arguments to a binary executable. In particular, passing

argv[0] (which corresponds to $0 in a script) lets the

executable be invoked by various names, and it can then carry

out different sets of actions, according to the name by which

it was called. What this amounts to is roundabout way of

passing options to an executable.

For example, the /usr/local/bin directory might contain a

binary called "aaa". Invoking doexec /usr/local/bin/aaa list

would list all those files in the current working directory

beginning with an "a", while invoking (the same executable

with) doexec /usr/local/bin/aaa delete would delete those

files.

Note

The various behaviors of the executable must be defined within the

code of the executable itself, analogous to something like the

following in a shell script:

case `basename $0` in

"name1" ) do_something;;

"name2" ) do_something_else;;

"name3" ) do_yet_another_thing;;

) bail_out;;

esac

dialog

The dialog family of tools provide a method of calling

interactive "dialog" boxes from a script. The more elaborate

variations of dialog -- gdialog, Xdialog, and kdialog --

actually invoke X-Windows widgets.

sox

The sox, or "sound exchange" command plays and performs

transformations on sound files. In fact, the /usr/bin/play

executable (now deprecated) is nothing but a shell wrapper for

sox.

For example, sox soundfile.wav soundfile.au changes a WAV

sound file into a (Sun audio format) AU sound file.

Shell scripts are ideally suited for batch-processing sox

operations on sound files. For examples, see the Linux Radio

Timeshift HOWTO and the MP3do Project.

________________________________________________________________

Chapter 17. System and Administrative Commands

The startup and shutdown scripts in /etc/rc.d illustrate the uses

(and usefulness) of many of these comands. These are usually invoked

by root and used for system maintenance or emergency filesystem

repairs. Use with caution, as some of these commands may damage your

system if misused.

Users and Groups

users

Show all logged on users. This is the approximate equivalent

of who -q.

groups

Lists the current user and the groups she belongs to. This

corresponds to the $GROUPS internal variable, but gives the

group names, rather than the numbers.

bash$ groups

bozita cdrom cdwriter audio xgrp

bash$ echo $GROUPS

501

chown, chgrp

The chown command changes the ownership of a file or files.

This command is a useful method that root can use to shift

file ownership from one user to another. An ordinary user may

not change the ownership of files, not even her own files.

[88]

root# chown bozo *.txt

The chgrp command changes the group ownership of a file or

files. You must be owner of the file(s) as well as a member of

the destination group (or root) to use this operation.

chgrp --recursive dunderheads *.data

The "dunderheads" group will now own all the "*.data" files

+ all the way down the $PWD directory tree (that's what "recursive" means).

useradd, userdel

The useradd administrative command adds a user account to the

system and creates a home directory for that particular user,

if so specified. The corresponding userdel command removes a

user account from the system [89] and deletes associated

files.

Note

The adduser command is a synonym for useradd and is usually a

symbolic link to it.

usermod

Modify a user account. Changes may be made to the password,

group membership, expiration date, and other attributes of a

given user's account. With this command, a user's password may

be locked, which has the effect of disabling the account.

groupmod

Modify a given group. The group name and/or ID number may be

changed using this command.

The id command lists the real and effective user IDs and the

group IDs of the user associated with the current process.

This is the counterpart to the $UID, $EUID, and $GROUPS

internal Bash variables.

bash$ id

uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)

bash$ echo $UID

501

Note

The id command shows the effective IDs only when they differ from the

real ones.

Also see Example 9-5.

lid

The lid (list ID) command shows the group(s) that a given user

belongs to, or alternately, the users belonging to a given

group. May be invoked only by root.

root# lid bozo

bozo(gid=500)

root# lid daemon

bin(gid=1)

daemon(gid=2)

adm(gid=4)

lp(gid=7)

who

Show all users logged on to the system.

bash$ who

bozo tty1 Apr 27 17:45

bozo pts/0 Apr 27 17:46

bozo pts/1 Apr 27 17:47

bozo pts/2 Apr 27 17:49

The -m gives detailed information about only the current user.

Passing any two arguments to who is the equivalent of who -m,

as in who am i or who The Man.

bash$ who -m

localhost.localdomain!bozo pts/2 Apr 27 17:49

whoami is similar to who -m, but only lists the user name.

bash$ whoami

bozo

Show all logged on users and the processes belonging to them.

This is an extended version of who. The output of w may be

piped to grep to find a specific user and/or process.

bash$ w | grep startx

bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx

logname

Show current user's login name (as found in /var/run/utmp).

This is a near-equivalent to whoami, above.

bash$ logname

bozo

bash$ whoami

bozo

However . . .

bash$ su

Password: ......

bash# whoami

root

bash# logname

bozo

Note

While logname prints the name of the logged in user, whoami gives the

name of the user attached to the current process. As we have just

seen, sometimes these are not the same.

Runs a program or script as a substitute user. su rjones

starts a shell as user rjones. A naked su defaults to root.

See Example A-14.

sudo

Runs a command as root (or another user). This may be used in

a script, thus permitting a regular user to run the script.

!/bin/bash

Some commands.

sudo cp /root/secretfile /home/bozo/secret

Some more commands.

The file /etc/sudoers holds the names of users permitted to

invoke sudo.

passwd

Sets, changes, or manages a user's password.

The passwd command can be used in a script, but probably

should not be.

Example 17-1. Setting a new password

!/bin/bash

setnew-password.sh: For demonstration purposes only.

Not a good idea to actually run this script.

This script must be run as root.

ROOT_UID=0 # Root has $UID 0.

E_WRONG_USER=65 # Not root?

E_NOSUCHUSER=70

SUCCESS=0

if [ "$UID" -ne "$ROOT_UID" ]

then

echo; echo "Only root can run this script."; echo

exit $E_WRONG_USER

else

echo

echo "You should know better than to run this script, root."

echo "Even root users get the blues... "

echo

username=bozo

NEWPASSWORD=security_violation

Check if bozo lives here.

grep -q "$username" /etc/passwd

if [ $? -ne $SUCCESS ]

then

echo "User $username does not exist."

echo "No password changed."

exit $E_NOSUCHUSER

echo "$NEWPASSWORD" | passwd --stdin "$username"

The '--stdin' option to 'passwd' permits

+ getting a new password from stdin (or a pipe).

echo; echo "User $username's password changed!"

Using the 'passwd' command in a script is dangerous.

exit 0

The passwd command's -l, -u, and -d options permit locking,

unlocking, and deleting a user's password. Only root may use

these options.

Show users' logged in time, as read from /var/log/wtmp. This

is one of the GNU accounting utilities.

bash$ ac

total 68.08

last

List last logged in users, as read from /var/log/wtmp. This

command can also show remote logins.

For example, to show the last few times the system rebooted:

bash$ last reboot

reboot system boot 2.6.9-1.667 Fri Feb 4 18:18 (00:02)

reboot system boot 2.6.9-1.667 Fri Feb 4 15:20 (01:27)

reboot system boot 2.6.9-1.667 Fri Feb 4 12:56 (00:49)

reboot system boot 2.6.9-1.667 Thu Feb 3 21:08 (02:17)

. . .

wtmp begins Tue Feb 1 12:50:09 2005

newgrp

Change user's group ID without logging out. This permits

access to the new group's files. Since users may be members of

multiple groups simultaneously, this command finds only

limited use.

Note

Kurt Glaesemann points out that the newgrp command could prove

helpful in setting the default group permissions for files a user

writes. However, the chgrp command might be more convenient for this

purpose.

Terminals

tty

Echoes the name (filename) of the current user's terminal.

Note that each separate xterm window counts as a different

terminal.

bash$ tty

/dev/pts/1

stty

Shows and/or changes terminal settings. This complex command,

used in a script, can control terminal behavior and the way

output displays. See the info page, and study it carefully.

Example 17-2. Setting an erase character

!/bin/bash

erase.sh: Using "stty" to set an erase character when reading input.

echo -n "What is your name? "

read name # Try to backspace

#+ to erase characters of input.

# Problems?

echo "Your name is $name."

stty erase '#' # Set "hashmark" (#) as erase character.

echo -n "What is your name? "

read name # Use # to erase last character typed.

echo "Your name is $name."

exit 0

Even after the script exits, the new key value remains set.

Exercise: How would you reset the erase character to the default value?

Example 17-3. secret password: Turning off terminal echoing

!/bin/bash

secret-pw.sh: secret password

echo

echo -n "Enter password "

read passwd

echo "password is $passwd"

echo -n "If someone had been looking over your shoulder, "

echo "your password would have been compromised."

echo && echo # Two line-feeds in an "and list."

stty -echo # Turns off screen echo.

May also be done with

read -sp passwd

A big Thank You to Leigh James for pointing this out.

echo -n "Enter password again "

read passwd

echo

echo "password is $passwd"

echo

stty echo # Restores screen echo.

exit 0

Do an 'info stty' for more on this useful-but-tricky command.

A creative use of stty is detecting a user keypress (without

hitting ENTER).

Example 17-4. Keypress detection

!/bin/bash

keypress.sh: Detect a user keypress ("hot keys").

echo

old_tty_settings=$(stty -g) # Save old settings (why?).

stty -icanon

Keypress=$(head -c1) # or $(dd bs=1 count=1 2> /dev/null)

# on non-GNU systems

echo

echo "Key pressed was \""$Keypress"\"."

echo

stty "$old_tty_settings" # Restore old settings.

Thanks, Stephane Chazelas.

exit 0

Also see Example 9-3 and Example A-43.

terminals and modes

Normally, a terminal works in the canonical mode. When a user hits a

key, the resulting character does not immediately go to the program

actually running in this terminal. A buffer local to the terminal

stores keystrokes. When the user hits the ENTER key, this sends all

the stored keystrokes to the program running. There is even a basic

line editor inside the terminal.

bash$ stty -a

speed 9600 baud; rows 36; columns 96; line = 0;

intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = <undef>; eol2 =

<undef>;

start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush

= ^O;

...

isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt

Using canonical mode, it is possible to redefine the special keys for

the local terminal line editor.

bash$ cat > filexxx

wha<ctl-W>I<ctl-H>foo bar<ctl-U>hello world<ENTER>

<ctl-D>

bash$ cat filexxx

hello world

bash$ wc -c < filexxx

The process controlling the terminal receives only 12 characters (11

alphabetic ones, plus a newline), although the user hit 26 keys.

In non-canonical ("raw") mode, every key hit (including special

editing keys such as ctl-H) sends a character immediately to the

controlling process.

The Bash prompt disables both icanon and echo, since it replaces the

basic terminal line editor with its own more elaborate one. For

example, when you hit ctl-A at the Bash prompt, there's no ^A echoed

by the terminal, but Bash gets a \1 character, interprets it, and

moves the cursor to the begining of the line.

Stéphane Chazelas

setterm

Set certain terminal attributes. This command writes to its

terminal's stdout a string that changes the behavior of that

terminal.

bash$ setterm -cursor off

bash$

The setterm command can be used within a script to change the

appearance of text written to stdout, although there are

certainly better tools available for this purpose.

setterm -bold on

echo bold hello

setterm -bold off

echo normal hello

tset

Show or initialize terminal settings. This is a less capable

version of stty.

bash$ tset -r

Terminal type is xterm-xfree86.

Kill is control-U (^U).

Interrupt is control-C (^C).

setserial

Set or display serial port parameters. This command must be

run by root and is usually found in a system setup script.

From /etc/pcmcia/serial script:

IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'`

setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ

getty, agetty

The initialization process for a terminal uses getty or agetty

to set it up for login by a user. These commands are not used

within user shell scripts. Their scripting counterpart is

stty.

mesg

Enables or disables write access to the current user's

terminal. Disabling access would prevent another user on the

network to write to the terminal.

Tip

It can be quite annoying to have a message about ordering pizza

suddenly appear in the middle of the text file you are editing. On a

multi-user network, you might therefore wish to disable write access

to your terminal when you need to avoid interruptions.

wall

This is an acronym for "write all," i.e., sending a message to

all users at every terminal logged into the network. It is

primarily a system administrator's tool, useful, for example,

when warning everyone that the system will shortly go down due

to a problem (see Example 19-1).

bash$ wall System going down for maintenance in 5 minutes!

Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001...

System going down for maintenance in 5 minutes!

Note

If write access to a particular terminal has been disabled with mesg,

then wall cannot send a message to that terminal.

Information and Statistics

uname

Output system specifications (OS, kernel version, etc.) to

stdout. Invoked with the -a option, gives verbose system info

(see Example 16-5). The -s option shows only the OS type.

bash$ uname

Linux

bash$ uname -s

Linux

bash$ uname -a

Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006

i686 i686 i386 GNU/Linux

arch

Show system architecture. Equivalent to uname -m. See Example

11-27.

bash$ arch

i686

bash$ uname -m

i686

lastcomm

Gives information about previous commands, as stored in the

/var/account/pacct file. Command name and user name can be

specified by options. This is one of the GNU accounting

utilities.

lastlog

List the last login time of all system users. This references

the /var/log/lastlog file.

bash$ lastlog

root tty1 Fri Dec 7 18:43:21 -0700 2001

bin **Never logged in**

daemon **Never logged in**

...

bozo tty1 Sat Dec 8 21:14:29 -0700 2001

bash$ lastlog | grep root

root tty1 Fri Dec 7 18:43:21 -0700 2001

Caution

This command will fail if the user invoking it does not have read

permission for the /var/log/lastlog file.

lsof

List open files. This command outputs a detailed table of all

currently open files and gives information about their owner,

size, the processes associated with them, and more. Of course,

lsof may be piped to grep and/or awk to parse and analyze its

results.

bash$ lsof

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

init 1 root mem REG 3,5 30748 30303 /sbin/init

init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.

init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.

3.so

cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr

...

The lsof command is a useful, if complex administrative tool.

If you are unable to dismount a filesystem and get an error

message that it is still in use, then running lsof helps

determine which files are still open on that filesystem. The

-i option lists open network socket files, and this can help

trace intrusion or hack attempts.

bash$ lsof -an -i tcp

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

firefox 2330 bozo 32u IPv4 9956 TCP 66.0.118.137:57596->67.112.7.104

:http ...

firefox 2330 bozo 38u IPv4 10535 TCP 66.0.118.137:57708->216.79.48.24

:http ...

See Example 30-2 for an effective use of lsof.

strace

System trace: diagnostic and debugging tool for tracing system

calls and signals. This command and ltrace, following, are

useful for diagnosing why a given program or package fails to

run . . . perhaps due to missing libraries or related causes.

bash$ strace df

execve("/bin/df", ["df"], [/* 45 vars */]) = 0

uname({sys="Linux", node="bozo.localdomain", ...}) = 0

brk(0) = 0x804f5e4

...

This is the Linux equivalent of the Solaris truss command.

ltrace

Library trace: diagnostic and debugging tool that traces

library calls invoked by a given command.

bash$ ltrace df

__libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 <unfinished .

..>:

setlocale(6, "") = "en_US.UTF-8"

bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale"

textdomain("coreutils") = "coreutils"

__cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0

getenv("DF_BLOCK_SIZE") = NULL

...

The nc (netcat) utility is a complete toolkit for connecting

to and listening to TCP and UDP ports. It is useful as a

diagnostic and testing tool and as a component in simple

script-based HTTP clients and servers.

bash$ nc localhost.localdomain 25

220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;

Thu, 31 Mar 2005 15:41:35 -0700

A real-life usage example.

Example 17-5. Checking a remote server for identd

! /bin/sh

Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed.

Args: target port [port port port ...]

Hose stdout _and_ stderr together.

Advantages: runs slower than ident-scan, giving remote inetd less cause

+ for alarm, and only hits the few known daemon ports you specify.

Disadvantages: requires numeric-only port args, the output sleazitude,

+ and won't work for r-services when coming from high source ports.

Script author: Hobbit <hobbit@avian.org>

Used in ABS Guide with permission.

---------------------------------------------------

E_BADARGS=65 # Need at least two args.

TWO_WINKS=2 # How long to sleep.

THREE_WINKS=3

IDPORT=113 # Authentication "tap ident" port.

RAND1=999

RAND2=31337

TIMEOUT0=9

TIMEOUT1=8

TIMEOUT2=4

---------------------------------------------------

case "${2}" in

"" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;;

esac

Ping 'em once and see if they are running identd.

nc -z -w $TIMEOUT0 "$1" $IDPORT || \

{ echo "Oops, $1 isn't running identd." ; exit 0 ; }

-z scans for listening daemons.

-w $TIMEOUT = How long to try to connect.

Generate a randomish base port.

RP=`expr $ % $RAND1 + $RAND2`

TRG="$1"

shift

while test "$1" ; do

nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &

PROC=$!

sleep $THREE_WINKS

echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1

sleep $TWO_WINKS

Does this look like a lamer script or what . . . ?

ABS Guide author comments: "Ain't really all that bad . . .

+ kinda clever, actually."

kill -HUP $PROC

RP=`expr ${RP} + 1`

shift

done

exit $?

Notes:

-----

Try commenting out line 30 and running this script

+ with "localhost.localdomain 25" as arguments.

For more of Hobbit's 'nc' example scripts,

+ look in the documentation:

+ the /usr/share/doc/nc-X.XX/scripts directory.

And, of course, there's Dr. Andrew Tridgell's notorious

one-line script in the BitKeeper Affair:

echo clone | nc thunk.org 5000 > e2fsprogs.dat

free

Shows memory and cache usage in tabular form. The output of

this command lends itself to parsing, using grep, awk or Perl.

The procinfo command shows all the information that free does,

and much more.

bash$ free

total used free shared buffers cached

Mem: 30504 28624 1880 15820 1608 16376

-/+ buffers/cache: 10640 19864

Swap: 68540 3128 65412

To show unused RAM memory:

bash$ free | grep Mem | awk '{ print $4 }'

1880

procinfo

Extract and list information and statistics from the /proc

pseudo-filesystem. This gives a very extensive and detailed

listing.

bash$ procinfo | grep Bootup

Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829

lsdev

List devices, that is, show installed hardware.

bash$ lsdev

Device DMA IRQ I/O Ports

------------------------------------------------

cascade 4 2

dma 0080-008f

dma1 0000-001f

dma2 00c0-00df

fpu 00f0-00ff

ide0 14 01f0-01f7 03f6-03f6

...

Show (disk) file usage, recursively. Defaults to current

working directory, unless otherwise specified.

bash$ du -ach

1.0k ./wi.sh

1.0k ./tst.sh

1.0k ./random.file

6.0k .

6.0k total

Shows filesystem usage in tabular form.

bash$ df

Filesystem 1k-blocks Used Available Use% Mounted on

/dev/hda5 273262 92607 166547 36% /

/dev/hda8 222525 123951 87085 59% /home

/dev/hda7 1408796 1075744 261488 80% /usr

dmesg

Lists all system bootup messages to stdout. Handy for

debugging and ascertaining which device drivers were installed

and which system interrupts in use. The output of dmesg may,

of course, be parsed with grep, sed, or awk from within a

script.

bash$ dmesg | grep hda

Kernel command line: ro root=/dev/hda2

hda: IBM-DLGA-23080, ATA DISK drive

hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63

hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4

stat

Gives detailed and verbose statistics on a given file (even a

directory or device file) or set of files.

bash$ stat test.cru

File: "test.cru"

Size: 49970 Allocated Blocks: 100 Filetype: Regular File

Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo)

Device: 3,8 Inode: 18185 Links: 1

Access: Sat Jun 2 16:40:24 2001

Modify: Sat Jun 2 16:40:24 2001

Change: Sat Jun 2 16:40:24 2001

If the target file does not exist, stat returns an error

message.

bash$ stat nonexistent-file

nonexistent-file: No such file or directory

In a script, you can use stat to extract information about

files (and filesystems) and set variables accordingly.

!/bin/bash

fileinfo2.sh

Per suggestion of Joël Bourquard and . . .

http://www.linuxquestions.org/questions/showthread.php?t=410766

FILENAME=testfile.txt

file_name=$(stat -c%n "$FILENAME") # Same as "$FILENAME" of course.

file_owner=$(stat -c%U "$FILENAME")

file_size=$(stat -c%s "$FILENAME")

Certainly easier than using "ls -l $FILENAME"

+ and then parsing with sed.

file_inode=$(stat -c%i "$FILENAME")

file_type=$(stat -c%F "$FILENAME")

file_access_rights=$(stat -c%A "$FILENAME")

echo "File name: $file_name"

echo "File owner: $file_owner"

echo "File size: $file_size"

echo "File inode: $file_inode"

echo "File type: $file_type"

echo "File access rights: $file_access_rights"

exit 0

sh fileinfo2.sh

File name: testfile.txt

File owner: bozo

File size: 418

File inode: 1730378

File type: regular file

File access rights: -rw-rw-r--

vmstat

Display virtual memory statistics.

bash$ vmstat

procs memory swap io system cpu

r b w swpd free buff cache si so bi bo in cs us sy id

0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89

uptime

Shows how long the system has been running, along with

associated statistics.

bash$ uptime

10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27

Note

A load average of 1 or less indicates that the system handles

processes immediately. A load average greater than 1 means that

processes are being queued. When the load average gets above 3 (on a

single-core processor), then system performance is significantly

degraded.

hostname

Lists the system's host name. This command sets the host name

in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or

similar). It is equivalent to uname -n, and a counterpart to

the $HOSTNAME internal variable.

bash$ hostname

localhost.localdomain

bash$ echo $HOSTNAME

localhost.localdomain

Similar to the hostname command are the domainname,

dnsdomainname, nisdomainname, and ypdomainname commands. Use

these to display or set the system DNS or NIS/YP domain name.

Various options to hostname also perform these functions.

hostid

Echo a 32-bit hexadecimal numerical identifier for the host

machine.

bash$ hostid

7f0100

Note

This command allegedly fetches a "unique" serial number for a

particular system. Certain product registration procedures use this

number to brand a particular user license. Unfortunately, hostid only

returns the machine network address in hexadecimal, with pairs of

bytes transposed.

The network address of a typical non-networked Linux machine, is

found in /etc/hosts.

bash$ cat /etc/hosts

127.0.0.1 localhost.localdomain localhost

As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0,

which translates in hex to 007f0100, the exact equivalent of what

hostid returns, above. There exist only a few million other Linux

machines with this identical hostid.

sar

Invoking sar (System Activity Reporter) gives a very detailed

rundown on system statistics. The Santa Cruz Operation ("Old"

SCO) released sar as Open Source in June, 1999.

This command is not part of the base Linux distribution, but

may be obtained as part of

the[http://perso.wanadoo.fr/sebastien.godard/] sysstat

utilities package, written by Sebastien Godard.

bash$ sar

Linux 2.4.9 (brooks.seringas.fr) 09/26/03

10:30:00 CPU %user %nice %system %iowait %idle

10:40:00 all 2.21 10.90 65.48 0.00 21.41

10:50:00 all 3.36 0.00 72.36 0.00 24.28

11:00:00 all 1.12 0.00 80.77 0.00 18.11

Average: all 2.23 3.63 72.87 0.00 21.27

14:32:30 LINUX RESTART

15:00:00 CPU %user %nice %system %iowait %idle

15:10:00 all 8.59 2.40 17.47 0.00 71.54

15:20:00 all 4.07 1.00 11.95 0.00 82.98

15:30:00 all 0.79 2.94 7.56 0.00 88.71

Average: all 6.33 1.70 14.71 0.00 77.26

readelf

Show information and statistics about a designated elf binary.

This is part of the binutils package.

bash$ readelf -h /bin/bash

ELF Header:

Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00

Class: ELF32

Data: 2's complement, little endian

Version: 1 (current)

OS/ABI: UNIX - System V

ABI Version: 0

Type: EXEC (Executable file)

. . .

size

The size [/path/to/binary] command gives the segment sizes of

a binary executable or archive file. This is mainly of use to

programmers.

bash$ size /bin/bash

text data bss dec hex filename

495971 22496 17392 535859 82d33 /bin/bash

System Logs

logger

Appends a user-generated message to the system log

(/var/log/messages). You do not have to be root to invoke

logger.

logger Experiencing instability in network connection at 23:10, 05/21.

Now, do a 'tail /var/log/messages'.

By embedding a logger command in a script, it is possible to

write debugging information to /var/log/messages.

logger -t $0 -i Logging at line "$LINENO".

The "-t" option specifies the tag for the logger entry.

The "-i" option records the process ID.

tail /var/log/message

...

Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.

logrotate

This utility manages the system log files, rotating,

compressing, deleting, and/or e-mailing them, as appropriate.

This keeps the /var/log from getting cluttered with old log

files. Usually cron runs logrotate on a daily basis.

Adding an appropriate entry to /etc/logrotate.conf makes it

possible to manage personal log files, as well as system-wide

ones.

Note

Stefano Falsetto has created [http://www.gnu.org/software/rottlog/]

rottlog, which he considers to be an improved version of logrotate.

Job Control

Process Statistics: lists currently executing processes by

owner and PID (process ID). This is usually invoked with ax or

aux options, and may be piped to grep or sed to search for a

specific process (see Example 15-14 and Example 29-3).

bash$ ps ax | grep sendmail

295 ? S 0:00 sendmail: accepting connections on port 25

To display system processes in graphical "tree" format: ps

afjx or ps ax --forest.

pgrep, pkill

Combining the ps command with grep or kill.

bash$ ps a | grep mingetty

2212 tty2 Ss+ 0:00 /sbin/mingetty tty2

2213 tty3 Ss+ 0:00 /sbin/mingetty tty3

2214 tty4 Ss+ 0:00 /sbin/mingetty tty4

2215 tty5 Ss+ 0:00 /sbin/mingetty tty5

2216 tty6 Ss+ 0:00 /sbin/mingetty tty6

4849 pts/2 S+ 0:00 grep mingetty

bash$ pgrep mingetty

2212 mingetty

2213 mingetty

2214 mingetty

2215 mingetty

2216 mingetty

Compare the action of pkill with killall.

pstree

Lists currently executing processes in "tree" format. The -p

option shows the PIDs, as well as the process names.

top

Continuously updated display of most cpu-intensive processes.

The -b option displays in text mode, so that the output may be

parsed or accessed from a script.

bash$ top -b

8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13

45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped

CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle

Mem: 78396K av, 65468K used, 12928K free, 0K shrd, 2352K buff

Swap: 157208K av, 0K used, 157208K free 37244K cach

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND

848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top

1 root 8 0 512 512 444 S 0.0 0.6 0:04 init

2 root 9 0 0 0 0 SW 0.0 0.0 0:00 keventd

...

nice

Run a background job with an altered priority. Priorities run

from 19 (lowest) to -20 (highest). Only root may set the

negative (higher) priorities. Related commands are renice and

snice, which change the priority of a running process or

processes, and skill, which sends a kill signal to a process

or processes.

nohup

Keeps a command running even after user logs off. The command

will run as a foreground process unless followed by &. If you

use nohup within a script, consider coupling it with a wait to

avoid creating an orphan or zombie process.

pidof

Identifies process ID (PID) of a running job. Since job

control commands, such as kill and renice act on the PID of a

process (not its name), it is sometimes necessary to identify

that PID. The pidof command is the approximate counterpart to

the $PPID internal variable.

bash$ pidof xclock

880

Example 17-6. pidof helps kill a process

!/bin/bash

kill-process.sh

NOPROCESS=2

process=xxxyyyzzz # Use nonexistent process.

For demo purposes only...

... don't want to actually kill any actual process with this script.

If, for example, you wanted to use this script to logoff the Internet,

process=pppd

t=`pidof $process` # Find pid (process id) of $process.

The pid is needed by 'kill' (can't 'kill' by program name).

if [ -z "$t" ] # If process not present, 'pidof' returns null.

then

echo "Process $process was not running."

echo "Nothing killed."

exit $NOPROCESS

kill $t # May need 'kill -9' for stubborn process.

Need a check here to see if process allowed itself to be killed.

Perhaps another " t=`pidof $process` " or ...

This entire script could be replaced by

kill $(pidof -x process_name)

or

killall process_name

but it would not be as instructive.

exit 0

fuser

Identifies the processes (by PID) that are accessing a given

file, set of files, or directory. May also be invoked with the

-k option, which kills those processes. This has interesting

implications for system security, especially in scripts

preventing unauthorized users from accessing system services.

bash$ fuser -u /usr/bin/vim

/usr/bin/vim: 3207e(bozo)

bash$ fuser -u /dev/null

/dev/null: 3009(bozo) 3010(bozo) 3197(bozo) 3199(bozo)

One important application for fuser is when physically

inserting or removing storage media, such as CD ROM disks or

USB flash drives. Sometimes trying a umount fails with a

device is busy error message. This means that some user(s)

and/or process(es) are accessing the device. An fuser -um

/dev/device_name will clear up the mystery, so you can kill

any relevant processes.

bash$ umount /mnt/usbdrive

umount: /mnt/usbdrive: device is busy

bash$ fuser -um /dev/usbdrive

/mnt/usbdrive: 1772c(bozo)

bash$ kill -9 1772

bash$ umount /mnt/usbdrive

The fuser command, invoked with the -n option identifies the

processes accessing a port. This is especially useful in

combination with nmap.

root# nmap localhost.localdomain

PORT STATE SERVICE

25/tcp open smtp

root# fuser -un tcp 25

25/tcp: 2095(root)

root# ps ax | grep 2095 | grep -v grep

2095 ? Ss 0:00 sendmail: accepting connections

cron

Administrative program scheduler, performing such duties as

cleaning up and deleting system log files and updating the

slocate database. This is the superuser version of at

(although each user may have their own crontab file which can

be changed with the crontab command). It runs as a daemon and

executes scheduled entries from /etc/crontab.

Note

Some flavors of Linux run crond, Matthew Dillon's version of cron.

Process Control and Booting

init

The init command is the parent of all processes. Called in the

final step of a bootup, init determines the runlevel of the

system from /etc/inittab. Invoked by its alias telinit, and by

root only.

telinit

Symlinked to init, this is a means of changing the system

runlevel, usually done for system maintenance or emergency

filesystem repairs. Invoked only by root. This command can be

dangerous -- be certain you understand it well before using!

runlevel

Shows the current and last runlevel, that is, whether the

system is halted (runlevel 0), in single-user mode (1), in

multi-user mode (2 or 3), in X Windows (5), or rebooting (6).

This command accesses the /var/run/utmp file.

halt, shutdown, reboot

Command set to shut the system down, usually just prior to a

power down.

Warning

On some Linux distros, the halt command has 755 permissions, so it

can be invoked by a non-root user. A careless halt in a terminal or a

script may shut down the system!

service

Starts or stops a system service. The startup scripts in

/etc/init.d and /etc/rc.d use this command to start services

at bootup.

root# /sbin/service iptables stop

Flushing firewall rules: [ OK ]

Setting chains to policy ACCEPT: filter [ OK ]

Unloading iptables modules: [ OK ]

Network

nmap

Network mapper and port scanner. This command scans a server

to locate open ports and the services associated with those

ports. It can also report information about packet filters and

firewalls. This is an important security tool for locking down

a network against hacking attempts.

!/bin/bash

SERVER=$HOST # localhost.localdomain (127.0.0.1).

PORT_NUMBER=25 # SMTP port.

nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open?

grep -w matches whole words only,

+ so this wouldn't match port 1025, for example.

exit 0

25/tcp open smtp

ifconfig

Network interface configuration and tuning utility.

bash$ ifconfig -a

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX packets:10 errors:0 dropped:0 overruns:0 frame:0

TX packets:10 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

RX bytes:700 (700.0 b) TX bytes:700 (700.0 b)

The ifconfig command is most often used at bootup to set up

the interfaces, or to shut them down when rebooting.

Code snippets from /etc/rc.d/init.d/network

...

Check that networking is up.

[ ${NETWORKING} = "no" ] && exit 0

[ -x /sbin/ifconfig ] || exit 0

...

for i in $interfaces ; do

if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then

action "Shutting down interface $i: " ./ifdown $i boot

The GNU-specific "-q" option to "grep" means "quiet", i.e.,

+ producing no output.

Redirecting output to /dev/null is therefore not strictly necessary.

...

echo "Currently active devices:"

echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'`

^^^^^ should be quoted to prevent globbing.

The following also work.

echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'

echo $(/sbin/ifconfig | sed -e 's/ .*//')

Thanks, S.C., for additional comments.

!/bin/bash

Script by Juan Nicolas Ruiz

Used with his kind permission.

Setting up (and stopping) a GRE tunnel.

--- start-tunnel.sh ---

LOCAL_IP="192.168.1.17"

REMOTE_IP="10.0.5.33"

OTHER_IFACE="192.168.0.100"

REMOTE_NET="192.168.3.0/24"

/sbin/ip tunnel add netb mode gre remote $REMOTE_IP \

local $LOCAL_IP ttl 255

/sbin/ip addr add $OTHER_IFACE dev netb

/sbin/ip link set netb up

/sbin/ip route add $REMOTE_NET dev netb

exit 0 #############################################

--- stop-tunnel.sh ---

REMOTE_NET="192.168.3.0/24"

/sbin/ip route del $REMOTE_NET dev netb

/sbin/ip link set netb down

/sbin/ip tunnel del netb

exit 0

route

Show info about or make changes to the kernel routing table.

bash$ route

Destination Gateway Genmask Flags MSS Window irtt Iface

pm3-67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0

127.0.0.0 * 255.0.0.0 U 40 0 0 lo

default pm3-67.bozosisp 0.0.0.0 UG 40 0 0 ppp0

iptables

The iptables command set is a packet filtering tool used

mainly for such security purposes as setting up network

firewalls. This is a complex tool, and a detailed explanation

of its use is beyond the scope of this document. Oskar

Andreasson's tutorial is a reasonable starting point.

See also shutting down iptables and Example 30-2.

chkconfig

Check network and system configuration. This command lists and

manages the network and system services started at bootup in

the /etc/rc?.d directory.

Originally a port from IRIX to Red Hat Linux, chkconfig may

not be part of the core installation of some Linux flavors.

bash$ chkconfig --list

atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off

rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off

...

tcpdump

Network packet "sniffer." This is a tool for analyzing and

troubleshooting traffic on a network by dumping packet headers

that match specified criteria.

Dump ip packet traffic between hosts bozoville and caduceus:

bash$ tcpdump ip host bozoville and caduceus

Of course, the output of tcpdump can be parsed with certain of

the previously discussed text processing utilities.

Filesystem

mount

Mount a filesystem, usually on an external device, such as a

floppy or CDROM. The file /etc/fstab provides a handy listing

of available filesystems, partitions, and devices, including

options, that may be automatically or manually mounted. The

file /etc/mtab shows the currently mounted filesystems and

partitions (including the virtual ones, such as /proc).

mount -a mounts all filesystems and partitions listed in

/etc/fstab, except those with a noauto option. At bootup, a

startup script in /etc/rc.d (rc.sysinit or something similar)

invokes this to get everything mounted.

mount -t iso9660 /dev/cdrom /mnt/cdrom

Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem.

mount /mnt/cdrom

Shortcut, if /mnt/cdrom listed in /etc/fstab

The versatile mount command can even mount an ordinary file on

a block device, and the file will act as if it were a

filesystem. Mount accomplishes that by associating the file

with a loopback device. One application of this is to mount

and examine an ISO9660 filesystem image before burning it onto

a CDR. [90]

Example 17-7. Checking a CD image

As root...

mkdir /mnt/cdtest # Prepare a mount point, if not already there.

mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest # Mount the image.

"-o loop" option equivalent to "losetup /dev/loop0"

cd /mnt/cdtest # Now, check the image.

ls -alR # List the files in the directory tree there.

# And so forth.

umount

Unmount a currently mounted filesystem. Before physically

removing a previously mounted floppy or CDROM disk, the device

must be umounted, else filesystem corruption may result.

umount /mnt/cdrom

You may now press the eject button and safely remove the disk.

Note

The automount utility, if properly installed, can mount and unmount

floppies or CDROM disks as they are accessed or removed. On

"multispindle" laptops with swappable floppy and optical drives, this

can cause problems, however.

gnome-mount

The newer Linux distros have deprecated mount and umount. The

successor, for command-line mounting of removable storage

devices, is gnome-mount. It can take the -d option to mount a

device file by its listing in /dev.

For example, to mount a USB flash drive:

bash$ gnome-mount -d /dev/sda1

gnome-mount 0.4

bash$ df

. . .

/dev/sda1 63584 12034 51550 19% /media/disk

sync

Forces an immediate write of all updated data from buffers to

hard drive (synchronize drive with buffers). While not

strictly necessary, a sync assures the sys admin or user that

the data just changed will survive a sudden power failure. In

the olden days, a sync; sync (twice, just to make absolutely

sure) was a useful precautionary measure before a system

reboot.

At times, you may wish to force an immediate buffer flush, as

when securely deleting a file (see Example 16-61) or when the

lights begin to flicker.

losetup

Sets up and configures loopback devices.

Example 17-8. Creating a filesystem in a file

SIZE=1000000 # 1 meg

head -c $SIZE < /dev/zero > file # Set up file of designated size.

losetup /dev/loop0 file # Set it up as loopback device.

mke2fs /dev/loop0 # Create filesystem.

mount -o loop /dev/loop0 /mnt # Mount it.

Thanks, S.C.

mkswap

Creates a swap partition or file. The swap area must

subsequently be enabled with swapon.

swapon, swapoff

Enable / disable swap partitition or file. These commands

usually take effect at bootup and shutdown.

mke2fs

Create a Linux ext2 filesystem. This command must be invoked

as root.

Example 17-9. Adding a new hard drive

!/bin/bash

Adding a second hard drive to system.

Software configuration. Assumes hardware already mounted.

From an article by the author of the ABS Guide.

In issue #38 of _Linux Gazette_, http://www.linuxgazette.com.

ROOT_UID=0 # This script must be run as root.

E_NOTROOT=67 # Non-root exit error.

if [ "$UID" -ne "$ROOT_UID" ]

then

echo "Must be root to run this script."

exit $E_NOTROOT

Use with extreme caution!

If something goes wrong, you may wipe out your current filesystem.

NEWDISK=/dev/hdb # Assumes /dev/hdb vacant. Check!

MOUNTPOINT=/mnt/newdisk # Or choose another mount point.

fdisk $NEWDISK

mke2fs -cv $NEWDISK1 # Check for bad blocks (verbose output).

Note: ^ /dev/hdb1, not /dev/hdb!

mkdir $MOUNTPOINT

chmod 777 $MOUNTPOINT # Makes new drive accessible to all users.

Now, test ...

mount -t ext2 /dev/hdb1 /mnt/newdisk

Try creating a directory.

If it works, umount it, and proceed.

Final step:

Add the following line to /etc/fstab.

/dev/hdb1 /mnt/newdisk ext2 defaults 1 1

exit

Creates a write-protected lockfile prefixed with the name of the script.

lockfile /home/bozo/lockfiles/${0##*/}.lock

A safer version of the above, as pointed out by E. Choroba.

Lock files are used in such applications as protecting system

mail folders from simultaneously being changed by multiple

users, indicating that a modem port is being accessed, and

showing that an instance of Firefox is using its cache.

Scripts may check for the existence of a lock file created by

a certain process to check if that process is running. Note

that if a script attempts to create a lock file that already

exists, the script will likely hang.

Normally, applications create and check for lock files in the

/var/lock directory. [92] A script can test for the presence

of a lock file by something like the following.

appname=xyzip

Application "xyzip" created lock file "/var/lock/xyzip.lock".

if [ -e "/var/lock/$appname.lock" ]

then #+ Prevent other programs & scripts

# from accessing files/resources used by xyzip.

...

flock

Much less useful than the lockfile command is flock. It sets

an "advisory" lock on a file and then executes a command while

the lock is on. This is to prevent any other process from

setting a lock on that file until completion of the specified

command.

flock $0 cat $0 > lockfile__$0

Set a lock on the script the above line appears in,

+ while listing the script to stdout.

Note

Unlike lockfile, flock does not automatically create a lock file.

mknod

Creates block or character device files (may be necessary when

installing new hardware on the system). The MAKEDEV utility

has virtually all of the functionality of mknod, and is easier

to use.

MAKEDEV

Utility for creating device files. It must be run as root, and

in the /dev directory. It is a sort of advanced version of

mknod.

tmpwatch

Automatically deletes files which have not been accessed

within a specified period of time. Usually invoked by cron to

remove stale log files.

Backup

dump, restore

The dump command is an elaborate filesystem backup utility,

generally used on larger installations and networks. [93] It

reads raw disk partitions and writes a backup file in a binary

format. Files to be backed up may be saved to a variety of

storage media, including disks and tape drives. The restore

command restores backups made with dump.

fdformat

Perform a low-level format on a floppy disk (/dev/fd0*).

System Resources

ulimit

Sets an upper limit on use of system resources. Usually

invoked with the -f option, which sets a limit on file size

(ulimit -f 1000 limits files to 1 meg maximum). [94] The -t

option limits the coredump size (ulimit -c 0 eliminates

coredumps). Normally, the value of ulimit would be set in

/etc/profile and/or ~/.bash_profile (see Appendix H).

Important

Judicious use of ulimit can protect a system against the dreaded fork

bomb.

!/bin/bash

This script is for illustrative purposes only.

Run it at your own peril -- it WILL freeze your system.

while true # Endless loop.

$0 & # This script invokes itself . . .

#+ forks an infinite number of times . . .

#+ until the system freezes up because all resources exhausted.

done # This is the notorious "sorcerer's appentice" scenario.

exit 0 # Will not exit here, because this script will never terminate.

A ulimit -Hu XX (where XX is the user process limit) in /etc/profile

would abort this script when it exceeded the preset limit.

quota

Display user or group disk quotas.

setquota

Set user or group disk quotas from the command-line.

umask

User file creation permissions mask. Limit the default file

attributes for a particular user. All files created by that

user take on the attributes specified by umask. The (octal)

value passed to umask defines the file permissions disabled.

For example, umask 022 ensures that new files will have at

most 755 permissions (777 NAND 022). [95] Of course, the user

may later change the attributes of particular files with

chmod. The usual practice is to set the value of umask in

/etc/profile and/or ~/.bash_profile (see Appendix H).

Example 17-10. Using umask to hide an output file from prying

eyes

!/bin/bash

rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file.

Usage: ./rot13a.sh filename

or ./rot13a.sh <filename

or ./rot13a.sh and supply keyboard input (stdin)

umask 177 # File creation mask.

# Files created by this script

#+ will have 600 permissions.

OUTFILE=decrypted.txt # Results output to file "decrypted.txt"

#+ which can only be read/written

# by invoker of script (or root).

cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' > $OUTFILE

^^ Input from stdin or a file. ^^^^^^^^^^ Output redirected to file.

exit 0

rdev

Get info about or make changes to root device, swap space, or

video mode. The functionality of rdev has generally been taken

over by lilo, but rdev remains useful for setting up a ram

disk. This is a dangerous command, if misused.

Modules

lsmod

List installed kernel modules.

bash$ lsmod

Module Size Used by

autofs 9456 2 (autoclean)

opl3 11376 0

serial_cs 5456 0 (unused)

sb 34752 0

uart401 6384 0 [sb]

sound 58368 0 [opl3 sb uart401]

soundlow 464 0 [sound]

soundcore 2800 6 [sb sound]

ds 6448 2 [serial_cs]

i82365 22928 2

pcmcia_core 45984 0 [serial_cs ds i82365]

Note

Doing a cat /proc/modules gives the same information.

insmod

Force installation of a kernel module (use modprobe instead,

when possible). Must be invoked as root.

rmmod

Force unloading of a kernel module. Must be invoked as root.

modprobe

Module loader that is normally invoked automatically in a

startup script. Must be invoked as root.

depmod

Creates module dependency file. Usually invoked from a startup

script.

modinfo

Output information about a loadable module.

bash$ modinfo hid

filename: /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o

description: "USB HID support drivers"

author: "Andreas Gal, Vojtech Pavlik <vojtech@suse.cz>"

license: "GPL"

Miscellaneous

env

Runs a program or script with certain environmental variables

set or changed (without changing the overall system

environment). The [varname=xxx] permits changing the

environmental variable varname for the duration of the script.

With no options specified, this command lists all the

environmental variable settings. [96]

Note

The first line of a script (the "sha-bang" line) may use env when the

path to the shell or interpreter is unknown.

! /usr/bin/env perl

print "This Perl script will run,\n";

print "even when I don't know where to find Perl.\n";

Good for portable cross-platform scripts,

where the Perl binaries may not be in the expected place.

Thanks, S.C.

Or even ...

!/bin/env bash

Queries the $PATH enviromental variable for the location of bash.

Therefore ...

This script will run where Bash is not in its usual place, in /bin.

...

ldd

Show shared lib dependencies for an executable file.

bash$ ldd /bin/ls

libc.so.6 => /lib/libc.so.6 (0x4000c000)

/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)

watch

Run a command repeatedly, at specified time intervals.

The default is two-second intervals, but this may be changed

with the -n option.

watch -n 5 tail /var/log/messages

Shows tail end of system log, /var/log/messages, every five seconds.

Note

Unfortunately, piping the output of watch command to grep does not

work.

strip

Remove the debugging symbolic references from an executable

binary. This decreases its size, but makes debugging it

impossible.

This command often occurs in a Makefile, but rarely in a shell

script.

List symbols in an unstripped compiled binary.

xrandr

Command-line tool for manipulating the root window of the

screen.

Example 17-11. Backlight: changes the brightness of the

(laptop) screen backlight

!/bin/bash

backlight.sh

reldate 02dec2011

A bug in Fedora Core 16/17 messes up the keyboard backlight controls.

This script is a quick-n-dirty workaround, essentially a shell wrapper

+ for xrandr. It gives more control than on-screen sliders and widgets.

OUTPUT=$(xrandr | grep LV | awk '{print $1}') # Get display name!

INCR=.05 # For finer-grained control, set INCR to .03 or .02.

old_brightness=$(xrandr --verbose | grep rightness | awk '{ print $2 }')

if [ -z "$1" ]

then

bright=1 # If no command-line arg, set brightness to 1.0 (default).

else

if [ "$1" = "+" ]

then

bright=$(echo "scale=2; $old_brightness + $INCR" | bc) # +.05

else

if [ "$1" = "-" ]

then

bright=$(echo "scale=2; $old_brightness - $INCR" | bc) # -.05

else

if [ "$1" = "#" ] # Echoes current brightness; does not change it.

then

bright=$old_brightness

else

if [[ "$1" = "h" || "$1" = "H" ]]

then

echo

echo "Usage:"

echo "$0 [No args] Sets/resets brightness to default (1.0)."

echo "$0 + Increments brightness by 0.5."

echo "$0 - Decrements brightness by 0.5."

echo "$0 # Echoes current brightness without changing it."

echo "$0 N (number) Sets brightness to N (useful range .7 - 1.2)."

echo "$0 h [H] Echoes this help message."

echo "$0 any-other Gives xrandr usage message."

bright=$old_brightness

else

bright="$1"

xrandr --output "$OUTPUT" --brightness "$bright" # See xrandr manpage.

# As root!

E_CHANGE0=$?

echo "Current brightness = $bright"

exit $E_CHANGE0

=========== Or, alternately . . . ==================== #

!/bin/bash

backlight2.sh

reldate 20jun2012

A bug in Fedora Core 16/17 messes up the keyboard backlight controls.

This is a quick-n-dirty workaround, an alternate to backlight.sh.

target_dir=\

/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0

Hardware directory.

actual_brightness=$(cat $target_dir/actual_brightness)

max_brightness=$(cat $target_dir/max_brightness)

Brightness=$target_dir/brightness

let "req_brightness = actual_brightness" # Requested brightness.

if [ "$1" = "-" ]

then # Decrement brightness 1 notch.

let "req_brightness = $actual_brightness - 1"

else

if [ "$1" = "+" ]

then # Increment brightness 1 notch.

let "req_brightness = $actual_brightness + 1"

if [ $req_brightness -gt $max_brightness ]

then

req_brightness=$max_brightness

fi # Do not exceed max. hardware design brightness.

echo

echo "Old brightness = $actual_brightness"

echo "Max brightness = $max_brightness"

echo "Requested brightness = $req_brightness"

echo

=====================================

echo $req_brightness > $Brightness

Must be root for this to take effect.

E_CHANGE1=$? # Successful?

=====================================

if [ "$?" -eq 0 ]

then

echo "Changed brightness!"

else

echo "Failed to change brightness!"

act_brightness=$(cat $Brightness)

echo "Actual brightness = $act_brightness"

scale0=2

sf=100 # Scale factor.

pct=$(echo "scale=$scale0; $act_brightness / $max_brightness * $sf" | bc)

echo "Percentage brightness = $pct%"

exit $E_CHANGE1

rdist

Remote distribution client: synchronizes, clones, or backs up

a file system on a remote server.

________________________________________________________________

17.1. Analyzing a System Script

Using our knowledge of administrative commands, let us examine a

system script. One of the shortest and simplest to understand scripts

is "killall," [97] used to suspend running processes at system

shutdown.

Example 17-12. killall, from /etc/rc.d/init.d

!/bin/sh

--> Comments added by the author of this document marked by "# -->".

--> This is part of the 'rc' script package

--> by Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>.

--> This particular script seems to be Red Hat / FC specific

--> (may not be present in other distributions).

Bring down all unneeded services that are still running

+ (there shouldn't be any, so this is just a sanity check)

for i in /var/lock/subsys/*; do

# --> Standard for/in loop, but since "do" is on same line,

# --> it is necessary to add ";".

# Check if the script is there.

[ ! -f $i ] && continue

# --> This is a clever use of an "and list", equivalent to:

# --> if [ ! -f "$i" ]; then continue

# Get the subsystem name.

subsys=${i#/var/lock/subsys/}

# --> Match variable name, which, in this case, is the file name.

# --> This is the exact equivalent of subsys=`basename $i`.

# --> It gets it from the lock file name

# -->+ (if there is a lock file,

# -->+ that's proof the process has been running).

# --> See the "lockfile" entry, above.

# Bring the subsystem down.

if [ -f /etc/rc.d/init.d/$subsys.init ]; then

/etc/rc.d/init.d/$subsys.init stop

else

/etc/rc.d/init.d/$subsys stop

# --> Suspend running jobs and daemons.

# --> Note that "stop" is a positional parameter,

# -->+ not a shell builtin.

done

That wasn't so bad. Aside from a little fancy footwork with variable

matching, there is no new material there.

Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit

longer than killall, but similar in concept. Make a copy of this

script somewhere in your home directory and experiment with it (do

not run it as root). Do a simulated run with the -vn flags (sh -vn

scriptname). Add extensive comments. Change the commands to echos.

Exercise 2. Look at some of the more complex scripts in

/etc/rc.d/init.d. Try to understand at least portions of them. Follow

the above procedure to analyze them. For some additional insight, you

might also examine the file sysvinitfiles in

/usr/share/doc/initscripts-?.??, which is part of the "initscripts"

documentation.

Part 5. Advanced Topics

At this point, we are ready to delve into certain of the difficult

and unusual aspects of scripting. Along the way, we will attempt to

"push the envelope" in various ways and examine boundary conditions

(what happens when we move into uncharted territory?).

Table of Contents

18. Regular Expressions

18.1. A Brief Introduction to Regular Expressions

18.2. Globbing

19. Here Documents

19.1. Here Strings

20. I/O Redirection

20.1. Using exec

20.2. Redirecting Code Blocks

20.3. Applications

21. Subshells

22. Restricted Shells

23. Process Substitution

24. Functions

24.1. Complex Functions and Function Complexities

24.2. Local Variables

24.3. Recursion Without Local Variables

25. Aliases

26. List Constructs

27. Arrays

28. Indirect References

29. /dev and /proc

29.1. /dev

29.2. /proc

30. Network Programming

31. Of Zeros and Nulls

32. Debugging

33. Options

34. Gotchas

35. Scripting With Style

35.1. Unofficial Shell Scripting Stylesheet

36. Miscellany

36.1. Interactive and non-interactive shells and scripts

36.2. Shell Wrappers

36.3. Tests and Comparisons: Alternatives

36.4. Recursion: a script calling itself

36.5. "Colorizing" Scripts

36.6. Optimizations

36.7. Assorted Tips

36.8. Security Issues

36.9. Portability Issues

36.10. Shell Scripting Under Windows

37. Bash, versions 2, 3, and 4

37.1. Bash, version 2

37.2. Bash, version 3

37.3. Bash, version 4

________________________________________________________________

Chapter 18. Regular Expressions

. . . the intellectual activity associated with software development

is largely one of gaining insight.

--Stowe Boyd

To fully utilize the power of shell scripting, you need to master

Regular Expressions. Certain commands and utilities commonly used in

scripts, such as grep, expr, sed and awk, interpret and use REs. As

of version 3, Bash has acquired its own RE-match operator: =~.

________________________________________________________________

18.1. A Brief Introduction to Regular Expressions

An expression is a string of characters. Those characters having an

interpretation above and beyond their literal meaning are called

metacharacters. A quote symbol, for example, may denote speech by a

person, ditto, or a meta-meaning [98] for the symbols that follow.

Regular Expressions are sets of characters and/or metacharacters that

match (or specify) patterns.

A Regular Expression contains one or more of the following:

* A character set. These are the characters retaining their literal

meaning. The simplest type of Regular Expression consists only of

a character set, with no metacharacters.

* An anchor. These designate (anchor) the position in the line of

text that the RE is to match. For example, ^, and $ are anchors.

* Modifiers. These expand or narrow (modify) the range of text the

RE is to match. Modifiers include the asterisk, brackets, and the

backslash.

The main uses for Regular Expressions (REs) are text searches and

string manipulation. An RE matches a single character or a set of

characters -- a string or a part of a string.

* The asterisk -- * -- matches any number of repeats of the

character string or RE preceding it, including zero instances.

"1133*" matches 11 + one or more 3's: 113, 1133, 1133333, and so

forth.

* The dot -- . -- matches any one character, except a newline. [99]

"13." matches 13 + at least one of any character (including a

space): 1133, 11333, but not 13 (additional character missing).

See Example 16-18 for a demonstration of dot single-character

matching.

* The caret -- ^ -- matches the beginning of a line, but sometimes,

depending on context, negates the meaning of a set of characters

in an RE.

* The dollar sign -- $ -- at the end of an RE matches the end of a

line.

"XXX$" matches XXX at the end of a line.

"^$" matches blank lines.

* Brackets -- [...] -- enclose a set of characters to match in a

single RE.

"[xyz]" matches any one of the characters x, y, or z.

"[c-n]" matches any one of the characters in the range c to n.

"[B-Pk-y]" matches any one of the characters in the ranges B to P

and k to y.

"[a-z0-9]" matches any single lowercase letter or any digit.

"[^b-d]" matches any character except those in the range b to d.

This is an instance of ^ negating or inverting the meaning of the

following RE (taking on a role similar to ! in a different

context).

Combined sequences of bracketed characters match common word

patterns. "[Yy][Ee][Ss]" matches yes, Yes, YES, yEs, and so

forth. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" matches

any Social Security number.

* The backslash -- \ -- escapes a special character, which means

that character gets interpreted literally (and is therefore no

longer special).

A "\$" reverts back to its literal meaning of "$", rather than

its RE meaning of end-of-line. Likewise a "\\" has the literal

meaning of "\".

* Escaped "angle brackets" -- \<...\> -- mark word boundaries.

The angle brackets must be escaped, since otherwise they have

only their literal character meaning.

"\<the\>" matches the word "the," but not the words "them,"

"there," "other," etc.

bash$ cat textfile

This is line 1, of which there is only one instance.

This is the only instance of line 2.

This is line 3, another line.

This is line 4.

bash$ grep 'the' textfile

This is line 1, of which there is only one instance.

This is the only instance of line 2.

This is line 3, another line.

bash$ grep '\<the\>' textfile

This is the only instance of line 2.

The only way to be certain that a particular RE works is to test it.

TEST FILE: tstfile # No match.

# No match.

Run grep "1133*" on this file. # Match.

# No match.

This line contains the number 113. # Match.

This line contains the number 13. # No match.

This line contains the number 133. # No match.

This line contains the number 1133. # Match.

This line contains the number 113312. # Match.

This line contains the number 1112. # No match.

This line contains the number 113312312. # Match.

This line contains no numbers at all. # No match.

bash$ grep "1133*" tstfile

Run grep "1133*" on this file. # Match.

This line contains the number 113. # Match.

This line contains the number 1133. # Match.

This line contains the number 113312. # Match.

This line contains the number 113312312. # Match.

* Extended REs. Additional metacharacters added to the basic set.

Used in egrep, awk, and Perl.

* The question mark -- ? -- matches zero or one of the previous RE.

It is generally used for matching single characters.

* The plus -- + -- matches one or more of the previous RE. It

serves a role similar to the *, but does not match zero

occurrences.

GNU versions of sed and awk can use "+",

but it needs to be escaped.

echo a111b | sed -ne '/a1\+b/p'

echo a111b | grep 'a1\+b'

echo a111b | gawk '/a1+b/'

All of above are equivalent.

Thanks, S.C.

* Escaped "curly brackets" -- \{ \} -- indicate the number of

occurrences of a preceding RE to match.

It is necessary to escape the curly brackets since they have only

their literal character meaning otherwise. This usage is

technically not part of the basic RE set.

"[0-9]\{5\}" matches exactly five digits (characters in the range

of 0 to 9).

Note

Curly brackets are not available as an RE in the "classic" (non-POSIX

compliant) version of awk. However, the GNU extended version of awk,

gawk, has the --re-interval option that permits them (without being

escaped).

bash$ echo 2222 | gawk --re-interval '/2{3}/'

2222

Perl and some egrep versions do not require escaping the curly

brackets.

* Parentheses -- ( ) -- enclose a group of REs. They are useful

with the following "|" operator and in substring extraction using

expr.

* The -- | -- "or" RE operator matches any of a set of alternate

characters.

bash$ egrep 're(a|e)d' misc.txt

People who read seem to be better informed than those who do not.

The clarinet produces sound by the vibration of its reed.

Note

Some versions of sed, ed, and ex support escaped versions of the

extended Regular Expressions described above, as do the GNU

utilities.

* POSIX Character Classes. [:class:]

This is an alternate method of specifying a range of characters

to match.

* [:alnum:] matches alphabetic or numeric characters. This is

equivalent to A-Za-z0-9.

* [:alpha:] matches alphabetic characters. This is equivalent to

A-Za-z.

* [:blank:] matches a space or a tab.

* [:cntrl:] matches control characters.

* [:digit:] matches (decimal) digits. This is equivalent to 0-9.

* [:graph:] (graphic printable characters). Matches characters in

the range of ASCII 33 - 126. This is the same as [:print:],

below, but excluding the space character.

* [:lower:] matches lowercase alphabetic characters. This is

equivalent to a-z.

* [:print:] (printable characters). Matches characters in the range

of ASCII 32 - 126. This is the same as [:graph:], above, but

adding the space character.

* [:space:] matches whitespace characters (space and horizontal

tab).

* [:upper:] matches uppercase alphabetic characters. This is

equivalent to A-Z.

* [:xdigit:] matches hexadecimal digits. This is equivalent to

0-9A-Fa-f.

Important

POSIX character classes generally require quoting or double brackets

([[ ]]).

bash$ grep [[:digit:]] test.file

abc=723

...

if [[ $arow =~ [[:digit:]] ]] # Numerical input?

then # POSIX char class

if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!

...

From ktour.sh example script.

These character classes may even be used with globbing, to a

limited extent.

bash$ ls -l ?[[:digit:]][[:digit:]]?

-rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b

POSIX character classes are used in Example 16-21 and Example

16-22.

Sed, awk, and Perl, used as filters in scripts, take REs as arguments

when "sifting" or transforming files or I/O streams. See Example A-12

and Example A-16 for illustrations of this.

The standard reference on this complex topic is Friedl's Mastering

Regular Expressions. Sed & Awk, by Dougherty and Robbins, also gives

a very lucid treatment of REs. See the Bibliography for more

information on these books.

________________________________________________________________

18.2. Globbing

Bash itself cannot recognize Regular Expressions. Inside scripts, it

is commands and utilities -- such as sed and awk -- that interpret

RE's.

Bash does carry out filename expansion [100] -- a process known as

globbing -- but this does not use the standard RE set. Instead,

globbing recognizes and expands wild cards. Globbing interprets the

standard wild card characters [101] -- * and ?, character lists in

square brackets, and certain other special characters (such as ^ for

negating the sense of a match). There are important limitations on

wild card characters in globbing, however. Strings containing * will

not match filenames that start with a dot, as, for example, .bashrc.

[102] Likewise, the ? has a different meaning in globbing than as

part of an RE.

bash$ ls -l

total 2

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1

-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh

-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt

bash$ ls -l t?.sh

-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh

bash$ ls -l [ab]*

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1

bash$ ls -l [a-c]*

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1

bash$ ls -l [^ab]*

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1

-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh

-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt

bash$ ls -l {b*,c*,*est*}

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1

-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1

-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt

Bash performs filename expansion on unquoted command-line arguments.

The echo command demonstrates this.

bash$ echo *

a.1 b.1 c.1 t2.sh test1.txt

bash$ echo t*

t2.sh test1.txt

bash$ echo t?.sh

t2.sh

Note

It is possible to modify the way Bash interprets special characters

in globbing. A set -f command disables globbing, and the nocaseglob

and nullglob options to shopt change globbing behavior.

Correct glob use:

Always use for-loop, prefix glob, check if exists file.

for file in ./* ; do # Use ./* ... NEVER bare *

if [ -e "$file" ] ; then # Check whether file exists.

COMMAND ... "$file" ...

done

This example taken from David Wheeler's site, with permission.

________________________________________________________________

Chapter 19. Here Documents

Here and now, boys.

--Aldous Huxley, Island

A here document is a special-purpose code block. It uses a form of

I/O redirection to feed a command list to an interactive program or a

command, such as ftp, cat, or the ex text editor.

COMMAND <<InputComesFromHERE

...

InputComesFromHERE

A limit string delineates (frames) the command list. The special

symbol << precedes the limit string. This has the effect of

redirecting the output of a command block into the stdin of the

program or command. It is similar to interactive-program <

command-file, where command-file contains

command #1

command #2

...

The here document equivalent looks like this:

interactive-program <<LimitString

command #1

command #2

...

LimitString

Choose a limit string sufficiently unusual that it will not occur

anywhere in the command list and confuse matters.

Note that here documents may sometimes be used to good effect with

non-interactive utilities and commands, such as, for example, wall.

Example 19-1. broadcast: Sends message to everyone logged in

!/bin/bash

wall <<zzz23EndOfMessagezzz23

E-mail your noontime orders for pizza to the system administrator.

(Add an extra dollar for anchovy or mushroom topping.)

Additional message text goes here.

Note: 'wall' prints comment lines.

zzz23EndOfMessagezzz23

Could have been done more efficiently by

wall <message-file

However, embedding the message template in a script

+ is a quick-and-dirty one-off solution.

exit

Even such unlikely candidates as the vi text editor lend themselves

to here documents.

Example 19-2. dummyfile: Creates a 2-line dummy file

!/bin/bash

Noninteractive use of 'vi' to edit a file.

Emulates 'sed'.

E_BADARGS=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

TARGETFILE=$1

Insert 2 lines in file, then save.

--------Begin here document-----------#

vi $TARGETFILE <<x23LimitStringx23

This is line 1 of the example file.

This is line 2 of the example file.

x23LimitStringx23

----------End here document-----------#

Note that ^[ above is a literal escape

+ typed by Control-V <Esc>.

Bram Moolenaar points out that this may not work with 'vim'

+ because of possible problems with terminal interaction.

exit

The above script could just as effectively have been implemented with

ex, rather than vi. Here documents containing a list of ex commands

are common enough to form their own category, known as ex scripts.

!/bin/bash

Replace all instances of "Smith" with "Jones"

+ in files with a ".txt" filename suffix.

ORIGINAL=Smith

REPLACEMENT=Jones

for word in $(fgrep -l $ORIGINAL *.txt)

# -------------------------------------

ex $word <<EOF

:%s/$ORIGINAL/$REPLACEMENT/g

:wq

EOF

# :%s is the "ex" substitution command.

# :wq is write-and-quit.

# -------------------------------------

done

Analogous to "ex scripts" are cat scripts.

Example 19-3. Multi-line message using cat

!/bin/bash

'echo' is fine for printing single line messages,

+ but somewhat problematic for for message blocks.

A 'cat' here document overcomes this limitation.

cat <<End-of-message

-------------------------------------

This is line 1 of the message.

This is line 2 of the message.

This is line 3 of the message.

This is line 4 of the message.

This is the last line of the message.

-------------------------------------

End-of-message

Replacing line 7, above, with

+ cat > $Newfile <<End-of-message

+ ^^^^^^^^^^

+ writes the output to the file $Newfile, rather than to stdout.

exit 0

--------------------------------------------

Code below disabled, due to "exit 0" above.

S.C. points out that the following also works.

echo "-------------------------------------

This is line 1 of the message.

This is line 2 of the message.

This is line 3 of the message.

This is line 4 of the message.

This is the last line of the message.

-------------------------------------"

However, text may not include double quotes unless they are escaped.

The - option to mark a here document limit string (<<-LimitString)

suppresses leading tabs (but not spaces) in the output. This may be

useful in making a script more readable.

Example 19-4. Multi-line message, with tabs suppressed

!/bin/bash

Same as previous example, but...

The - option to a here document <<-

+ suppresses leading tabs in the body of the document,

+ but not spaces.

cat <<-ENDOFMESSAGE

This is line 1 of the message.

This is line 2 of the message.

This is line 3 of the message.

This is line 4 of the message.

This is the last line of the message.

ENDOFMESSAGE

The output of the script will be flush left.

Leading tab in each line will not show.

Above 5 lines of "message" prefaced by a tab, not spaces.

Spaces not affected by <<- .

Note that this option has no effect on embedded tabs.

exit 0

A here document supports parameter and command substitution. It is

therefore possible to pass different parameters to the body of the

here document, changing its output accordingly.

Example 19-5. Here document with replaceable parameters

!/bin/bash

Another 'cat' here document, using parameter substitution.

Try it with no command-line parameters, ./scriptname

Try it with one command-line parameter, ./scriptname Mortimer

Try it with one two-word quoted command-line parameter,

./scriptname "Mortimer Jones"

CMDLINEPARAM=1 # Expect at least command-line parameter.

if [ $# -ge $CMDLINEPARAM ]

then

NAME=$1 # If more than one command-line param,

#+ then just take the first.

else

NAME="John Doe" # Default, if no command-line parameter.

RESPONDENT="the author of this fine script"

cat <<Endofmessage

Hello, there, $NAME.

Greetings to you, $NAME, from $RESPONDENT.

This comment shows up in the output (why?).

Endofmessage

Note that the blank lines show up in the output.

So does the comment.

exit

This is a useful script containing a here document with parameter

substitution.

Example 19-6. Upload a file pair to Sunsite incoming directory

!/bin/bash

upload.sh

Upload file pair (Filename.lsm, Filename.tar.gz)

+ to incoming directory at Sunsite/UNC (ibiblio.org).

Filename.tar.gz is the tarball itself.

Filename.lsm is the descriptor file.

Sunsite requires "lsm" file, otherwise will bounce contributions.

E_ARGERROR=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` Filename-to-upload"

exit $E_ARGERROR

Filename=`basename $1` # Strips pathname out of file name.

Server="ibiblio.org"

Directory="/incoming/Linux"

These need not be hard-coded into script,

+ but may instead be changed to command-line argument.

Password="your.e-mail.address" # Change above to suit.

ftp -n $Server <<End-Of-Session

-n option disables auto-logon

user anonymous "$Password" # If this doesn't work, then try:

# quote user anonymous "$Password"

binary

bell # Ring 'bell' after each file transfer.

cd $Directory

put "$Filename.lsm"

put "$Filename.tar.gz"

bye

End-Of-Session

exit 0

Quoting or escaping the "limit string" at the head of a here document

disables parameter substitution within its body. The reason for this

is that quoting/escaping the limit string effectively escapes the $,

`, and \ special characters, and causes them to be interpreted

literally. (Thank you, Allen Halsey, for pointing this out.)

Example 19-7. Parameter substitution turned off

!/bin/bash

A 'cat' here-document, but with parameter substitution disabled.

NAME="John Doe"

RESPONDENT="the author of this fine script"

cat <<'Endofmessage'

Hello, there, $NAME.

Greetings to you, $NAME, from $RESPONDENT.

Endofmessage

No parameter substitution when the "limit string" is quoted or escaped.

Either of the following at the head of the here document would have

+ the same effect.

cat <<"Endofmessage"

cat <<\Endofmessage

And, likewise:

cat <<"SpecialCharTest"

Directory listing would follow

if limit string were not quoted.

`ls -l`

Arithmetic expansion would take place

if limit string were not quoted.

$((5 + 3))

A a single backslash would echo

if limit string were not quoted.

SpecialCharTest

exit

Disabling parameter substitution permits outputting literal text.

Generating scripts or even program code is one use for this.

Example 19-8. A script that generates another script

!/bin/bash

generate-script.sh

Based on an idea by Albert Reiner.

OUTFILE=generated.sh # Name of the file to generate.

-----------------------------------------------------------

'Here document containing the body of the generated script.

(

cat <<'EOF'

!/bin/bash

echo "This is a generated shell script."

Note that since we are inside a subshell,

+ we can't access variables in the "outside" script.

echo "Generated file will be named: $OUTFILE"

Above line will not work as normally expected

+ because parameter expansion has been disabled.

Instead, the result is literal output.

a=7

b=3

let "c = $a * $b"

echo "c = $c"

exit 0

EOF

) > $OUTFILE

-----------------------------------------------------------

Quoting the 'limit string' prevents variable expansion

+ within the body of the above 'here document.'

This permits outputting literal strings in the output file.

if [ -f "$OUTFILE" ]

then

chmod 755 $OUTFILE

# Make the generated file executable.

else

echo "Problem in creating file: \"$OUTFILE\""

This method also works for generating

+ C programs, Perl programs, Python programs, Makefiles,

+ and the like.

exit 0

It is possible to set a variable from the output of a here document.

This is actually a devious form of command substitution.

variable=$(cat <<SETVAR

This variable

runs over multiple lines.

SETVAR

)

echo "$variable"

A here document can supply input to a function in the same script.

Example 19-9. Here documents and functions

!/bin/bash

here-function.sh

GetPersonalData ()

{

read firstname

read lastname

read address

read city

read state

read zipcode

} # This certainly appears to be an interactive function, but . . .

Supply input to the above function.

GetPersonalData <<RECORD001

Bozo

Bozeman

2726 Nondescript Dr.

Bozeman

21226

RECORD001

echo

echo "$firstname $lastname"

echo "$address"

echo "$city, $state $zipcode"

echo

exit 0

It is possible to use : as a dummy command accepting output from a

here document. This, in effect, creates an "anonymous" here document.

Example 19-10. "Anonymous" Here Document

!/bin/bash

: <<TESTVARIABLES

${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables no

t set.

TESTVARIABLES

exit $?

Tip

A variation of the above technique permits "commenting out" blocks of

code.

Example 19-11. Commenting out a block of code

!/bin/bash

commentblock.sh

: <<COMMENTBLOCK

echo "This line will not echo."

This is a comment line missing the "#" prefix.

This is another comment line missing the "#" prefix.

&*@!!++=

The above line will cause no error message,

because the Bash interpreter will ignore it.

COMMENTBLOCK

echo "Exit value of above \"COMMENTBLOCK\" is $?." # 0

No error shown.

echo

The above technique also comes in useful for commenting out

+ a block of working code for debugging purposes.

This saves having to put a "#" at the beginning of each line,

+ then having to go back and delete each "#" later.

Note that the use of of colon, above, is optional.

echo "Just before commented-out code block."

The lines of code between the double-dashed lines will not execute.

===================================================================

: <<DEBUGXXX

for file in *

cat "$file"

done

DEBUGXXX

===================================================================

echo "Just after commented-out code block."

exit 0

Note, however, that if a bracketed variable is contained within

+ the commented-out code block,

+ then this could cause problems.

for example:

/!/bin/bash

: <<COMMENTBLOCK

echo "This line will not echo."

&*@!!++=

${foo_bar_bazz?}

$(rm -rf /tmp/foobar/)

$(touch my_build_directory/cups/Makefile)

COMMENTBLOCK

$ sh commented-bad.sh

commented-bad.sh: line 3: foo_bar_bazz: parameter null or not set

The remedy for this is to strong-quote the 'COMMENTBLOCK' in line 49, above.

: <<'COMMENTBLOCK'

Thank you, Kurt Pfeifle, for pointing this out.

Tip

Yet another twist of this nifty trick makes "self-documenting"

scripts possible.

Example 19-12. A self-documenting script

!/bin/bash

self-document.sh: self-documenting script

Modification of "colm.sh".

DOC_REQUEST=70

if [ "$1" = "-h" -o "$1" = "--help" ] # Request help.

then

echo; echo "Usage: $0 [directory-name]"; echo

sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" |

sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi

: <<DOCUMENTATIONXX

List the statistics of a specified directory in tabular format.

---------------------------------------------------------------

The command-line parameter gives the directory to be listed.

If no directory specified or directory specified cannot be read,

then list the current working directory.

DOCUMENTATIONXX

if [ -z "$1" -o ! -r "$1" ]

then

directory=.

else

directory="$1"

echo "Listing of "$directory":"; echo

(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \

; ls -l "$directory" | sed 1d) | column -t

exit 0

Using a cat script is an alternate way of accomplishing this.

DOC_REQUEST=70

if [ "$1" = "-h" -o "$1" = "--help" ] # Request help.

then # Use a "cat script" . . .

cat <<DOCUMENTATIONXX

List the statistics of a specified directory in tabular format.

---------------------------------------------------------------

The command-line parameter gives the directory to be listed.

If no directory specified or directory specified cannot be read,

then list the current working directory.

DOCUMENTATIONXX

exit $DOC_REQUEST

See also Example A-28, Example A-40, Example A-41, and Example A-42

for more examples of self-documenting scripts.

Note

Here documents create temporary files, but these files are deleted

after opening and are not accessible to any other process.

bash$ bash -c 'lsof -a -p $ -d0' << EOF

EOF

lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213-0-sh (deleted)

Caution

Some utilities will not work inside a here document.

Warning

The closing limit string, on the final line of a here document, must

start in the first character position. There can be no leading

whitespace. Trailing whitespace after the limit string likewise

causes unexpected behavior. The whitespace prevents the limit string

from being recognized. [103]

!/bin/bash

echo "----------------------------------------------------------------------"

cat <<LimitString

echo "This is line 1 of the message inside the here document."

echo "This is line 2 of the message inside the here document."

echo "This is the final line of the message inside the here document."

LimitString

^^^^Indented limit string. Error! This script will not behave as expected.

echo "----------------------------------------------------------------------"

These comments are outside the 'here document',

+ and should not echo.

echo "Outside the here document."

exit 0

echo "This line had better not echo." # Follows an 'exit' command.

Caution

Some people very cleverly use a single ! as a limit string. But,

that's not necessarily a good idea.

This works.

cat <<!

Hello!

! Three more exclamations !!!

But . . .

cat <<!

Hello!

Single exclamation point follows!

Crashes with an error message.

However, the following will work.

cat <<EOF

Hello!

Single exclamation point follows!

EOF

It's safer to use a multi-character limit string.

For those tasks too complex for a here document, consider using the

expect scripting language, which was specifically designed for

feeding input into interactive programs.

________________________________________________________________

19.1. Here Strings

A here string can be considered as a stripped-down form of a here

document.

It consists of nothing more than COMMAND <<< $WORD,

where $WORD is expanded and fed to the stdin of COMMAND.

As a simple example, consider this alternative to the echo-grep

construction.

Instead of:

if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]]

etc.

Try:

if grep -q "txt" <<< "$VAR"

then # ^^^

echo "$VAR contains the substring sequence \"txt\""

Thank you, Sebastian Kaminski, for the suggestion.

Or, in combination with read:

String="This is a string of words."

read -r -a Words <<< "$String"

The -a option to "read"

+ assigns the resulting values to successive members of an array.

echo "First word in String is: ${Words[0]}" # This

echo "Second word in String is: ${Words[1]}" # is

echo "Third word in String is: ${Words[2]}" # a

echo "Fourth word in String is: ${Words[3]}" # string

echo "Fifth word in String is: ${Words[4]}" # of

echo "Sixth word in String is: ${Words[5]}" # words.

echo "Seventh word in String is: ${Words[6]}" # (null)

# Past end of $String.

Thank you, Francisco Lobo, for the suggestion.

It is, of course, possible to feed the output of a here string into

the stdin of a loop.

As Seamus points out . . .

ArrayVar=( element0 element1 element2 {A..D} )

while read element ; do

echo "$element" 1>&2

done <<< $(echo ${ArrayVar[*]})

element0 element1 element2 A B C D

Example 19-13. Prepending a line to a file

!/bin/bash

prepend.sh: Add text at beginning of file.

Example contributed by Kenny Stauffer,

+ and slightly modified by document author.

E_NOSUCHFILE=85

read -p "File: " file # -p arg to 'read' displays prompt.

if [ ! -e "$file" ]

then # Bail out if no such file.

echo "File $file not found."

exit $E_NOSUCHFILE

read -p "Title: " title

cat - $file <<<$title > $file.new

echo "Modified file is $file.new"

exit # Ends script execution.

from 'man bash':

Here Strings

A variant of here documents, the format is:

<<<word

The word is expanded and supplied to the command on its standard input

Of course, the following also works:

sed -e '1i\

Title: ' $file

Example 19-14. Parsing a mailbox

!/bin/bash

Script by Francisco Lobo,

+ and slightly modified and commented by ABS Guide author.

Used in ABS Guide with permission. (Thank you!)

This script will not run under Bash versions -lt 3.0.

E_MISSING_ARG=87

if [ -z "$1" ]

then

echo "Usage: $0 mailbox-file"

exit $E_MISSING_ARG

mbox_grep() # Parse mailbox file.

{

declare -i body=0 match=0

declare -a date sender

declare mail header value

while IFS= read -r mail

^^^^ Reset $IFS.

Otherwise "read" will strip leading & trailing space from its input.

if [[ $mail =~ ^From ]] # Match "From" field in message.

then

(( body = 0 )) # "Zero out" variables.

(( match = 0 ))

unset date

elif (( body ))

then

(( match ))

# echo "$mail"

# Uncomment above line if you want entire body

#+ of message to display.

elif [[ $mail ]]; then

IFS=: read -r header value <<< "$mail"

# ^^^ "here string"

case "$header" in

[Ff][Rr][Oo][Mm] ) [[ $value =~ "$2" ]] && (( match++ )) ;;

# Match "From" line.

[Dd][Aa][Tt][Ee] ) read -r -a date <<< "$value" ;;

# ^^^

# Match "Date" line.

[Rr][Ee][Cc][Ee][Ii][Vv][Ee][Dd] ) read -r -a sender <<< "$value" ;;

# ^^^

# Match IP Address (may be spoofed).

esac

else

(( body++ ))

(( match )) &&

echo "MESSAGE ${date:+of: ${date[*]} }"

# Entire $date array ^

echo "IP address of sender: ${sender[1]}"

# Second field of "Received" line ^

done < "$1" # Redirect stdout of file into loop.

}

mbox_grep "$1" # Send mailbox file to function.

exit $?

Exercises:

---------

1) Break the single function, above, into multiple functions,

+ for the sake of readability.

2) Add additional parsing to the script, checking for various keywords.

$ mailbox_grep.sh scam_mail

MESSAGE of Thu, 5 Jan 2006 08:00:56 -0500 (EST)

IP address of sender: 196.3.62.4

Exercise: Find other uses for here strings, such as, for example,

feeding input to dc.

________________________________________________________________

Chapter 20. I/O Redirection

There are always three default files [104] open, stdin (the

keyboard), stdout (the screen), and stderr (error messages output to

the screen). These, and any other open files, can be redirected.

Redirection simply means capturing output from a file, command,

program, script, or even code block within a script (see Example 3-1

and Example 3-2) and sending it as input to another file, command,

program, or script.

Each open file gets assigned a file descriptor. [105] The file

descriptors for stdin, stdout, and stderr are 0, 1, and 2,

respectively. For opening additional files, there remain descriptors

3 to 9. It is sometimes useful to assign one of these additional file

descriptors to stdin, stdout, or stderr as a temporary duplicate

link. [106] This simplifies restoration to normal after complex

redirection and reshuffling (see Example 20-1).

COMMAND_OUTPUT >

# Redirect stdout to a file.

# Creates the file if not present, otherwise overwrites it.

ls -lR > dir-tree.list

# Creates a file containing a listing of the directory tree.

: > filename

# The > truncates file "filename" to zero length.

# If file not present, creates zero-length file (same effect as 'touch')

# The : serves as a dummy placeholder, producing no output.

> filename

# The > truncates file "filename" to zero length.

# If file not present, creates zero-length file (same effect as 'touch')

# (Same result as ": >", above, but this does not work with some shells.

)

COMMAND_OUTPUT >>

# Redirect stdout to a file.

# Creates the file if not present, otherwise appends to it.

# Single-line redirection commands (affect only the line they are on):

# --------------------------------------------------------------------

1>filename

# Redirect stdout to file "filename."

1>>filename

# Redirect and append stdout to file "filename."

2>filename

# Redirect stderr to file "filename."

2>>filename

# Redirect and append stderr to file "filename."

&>filename

# Redirect both stdout and stderr to file "filename."

# This operator is now functional, as of Bash 4, final release.

M>N

# "M" is a file descriptor, which defaults to 1, if not explicitly set.

# "N" is a filename.

# File descriptor "M" is redirect to file "N."

M>&N

# "M" is a file descriptor, which defaults to 1, if not set.

# "N" is another file descriptor.

#=======================================================================

=======

# Redirecting stdout, one line at a time.

LOGFILE=script.log

echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE

echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE

echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE

echo "This statement is echoed to stdout, and will not appear in \"$LOGF

ILE\"."

# These redirection commands automatically "reset" after each line.

# Redirecting stderr, one line at a time.

ERRORFILE=script.errors

bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE.

bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE.

bad_command3 # Error message echoed to stderr,

#+ and does not appear in $ERRORFILE.

# These redirection commands also automatically "reset" after each line.

#=======================================================================

2>&1

# Redirects stderr to stdout.

# Error messages get sent to same place as standard output.

>>filename 2>&1

bad_command >>filename 2>&1

# Appends both stdout and stderr to the file "filename" ...

2>&1 | [command(s)]

bad_command 2>&1 | awk '{print $5}' # found

# Sends stderr through a pipe.

# |& was added to Bash 4 as an abbreviation for 2>&1 |.

i>&j

# Redirects file descriptor i to j.

# All output of file pointed to by i gets sent to file pointed to by j.

>&j

# Redirects, by default, file descriptor 1 (stdout) to j.

# All stdout gets sent to file pointed to by j.

0< FILENAME

< FILENAME

# Accept input from a file.

# Companion command to ">", and often used in combination with it.

# grep search-word <filename

[j]<>filename

# Open file "filename" for reading and writing,

#+ and assign file descriptor "j" to it.

# If "filename" does not exist, create it.

# If file descriptor "j" is not specified, default to fd 0, stdin.

# An application of this is writing at a specified place in a file.

echo 1234567890 > File # Write string to "File".

exec 3<> File # Open "File" and assign fd 3 to it.

read -n 4 <&3 # Read only 4 characters.

echo -n . >&3 # Write a decimal point there.

exec 3>&- # Close fd 3.

cat File # ==> 1234.67890

# Random access, by golly.

# Pipe.

# General purpose process and command chaining tool.

# Similar to ">", but more general in effect.

# Useful for chaining commands, scripts, files, and programs together.

cat *.txt | sort | uniq > result-file

# Sorts the output of all the .txt files and deletes duplicate lines,

# finally saves results to "result-file".

Multiple instances of input and output redirection and/or pipes can

be combined in a single command line.

command < input-file > output-file

Or the equivalent:

< input-file command > output-file # Although this is non-standard.

command1 | command2 | command3 > output-file

See Example 16-31 and Example A-14.

Multiple output streams may be redirected to one file.

ls -yz >> command.log 2>&1

Capture result of illegal options "yz" in file "command.log."

Because stderr is redirected to the file,

+ any error messages will also be there.

Note, however, that the following does not give the same result.

ls -yz 2>&1 >> command.log

Outputs an error message, but does not write to file.

More precisely, the command output (in this case, null)

+ writes to the file, but the error message goes only to stdout.

If redirecting both stdout and stderr,

+ the order of the commands makes a difference.

Closing File Descriptors

n<&-

Close input file descriptor n.

0<&-, <&-

Close stdin.

n>&-

Close output file descriptor n.

1>&-, >&-

Close stdout.

Child processes inherit open file descriptors. This is why pipes

work. To prevent an fd from being inherited, close it.

Redirecting only stderr to a pipe.

exec 3>&1 # Save current "value" of stdout.

ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls').

^^^^ ^^^^

exec 3>&- # Now close it for the remainder of the

script.

Thanks, S.C.

For a more detailed introduction to I/O redirection see Appendix F.

________________________________________________________________

20.1. Using exec

An exec <filename command redirects stdin to a file. From that point

on, all stdin comes from that file, rather than its normal source

(usually keyboard input). This provides a method of reading a file

line by line and possibly parsing each line of input using sed and/or

awk.

Example 20-1. Redirecting stdin using exec

!/bin/bash

Redirecting stdin using 'exec'.

exec 6<&0 # Link file descriptor #6 with stdin.

# Saves stdin.

exec < data-file # stdin replaced by file "data-file"

read a1 # Reads first line of file "data-file".

read a2 # Reads second line of file "data-file."

echo

echo "Following lines read from file."

echo "-------------------------------"

echo $a1

echo $a2

echo; echo; echo

exec 0<&6 6<&-

Now restore stdin from fd #6, where it had been saved,

+ and close fd #6 ( 6<&- ) to free it for other processes to use.

<&6 6<&- also works.

echo -n "Enter data "

read b1 # Now "read" functions as expected, reading from normal stdin.

echo "Input read from stdin."

echo "----------------------"

echo "b1 = $b1"

echo

exit 0

Similarly, an exec >filename command redirects stdout to a designated

file. This sends all command output that would normally go to stdout

to that file.

Important

exec N > filename affects the entire script or current shell.

Redirection in the PID of the script or shell from that point on has

changed. However . . .

N > filename affects only the newly-forked process, not the entire

script or shell.

Thank you, Ahmed Darwish, for pointing this out.

Example 20-2. Redirecting stdout using exec

!/bin/bash

reassign-stdout.sh

LOGFILE=logfile.txt

exec 6>&1 # Link file descriptor #6 with stdout.

# Saves stdout.

exec > $LOGFILE # stdout replaced with file "logfile.txt".

----------------------------------------------------------- #

All output from commands in this block sent to file $LOGFILE.

echo -n "Logfile: "

date

echo "-------------------------------------"

echo

echo "Output of \"ls -al\" command"

echo

ls -al

echo; echo

echo "Output of \"df\" command"

echo

----------------------------------------------------------- #

exec 1>&6 6>&- # Restore stdout and close file descriptor #6.

echo

echo "== stdout now restored to default == "

echo

ls -al

echo

exit 0

Example 20-3. Redirecting both stdin and stdout in the same script

with exec

!/bin/bash

upperconv.sh

Converts a specified input file to uppercase.

E_FILE_ACCESS=70

E_WRONG_ARGS=71

if [ ! -r "$1" ] # Is specified input file readable?

then

echo "Can't read from input file!"

echo "Usage: $0 input-file output-file"

exit $E_FILE_ACCESS

fi # Will exit with same error

#+ even if input file ($1) not specified (why?).

if [ -z "$2" ]

then

echo "Need to specify output file."

echo "Usage: $0 input-file output-file"

exit $E_WRONG_ARGS

exec 4<&0

exec < $1 # Will read from input file.

exec 7>&1

exec > $2 # Will write to output file.

# Assumes output file writable (add check?).

-----------------------------------------------

cat - | tr a-z A-Z # Uppercase conversion.

^^^^^ # Reads from stdin.

^^^^^^^^^^ # Writes to stdout.

However, both stdin and stdout were redirected.

Note that the 'cat' can be omitted.

-----------------------------------------------

exec 1>&7 7>&- # Restore stout.

exec 0<&4 4<&- # Restore stdin.

After restoration, the following line prints to stdout as expected.

echo "File \"$1\" written to \"$2\" as uppercase conversion."

exit 0

I/O redirection is a clever way of avoiding the dreaded inaccessible

variables within a subshell problem.

Example 20-4. Avoiding a subshell

!/bin/bash

avoid-subshell.sh

Suggested by Matthew Walker.

Lines=0

echo

cat myfile.txt | while read line;

do {

echo $line

(( Lines++ )); # Incremented values of this variable

#+ inaccessible outside loop.

# Subshell problem.

}

done

echo "Number of lines read = $Lines" # 0

# Wrong!

echo "------------------------"

exec 3<> myfile.txt

while read line <&3

do {

echo "$line"

(( Lines++ )); # Incremented values of this variable

#+ accessible outside loop.

# No subshell, no problem.

}

done

exec 3>&-

echo "Number of lines read = $Lines" # 8

echo

exit 0

Lines below not seen by script.

$ cat myfile.txt

Line 1.

Line 2.

Line 3.

Line 4.

Line 5.

Line 6.

Line 7.

Line 8.

________________________________________________________________

20.2. Redirecting Code Blocks

Blocks of code, such as while, until, and for loops, even if/then

test blocks can also incorporate redirection of stdin. Even a

function may use this form of redirection (see Example 24-11). The <

operator at the end of the code block accomplishes this.

Example 20-5. Redirected while loop

!/bin/bash

redir2.sh

if [ -z "$1" ]

then

Filename=names.data # Default, if no filename specified.

else

Filename=$1

+ Filename=${1:-names.data}

can replace the above test (parameter substitution).

count=0

echo

while [ "$name" != Smith ] # Why is variable $name in quotes?

read name # Reads from $Filename, rather than stdin.

echo $name

let "count += 1"

done <"$Filename" # Redirects stdin to file $Filename.

^^^^^^^^^^^^

echo; echo "$count names read"; echo

exit 0

Note that in some older shell scripting languages,

+ the redirected loop would run as a subshell.

Therefore, $count would return 0, the initialized value outside the loop.

Bash and ksh avoid starting a subshell whenever possible,

+ so that this script, for example, runs correctly.

(Thanks to Heiner Steven for pointing this out.)

However . . .

Bash can sometimes start a subshell in a PIPED "while-read" loop,

+ as distinct from a REDIRECTED "while" loop.

abc=hi

echo -e "1\n2\n3" | while read l

do abc="$l"

echo $abc

done

echo $abc

Thanks, Bruno de Oliveira Schneider, for demonstrating this

+ with the above snippet of code.

And, thanks, Brian Onn, for correcting an annotation error.

Example 20-6. Alternate form of redirected while loop

!/bin/bash

This is an alternate form of the preceding script.

Suggested by Heiner Steven

+ as a workaround in those situations when a redirect loop

+ runs as a subshell, and therefore variables inside the loop

+do not keep their values upon loop termination.

if [ -z "$1" ]

then

Filename=names.data # Default, if no filename specified.

else

Filename=$1

exec 3<&0 # Save stdin to file descriptor 3.

exec 0<"$Filename" # Redirect standard input.

count=0

echo

while [ "$name" != Smith ]

read name # Reads from redirected stdin ($Filename).

echo $name

let "count += 1"

done # Loop reads from file $Filename

#+ because of line 20.

The original version of this script terminated the "while" loop with

+ done <"$Filename"

Exercise:

Why is this unnecessary?

exec 0<&3 # Restore old stdin.

exec 3<&- # Close temporary fd 3.

echo; echo "$count names read"; echo

exit 0

Example 20-7. Redirected until loop

!/bin/bash

Same as previous example, but with "until" loop.

if [ -z "$1" ]

then

Filename=names.data # Default, if no filename specified.

else

Filename=$1

while [ "$name" != Smith ]

until [ "$name" = Smith ] # Change != to =.

read name # Reads from $Filename, rather than stdin.

echo $name

done <"$Filename" # Redirects stdin to file $Filename.

^^^^^^^^^^^^

Same results as with "while" loop in previous example.

exit 0

Example 20-8. Redirected for loop

!/bin/bash

if [ -z "$1" ]

then

Filename=names.data # Default, if no filename specified.

else

Filename=$1

line_count=`wc $Filename | awk '{ print $1 }'`

Number of lines in target file.

Very contrived and kludgy, nevertheless shows that

+ it's possible to redirect stdin within a "for" loop...

+ if you're clever enough.

More concise is line_count=$(wc -l < "$Filename")

for name in `seq $line_count` # Recall that "seq" prints sequence of numbers.

while [ "$name" != Smith ] -- more complicated than a "while" loop --

read name # Reads from $Filename, rather than stdin.

echo $name

if [ "$name" = Smith ] # Need all this extra baggage here.

then

break

done <"$Filename" # Redirects stdin to file $Filename.

^^^^^^^^^^^^

exit 0

We can modify the previous example to also redirect the output of the

loop.

Example 20-9. Redirected for loop (both stdin and stdout redirected)

!/bin/bash

if [ -z "$1" ]

then

Filename=names.data # Default, if no filename specified.

else

Filename=$1

Savefile=$Filename.new # Filename to save results in.

FinalName=Jonah # Name to terminate "read" on.

line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target fi

le.

for name in `seq $line_count`

read name

echo "$name"

if [ "$name" = "$FinalName" ]

then

break

done < "$Filename" > "$Savefile" # Redirects stdin to file $Filename,

^^^^^^^^^^^^^^^^^^^^^^^^^^^ and saves it to backup file.

exit 0

Example 20-10. Redirected if/then test

!/bin/bash

if [ -z "$1" ]

then

Filename=names.data # Default, if no filename specified.

else

Filename=$1

TRUE=1

if [ "$TRUE" ] # if true and if : also work.

then

read name

echo $name

fi <"$Filename"

^^^^^^^^^^^^

Reads only first line of file.

An "if/then" test has no way of iterating unless embedded in a loop.

exit 0

Example 20-11. Data file names.data for above examples

Aristotle

Arrhenius

Belisarius

Capablanca

Dickens

Euler

Goethe

Hegel

Jonah

Laplace

Maroczy

Purcell

Schmidt

Schopenhauer

Semmelweiss

Smith

Steinmetz

Tukhashevsky

Turing

Venn

Warshawski

Znosko-Borowski

This is a data file for

+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".

Redirecting the stdout of a code block has the effect of saving its

output to a file. See Example 3-2.

Here documents are a special case of redirected code blocks. That

being the case, it should be possible to feed the output of a here

document into the stdin for a while loop.

This example by Albert Siersema

Used with permission (thanks!).

function doesOutput()

# Could be an external command too, of course.

# Here we show you can use a function as well.

{

ls -al *.jpg | awk '{print $5,$9}'

}

nr=0 # We want the while loop to be able to manipulate these and

totalSize=0 #+ to be able to see the changes after the 'while' finished.

while read fileSize fileName ; do

echo "$fileName is $fileSize bytes"

let nr++

totalSize=$((totalSize+fileSize)) # Or: "let totalSize+=fileSize"

done<<EOF

$(doesOutput)

EOF

echo "$nr files totaling $totalSize bytes"

________________________________________________________________

20.3. Applications

Clever use of I/O redirection permits parsing and stitching together

snippets of command output (see Example 15-7). This permits

generating report and log files.

Example 20-12. Logging events

!/bin/bash

logevents.sh

Author: Stephane Chazelas.

Used in ABS Guide with permission.

Event logging to a file.

Must be run as root (for write access in /var/log).

ROOT_UID=0 # Only users with $UID 0 have root privileges.

E_NOTROOT=67 # Non-root exit error.

if [ "$UID" -ne "$ROOT_UID" ]

then

echo "Must be root to run this script."

exit $E_NOTROOT

FD_DEBUG1=3

FD_DEBUG2=4

FD_DEBUG3=5

=== Uncomment one of the two lines below to activate script. ===

LOG_EVENTS=1

LOG_VARS=1

log() # Writes time and date to log file.

{

echo "$(date) $*" >&7 # This *appends* the date to the file.

^^^^^^^ command substitution

# See below.

}

case $LOG_LEVEL in

1) exec 3>&2 4> /dev/null 5> /dev/null;;

2) exec 3>&2 4>&2 5> /dev/null;;

3) exec 3>&2 4>&2 5>&2;;

*) exec 3> /dev/null 4> /dev/null 5> /dev/null;;

esac

FD_LOGVARS=6

if [[ $LOG_VARS ]]

then exec 6>> /var/log/vars.log

else exec 6> /dev/null # Bury output.

FD_LOGEVENTS=7

if [[ $LOG_EVENTS ]]

then

# exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)

# Above line fails in versions of Bash more recent than 2.04. Why?

exec 7>> /var/log/event.log # Append to "event.log".

log # Write time and date.

else exec 7> /dev/null # Bury output.

echo "DEBUG3: beginning" >&${FD_DEBUG3}

ls -l >&5 2>&4 # command1 >&5 2>&4

echo "Done" # command2

echo "sending mail" >&${FD_LOGEVENTS}

Writes "sending mail" to file descriptor #7.

exit 0

________________________________________________________________

Chapter 21. Subshells

Running a shell script launches a new process, a subshell.

Definition: A subshell is a child process launched by a shell (or

shell script).

A subshell is a separate instance of the command processor -- the

shell that gives you the prompt at the console or in an xterm window.

Just as your commands are interpreted at the command-line prompt,

similarly does a script batch-process a list of commands. Each shell

script running is, in effect, a subprocess (child process) of the

parent shell.

A shell script can itself launch subprocesses. These subshells let

the script do parallel processing, in effect executing multiple

subtasks simultaneously.

!/bin/bash

subshell-test.sh

(

Inside parentheses, and therefore a subshell . . .

while [ 1 ] # Endless loop.

echo "Subshell running . . ."

done

)

Script will run forever,

+ or at least until terminated by a Ctl-C.

exit $? # End of script (but will never get here).

Now, run the script:

sh subshell-test.sh

And, while the script is running, from a different xterm:

ps -ef | grep subshell-test.sh

UID PID PPID C STIME TTY TIME CMD

500 2698 2502 0 14:26 pts/4 00:00:00 sh subshell-test.sh

500 2699 2698 21 14:26 pts/4 00:00:24 sh subshell-test.sh

^^^^

Analysis:

PID 2698, the script, launched PID 2699, the subshell.

Note: The "UID ..." line would be filtered out by the "grep" command,

but is shown here for illustrative purposes.

In general, an external command in a script forks off a subprocess,

[107] whereas a Bash builtin does not. For this reason, builtins

execute more quickly and use fewer system resources than their

external command equivalents.

Command List within Parentheses

( command1; command2; command3; ... )

A command list embedded between parentheses runs as a

subshell.

Variables in a subshell are not visible outside the block of code in

the subshell. They are not accessible to the parent process, to the

shell that launched the subshell. These are, in effect, variables

local to the child process.

Example 21-1. Variable scope in a subshell

!/bin/bash

subshell.sh

echo

echo "We are outside the subshell."

echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"

Bash, version 3, adds the new $BASH_SUBSHELL variable.

echo; echo

outer_variable=Outer

global_variable=

Define global variable for "storage" of

+ value of subshell variable.

(

echo "We are inside the subshell."

echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"

inner_variable=Inner

echo "From inside subshell, \"inner_variable\" = $inner_variable"

echo "From inside subshell, \"outer\" = $outer_variable"

global_variable="$inner_variable" # Will this allow "exporting"

#+ a subshell variable?

)

echo; echo

echo "We are outside the subshell."

echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"

echo

if [ -z "$inner_variable" ]

then

echo "inner_variable undefined in main body of shell"

else

echo "inner_variable defined in main body of shell"

echo "From main body of shell, \"inner_variable\" = $inner_variable"

$inner_variable will show as blank (uninitialized)

+ because variables defined in a subshell are "local variables".

Is there a remedy for this?

echo "global_variable = "$global_variable"" # Why doesn't this work?

echo

=======================================================================

Additionally ...

echo "-----------------"; echo

var=41 # Global variable.

( let "var+=1"; echo "\$var INSIDE subshell = $var" ) # 42

echo "\$var OUTSIDE subshell = $var" # 41

Variable operations inside a subshell, even to a GLOBAL variable

+ do not affect the value of the variable outside the subshell!

exit 0

Question:

--------

Once having exited a subshell,

+ is there any way to reenter that very same subshell

+ to modify or access the subshell variables?

^ ^ * nested * ^ ^

echo

echo " \$SHLVL outside subshell = $SHLVL" # 3

( echo " \$SHLVL inside subshell = $SHLVL" ) # 3 (No change!)

Directory changes made in a subshell do not carry over to the parent

shell.

Example 21-2. List User Profiles

!/bin/bash

allprofs.sh: Print all user profiles.

This script written by Heiner Steven, and modified by the document author.

FILE=.bashrc # File containing user profile,

#+ was ".profile" in original script.

for home in `awk -F: '{print $6}' /etc/passwd`

[ -d "$home" ] || continue # If no home directory, go to next.

[ -r "$home" ] || continue # If not readable, go to next.

(cd $home; [ -e $FILE ] && less $FILE)

done

When script terminates, there is no need to 'cd' back to original directory

+ because 'cd $home' takes place in a subshell.

exit 0

A subshell may be used to set up a "dedicated environment" for a

command group.

COMMAND1

COMMAND2

COMMAND3

(

IFS=:

PATH=/bin

unset TERMINFO

set -C

shift 5

COMMAND4

COMMAND5

exit 3 # Only exits the subshell!

)

The parent shell has not been affected, and the environment is preserved.

COMMAND6

COMMAND7

As seen here, the exit command only terminates the subshell in which

it is running, not the parent shell or script.

One application of such a "dedicated environment" is testing whether

a variable is defined.

if (set -u; : $variable) 2> /dev/null

then

echo "Variable is set."

fi # Variable has been set in current script,

#+ or is an an internal Bash variable,

#+ or is present in environment (has been exported).

Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]

or [[ ${variable-x} != x$variable ]]

or [[ ${variable+x} = x ]]

or [[ ${variable-x} != x ]]

Another application is checking for a lock file:

if (set -C; : > lock_file) 2> /dev/null

then

: # lock_file didn't exist: no user running the script

else

echo "Another user is already running that script."

exit 65

Code snippet by Stéphane Chazelas,

+ with modifications by Paulo Marcel Coelho Aragao.

Processes may execute in parallel within different subshells. This

permits breaking a complex task into subcomponents processed

concurrently.

Example 21-3. Running parallel processes in subshells

(cat list1 list2 list3 | sort | uniq > list123) &

(cat list4 list5 list6 | sort | uniq > list456) &

# Merges and sorts both sets of lists simultaneously.

# Running in background ensures parallel execution.

# Same effect as

# cat list1 list2 list3 | sort | uniq > list123 &

# cat list4 list5 list6 | sort | uniq > list456 &

wait # Don't execute the next command until subshells finish.

diff list123 list456

Redirecting I/O to a subshell uses the "|" pipe operator, as in ls

-al | (command).

Note

A code block between curly brackets does not launch a subshell.

{ command1; command2; command3; . . . commandN; }

var1=23

echo "$var1" # 23

{ var1=76; }

echo "$var1" # 76

________________________________________________________________

Chapter 22. Restricted Shells

Disabled commands in restricted shells

. Running a script or portion of a script in restricted mode

disables certain commands that would otherwise be available.

This is a security measure intended to limit the privileges of

the script user and to minimize possible damage from running

the script.

The following commands and actions are disabled:

* Using cd to change the working directory.

* Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV

environmental variables.

* Reading or changing the $SHELLOPTS, shell environmental options.

* Output redirection.

* Invoking commands containing one or more /'s.

* Invoking exec to substitute a different process for the shell.

* Various other commands that would enable monkeying with or

attempting to subvert the script for an unintended purpose.

* Getting out of restricted mode within the script.

Example 22-1. Running a script in restricted mode

!/bin/bash

Starting the script with "#!/bin/bash -r"

+ runs entire script in restricted mode.

echo

echo "Changing directory."

cd /usr/local

echo "Now in `pwd`"

echo "Coming back home."

echo "Now in `pwd`"

echo

Everything up to here in normal, unrestricted mode.

set -r

set --restricted has same effect.

echo "==> Now in restricted mode. <=="

echo

echo "Attempting directory change in restricted mode."

cd ..

echo "Still in `pwd`"

echo

echo "\$SHELL = $SHELL"

echo "Attempting to change shell in restricted mode."

SHELL="/bin/ash"

echo

echo "\$SHELL= $SHELL"

echo

echo "Attempting to redirect output in restricted mode."

ls -l /usr/bin > bin.files

ls -l bin.files # Try to list attempted file creation effort.

echo

exit 0

________________________________________________________________

Chapter 23. Process Substitution

Piping the stdout of a command into the stdin of another is a

powerful technique. But, what if you need to pipe the stdout of

multiple commands? This is where process substitution comes in.

Process substitution feeds the output of a process (or processes)

into the stdin of another process.

Template

Command list enclosed within parentheses

>(command_list)

<(command_list)

Process substitution uses /dev/fd/<n> files to send the

results of the process(es) within parentheses to another

process. [108]

Caution

There is no space between the the "<" or ">" and the parentheses.

Space there would give an error message.

bash$ echo >(true)

/dev/fd/63

bash$ echo <(true)

/dev/fd/63

bash$ echo >(true) <(true)

/dev/fd/63 /dev/fd/62

bash$ wc <(cat /usr/share/dict/linux.words)

483523 483523 4992010 /dev/fd/63

bash$ grep script /usr/share/dict/linux.words | wc

262 262 3601

bash$ wc <(grep script /usr/share/dict/linux.words)

262 262 3601 /dev/fd/63

Note

Bash creates a pipe with two file descriptors, --fIn and fOut--. The

stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a

/dev/fd/fIn argument to echo. On systems lacking /dev/fd/<n> files,

Bash may use temporary files. (Thanks, S.C.)

Process substitution can compare the output of two different

commands, or even the output of different options to the same

command.

bash$ comm <(ls -l) <(ls -al)

total 12

-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0

-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2

-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh

total 20

drwxrwxrwx 2 bozo bozo 4096 Mar 10 18:10 .

drwx------ 72 bozo bozo 4096 Mar 10 17:58 ..

-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0

-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2

-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh

Process substitution can compare the contents of two directories --

to see which filenames are in one, but not the other.

diff <(ls $first_directory) <(ls $second_directory)

Some other usages and uses of process substitution:

read -a list < <( od -Ad -w24 -t u2 /dev/urandom )

Read a list of random numbers from /dev/urandom,

+ process with "od"

+ and feed into stdin of "read" . . .

From "insertion-sort.bash" example script.

Courtesy of JuanJo Ciarlante.

PORT=6881 # bittorrent

Scan the port to make sure nothing nefarious is going on.

netcat -l $PORT | tee>(md5sum ->mydata-orig.md5) |

gzip | tee>(md5sum - | sed 's/-$/mydata.lz2/'>mydata-gz.md5)>mydata.gz

Check the decompression:

gzip -d<mydata.gz | md5sum -c mydata-orig.md5)

The MD5sum of the original checks stdin and detects compression issues.

Bill Davidsen contributed this example

+ (with light edits by the ABS Guide author).

cat <(ls -l)

Same as ls -l | cat

sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)

Lists all the files in the 3 main 'bin' directories, and sorts by filename.

Note that three (count 'em) distinct commands are fed to 'sort'.

diff <(command1) <(command2) # Gives difference in command output.

tar cf >(bzip2 -c > file.tar.bz2) $directory_name

Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2".

Because of the /dev/fd/<n> system feature,

the pipe between both commands does not need to be named.

This can be emulated.

bzip2 -c < pipe > file.tar.bz2&

tar cf pipe $directory_name

rm pipe

or

exec 3>&1

tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-

exec 3>&-

Thanks, Stéphane Chazelas

Here is a method of circumventing the problem of an echo piped to a

while-read loop running in a subshell.

Example 23-1. Code block redirection without forking

!/bin/bash

wr-ps.bash: while-read loop with process substitution.

This example contributed by Tomas Pospisek.

(Heavily edited by the ABS Guide author.)

echo

echo "random input" | while read i

global=3D": Not available outside the loop."

# ... because it runs in a subshell.

done

echo "\$global (from outside the subprocess) = $global"

$global (from outside the subprocess) =

echo; echo "--"; echo

while read i

echo $i

global=3D": Available outside the loop."

# ... because it does NOT run in a subshell.

done < <( echo "random input" )

^ ^

echo "\$global (using process substitution) = $global"

Random input

$global (using process substitution) = 3D: Available outside the loop.

echo; echo "##########"; echo

And likewise . . .

declare -a inloop

index=0

cat $0 | while read line

inloop[$index]="$line"

((index++))

# It runs in a subshell, so ...

done

echo "OUTPUT = "

echo ${inloop[*]} # ... nothing echoes.

echo; echo "--"; echo

declare -a outloop

index=0

while read line

outloop[$index]="$line"

((index++))

# It does NOT run in a subshell, so ...

done < <( cat $0 )

echo "OUTPUT = "

echo ${outloop[*]} # ... the entire script echoes.

exit $?

This is a similar example.

Example 23-2. Redirecting the output of process substitution into a

loop.

!/bin/bash

psub.bash

As inspired by Diego Molina (thanks!).

declare -a array0

while read

array0[${#array0[@]}]="$REPLY"

done < <( sed -e 's/bash/CRASH-BANG!/' $0 | grep bin | awk '{print $1}' )

Sets the default 'read' variable, $REPLY, by process substitution,

+ then copies it into an array.

echo "${array0[@]}"

exit $?

====================================== #

bash psub.bash

!/bin/CRASH-BANG! done #!/bin/CRASH-BANG!

A reader sent in the following interesting example of process

substitution.

Script fragment taken from SuSE distribution:

--------------------------------------------------------------#

while read des what mask iface; do

Some commands ...

done < <(route -n)

^ ^ First < is redirection, second is process substitution.

To test it, let's make it do something.

while read des what mask iface; do

echo $des $what $mask $iface

done < <(route -n)

Output:

Kernel IP routing table

Destination Gateway Genmask Flags Metric Ref Use Iface

127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo

--------------------------------------------------------------#

As Stéphane Chazelas points out,

+ an easier-to-understand equivalent is:

route -n |

while read des what mask iface; do # Variables set from output of pipe.

echo $des $what $mask $iface

done # This yields the same output as above.

# However, as Ulrich Gayer points out . . .

#+ this simplified equivalent uses a subshell for the while loop,

#+ and therefore the variables disappear when the pipe terminates.

--------------------------------------------------------------#

However, Filip Moritz comments that there is a subtle difference

+ between the above two examples, as the following shows.

(

route -n | while read x; do ((y++)); done

echo $y # $y is still unset

while read x; do ((y++)); done < <(route -n)

echo $y # $y has the number of lines of output of route -n

)

More generally spoken

(

: | x=x

seems to start a subshell like

: | ( x=x )

while

x=x < <(:)

does not

)

This is useful, when parsing csv and the like.

That is, in effect, what the original SuSE code fragment does.

________________________________________________________________

Chapter 24. Functions

Like "real" programming languages, Bash has functions, though in a

somewhat limited implementation. A function is a subroutine, a code

block that implements a set of operations, a "black box" that

performs a specified task. Wherever there is repetitive code, when a

task repeats with only slight variations in procedure, then consider

using a function.

function function_name {

command...

}

function_name () {

command...

}

This second form will cheer the hearts of C programmers (and is more

portable).

As in C, the function's opening bracket may optionally appear on the

second line.

function_name ()

{

command...

}

Note

A function may be "compacted" into a single line.

fun () { echo "This is a function"; echo; }

^ ^

In this case, however, a semicolon must follow the final command in

the function.

fun () { echo "This is a function"; echo } # Error!

^

fun2 () { echo "Even a single-command function? Yes!"; }

^

Functions are called, triggered, simply by invoking their names. A

function call is equivalent to a command.

Example 24-1. Simple functions

!/bin/bash

ex59.sh: Exercising functions (simple).

JUST_A_SECOND=1

funky ()

{ # This is about as simple as functions get.

echo "This is a funky function."

echo "Now exiting funky function."

} # Function declaration must precede call.

fun ()

{ # A somewhat more complex function.

i=0

REPEATS=30

echo

echo "And now the fun really begins."

echo

sleep $JUST_A_SECOND # Hey, wait a second!

while [ $i -lt $REPEATS ]

echo "----------FUNCTIONS---------->"

echo "<------------ARE-------------"

echo "<------------FUN------------>"

echo

let "i+=1"

done

}

# Now, call the functions.

funky

fun

exit $?

The function definition must precede the first call to it. There is

no method of "declaring" the function, as, for example, in C.

Will give an error message, since function "f1" not yet defined.

declare -f f1 # This doesn't help either.

f1 # Still an error message.

However...

f1 ()

{

echo "Calling function \"f2\" from within function \"f1\"."

}

f2 ()

{

echo "Function \"f2\"."

}

f1 # Function "f2" is not actually called until this point,

#+ although it is referenced before its definition.

# This is permissible.

# Thanks, S.C.

Note

Functions may not be empty!

!/bin/bash

empty-function.sh

empty ()

{

}

exit 0 # Will not exit here!

$ sh empty-function.sh

empty-function.sh: line 6: syntax error near unexpected token `}'

empty-function.sh: line 6: `}'

$ echo $?

2 Note that a function containing only comments is empty.

func ()

{

# Comment 1.

# Comment 2.

# This is still an empty function.

# Thank you, Mark Bova, for pointing this out.

}

Results in same error message as above.

However ...

not_quite_empty ()

{

illegal_command

} # A script containing this function will *not* bomb

#+ as long as the function is not called.

not_empty ()

{

} # Contains a : (null command), and this is okay.

Thank you, Dominick Geyer and Thiemo Kellner.

It is even possible to nest a function within another function,

although this is not very useful.

f1 ()

{

f2 () # nested

{

echo "Function \"f2\", inside \"f1\"."

}

f2 # Gives an error message.

# Even a preceding "declare -f f2" wouldn't help.

echo

f1 # Does nothing, since calling "f1" does not automatically call "f2".

f2 # Now, it's all right to call "f2",

#+ since its definition has been made visible by calling "f1".

# Thanks, S.C.

Function declarations can appear in unlikely places, even where a

command would otherwise go.

ls -l | foo() { echo "foo"; } # Permissible, but useless.

if [ "$USER" = bozo ]

then

bozo_greet () # Function definition embedded in an if/then construct.

{

echo "Hello, Bozo."

}

bozo_greet # Works only for Bozo, and other users get an error.

Something like this might be useful in some contexts.

NO_EXIT=1 # Will enable function definition below.

[[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-

list".

If $NO_EXIT is 1, declares "exit ()".

This disables the "exit" builtin by aliasing it to "true".

exit # Invokes "exit ()" function, not "exit" builtin.

Or, similarly:

filename=file1

[ -f "$filename" ] &&

foo () { rm -f "$filename"; echo "File "$filename" deleted."; } ||

foo () { echo "File "$filename" not found."; touch bar; }

foo

Thanks, S.C. and Christopher Head

Function names can take strange forms.

_(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; }

^^^ No space between function name and parentheses.

This doesn't always work. Why not?

Now, let's invoke the function.

_ # __________

^^^^^^^^^^ 10 underscores (10 x function name)!

A "naked" underscore is an acceptable function name.

In fact, a colon is likewise an acceptable function name.

:(){ echo ":"; }; :

Of what use is this?

It's a devious way to obfuscate the code in a script.

As Yan Chen points out,

when a function is defined multiple times,

the final version is what is invoked.

This is not, however, particularly useful.

func ()

{

echo "First version of func ()."

}

func ()

{

echo "Second version of func ()."

}

func # Second version of func ().

exit $?

It is even possible to use functions to override

+ or preempt system commands.

Of course, this is not advisable.

________________________________________________________________

24.1. Complex Functions and Function Complexities

Functions may process arguments passed to them and return an exit

status to the script for further processing.

function_name $arg1 $arg2

The function refers to the passed arguments by position (as if they

were positional parameters), that is, $1, $2, and so forth.

Example 24-2. Function Taking Parameters

!/bin/bash

Functions and parameters

DEFAULT=default # Default param value.

func2 () {

if [ -z "$1" ] # Is parameter #1 zero length?

then

echo "-Parameter #1 is zero length.-" # Or no parameter passed.

else

echo "-Parameter #1 is \"$1\".-"

variable=${1-$DEFAULT} # What does

echo "variable = $variable" #+ parameter substitution show?

# ---------------------------

# It distinguishes between

#+ no param and a null param.

if [ "$2" ]

then

echo "-Parameter #2 is \"$2\".-"

return 0

}

echo

echo "Nothing passed."

func2 # Called with no params

echo

echo "Zero-length parameter passed."

func2 "" # Called with zero-length param

echo

echo "Null parameter passed."

func2 "$uninitialized_param" # Called with uninitialized param

echo

echo "One parameter passed."

func2 first # Called with one param

echo

echo "Two parameters passed."

func2 first second # Called with two params

echo

echo "\"\" \"second\" passed."

func2 "" second # Called with zero-length first parameter

echo # and ASCII string as a second one.

exit 0

Important

The shift command works on arguments passed to functions (see Example

36-18).

But, what about command-line arguments passed to the script? Does a

function see them? Well, let's clear up the confusion.

Example 24-3. Functions and command-line args passed to the script

!/bin/bash

func-cmdlinearg.sh

Call this script with a command-line argument,

+ something like $0 arg1.

func ()

{

echo "$1" # Echoes first arg passed to the function.

} # Does a command-line arg qualify?

echo "First call to function: no arg passed."

echo "See if command-line arg is seen."

func

No! Command-line arg not seen.

echo "============================================================"

echo

echo "Second call to function: command-line arg passed explicitly."

func $1

Now it's seen!

exit 0

In contrast to certain other programming languages, shell scripts

normally pass only value parameters to functions. Variable names

(which are actually pointers), if passed as parameters to functions,

will be treated as string literals. Functions interpret their

arguments literally.

Indirect variable references (see Example 37-2) provide a clumsy sort

of mechanism for passing variable pointers to functions.

Example 24-4. Passing an indirect reference to a function

!/bin/bash

ind-func.sh: Passing an indirect reference to a function.

echo_var ()

{

echo "$1"

}

message=Hello

Hello=Goodbye

echo_var "$message" # Hello

Now, let's pass an indirect reference to the function.

echo_var "${!message}" # Goodbye

echo "-------------"

What happens if we change the contents of "hello" variable?

Hello="Hello, again!"

echo_var "$message" # Hello

echo_var "${!message}" # Hello, again!

exit 0

The next logical question is whether parameters can be dereferenced

after being passed to a function.

Example 24-5. Dereferencing a parameter passed to a function

!/bin/bash

dereference.sh

Dereferencing parameter passed to a function.

Script by Bruce W. Clare.

dereference ()

{

y=\$"$1" # Name of variable (not value!).

echo $y # $Junk

x=`eval "expr \"$y\" "`

echo $1=$x

eval "$1=\"Some Different Text \"" # Assign new value.

}

Junk="Some Text"

echo $Junk "before" # Some Text before

dereference Junk

echo $Junk "after" # Some Different Text after

exit 0

Example 24-6. Again, dereferencing a parameter passed to a function

!/bin/bash

ref-params.sh: Dereferencing a parameter passed to a function.

(Complex Example)

ITERATIONS=3 # How many times to get input.

icount=1

my_read () {

# Called with my_read varname,

#+ outputs the previous value between brackets as the default value,

#+ then asks for a new value.

local local_var

echo -n "Enter a value "

eval 'echo -n "[ $1'] "' # Previous value.

eval echo -n "[\$1] " # Easier to understand,

#+ but loses trailing space in user prompt.

read local_var

[ -n "$local_var" ] && eval $1=\$local_var

# "And-list": if "local_var" then set "$1" to its value.

}

echo

while [ "$icount" -le "$ITERATIONS" ]

my_read var

echo "Entry #$icount = $var"

let "icount += 1"

echo

done

Thanks to Stephane Chazelas for providing this instructive example.

exit 0

Exit and Return

exit status

Functions return a value, called an exit status. This is

analogous to the exit status returned by a command. The exit

status may be explicitly specified by a return statement,

otherwise it is the exit status of the last command in the

function (0 if successful, and a non-zero error code if not).

This exit status may be used in the script by referencing it

as $?. This mechanism effectively permits script functions to

have a "return value" similar to C functions.

return

Terminates a function. A return command [109] optionally takes

an integer argument, which is returned to the calling script

as the "exit status" of the function, and this exit status is

assigned to the variable $?.

Example 24-7. Maximum of two numbers

!/bin/bash

max.sh: Maximum of two integers.

E_PARAM_ERR=250 # If less than 2 params passed to function.

EQUAL=251 # Return value if both params equal.

Error values out of range of any

+ params that might be fed to the function.

max2 () # Returns larger of two numbers.

{ # Note: numbers compared must be less than 250.

if [ -z "$2" ]

then

return $E_PARAM_ERR

if [ "$1" -eq "$2" ]

then

return $EQUAL

else

if [ "$1" -gt "$2" ]

then

return $1

else

return $2

}

max2 33 34

return_val=$?

if [ "$return_val" -eq $E_PARAM_ERR ]

then

echo "Need to pass two parameters to the function."

elif [ "$return_val" -eq $EQUAL ]

then

echo "The two numbers are equal."

else

echo "The larger of the two numbers is $return_val."

exit 0

Exercise (easy):

---------------

Convert this to an interactive script,

+ that is, have the script ask for input (two numbers).

Tip

For a function to return a string or array, use a dedicated variable.

count_lines_in_etc_passwd()

{

[[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd))

# If /etc/passwd is readable, set REPLY to line count.

# Returns both a parameter value and status information.

# The 'echo' seems unnecessary, but . . .

#+ it removes excess whitespace from the output.

}

if count_lines_in_etc_passwd

then

echo "There are $REPLY lines in /etc/passwd."

else

echo "Cannot count lines in /etc/passwd."

Thanks, S.C.

Example 24-8. Converting numbers to Roman numerals

!/bin/bash

Arabic number to Roman numeral conversion

Range: 0 - 200

It's crude, but it works.

Extending the range and otherwise improving the script is left as an exercis

Usage: roman number-to-convert

LIMIT=200

E_ARG_ERR=65

E_OUT_OF_RANGE=66

if [ -z "$1" ]

then

echo "Usage: `basename $0` number-to-convert"

exit $E_ARG_ERR

num=$1

if [ "$num" -gt $LIMIT ]

then

echo "Out of range!"

exit $E_OUT_OF_RANGE

to_roman () # Must declare function before first call to it.

{

number=$1

factor=$2

rchar=$3

let "remainder = number - factor"

while [ "$remainder" -ge 0 ]

echo -n $rchar

let "number -= factor"

let "remainder = number - factor"

done

return $number

# Exercises:

# ---------

# 1) Explain how this function works.

# Hint: division by successive subtraction.

# 2) Extend to range of the function.

# Hint: use "echo" and command-substitution capture.

}

to_roman $num 100 C

num=$?

to_roman $num 90 LXXXX

num=$?

to_roman $num 50 L

num=$?

to_roman $num 40 XL

num=$?

to_roman $num 10 X

num=$?

to_roman $num 9 IX

num=$?

to_roman $num 5 V

num=$?

to_roman $num 4 IV

num=$?

to_roman $num 1 I

Successive calls to conversion function!

Is this really necessary??? Can it be simplified?

echo

exit

!/bin/bash

return-test.sh

The largest positive value a function can return is 255.

return_test () # Returns whatever passed to it.

{

return $1

}

return_test 27 # o.k.

echo $? # Returns 27.

return_test 255 # Still o.k.

echo $? # Returns 255.

return_test 257 # Error!

echo $? # Returns 1 (return code for miscellaneous error).

=========================================================

return_test -151896 # Do large negative numbers work?

echo $? # Will this return -151896?

# No! It returns 168.

Version of Bash before 2.05b permitted

+ large negative integer return values.

It happened to be a useful feature.

Newer versions of Bash unfortunately plug this loophole.

This may break older scripts.

Caution!

=========================================================

exit 0

A workaround for obtaining large integer "return values" is to simply

assign the "return value" to a global variable.

Return_Val= # Global variable to hold oversize return value of function.

alt_return_test ()

{

fvar=$1

Return_Val=$fvar

return # Returns 0 (success).

}

alt_return_test 1

echo $? # 0

echo "return value = $Return_Val" # 1

alt_return_test 256

echo "return value = $Return_Val" # 256

alt_return_test 257

echo "return value = $Return_Val" # 257

alt_return_test 25701

echo "return value = $Return_Val" #25701

A more elegant method is to have the function echo its "return value

to stdout," and then capture it by command substitution. See the

discussion of this in Section 36.7.

Example 24-10. Comparing two large integers

!/bin/bash

max2.sh: Maximum of two LARGE integers.

This is the previous "max.sh" example,

+ modified to permit comparing large integers.

EQUAL=0 # Return value if both params equal.

E_PARAM_ERR=-99999 # Not enough params passed to function.

^^^^^^ Out of range of any params that might be passed.

max2 () # "Returns" larger of two numbers.

{

if [ -z "$2" ]

then

echo $E_PARAM_ERR

return

if [ "$1" -eq "$2" ]

then

echo $EQUAL

return

else

if [ "$1" -gt "$2" ]

then

retval=$1

else

retval=$2

echo $retval # Echoes (to stdout), rather than returning value.

# Why?

}

return_val=$(max2 33001 33997)

^^^^ Function name

^^^^^ ^^^^^ Params passed

This is actually a form of command substitution:

+ treating a function as if it were a command,

+ and assigning the stdout of the function to the variable "return_val."

========================= OUTPUT ========================

if [ "$return_val" -eq "$E_PARAM_ERR" ]

then

echo "Error in parameters passed to comparison function!"

elif [ "$return_val" -eq "$EQUAL" ]

then

echo "The two numbers are equal."

else

echo "The larger of the two numbers is $return_val."

=========================================================

exit 0

Exercises:

---------

1) Find a more elegant way of testing

+ the parameters passed to the function.

2) Simplify the if/then structure at "OUTPUT."

3) Rewrite the script to take input from command-line parameters.

Here is another example of capturing a function "return value."

Understanding it requires some knowledge of awk.

month_length () # Takes month number as an argument.

{ # Returns number of days in month.

monthD="31 28 31 30 31 30 31 31 30 31 30 31" # Declare as local?

echo "$monthD" | awk '{ print "${1}"' }' # Tricky.

^^^^^^^^^

Parameter passed to function ($1 -- month number), then to awk.

Awk sees this as "print $1 . . . print $12" (depending on month number)

Template for passing a parameter to embedded awk script:

"${script_parameter}"'

Here's a slightly simpler awk construct:

echo $monthD | awk -v month=$1 '{print $(month)}'

Uses the -v awk option, which assigns a variable value

+ prior to execution of the awk program block.

Thank you, Rich.

Needs error checking for correct parameter range (1-12)

+ and for February in leap year.

}

----------------------------------------------

Usage example:

month=4 # April, for example (4th month).

days_in=$(month_length $month)

echo $days_in # 30

----------------------------------------------

!/bin/bash

realname.sh

From username, gets "real name" from /etc/passwd.

ARGCOUNT=1 # Expect one arg.

E_WRONGARGS=85

file=/etc/passwd

pattern=$1

if [ $# -ne "$ARGCOUNT" ]

then

echo "Usage: `basename $0` USERNAME"

exit $E_WRONGARGS

file_excerpt () # Scan file for pattern,

{ #+ then print relevant portion of line.

while read line # "while" does not necessarily need [ condition ]

echo "$line" | grep $1 | awk -F":" '{ print $5 }'

# Have awk use ":" delimiter.

done

} <$file # Redirect into function's stdin.

file_excerpt $pattern

Yes, this entire script could be reduced to

grep PATTERN /etc/passwd | awk -F":" '{ print $5 }'

or

awk -F: '/PATTERN/ {print $5}'

or

awk -F: '($1 == "username") { print $5 }' # real name from username

However, it might not be as instructive.

exit 0

There is an alternate, and perhaps less confusing method of

redirecting a function's stdin. This involves redirecting the

stdin to an embedded bracketed code block within the function.

Instead of:

Function ()

{

...

} < file

Try this:

Function ()

{

...

} < file

}

Similarly,

Function () # This works.

{

echo $*

} | tr a b

}

Function () # This doesn't work.

{

echo $*

} | tr a b # A nested code block is mandatory here.

Thanks, S.C.

Note

Emmanuel Rouat's sample bashrc file contains some instructive

examples of functions.

________________________________________________________________

24.2. Local Variables

What makes a variable local?

local variables

A variable declared as local is one that is visible only

within the block of code in which it appears. It has local

scope. In a function, a local variable has meaning only within

that function block. [110]

Example 24-12. Local variable visibility

!/bin/bash

ex62.sh: Global and local variables inside a function.

func ()

{

local loc_var=23 # Declared as local variable.

echo # Uses the 'local' builtin.

echo "\"loc_var\" in function = $loc_var"

global_var=999 # Not declared as local.

# Therefore, defaults to global.

echo "\"global_var\" in function = $global_var"

}

func

Now, to see if local variable "loc_var" exists outside the function.

echo

echo "\"loc_var\" outside function = $loc_var"

# $loc_var outside function =

# No, $loc_var not visible globally.

echo "\"global_var\" outside function = $global_var"

# $global_var outside function = 999

# $global_var is visible globally.

echo

exit 0

In contrast to C, a Bash variable declared inside a function

+ is local ONLY if declared as such.

Caution

Before a function is called, all variables declared within the

function are invisible outside the body of the function, not just

those explicitly declared as local.

!/bin/bash

func ()

{

global_var=37 # Visible only within the function block

#+ before the function has been called.

} # END OF FUNCTION

echo "global_var = $global_var" # global_var =

# Function "func" has not yet been called,

#+ so $global_var is not visible here.

func

echo "global_var = $global_var" # global_var = 37

# Has been set by function call.

Note

As Evgeniy Ivanov points out, when declaring and setting a local

variable in a single command, apparently the order of operations is

to first set the variable, and only afterwards restrict it to local

scope. This is reflected in the return value.

!/bin/bash

echo "==OUTSIDE Function (global)=="

t=$(exit 1)

echo $? # 1

# As expected.

echo

function0 ()

{

echo "==INSIDE Function=="

echo "Global"

t0=$(exit 1)

echo $? # 1

# As expected.

echo

echo "Local declared & assigned in same command."

local t1=$(exit 1)

echo $? # 0

# Unexpected!

Apparently, the variable assignment takes place before

+ the local declaration.

+ The return value is for the latter.

echo

echo "Local declared, then assigned (separate commands)."

local t2

t2=$(exit 1)

echo $? # 1

# As expected.

}

function0

________________________________________________________________

24.2.1. Local variables and recursion.

Recursion is an interesting and sometimes useful form of

self-reference. Herbert Mayer defines it as ". . . expressing an

algorithm by using a simpler version of that same algorithm . . ."

Consider a definition defined in terms of itself, [111] an expression

implicit in its own expression, [112] a snake swallowing its own

tail, [113] or . . . a function that calls itself. [114]

Example 24-13. Demonstration of a simple recursive function

!/bin/bash

recursion-demo.sh

Demonstration of recursion.

RECURSIONS=9 # How many times to recurse.

r_count=0 # Must be global. Why?

recurse ()

{

var="$1"

while [ "$var" -ge 0 ]

echo "Recursion count = "$r_count" +-+ \$var = "$var""

(( var-- )); (( r_count++ ))

recurse "$var" # Function calls itself (recurses)

done #+ until what condition is met?

}

recurse $RECURSIONS

exit $?

Example 24-14. Another simple demonstration

!/bin/bash

recursion-def.sh

A script that defines "recursion" in a rather graphic way.

RECURSIONS=10

r_count=0

sp=" "

define_recursion ()

{

((r_count++))

sp="$sp"" "

echo -n "$sp"

echo "\"The act of recurring ... \"" # Per 1913 Webster's dictionary.

while [ $r_count -le $RECURSIONS ]

define_recursion

done

}

echo

echo "Recursion: "

define_recursion

echo

exit $?

Local variables are a useful tool for writing recursive code, but

this practice generally involves a great deal of computational

overhead and is definitely not recommended in a shell script. [115]

Example 24-15. Recursion, using a local variable

!/bin/bash

factorial

---------

Does bash permit recursion?

Well, yes, but...

It's so slow that you gotta have rocks in your head to try it.

MAX_ARG=5

E_WRONG_ARGS=85

E_RANGE_ERR=86

if [ -z "$1" ]

then

echo "Usage: `basename $0` number"

exit $E_WRONG_ARGS

if [ "$1" -gt $MAX_ARG ]

then

echo "Out of range ($MAX_ARG is maximum)."

# Let's get real now.

# If you want greater range than this,

#+ rewrite it in a Real Programming Language.

exit $E_RANGE_ERR

fact ()

{

local number=$1

# Variable "number" must be declared as local,

#+ otherwise this doesn't work.

if [ "$number" -eq 0 ]

then

factorial=1 # Factorial of 0 = 1.

else

let "decrnum = number - 1"

fact $decrnum # Recursive function call (the function calls itself).

let "factorial = $number * $?"

return $factorial

}

fact $1

echo "Factorial of $1 is $?."

exit 0

Also see Example A-15 for an example of recursion in a script. Be

aware that recursion is resource-intensive and executes slowly, and

is therefore generally not appropriate in a script.

________________________________________________________________

24.3. Recursion Without Local Variables

A function may recursively call itself even without use of local

variables.

Example 24-16. The Fibonacci Sequence

!/bin/bash

fibo.sh : Fibonacci sequence (recursive)

Author: M. Cooper

License: GPL3

----------algorithm--------------

Fibo(0) = 0

Fibo(1) = 1

else

Fibo(j) = Fibo(j-1) + Fibo(j-2)

---------------------------------

MAXTERM=15 # Number of terms (+1) to generate.

MINIDX=2 # If idx is less than 2, then Fibo(idx) = idx.

Fibonacci ()

{

idx=$1 # Doesn't need to be local. Why not?

if [ "$idx" -lt "$MINIDX" ]

then

echo "$idx" # First two terms are 0 1 ... see above.

else

(( --idx )) # j-1

term1=$( Fibonacci $idx ) # Fibo(j-1)

(( --idx )) # j-2

term2=$( Fibonacci $idx ) # Fibo(j-2)

echo $(( term1 + term2 ))

# An ugly, ugly kludge.

# The more elegant implementation of recursive fibo in C

#+ is a straightforward translation of the algorithm in lines 7 - 10.

}

for i in $(seq 0 $MAXTERM)

do # Calculate $MAXTERM+1 terms.

FIBO=$(Fibonacci $i)

echo -n "$FIBO "

done

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610

Takes a while, doesn't it? Recursion in a script is slow.

echo

exit 0

Example 24-17. The Towers of Hanoi

! /bin/bash

The Towers Of Hanoi

Bash script

Copyright (C) 2000 Amit Singh. All Rights Reserved.

http://hanoi.kernelthread.com

Tested under Bash version 2.05b.0(13)-release.

Also works under Bash version 3.x.

Used in "Advanced Bash Scripting Guide"

+ with permission of script author.

Slightly modified and commented by ABS author.

=================================================================#

The Tower of Hanoi is a mathematical puzzle attributed to

+ Edouard Lucas, a nineteenth-century French mathematician.

There are three vertical posts set in a base.

The first post has a set of annular rings stacked on it.

These rings are disks with a hole drilled out of the center,

+ so they can slip over the posts and rest flat.

The rings have different diameters, and they stack in ascending

+ order, according to size.

The smallest ring is on top, and the largest on the bottom.

The task is to transfer the stack of rings

+ to one of the other posts.

You can move only one ring at a time to another post.

You are permitted to move rings back to the original post.

You may place a smaller ring atop a larger one,

+ but not vice versa.

Again, it is forbidden to place a larger ring atop a smaller one.

For a small number of rings, only a few moves are required.

+ For each additional ring,

+ the required number of moves approximately doubles,

+ and the "strategy" becomes increasingly complicated.

For more information, see http://hanoi.kernelthread.com

+ or pp. 186-92 of _The Armchair Universe_ by A.K. Dewdney.

... ... ...

| | | | | |

_|_|_ | | | |

|_____| | | | |

|_______| | | | |

|_________| | | | |

|___________| | | | |

| | | | | |

.--------------------------------------------------------------.

|**************************************************************|

#1 #2 #3

=================================================================#

E_NOPARAM=66 # No parameter passed to script.

E_BADPARAM=67 # Illegal number of disks passed to script.

Moves= # Global variable holding number of moves.

# Modification to original script.

dohanoi() { # Recursive function.

case $1 in

;;

dohanoi "$(($1-1))" $2 $4 $3

echo move $2 "-->" $3

((Moves++)) # Modification to original script.

dohanoi "$(($1-1))" $4 $3 $2

;;

esac

}

case $# in

1) case $(($1>0)) in # Must have at least one disk.

1) # Nested case statement.

dohanoi $1 1 3 2

echo "Total moves = $Moves" # 2^n - 1, where n = # of disks.

exit 0;

;;

echo "$0: illegal value for number of disks";

exit $E_BADPARAM;

;;

esac

;;

echo "usage: $0 N"

echo " Where \"N\" is the number of disks."

exit $E_NOPARAM;

;;

esac

Exercises:

---------

1) Would commands beyond this point ever be executed?

Why not? (Easy)

2) Explain the workings of the workings of the "dohanoi" function.

(Difficult -- see the Dewdney reference, above.)

________________________________________________________________

Chapter 25. Aliases

A Bash alias is essentially nothing more than a keyboard shortcut, an

abbreviation, a means of avoiding typing a long command sequence. If,

for example, we include alias lm="ls -l | more" in the ~/.bashrc

file, then each lm [116] typed at the command-line will automatically

be replaced by a ls -l | more. This can save a great deal of typing

at the command-line and avoid having to remember complex combinations

of commands and options. Setting alias rm="rm -i" (interactive mode

delete) may save a good deal of grief, since it can prevent

inadvertently deleting important files.

In a script, aliases have very limited usefulness. It would be nice

if aliases could assume some of the functionality of the C

preprocessor, such as macro expansion, but unfortunately Bash does

not expand arguments within the alias body. [117] Moreover, a script

fails to expand an alias itself within "compound constructs," such as

if/then statements, loops, and functions. An added limitation is that

an alias will not expand recursively. Almost invariably, whatever we

would like an alias to do could be accomplished much more effectively

with a function.

Example 25-1. Aliases within a script

!/bin/bash

alias.sh

shopt -s expand_aliases

Must set this option, else script will not expand aliases.

First, some fun.

alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob

Hope."'

Jesse_James

echo; echo; echo;

alias ll="ls -l"

May use either single (') or double (") quotes to define an alias.

echo "Trying aliased \"ll\":"

ll /usr/X11R6/bin/mk* #* Alias works.

echo

directory=/usr/X11R6/bin/

prefix=mk* # See if wild card causes problems.

echo "Variables \"directory\" + \"prefix\" = $directory$prefix"

echo

alias lll="ls -l $directory$prefix"

echo "Trying aliased \"lll\":"

lll # Long listing of all files in /usr/X11R6/bin stating with mk.

An alias can handle concatenated variables -- including wild card -- o.k.

TRUE=1

echo

if [ TRUE ]

then

alias rr="ls -l"

echo "Trying aliased \"rr\" within if/then statement:"

rr /usr/X11R6/bin/mk* #* Error message results!

# Aliases not expanded within compound statements.

echo "However, previously expanded alias still recognized:"

ll /usr/X11R6/bin/mk*

echo

count=0

while [ $count -lt 3 ]

alias rrr="ls -l"

echo "Trying aliased \"rrr\" within \"while\" loop:"

rrr /usr/X11R6/bin/mk* #* Alias will not expand here either.

# alias.sh: line 57: rrr: command not found

let count+=1

done

echo; echo

alias xyz='cat $0' # Script lists itself.

# Note strong quotes.

xyz

This seems to work,

+ although the Bash documentation suggests that it shouldn't.

However, as Steve Jacobson points out,

+ the "$0" parameter expands immediately upon declaration of the alias.

exit 0

The unalias command removes a previously set alias.

Example 25-2. unalias: Setting and unsetting an alias

!/bin/bash

unalias.sh

shopt -s expand_aliases # Enables alias expansion.

alias llm='ls -al | more'

llm

echo

unalias llm # Unset alias.

llm

Error message results, since 'llm' no longer recognized.

exit 0

bash$ ./unalias.sh

total 6

drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 .

drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 ..

-rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh

./unalias.sh: llm: command not found

________________________________________________________________

Chapter 26. List Constructs

The and list and or list constructs provide a means of processing a

number of commands consecutively. These can effectively replace

complex nested if/then or even case statements.

Chaining together commands

and list

command-1 && command-2 && command-3 && ... command-n

Each command executes in turn, provided that the previous

command has given a return value of true (zero). At the first

false (non-zero) return, the command chain terminates (the

first command returning false is the last one to execute).

An interesting use of a two-condition and list from an early

version of YongYe's Tetris game script:

equation()

{ # core algorithm used for doubling and halving the coordinates

[[ ${cdx} ]] && ((y=cy+(ccy-cdy)${2}2))

eval ${1}+=\"${x} ${y} \"

}

Example 26-1. Using an and list to test for command-line

arguments

!/bin/bash

and list

if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && \

^^ ^^ ^^

echo "Argument #2 = $2"

then

echo "At least 2 arguments passed to script."

# All the chained commands return true.

else

echo "Fewer than 2 arguments passed to script."

# At least one of the chained commands returns false.

Note that "if [ ! -z $1 ]" works, but its alleged equivalent,

"if [ -n $1 ]" does not.

However, quoting fixes this.

if "[ -n "$1" ]" works.

^ ^ Careful!

It is always best to QUOTE the variables being tested.

This accomplishes the same thing, using "pure" if/then statements.

if [ ! -z "$1" ]

then

echo "Argument #1 = $1"

if [ ! -z "$2" ]

then

echo "Argument #2 = $2"

echo "At least 2 arguments passed to script."

else

echo "Fewer than 2 arguments passed to script."

It's longer and more ponderous than using an "and list".

exit $?

Example 26-2. Another command-line arg test using an and list

!/bin/bash

ARGS=1 # Number of arguments expected.

E_BADARGS=85 # Exit value if incorrect number of args passed.

test $# -ne $ARGS && \

^^^^^^^^^^^^ condition #1

echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS

^^

If condition #1 tests true (wrong number of args passed to script),

+ then the rest of the line executes, and script terminates.

Line below executes only if the above test fails.

echo "Correct number of arguments passed to this script."

exit 0

To check exit value, do a "echo $?" after script termination.

Of course, an and list can also set variables to a default

value.

arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT

# Set $arg1 to command-line arguments, if any.

# But . . . set to DEFAULT if not specified on command-line.

or list

command-1 || command-2 || command-3 || ... command-n

Each command executes in turn for as long as the previous

command returns false. At the first true return, the command

chain terminates (the first command returning true is the last

one to execute). This is obviously the inverse of the "and

list".

Example 26-3. Using or lists in combination with an and list

!/bin/bash

delete.sh, a not-so-cunning file deletion utility.

Usage: delete filename

E_BADARGS=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS # No arg? Bail out.

else

file=$1 # Set filename.

[ ! -f "$file" ] && echo "File \"$file\" not found. \

Cowardly refusing to delete a nonexistent file."

AND LIST, to give error message if file not present.

Note echo message continuing on to a second line after an escape.

[ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.")

OR LIST, to delete file if present.

Note logic inversion above.

AND LIST executes on true, OR LIST on false.

exit $?

Caution

If the first command in an or list returns true, it will execute.

==> The following snippets from the /etc/rc.d/init.d/single

+==> script by Miquel van Smoorenburg

+==> illustrate use of "and" and "or" lists.

==> "Arrowed" comments added by document author.

[ -x /usr/bin/clear ] && /usr/bin/clear

# ==> If /usr/bin/clear exists, then invoke it.

# ==> Checking for the existence of a command before calling it

#+==> avoids error messages and other awkward consequences.

# ==> . . .

If they want to run something in single user mode, might as well run it...

for i in /etc/rc1.d/S[0-9][0-9]* ; do

# Check if the script is there.

[ -x "$i" ] || continue

# ==> If corresponding file in $PWD *not* found,

#+==> then "continue" by jumping to the top of the loop.

# Reject backup files and files generated by rpm.

case "$1" in

*.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)

continue;;

esac

[ "$i" = "/etc/rc1.d/S00single" ] && continue

# ==> Set script name, but don't execute it yet.

$i start

done

# ==> . . .

Important

The exit status of an and list or an or list is the exit status of

the last command executed.

Clever combinations of and and or lists are possible, but the logic

may easily become convoluted and require close attention to operator

precedence rules, and possibly extensive debugging.

false && true || echo false # false

Same result as

( false && true ) || echo false # false

But NOT

false && ( true || echo false ) # (nothing echoed)

Note left-to-right grouping and evaluation of statements.

It's usually best to avoid such complexities.

Thanks, S.C.

See Example A-7 and Example 7-4 for illustrations of using and / or

list constructs to test variables.

________________________________________________________________

Chapter 27. Arrays

Newer versions of Bash support one-dimensional arrays. Array elements

may be initialized with the variable[xx] notation. Alternatively, a

script may introduce the entire array by an explicit declare -a

variable statement. To dereference (retrieve the contents of) an

array element, use curly bracket notation, that is, ${element[xx]}.

Example 27-1. Simple array usage

!/bin/bash

area[11]=23

area[13]=37

area[51]=UFOs

Array members need not be consecutive or contiguous.

Some members of the array can be left uninitialized.

Gaps in the array are okay.

In fact, arrays with sparse data ("sparse arrays")

+ are useful in spreadsheet-processing software.

echo -n "area[11] = "

echo ${area[11]} # {curly brackets} needed.

echo -n "area[13] = "

echo ${area[13]}

echo "Contents of area[51] are ${area[51]}."

Contents of uninitialized array variable print blank (null variable).

echo -n "area[43] = "

echo ${area[43]}

echo "(area[43] unassigned)"

echo

Sum of two array variables assigned to third

area[5]=`expr ${area[11]} + ${area[13]}`

echo "area[5] = area[11] + area[13]"

echo -n "area[5] = "

echo ${area[5]}

area[6]=`expr ${area[11]} + ${area[51]}`

echo "area[6] = area[11] + area[51]"

echo -n "area[6] = "

echo ${area[6]}

This fails because adding an integer to a string is not permitted.

echo; echo; echo

-----------------------------------------------------------------

Another array, "area2".

Another way of assigning array variables...

array_name=( XXX YYY ZZZ ... )

area2=( zero one two three four )

echo -n "area2[0] = "

echo ${area2[0]}

Aha, zero-based indexing (first element of array is [0], not [1]).

echo -n "area2[1] = "

echo ${area2[1]} # [1] is second element of array.

-----------------------------------------------------------------

echo; echo; echo

-----------------------------------------------

Yet another array, "area3".

Yet another way of assigning array variables...

array_name=([xx]=XXX [yy]=YYY ...)

area3=([17]=seventeen [24]=twenty-four)

echo -n "area3[17] = "

echo ${area3[17]}

echo -n "area3[24] = "

echo ${area3[24]}

-----------------------------------------------

exit 0

As we have seen, a convenient way of initializing an entire array is

the array=( element1 element2 ... elementN ) notation.

base64_charset=( {A..Z} {a..z} {0..9} + / = )

# Using extended brace expansion

#+ to initialize the elements of the array.

# Excerpted from vladz's "base64.sh" script

#+ in the "Contributed Scripts" appendix.

Bash permits array operations on variables, even if the variables are

not explicitly declared as arrays.

string=abcABC123ABCabc

echo ${string[@]} # abcABC123ABCabc

echo ${string[*]} # abcABC123ABCabc

echo ${string[0]} # abcABC123ABCabc

echo ${string[1]} # No output!

# Why?

echo ${#string[@]} # 1

# One element in the array.

# The string itself.

Thank you, Michael Zick, for pointing this out.

Once again this demonstrates that Bash variables are untyped.

Example 27-2. Formatting a poem

!/bin/bash

poem.sh: Pretty-prints one of the ABS Guide author's favorite poems.

Lines of the poem (single stanza).

Line[1]="I do not know which to prefer,"

Line[2]="The beauty of inflections"

Line[3]="Or the beauty of innuendoes,"

Line[4]="The blackbird whistling"

Line[5]="Or just after."

Note that quoting permits embedding whitespace.

Attribution.

Attrib[1]=" Wallace Stevens"

Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\""

This poem is in the Public Domain (copyright expired).

echo

tput bold # Bold print.

for index in 1 2 3 4 5 # Five lines.

printf " %s\n" "${Line[index]}"

done

for index in 1 2 # Two attribution lines.

printf " %s\n" "${Attrib[index]}"

done

tput sgr0 # Reset terminal.

# See 'tput' docs.

echo

exit 0

Exercise:

--------

Modify this script to pretty-print a poem from a text data file.

Array variables have a syntax all their own, and even standard Bash

commands and operators have special options adapted for array use.

Example 27-3. Various array operations

!/bin/bash

array-ops.sh: More fun with arrays.

array=( zero one two three four five )

Element 0 1 2 3 4 5

echo ${array[0]} # zero

echo ${array:0} # zero

# Parameter expansion of first element,

#+ starting at position # 0 (1st character).

echo ${array:1} # ero

# Parameter expansion of first element,

#+ starting at position # 1 (2nd character).

echo "--------------"

echo ${#array[0]} # 4

# Length of first element of array.

echo ${#array} # 4

# Length of first element of array.

# (Alternate notation)

echo ${#array[1]} # 3

# Length of second element of array.

# Arrays in Bash have zero-based indexing.

echo ${#array[*]} # 6

# Number of elements in array.

echo ${#array[@]} # 6

# Number of elements in array.

echo "--------------"

array2=( [0]="first element" [1]="second element" [3]="fourth element" )

^ ^ ^ ^ ^ ^ ^ ^ ^

Quoting permits embedding whitespace within individual array elements.

echo ${array2[0]} # first element

echo ${array2[1]} # second element

echo ${array2[2]} #

# Skipped in initialization, and therefore null.

echo ${array2[3]} # fourth element

echo ${#array2[0]} # 13 (length of first element)

echo ${#array2[*]} # 3 (number of elements in array)

exit

Many of the standard string operations work on arrays.

Example 27-4. String operations on arrays

!/bin/bash

array-strops.sh: String operations on arrays.

Script by Michael Zick.

Used in ABS Guide with permission.

Fixups: 05 May 08, 04 Aug 08.

In general, any string operation using the ${name ... } notation

+ can be applied to all string elements in an array,

+ with the ${name[@] ... } or ${name[*] ...} notation.

arrayZ=( one two three four five five )

echo

Trailing Substring Extraction

echo ${arrayZ[@]:0} # one two three four five five

^ All elements.

echo ${arrayZ[@]:1} # two three four five five

^ All elements following element[0].

echo ${arrayZ[@]:1:2} # two three

^ Only the two elements after element[0].

echo "---------"

Substring Removal

Removes shortest match from front of string(s).

echo ${arrayZ[@]#f*r} # one two three five five

^ # Applied to all elements of the array.

# Matches "four" and removes it.

Longest match from front of string(s)

echo ${arrayZ[@]##t*e} # one two four five five

^^ # Applied to all elements of the array.

# Matches "three" and removes it.

Shortest match from back of string(s)

echo ${arrayZ[@]%h*e} # one two t four five five

^ # Applied to all elements of the array.

# Matches "hree" and removes it.

Longest match from back of string(s)

echo ${arrayZ[@]%%t*e} # one two four five five

^^ # Applied to all elements of the array.

# Matches "three" and removes it.

echo "----------------------"

Substring Replacement

Replace first occurrence of substring with replacement.

echo ${arrayZ[@]/fiv/XYZ} # one two three four XYZe XYZe

^ # Applied to all elements of the array.

Replace all occurrences of substring.

echo ${arrayZ[@]//iv/YY} # one two three four fYYe fYYe

# Applied to all elements of the array.

Delete all occurrences of substring.

Not specifing a replacement defaults to 'delete' ...

echo ${arrayZ[@]//fi/} # one two three four ve ve

^^ # Applied to all elements of the array.

Replace front-end occurrences of substring.

echo ${arrayZ[@]/#fi/XY} # one two three four XYve XYve

^ # Applied to all elements of the array.

Replace back-end occurrences of substring.

echo ${arrayZ[@]/%ve/ZZ} # one two three four fiZZ fiZZ

^ # Applied to all elements of the array.

echo ${arrayZ[@]/%o/XX} # one twXX three four five five

^ # Why?

echo "-----------------------------"

replacement() {

echo -n "!!!"

}

echo ${arrayZ[@]/%e/$(replacement)}

^ ^^^^^^^^^^^^^^

on!!! two thre!!! four fiv!!! fiv!!!

The stdout of replacement() is the replacement string.

Q.E.D: The replacement action is, in effect, an 'assignment.'

echo "------------------------------------"

Accessing the "for-each":

echo ${arrayZ[@]//*/$(replacement optional_arguments)}

^^ ^^^^^^^^^^^^^

!!! !!! !!! !!! !!! !!!

Now, if Bash would only pass the matched string

+ to the function being called . . .

echo

exit 0

Before reaching for a Big Hammer -- Perl, Python, or all the rest --

recall:

$( ... ) is command substitution.

A function runs as a sub-process.

A function writes its output (if echo-ed) to stdout.

Assignment, in conjunction with "echo" and command substitution,

+ can read a function's stdout.

The name[@] notation specifies (the equivalent of) a "for-each"

+ operation.

Bash is more powerful than you think!

Command substitution can construct the individual elements of an

array.

Example 27-5. Loading the contents of a script into an array

!/bin/bash

script-array.sh: Loads this script into an array.

Inspired by an e-mail from Chris Martin (thanks!).

script_contents=( $(cat "$0") ) # Stores contents of this script ($0)

#+ in an array.

for element in $(seq 0 $((${#script_contents[@]} - 1)))

do # ${#script_contents[@]}

#+ gives number of elements in the array.

# Question:

# Why is seq 0 necessary?

# Try changing it to seq 1.

echo -n "${script_contents[$element]}"

# List each field of this script on a single line.

echo -n "${script_contents[element]}" also works because of ${ ... }.

echo -n " -- " # Use " -- " as a field separator.

done

echo

exit 0

Exercise:

--------

Modify this script so it lists itself

+ in its original format,

+ complete with whitespace, line breaks, etc.

In an array context, some Bash builtins have a slightly altered

meaning. For example, unset deletes array elements, or even an entire

array.

Example 27-6. Some special properties of arrays

!/bin/bash

declare -a colors

All subsequent commands in this script will treat

+ the variable "colors" as an array.

echo "Enter your favorite colors (separated from each other by a space)."

read -a colors # Enter at least 3 colors to demonstrate features below.

Special option to 'read' command,

+ allowing assignment of elements in an array.

echo

element_count=${#colors[@]}

Special syntax to extract number of elements in array.

element_count=${#colors[*]} works also.

The "@" variable allows word splitting within quotes

+ (extracts variables separated by whitespace).

This corresponds to the behavior of "$@" and "$*"

+ in positional parameters.

index=0

while [ "$index" -lt "$element_count" ]

do # List all the elements in the array.

echo ${colors[$index]}

# ${colors[index]} also works because it's within ${ ... } brackets.

let "index = $index + 1"

# Or:

# ((index++))

done

Each array element listed on a separate line.

If this is not desired, use echo -n "${colors[$index]} "

Doing it with a "for" loop instead:

for i in "${colors[@]}"

do

echo "$i"

done

(Thanks, S.C.)

echo

Again, list all the elements in the array, but using a more elegant method.

echo ${colors[@]} # echo ${colors[*]} also works.

echo

The "unset" command deletes elements of an array, or entire array.

unset colors[1] # Remove 2nd element of array.

# Same effect as colors[1]=

echo ${colors[@]} # List array again, missing 2nd element.

unset colors # Delete entire array.

# unset colors[*] and

#+ unset colors[@] also work.

echo; echo -n "Colors gone."

echo ${colors[@]} # List array again, now empty.

exit 0

As seen in the previous example, either ${array_name[@]} or

${array_name[*]} refers to all the elements of the array. Similarly,

to get a count of the number of elements in an array, use either

${#array_name[@]} or ${#array_name[*]}. ${#array_name} is the length

(number of characters) of ${array_name[0]}, the first element of the

array.

Example 27-7. Of empty arrays and empty elements

!/bin/bash

empty-array.sh

Thanks to Stephane Chazelas for the original example,

+ and to Michael Zick and Omair Eshkenazi, for extending it.

And to Nathan Coulter for clarifications and corrections.

An empty array is not the same as an array with empty elements.

array0=( first second third )

array1=( '' ) # "array1" consists of one empty element.

array2=( ) # No elements . . . "array2" is empty.

array3=( ) # What about this array?

echo

ListArray()

{

echo

echo "Elements in array0: ${array0[@]}"

echo "Elements in array1: ${array1[@]}"

echo "Elements in array2: ${array2[@]}"

echo "Elements in array3: ${array3[@]}"

echo

echo "Length of first element in array0 = ${#array0}"

echo "Length of first element in array1 = ${#array1}"

echo "Length of first element in array2 = ${#array2}"

echo "Length of first element in array3 = ${#array3}"

echo

echo "Number of elements in array0 = ${#array0[*]}" # 3

echo "Number of elements in array1 = ${#array1[*]}" # 1 (Surprise!)

echo "Number of elements in array2 = ${#array2[*]}" # 0

echo "Number of elements in array3 = ${#array3[*]}" # 0

}

===================================================================

ListArray

Try extending those arrays.

Adding an element to an array.

array0=( "${array0[@]}" "new1" )

array1=( "${array1[@]}" "new1" )

array2=( "${array2[@]}" "new1" )

array3=( "${array3[@]}" "new1" )

ListArray

or

array0[${#array0[*]}]="new2"

array1[${#array1[*]}]="new2"

array2[${#array2[*]}]="new2"

array3[${#array3[*]}]="new2"

ListArray

When extended as above, arrays are 'stacks' ...

Above is the 'push' ...

The stack 'height' is:

height=${#array2[@]}

echo

echo "Stack height for array2 = $height"

The 'pop' is:

unset array2[${#array2[@]}-1] # Arrays are zero-based,

height=${#array2[@]} #+ which means first element has index 0.

echo

echo "POP"

echo "New stack height for array2 = $height"

ListArray

List only 2nd and 3rd elements of array0.

from=1 # Zero-based numbering.

to=2

array3=( ${array0[@]:1:2} )

echo

echo "Elements in array3: ${array3[@]}"

Works like a string (array of characters).

Try some other "string" forms.

Replacement:

array4=( ${array0[@]/second/2nd} )

echo

echo "Elements in array4: ${array4[@]}"

Replace all matching wildcarded string.

array5=( ${array0[@]//new?/old} )

echo

echo "Elements in array5: ${array5[@]}"

Just when you are getting the feel for this . . .

array6=( ${array0[@]#*new} )

echo # This one might surprise you.

echo "Elements in array6: ${array6[@]}"

array7=( ${array0[@]#new1} )

echo # After array6 this should not be a surprise.

echo "Elements in array7: ${array7[@]}"

Which looks a lot like . . .

array8=( ${array0[@]/new1/} )

echo

echo "Elements in array8: ${array8[@]}"

So what can one say about this?

The string operations are performed on

+ each of the elements in var[@] in succession.

Therefore : Bash supports string vector operations.

If the result is a zero length string,

+ that element disappears in the resulting assignment.

However, if the expansion is in quotes, the null elements remain.

Michael Zick: Question, are those strings hard or soft quotes?

Nathan Coulter: There is no such thing as "soft quotes."

! What's really happening is that

!+ the pattern matching happens after

!+ all the other expansions of [word]

!+ in cases like ${parameter#word}.

zap='new*'

array9=( ${array0[@]/$zap/} )

echo

echo "Number of elements in array9: ${#array9[@]}"

array9=( "${array0[@]/$zap/}" )

echo "Elements in array9: ${array9[@]}"

This time the null elements remain.

echo "Number of elements in array9: ${#array9[@]}"

Just when you thought you were still in Kansas . . .

array10=( ${array0[@]#$zap} )

echo

echo "Elements in array10: ${array10[@]}"

But, the asterisk in zap won't be interpreted if quoted.

array10=( ${array0[@]#"$zap"} )

echo

echo "Elements in array10: ${array10[@]}"

Well, maybe we _are_ still in Kansas . . .

(Revisions to above code block by Nathan Coulter.)

Compare array7 with array10.

Compare array8 with array9.

Reiterating: No such thing as soft quotes!

Nathan Coulter explains:

Pattern matching of 'word' in ${parameter#word} is done after

+ parameter expansion and before quote removal.

In the normal case, pattern matching is done after quote removal.

exit

The relationship of ${array_name[@]} and ${array_name[*]} is

analogous to that between $@ and $*. This powerful array notation has

a number of uses.

Copying an array.

array2=( "${array1[@]}" )

or

array2="${array1[@]}"

However, this fails with "sparse" arrays,

+ arrays with holes (missing elements) in them,

+ as Jochen DeSmet points out.

------------------------------------------

array1[0]=0

array1[1] not assigned

array1[2]=2

array2=( "${array1[@]}" ) # Copy it?

echo ${array2[0]} # 0

echo ${array2[2]} # (null), should be 2

------------------------------------------

Adding an element to an array.

array=( "${array[@]}" "new element" )

or

array[${#array[*]}]="new element"

Thanks, S.C.

Tip

The array=( element1 element2 ... elementN ) initialization

operation, with the help of command substitution, makes it possible

to load the contents of a text file into an array.

!/bin/bash

filename=sample_file

cat sample_file

1 a b c

2 d e fg

declare -a array1

array1=( `cat "$filename"`) # Loads contents

List file to stdout #+ of $filename into array1.

array1=( `cat "$filename" | tr '\n' ' '`)

change linefeeds in file to spaces.

Not necessary because Bash does word splitting,

+ changing linefeeds to spaces.

echo ${array1[@]} # List the array.

1 a b c 2 d e fg

Each whitespace-separated "word" in the file

+ has been assigned to an element of the array.

element_count=${#array1[*]}

echo $element_count # 8

Clever scripting makes it possible to add array operations.

Example 27-8. Initializing arrays

! /bin/bash

array-assign.bash

Array operations are Bash-specific,

+ hence the ".bash" in the script name.

Copyright (c) Michael S. Zick, 2003, All rights reserved.

License: Unrestricted reuse in any form, for any purpose.

Version: $ID$

Clarification and additional comments by William Park.

Based on an example provided by Stephane Chazelas

+ which appeared in an earlier version of the

+ Advanced Bash Scripting Guide.

Output format of the 'times' command:

User CPU <space> System CPU

User CPU of dead children <space> System CPU of dead children

Bash has two versions of assigning all elements of an array

+ to a new array variable.

Both drop 'null reference' elements

+ in Bash versions 2.04 and later.

An additional array assignment that maintains the relationship of

+ [subscript]=value for arrays may be added to newer versions.

Constructs a large array using an internal command,

+ but anything creating an array of several thousand elements

+ will do just fine.

declare -a bigOne=( /dev/* ) # All the files in /dev . . .

echo

echo 'Conditions: Unquoted, default IFS, All-Elements-Of'

echo "Number of elements in array is ${#bigOne[@]}"

set -vx

echo

echo '- - testing: =( ${array[@]} ) - -'

times

declare -a bigTwo=( ${bigOne[@]} )

Note parens: ^ ^

times

echo

echo '- - testing: =${array[@]} - -'

times

declare -a bigThree=${bigOne[@]}

No parentheses this time.

times

Comparing the numbers shows that the second form, pointed out

+ by Stephane Chazelas, is faster.

As William Park explains:

+ The bigTwo array assigned element by element (because of parentheses),

+ whereas bigThree assigned as a single string.

So, in essence, you have:

bigTwo=( [0]="..." [1]="..." [2]="..." ... )

bigThree=( [0]="... ... ..." )

Verify this by: echo ${bigTwo[0]}

echo ${bigThree[0]}

I will continue to use the first form in my example descriptions

+ because I think it is a better illustration of what is happening.

The reusable portions of my examples will actual contain

+ the second form where appropriate because of the speedup.

MSZ: Sorry about that earlier oversight folks.

Note:

----

The "declare -a" statements in lines 32 and 44

+ are not strictly necessary, since it is implicit

+ in the Array=( ... ) assignment form.

However, eliminating these declarations slows down

+ the execution of the following sections of the script.

Try it, and see.

exit 0

Note

Adding a superfluous declare -a statement to an array declaration may

speed up execution of subsequent operations on the array.

Example 27-9. Copying and concatenating arrays

! /bin/bash

CopyArray.sh

This script written by Michael Zick.

Used here with permission.

How-To "Pass by Name & Return by Name"

+ or "Building your own assignment statement".

CpArray_Mac() {

Assignment Command Statement Builder

echo -n 'eval '

echo -n "$2" # Destination name

echo -n '=( ${'

echo -n "$1" # Source name

echo -n '[@]} )'

That could all be a single command.

Matter of style only.

}

declare -f CopyArray # Function "Pointer"

CopyArray=CpArray_Mac # Statement Builder

Hype()

{

Hype the array named $1.

(Splice it together with array containing "Really Rocks".)

Return in array named $2.

local -a TMP

local -a hype=( Really Rocks )

$($CopyArray $1 TMP)

TMP=( ${TMP[@]} ${hype[@]} )

$($CopyArray TMP $2)

}

declare -a before=( Advanced Bash Scripting )

declare -a after

echo "Array Before = ${before[@]}"

Hype before after

echo "Array After = ${after[@]}"

Too much hype?

echo "What ${after[@]:3:2}?"

declare -a modest=( ${after[@]:2:1} ${after[@]:3:2} )

---- substring extraction ----

echo "Array Modest = ${modest[@]}"

What happened to 'before' ?

echo "Array Before = ${before[@]}"

exit 0

Example 27-10. More on concatenating arrays

! /bin/bash

array-append.bash

Copyright (c) Michael S. Zick, 2003, All rights reserved.

License: Unrestricted reuse in any form, for any purpose.

Version: $ID$

Slightly modified in formatting by M.C.

Array operations are Bash-specific.

Legacy UNIX /bin/sh lacks equivalents.

Pipe the output of this script to 'more'

+ so it doesn't scroll off the terminal.

Or, redirect output to a file.

declare -a array1=( zero1 one1 two1 )

Subscript packed.

declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 )

Subscript sparse -- [1] is not defined.

echo

echo '- Confirm that the array is really subscript sparse. -'

echo "Number of elements: 4" # Hard-coded for illustration.

for (( i = 0 ; i < 4 ; i++ ))

echo "Element [$i]: ${array2[$i]}"

done

Combine (append) two arrays into a third array.

echo

echo 'Conditions: Unquoted, default IFS, All-Elements-Of operator'

echo '- Undefined elements not present, subscripts not maintained. -'

# The undefined elements do not exist; they are not being dropped.

dest=( ${array1[@]} ${array2[@]} )

dest=${array1[@]}${array2[@]} # Strange results, possibly a bug.

Now, list the result.

echo

echo '- - Testing Array Append - -'

cnt=${#dest[@]}

echo "Number of elements: $cnt"

for (( i = 0 ; i < cnt ; i++ ))

echo "Element [$i]: ${dest[$i]}"

done

Assign an array to a single array element (twice).

dest[0]=${array1[@]}

dest[1]=${array2[@]}

List the result.

echo

echo '- - Testing modified array - -'

cnt=${#dest[@]}

echo "Number of elements: $cnt"

for (( i = 0 ; i < cnt ; i++ ))

echo "Element [$i]: ${dest[$i]}"

done

Examine the modified second element.

echo

echo '- - Reassign and list second element - -'

declare -a subArray=${dest[1]}

cnt=${#subArray[@]}

echo "Number of elements: $cnt"

for (( i = 0 ; i < cnt ; i++ ))

echo "Element [$i]: ${subArray[$i]}"

done

The assignment of an entire array to a single element

+ of another array using the '=${ ... }' array assignment

+ has converted the array being assigned into a string,

+ with the elements separated by a space (the first character of IFS).

If the original elements didn't contain whitespace . . .

If the original array isn't subscript sparse . . .

Then we could get the original array structure back again.

Restore from the modified second element.

echo

echo '- - Listing restored element - -'

declare -a subArray=( ${dest[1]} )

cnt=${#subArray[@]}

echo "Number of elements: $cnt"

for (( i = 0 ; i < cnt ; i++ ))

echo "Element [$i]: ${subArray[$i]}"

done

echo '- - Do not depend on this behavior. - -'

echo '- - This behavior is subject to change - -'

echo '- - in versions of Bash newer than version 2.05b - -'

MSZ: Sorry about any earlier confusion folks.

exit 0

Arrays permit deploying old familiar algorithms as shell scripts.

Whether this is necessarily a good idea is left for the reader to

decide.

Example 27-11. The Bubble Sort

!/bin/bash

bubble.sh: Bubble sort, of sorts.

Recall the algorithm for a bubble sort. In this particular version...

With each successive pass through the array to be sorted,

+ compare two adjacent elements, and swap them if out of order.

At the end of the first pass, the "heaviest" element has sunk to bottom.

At the end of the second pass, the next "heaviest" one has sunk next to bot

tom.

And so forth.

This means that each successive pass needs to traverse less of the array.

You will therefore notice a speeding up in the printing of the later passes

exchange()

{

# Swaps two members of the array.

local temp=${Countries[$1]} # Temporary storage

#+ for element getting swapped out.

Countries[$1]=${Countries[$2]}

Countries[$2]=$temp

return

}

declare -a Countries # Declare array,

#+ optional here since it's initialized below.

Is it permissable to split an array variable over multiple lines

+ using an escape (\)?

Yes.

Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \

Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \

Israel Peru Canada Oman Denmark Wales France Kenya \

Xanadu Qatar Liechtenstein Hungary)

"Xanadu" is the mythical place where, according to Coleridge,

+ Kubla Khan did a pleasure dome decree.

clear # Clear the screen to start with.

echo "0: ${Countries[*]}" # List entire array at pass 0.

number_of_elements=${#Countries[@]}

let "comparisons = $number_of_elements - 1"

count=1 # Pass number.

while [ "$comparisons" -gt 0 ] # Beginning of outer loop

index=0 # Reset index to start of array after each pass.

while [ "$index" -lt "$comparisons" ] # Beginning of inner loop

if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ]

# If out of order...

# Recalling that \> is ASCII comparison operator

#+ within single brackets.

# if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]]

#+ also works.

then

exchange $index `expr $index + 1` # Swap.

let "index += 1" # Or, index+=1 on Bash, ver. 3.1 or newer.

done # End of inner loop

----------------------------------------------------------------------

Paulo Marcel Coelho Aragao suggests for-loops as a simpler altenative.

for (( last = $number_of_elements - 1 ; last > 0 ; last-- ))

Fix by C.Y. Hunt ^ (Thanks!)

do

for (( i = 0 ; i < last ; i++ ))

do

[[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \

&& exchange $i $((i+1))

done

----------------------------------------------------------------------

let "comparisons -= 1" # Since "heaviest" element bubbles to bottom,

#+ we need do one less comparison each pass.

echo

echo "$count: ${Countries[@]}" # Print resultant array at end of each pass.

echo

let "count += 1" # Increment pass count.

done # End of outer loop

# All done.

exit 0

Is it possible to nest arrays within arrays?

!/bin/bash

"Nested" array.

Michael Zick provided this example,

+ with corrections and clarifications by William Park.

AnArray=( $(ls --inode --ignore-backups --almost-all \

--directory --full-time --color=none --time=status \

--sort=time -l ${PWD} ) ) # Commands and options.

Spaces are significant . . . and don't quote anything in the above.

SubArray=( ${AnArray[@]:11:1} ${AnArray[@]:6:5} )

This array has six elements:

+ SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]}

[3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} )

Arrays in Bash are (circularly) linked lists

+ of type string (char *).

So, this isn't actually a nested array,

+ but it's functionally similar.

echo "Current directory and date of last status change:"

echo "${SubArray[@]}"

exit 0

Embedded arrays in combination with indirect references create some

fascinating possibilities

Example 27-12. Embedded arrays and indirect references

!/bin/bash

embedded-arrays.sh

Embedded arrays and indirect references.

This script by Dennis Leeuw.

Used with permission.

Modified by document author.

ARRAY1=(

VAR1_1=value11

VAR1_2=value12

VAR1_3=value13

)

ARRAY2=(

VARIABLE="test"

STRING="VAR1=value1 VAR2=value2 VAR3=value3"

ARRAY21=${ARRAY1[*]}

) # Embed ARRAY1 within this second array.

function print () {

OLD_IFS="$IFS"

IFS= \n' # To print each array element

#+ on a separate line.

TEST1="ARRAY2[*]"

local ${!TEST1} # See what happens if you delete this line.

# Indirect reference.

# This makes the components of $TEST1

#+ accessible to this function.

# Let's see what we've got so far.

echo

echo "\$TEST1 = $TEST1" # Just the name of the variable.

echo; echo

echo "{\$TEST1} = ${!TEST1}" # Contents of the variable.

# That's what an indirect

#+ reference does.

echo

echo "-------------------------------------------"; echo

echo

# Print variable

echo "Variable VARIABLE: $VARIABLE"

# Print a string element

IFS="$OLD_IFS"

TEST2="STRING[*]"

local ${!TEST2} # Indirect reference (as above).

echo "String element VAR2: $VAR2 from STRING"

# Print an array element

TEST2="ARRAY21[*]"

local ${!TEST2} # Indirect reference (as above).

echo "Array element VAR1_1: $VAR1_1 from ARRAY21"

}

echo

exit 0

As the author of the script notes,

+ "you can easily expand it to create named-hashes in bash."

(Difficult) exercise for the reader: implement this.

Arrays enable implementing a shell script version of the Sieve of

Eratosthenes. Of course, a resource-intensive application of this

nature should really be written in a compiled language, such as C. It

runs excruciatingly slowly as a script.

Example 27-13. The Sieve of Eratosthenes

!/bin/bash

sieve.sh (ex68.sh)

Sieve of Eratosthenes

Ancient algorithm for finding prime numbers.

This runs a couple of orders of magnitude slower

+ than the equivalent program written in C.

LOWER_LIMIT=1 # Starting with 1.

UPPER_LIMIT=1000 # Up to 1000.

(You may set this higher . . . if you have time on your hands.)

PRIME=1

NON_PRIME=0

let SPLIT=UPPER_LIMIT/2

Optimization:

Need to test numbers only halfway to upper limit. Why?

declare -a Primes

Primes[] is an array.

initialize ()

{

Initialize the array.

i=$LOWER_LIMIT

until [ "$i" -gt "$UPPER_LIMIT" ]

Primes[i]=$PRIME

let "i += 1"

done

Assume all array members guilty (prime)

+ until proven innocent.

}

print_primes ()

{

Print out the members of the Primes[] array tagged as prime.

i=$LOWER_LIMIT

until [ "$i" -gt "$UPPER_LIMIT" ]

if [ "${Primes[i]}" -eq "$PRIME" ]

then

printf "%8d" $i

# 8 spaces per number gives nice, even columns.

let "i += 1"

done

}

sift () # Sift out the non-primes.

{

let i=$LOWER_LIMIT+1

Let's start with 2.

until [ "$i" -gt "$UPPER_LIMIT" ]

if [ "${Primes[i]}" -eq "$PRIME" ]

Don't bother sieving numbers already sieved (tagged as non-prime).

then

t=$i

while [ "$t" -le "$UPPER_LIMIT" ]

let "t += $i "

Primes[t]=$NON_PRIME

# Tag as non-prime all multiples.

done

let "i += 1"

done

}

==============================================

main ()

Invoke the functions sequentially.

initialize

sift

print_primes

This is what they call structured programming.

==============================================

echo

exit 0

-------------------------------------------------------- #

Code below line will not execute, because of 'exit.'

This improved version of the Sieve, by Stephane Chazelas,

+ executes somewhat faster.

Must invoke with command-line argument (limit of primes).

UPPER_LIMIT=$1 # From command-line.

let SPLIT=UPPER_LIMIT/2 # Halfway to max number.

Primes=( '' $(seq $UPPER_LIMIT) )

i=1

until (( ( i += 1 ) > SPLIT )) # Need check only halfway.

if [[ -n ${Primes[i]} ]]

then

t=$i

until (( ( t += i ) > UPPER_LIMIT ))

Primes[t]=

done

echo ${Primes[*]}

exit $?

Example 27-14. The Sieve of Eratosthenes, Optimized

!/bin/bash

Optimized Sieve of Eratosthenes

Script by Jared Martin, with very minor changes by ABS Guide author.

Used in ABS Guide with permission (thanks!).

Based on script in Advanced Bash Scripting Guide.

http://tldp.org/LDP/abs/html/arrays.html#PRIMES0 (ex68.sh).

http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf (reference)

Check results against http://primes.utm.edu/lists/small/1000.txt

Necessary but not sufficient would be, e.g.,

(($(sieve 7919 | wc -w) == 1000)) && echo "7919 is the 1000th prime"

UPPER_LIMIT=${1:?"Need an upper limit of primes to search."}

Primes=( '' $(seq ${UPPER_LIMIT}) )

typeset -i i t

Primes[i=1]='' # 1 is not a prime.

until (( ( i += 1 ) > (${UPPER_LIMIT}/i) )) # Need check only ith-way.

do # Why?

if ((${Primes[t=i*(i-1), i]}))

# Obscure, but instructive, use of arithmetic expansion in subscript.

then

until (( ( t += i ) > ${UPPER_LIMIT} ))

do Primes[t]=; done

done

echo ${Primes[*]}

echo # Change to original script for pretty-printing (80-col. display).

printf "%8d" ${Primes[*]}

echo; echo

exit $?

Compare these array-based prime number generators with alternatives

that do not use arrays, Example A-15, and Example 16-46.

Arrays lend themselves, to some extent, to emulating data structures

for which Bash has no native support.

Example 27-15. Emulating a push-down stack

!/bin/bash

stack.sh: push-down stack simulation

Similar to the CPU stack, a push-down stack stores data items

+ sequentially, but releases them in reverse order, last-in first-out.

BP=100 # Base Pointer of stack array.

# Begin at element 100.

SP=$BP # Stack Pointer.

# Initialize it to "base" (bottom) of stack.

Data= # Contents of stack location.

# Must use global variable,

#+ because of limitation on function return range.

# 100 Base pointer <-- Base Pointer

# 99 First data item

# 98 Second data item

# ... More data

# Last data item <-- Stack pointer

declare -a stack

push() # Push item on stack.

{

if [ -z "$1" ] # Nothing to push?

then

return

let "SP -= 1" # Bump stack pointer.

stack[$SP]=$1

return

}

pop() # Pop item off stack.

{

Data= # Empty out data item.

if [ "$SP" -eq "$BP" ] # Stack empty?

then

return

fi # This also keeps SP from getting past 100,

#+ i.e., prevents a runaway stack.

Data=${stack[$SP]}

let "SP += 1" # Bump stack pointer.

return

}

status_report() # Find out what's happening.

{

echo "-------------------------------------"

echo "REPORT"

echo "Stack Pointer = $SP"

echo "Just popped \""$Data"\" off the stack."

echo "-------------------------------------"

echo

}

=======================================================

Now, for some fun.

echo

See if you can pop anything off empty stack.

pop

status_report

echo

push garbage

pop

status_report # Garbage in, garbage out.

value1=23; push $value1

value2=skidoo; push $value2

value3=LAST; push $value3

pop # LAST

status_report

pop # skidoo

status_report

pop # 23

status_report # Last-in, first-out!

Notice how the stack pointer decrements with each push,

+ and increments with each pop.

echo

exit 0

=======================================================

Exercises:

---------

1) Modify the "push()" function to permit pushing

+ multiple element on the stack with a single function call.

2) Modify the "pop()" function to permit popping

+ multiple element from the stack with a single function call.

3) Add error checking to the critical functions.

That is, return an error code, depending on

+ successful or unsuccessful completion of the operation,

+ and take appropriate action.

4) Using this script as a starting point,

+ write a stack-based 4-function calculator.

Fancy manipulation of array "subscripts" may require intermediate

variables. For projects involving this, again consider using a more

powerful programming language, such as Perl or C.

Example 27-16. Complex array application: Exploring a weird

mathematical series

!/bin/bash

Douglas Hofstadter's notorious "Q-series":

Q(1) = Q(2) = 1

Q(n) = Q(n - Q(n-1)) + Q(n - Q(n-2)), for n>2

This is a "chaotic" integer series with strange

+ and unpredictable behavior.

The first 20 terms of the series are:

1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12

See Hofstadter's book, _Goedel, Escher, Bach: An Eternal Golden Braid_,

+ p. 137, ff.

LIMIT=100 # Number of terms to calculate.

LINEWIDTH=20 # Number of terms printed per line.

Q[1]=1 # First two terms of series are 1.

Q[2]=1

echo

echo "Q-series [$LIMIT terms]:"

echo -n "${Q[1]} " # Output first two terms.

echo -n "${Q[2]} "

for ((n=3; n <= $LIMIT; n++)) # C-like loop expression.

do # Q[n] = Q[n - Q[n-1]] + Q[n - Q[n-2]] for n>2

Need to break the expression into intermediate terms,

+ since Bash doesn't handle complex array arithmetic very well.

let "n1 = $n - 1" # n-1

let "n2 = $n - 2" # n-2

t0=`expr $n - ${Q[n1]}` # n - Q[n-1]

t1=`expr $n - ${Q[n2]}` # n - Q[n-2]

T0=${Q[t0]} # Q[n - Q[n-1]]

T1=${Q[t1]} # Q[n - Q[n-2]]

Q[n]=`expr $T0 + $T1` # Q[n - Q[n-1]] + Q[n - Q[n-2]]

echo -n "${Q[n]} "

if [ `expr $n % $LINEWIDTH` -eq 0 ] # Format output.

then # ^ modulo

echo # Break lines into neat chunks.

done

echo

exit 0

This is an iterative implementation of the Q-series.

The more intuitive recursive implementation is left as an exercise.

Warning: calculating this series recursively takes a VERY long time

+ via a script. C/C++ would be orders of magnitude faster.

Bash supports only one-dimensional arrays, though a little trickery

permits simulating multi-dimensional ones.

Example 27-17. Simulating a two-dimensional array, then tilting it

!/bin/bash

twodim.sh: Simulating a two-dimensional array.

A one-dimensional array consists of a single row.

A two-dimensional array stores rows sequentially.

Rows=5

Columns=5

5 X 5 Array.

declare -a alpha # char alpha [Rows] [Columns];

# Unnecessary declaration. Why?

load_alpha ()

{

local rc=0

local index

for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y

do # Use different symbols if you like.

local row=`expr $rc / $Columns`

local column=`expr $rc % $Rows`

let "index = $row * $Rows + $column"

alpha[$index]=$i

alpha[$row][$column]

let "rc += 1"

done

Simpler would be

+ declare -a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y )

+ but this somehow lacks the "flavor" of a two-dimensional array.

}

print_alpha ()

{

local row=0

local index

echo

while [ "$row" -lt "$Rows" ] # Print out in "row major" order:

do #+ columns vary,

#+ while row (outer loop) remains the same.

local column=0

echo -n " " # Lines up "square" array with rotated one.

while [ "$column" -lt "$Columns" ]

let "index = $row * $Rows + $column"

echo -n "${alpha[index]} " # alpha[$row][$column]

let "column += 1"

done

let "row += 1"

echo

done

The simpler equivalent is

echo ${alpha[*]} | xargs -n $Columns

echo

}

filter () # Filter out negative array indices.

{

echo -n " " # Provides the tilt.

# Explain how.

if [[ "$1" -ge 0 && "$1" -lt "$Rows" && "$2" -ge 0 && "$2" -lt "$Columns" ]]

then

let "index = $1 * $Rows + $2"

# Now, print it rotated.

echo -n " ${alpha[index]}"

# alpha[$row][$column]

}

rotate () # Rotate the array 45 degrees --

{ #+ "balance" it on its lower lefthand corner.

local row

local column

for (( row = Rows; row > -Rows; row-- ))

do # Step through the array backwards. Why?

for (( column = 0; column < Columns; column++ ))

if [ "$row" -ge 0 ]

then

let "t1 = $column - $row"

let "t2 = $column"

else

let "t1 = $column"

let "t2 = $column + $row"

filter $t1 $t2 # Filter out negative array indices.

# What happens if you don't do this?

done

echo; echo

done

Array rotation inspired by examples (pp. 143-146) in

+ "Advanced C Programming on the IBM PC," by Herbert Mayer

+ (see bibliography).

This just goes to show that much of what can be done in C

+ can also be done in shell scripting.

}

--------------- Now, let the show begin. ------------#

load_alpha # Load the array.

print_alpha # Print it out.

rotate # Rotate it 45 degrees counterclockwise.

-----------------------------------------------------#

exit 0

This is a rather contrived, not to mention inelegant simulation.

Exercises:

---------

1) Rewrite the array loading and printing functions

in a more intuitive and less kludgy fashion.

2) Figure out how the array rotation functions work.

Hint: think about the implications of backwards-indexing an array.

3) Rewrite this script to handle a non-square array,

such as a 6 X 4 one.

Try to minimize "distortion" when the array is rotated.

A two-dimensional array is essentially equivalent to a

one-dimensional one, but with additional addressing modes for

referencing and manipulating the individual elements by row and

column position.

For an even more elaborate example of simulating a two-dimensional

array, see Example A-10.

For more interesting scripts using arrays, see:

* Example 12-3

* Example 16-46

* Example A-22

* Example A-44

* Example A-41

* Example A-42

________________________________________________________________

Chapter 28. Indirect References

We have seen that referencing a variable, $var, fetches its value.

But, what about the value of a value? What about $var?

The actual notation is \$var, usually preceded by an eval (and

sometimes an echo). This is called an indirect reference.

Example 28-1. Indirect Variable References

!/bin/bash

ind-ref.sh: Indirect variable referencing.

Accessing the contents of the contents of a variable.

First, let's fool around a little.

var=23

echo "\$var = $var" # $var = 23

So far, everything as expected. But ...

echo "\$\$var = $var" # $var = 4570var

Not useful ...

\$\$ expanded to PID of the script

-- refer to the entry on the $ variable --

+ and "var" is echoed as plain text.

(Thank you, Jakob Bohm, for pointing this out.)

echo "\\\$\$var = \$var" # \$var = $23

As expected. The first $ is escaped and pasted on to

+ the value of var ($var = 23 ).

Meaningful, but still not useful.

Now, let's start over and do it the right way.

============================================== #

a=letter_of_alphabet # Variable "a" holds the name of another variable.

letter_of_alphabet=z

echo

Direct reference.

echo "a = $a" # a = letter_of_alphabet

Indirect reference.

eval a=\$a

^^^ Forcing an eval(uation), and ...

^ Escaping the first $ ...

------------------------------------------------------------------------

The 'eval' forces an update of $a, sets it to the updated value of \$a.

So, we see why 'eval' so often shows up in indirect reference notation.

------------------------------------------------------------------------

echo "Now a = $a" # Now a = z

echo

Now, let's try changing the second-order reference.

t=table_cell_3

table_cell_3=24

echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24

echo -n "dereferenced \"t\" = "; eval echo \$t # dereferenced "t" = 24

In this simple case, the following also works (why?).

eval t=\$t; echo "\"t\" = $t"

echo

t=table_cell_3

NEW_VAL=387

table_cell_3=$NEW_VAL

echo "Changing value of \"table_cell_3\" to $NEW_VAL."

echo "\"table_cell_3\" now $table_cell_3"

echo -n "dereferenced \"t\" now "; eval echo \$t

"eval" takes the two arguments "echo" and "\$t" (set equal to $table_cell_3

)

echo

(Thanks, Stephane Chazelas, for clearing up the above behavior.)

A more straightforward method is the ${!t} notation, discussed in the

+ "Bash, version 2" section.

!/bin/bash

---------------------------------------------

This could be "sourced" from a separate file.

isdnMyProviderRemoteNet=172.16.0.100

isdnYourProviderRemoteNet=10.0.0.10

isdnOnlineService="MyProvider"

---------------------------------------------

remoteNet=$(eval "echo \$(echo isdn${isdnOnlineService}RemoteNet)")

remoteNet=$(eval "echo \$(echo isdnMyProviderRemoteNet)")

remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")

remoteNet=$(eval "echo $isdnMyProviderRemoteNet")

echo "$remoteNet" # 172.16.0.100

================================================================

And, it gets even better.

Consider the following snippet given a variable named getSparc,

+ but no such variable getIa64:

chkMirrorArchs () {

arch="$1";

if [ "$(eval "echo \${$(echo get$(echo -ne $arch |

sed 's/^$.$.*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |

sed 's/^.$.*$/\1/g')):-false}")" = true ]

then

return 0;

else

return 1;

fi;

}

getSparc="true"

unset getIa64

chkMirrorArchs sparc

echo $? # 0

# True

chkMirrorArchs Ia64

echo $? # 1

# False

Notes:

-----

Even the to-be-substituted variable name part is built explicitly.

The parameters to the chkMirrorArchs calls are all lower case.

The variable name is composed of two parts: "get" and "Sparc" . . .

Example 28-2. Passing an indirect reference to awk

!/bin/bash

Another version of the "column totaler" script

+ that adds up a specified column (of numbers) in the target file.

This one uses indirect references.

ARGS=2

E_WRONGARGS=85

if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.

then

echo "Usage: `basename $0` filename column-number"

exit $E_WRONGARGS

filename=$1 # Name of file to operate on.

column_number=$2 # Which column to total up.

===== Same as original script, up to this point =====#

A multi-line awk script is invoked by

awk "

...

"

Begin awk script.

-------------------------------------------------

awk "

{ total += \${column_number} # Indirect reference

}

END {

print total

}

" "$filename"

Note that awk doesn't need an eval preceding \$.

-------------------------------------------------

End awk script.

Indirect variable reference avoids the hassles

+ of referencing a shell variable within the embedded awk script.

Thanks, Stephane Chazelas.

exit $?

Caution

This method of indirect referencing is a bit tricky. If the second

order variable changes its value, then the first order variable must

be properly dereferenced (as in the above example). Fortunately, the

${!variable} notation introduced with version 2 of Bash (see Example

37-2 and Example A-22) makes indirect referencing more intuitive.

Bash does not support pointer arithmetic, and this severely limits

the usefulness of indirect referencing. In fact, indirect referencing

in a scripting language is, at best, something of an afterthought.

________________________________________________________________

Chapter 29. /dev and /proc

A Linux or UNIX filesystem typically has the /dev and /proc

special-purpose directories.

________________________________________________________________

29.1. /dev

The /dev directory contains entries for the physical devices that may

or may not be present in the hardware. [118] Appropriately enough,

these are called device files. As an example, the hard drive

partitions containing the mounted filesystem(s) have entries in /dev,

as df shows.

bash$ df

Filesystem 1k-blocks Used Available Use%

Mounted on

/dev/hda6 495876 222748 247527 48% /

/dev/hda1 50755 3887 44248 9% /boot

/dev/hda8 367013 13262 334803 4% /home

/dev/hda5 1714416 1123624 503704 70% /usr

Among other things, the /dev directory contains loopback devices,

such as /dev/loop0. A loopback device is a gimmick that allows an

ordinary file to be accessed as if it were a block device. [119] This

permits mounting an entire filesystem within a single large file. See

Example 17-8 and Example 17-7.

A few of the pseudo-devices in /dev have other specialized uses, such

as /dev/null, /dev/zero, /dev/urandom, /dev/sda1 (hard drive

partition), /dev/udp (User Datagram Packet port), and /dev/tcp.

For instance:

To manually mount a USB flash drive, append the following line to

/etc/fstab. [120]

/dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0

(See also Example A-23.)

Checking whether a disk is in the CD-burner (soft-linked to

/dev/hdc):

head -1 /dev/hdc

head: cannot open '/dev/hdc' for reading: No medium found

(No disc in the drive.)

head: error reading '/dev/hdc': Input/output error

(There is a disk in the drive, but it can't be read;

+ possibly it's an unrecorded CDR blank.)

Stream of characters and assorted gibberish

(There is a pre-recorded disk in the drive,

+ and this is raw output -- a stream of ASCII and binary data.)

Here we see the wisdom of using 'head' to limit the output

+ to manageable proportions, rather than 'cat' or something similar.

Now, it's just a matter of checking/parsing the output and taking

+ appropriate action.

When executing a command on a /dev/tcp/$host/$port pseudo-device

file, Bash opens a TCP connection to the associated socket.

A socket is a communications node associated with a specific I/O

port. (This is analogous to a hardware socket, or receptacle, for a

connecting cable.) It permits data transfer between hardware devices

on the same machine, between machines on the same network, between

machines across different networks, and, of course, between machines

at different locations on the Internet.

The following examples assume an active Internet connection.

Getting the time from nist.gov:

bash$ cat </dev/tcp/time.nist.gov/13

53082 04-03-18 04:26:54 68 0 0 502.3 UTC(NIST) *

[Mark contributed this example.]

Generalizing the above into a script:

!/bin/bash

This script must run with root permissions.

URL="time.nist.gov/13"

Time=$(cat </dev/tcp/"$URL")

UTC=$(echo "$Time" | awk '{print$3}') # Third field is UTC (GMT) time.

Exercise: modify this for different time zones.

echo "UTC Time = "$UTC""

Downloading a URL:

bash$ exec 5<>/dev/tcp/www.net.cn/80

bash$ echo -e "GET / HTTP/1.0\n" >&5

bash$ cat <&5

[Thanks, Mark and Mihai Maties.]

Example 29-1. Using /dev/tcp for troubleshooting

!/bin/bash

dev-tcp.sh: /dev/tcp redirection to check Internet connection.

Script by Troy Engel.

Used with permission.

TCP_HOST=news-15.net # A known spam-friendly ISP.

TCP_PORT=80 # Port 80 is http.

Try to connect. (Somewhat similar to a 'ping' . . .)

echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT}

MYEXIT=$?

: <<EXPLANATION

If bash was compiled with --enable-net-redirections, it has the capability of

using a special character device for both TCP and UDP redirections. These

redirections are used identically as STDIN/STDOUT/STDERR. The device entries

are 30,36 for /dev/tcp:

mknod /dev/tcp c 30 36

From the bash reference:

/dev/tcp/host/port

If host is a valid hostname or Internet address, and port is an integer

port number or service name, Bash attempts to open a TCP connection to the

corresponding socket.

EXPLANATION

if [ "X$MYEXIT" = "X0" ]; then

echo "Connection successful. Exit code: $MYEXIT"

else

echo "Connection unsuccessful. Exit code: $MYEXIT"

exit $MYEXIT

Example 29-2. Playing music

!/bin/bash

music.sh

Music without external files

Author: Antonio Macchi

Used in ABS Guide with permission.

/dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte),

+ 1 channel (mono)

duration=2000 # If 8000 bytes = 1 second, then 2000 = 1/4 second.

volume= \xc0' # Max volume = \xff (or \x00).

mute= \x80' # No volume = \x80 (the middle).

function mknote () # $1=Note Hz in bytes (e.g. A = 440Hz ::

{ #+ 8000 fps / 440 = 16 :: A = 16 bytes per second)

for t in `seq 0 $duration`

test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute

done

}

e=`mknote 49`

g=`mknote 41`

a=`mknote 36`

b=`mknote 32`

c=`mknote 30`

cis=`mknote 29`

d=`mknote 27`

e2=`mknote 24`

n=`mknote 32767`

European notation.

echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \

$n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp

dsp = Digital Signal Processor

exit # A "bonny" example of an elegant shell script!

________________________________________________________________

29.2. /proc

The /proc directory is actually a pseudo-filesystem. The files in

/proc mirror currently running system and kernel processes and

contain information and statistics about them.

bash$ cat /proc/devices

Character devices:

1 mem

2 pty

3 ttyp

4 ttyS

5 cua

7 vcs

10 misc

14 sound

29 fb

36 netlink

128 ptm

136 pts

162 raw

254 pcmcia

Block devices:

1 ramdisk

2 fd

3 ide0

9 md

bash$ cat /proc/interrupts

CPU0

0: 84505 XT-PIC timer

1: 3375 XT-PIC keyboard

2: 0 XT-PIC cascade

5: 1 XT-PIC soundblaster

8: 1 XT-PIC rtc

12: 4231 XT-PIC PS/2 Mouse

14: 109373 XT-PIC ide0

NMI: 0

ERR: 0

bash$ cat /proc/partitions

major minor #blocks name rio rmerge rsect ruse wio wmerge wsect wuse run

ning use aveq

3 0 3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 1

11550 644030

3 1 52416 hda1 27 395 844 960 4 2 14 180 0 800 1140

3 2 1 hda2 0 0 0 0 0 0 0 0 0 0 0

3 4 165280 hda4 10 0 20 210 0 0 0 0 0 210 210

...

bash$ cat /proc/loadavg

0.13 0.42 0.27 2/44 1119

bash$ cat /proc/apm

1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?

bash$ cat /proc/acpi/battery/BAT0/info

present: yes

design capacity: 43200 mWh

last full capacity: 36640 mWh

battery technology: rechargeable

design voltage: 10800 mV

design capacity warning: 1832 mWh

design capacity low: 200 mWh

capacity granularity 1: 1 mWh

capacity granularity 2: 1 mWh

model number: IBM-02K6897

serial number: 1133

battery type: LION

OEM info: Panasonic

bash$ fgrep Mem /proc/meminfo

MemTotal: 515216 kB

MemFree: 266248 kB

Shell scripts may extract data from certain of the files in /proc.

[121]

FS=iso # ISO filesystem support in kernel?

grep $FS /proc/filesystems # iso9660

kernel_version=$( awk '{ print $3 }' /proc/version )

CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo )

if [ "$CPU" = "Pentium(R)" ]

then

run_some_commands

...

else

run_other_commands

...

cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' )

Current operating speed (in MHz) of the cpu on your machine.

On a laptop this may vary, depending on use of battery

+ or AC power.

!/bin/bash

get-commandline.sh

Get the command-line parameters of a process.

OPTION=cmdline

Identify PID.

pid=$( echo $(pidof "$1") | awk '{ print $1 }' )

Get only first ^^^^^^^^^^^^^^^^^^ of multiple instances.

echo

echo "Process ID of (first instance of) "$1" = $pid"

echo -n "Command-line arguments: "

cat /proc/"$pid"/"$OPTION" | xargs -0 echo

Formats output: ^^^^^^^^^^^^^^^

(Thanks, Han Holl, for the fixup!)

echo; echo

For example:

sh get-commandline.sh xterm

devfile="/proc/bus/usb/devices"

text="Spd"

USB1="Spd=12"

USB2="Spd=480"

bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}')

^^^^ Stop after first match.

if [ "$bus_speed" = "$USB1" ]

then

echo "USB 1.1 port found."

# Do something appropriate for USB 1.1.

Note

It is even possible to control certain peripherals with commands sent

to the /proc directory.

root# echo on > /proc/acpi/ibm/light

This turns on the Thinklight in certain models of IBM/Lenovo

Thinkpads. (May not work on all Linux distros.)

Of course, caution is advised when writing to /proc.

The /proc directory contains subdirectories with unusual numerical

names. Every one of these names maps to the process ID of a currently

running process. Within each of these subdirectories, there are a

number of files that hold useful information about the corresponding

process. The stat and status files keep running statistics on the

process, the cmdline file holds the command-line arguments the

process was invoked with, and the exe file is a symbolic link to the

complete path name of the invoking process. There are a few more such

files, but these seem to be the most interesting from a scripting

standpoint.

Example 29-3. Finding the process associated with a PID

!/bin/bash

pid-identifier.sh:

Gives complete path name to process associated with pid.

ARGNO=1 # Number of arguments the script expects.

E_WRONGARGS=65

E_BADPID=66

E_NOSUCHPROCESS=67

E_NOPERMISSION=68

PROCFILE=exe

if [ $# -ne $ARGNO ]

then

echo "Usage: `basename $0` PID-number" >&2 # Error message >stderr.

exit $E_WRONGARGS

pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )

Checks for pid in "ps" listing, field #1.

Then makes sure it is the actual process, not the process invoked by this sc

ript.

The last "grep $1" filters out this possibility.

pidno=$( ps ax | awk '{ print $1 }' | grep $1 )

also works, as Teemu Huovila, points out.

if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-lengt

h string,

then #+ no running process corresponds to the pid given.

echo "No such process running."

exit $E_NOSUCHPROCESS

Alternatively:

if ! ps $1 > /dev/null 2>&1

then # no running process corresponds to the pid given.

echo "No such process running."

exit $E_NOSUCHPROCESS

fi

To simplify the entire process, use "pidof".

if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission.

then

echo "Process $1 running, but..."

echo "Can't get read permission on /proc/$1/$PROCFILE."

exit $E_NOPERMISSION # Ordinary user can't access some files in /proc.

The last two tests may be replaced by:

if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but

# this will test whether it is possible

# to send a signal to the process.

then echo "PID doesn't exist or you're not its owner" >&2

exit $E_BADPID

fi

exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )

Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )

/proc/pid-number/exe is a symbolic link

+ to the complete path name of the invoking process.

if [ -e "$exe_file" ] # If /proc/pid-number/exe exists,

then #+ then the corresponding process exists.

echo "Process #$1 invoked by $exe_file."

else

echo "No such process running."

This elaborate script can almost be replaced by

ps ax | grep $1 | awk '{ print $5 }'

However, this will not work...

+ because the fifth field of 'ps' is argv[0] of the process,

+ not the executable file path.

However, either of the following would work.

find /proc/$1/exe -printf '%l\n'

lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'

Additional commentary by Stephane Chazelas.

exit 0

Example 29-4. On-line connect status

!/bin/bash

connect-stat.sh

Note that this script may need modification

+ to work with a wireless connection.

PROCNAME=pppd # ppp daemon

PROCFILENAME=status # Where to look.

NOTCONNECTED=85

INTERVAL=2 # Update every 2 seconds.

pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME |

awk '{ print $1 }' )

Finding the process number of 'pppd', the 'ppp daemon'.

Have to filter out the process lines generated by the search itself.

However, as Oleg Philon points out,

+ this could have been considerably simplified by using "pidof".

pidno=$( pidof $PROCNAME )

Moral of the story:

+ When a command sequence gets too complex, look for a shortcut.

if [ -z "$pidno" ] # If no pid, then process is not running.

then

echo "Not connected."

exit $NOTCONNECTED

else

echo "Connected."; echo

while [ true ] # Endless loop, script can be improved here.

if [ ! -e "/proc/$pidno/$PROCFILENAME" ]

# While process running, then "status" file exists.

then

echo "Disconnected."

exit $NOTCONNECTED

netstat -s | grep "packets received" # Get some connect statistics.

netstat -s | grep "packets delivered"

sleep $INTERVAL

echo; echo

done

exit 0

As it stands, this script must be terminated with a Control-C.

Exercises:

---------

Improve the script so it exits on a "q" keystroke.

Make the script more user-friendly in other ways.

Fix the script to work with wireless/DSL connections.

Warning

In general, it is dangerous to write to the files in /proc, as this

can corrupt the filesystem or crash the machine.

________________________________________________________________

Chapter 30. Network Programming

The Net's a cross between an elephant and a white elephant sale: it

never forgets, and it's always crap.

--Nemo

A Linux system has quite a number of tools for accessing,

manipulating, and troubleshooting network connections. We can

incorporate some of these tools into scripts -- scripts that expand

our knowledge of networking, useful scripts that can facilitate the

administration of a network.

Here is a simple CGI script that demonstrates connecting to a remote

server.

Example 30-1. Print the server environment

!/bin/bash

test-cgi.sh

by Michael Zick

Used with permission

May have to change the location for your site.

(At the ISP's servers, Bash may not be in the usual place.)

Other places: /usr/bin or /usr/local/bin

Might even try it without any path in sha-bang.

Disable filename globbing.

set -f

Header tells browser what to expect.

echo Content-type: text/plain

echo

echo CGI/1.0 test script report:

echo

echo environment settings:

set

echo

echo whereis bash?

whereis bash

echo

echo who are we?

echo ${BASH_VERSINFO[*]}

echo

echo argc is $#. argv is "$*".

echo

CGI/1.0 expected environment variables.

echo SERVER_SOFTWARE = $SERVER_SOFTWARE

echo SERVER_NAME = $SERVER_NAME

echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE

echo SERVER_PROTOCOL = $SERVER_PROTOCOL

echo SERVER_PORT = $SERVER_PORT

echo REQUEST_METHOD = $REQUEST_METHOD

echo HTTP_ACCEPT = "$HTTP_ACCEPT"

echo PATH_INFO = "$PATH_INFO"

echo PATH_TRANSLATED = "$PATH_TRANSLATED"

echo SCRIPT_NAME = "$SCRIPT_NAME"

echo QUERY_STRING = "$QUERY_STRING"

echo REMOTE_HOST = $REMOTE_HOST

echo REMOTE_ADDR = $REMOTE_ADDR

echo REMOTE_USER = $REMOTE_USER

echo AUTH_TYPE = $AUTH_TYPE

echo CONTENT_TYPE = $CONTENT_TYPE

echo CONTENT_LENGTH = $CONTENT_LENGTH

exit 0

Here document to give short instructions.

:<<-'_test_CGI_'

1) Drop this in your http://domain.name/cgi-bin directory.

2) Then, open http://domain.name/cgi-bin/test-cgi.sh.

_test_CGI_

For security purposes, it may be helpful to identify the IP addresses

a computer is accessing.

Example 30-2. IP addresses

!/bin/bash

ip-addresses.sh

List the IP addresses your computer is connected to.

Inspired by Greg Bledsoe's ddos.sh script,

Linux Journal, 09 March 2011.

URL:

http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos

Greg licensed his script under the GPL2,

+ and as a derivative, this script is likewise GPL2.

connection_type=TCP # Also try UDP.

field=2 # Which field of the output we're interested in.

no_match=LISTEN # Filter out records containing this. Why?

lsof_args=-ni # -i lists Internet-associated files.

# -n preserves numerical IP addresses.

# What happens without the -n option? Try it.

router="[0-9][0-9][0-9][0-9][0-9]->"

Delete the router info.

lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |

awk '{print $9}' | cut -d : -f $field | sort | uniq |

sed s/"^$router"//

Bledsoe's script assigns the output of a filtered IP list,

(similar to lines 19-22, above) to a variable.

He checks for multiple connections to a single IP address,

then uses:

iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset

... within a 60-second delay loop to bounce packets from DDOS attacks.

Exercise:

--------

Use the 'iptables' command to extend this script

+ to reject connection attempts from well-known spammer IP domains.

More examples of network programming:

1. Getting the time from nist.gov

2. Downloading a URL

3. A GRE tunnel

4. Checking if an Internet server is up

5. Example 16-41

6. Example A-28

7. Example A-29

8. Example 29-1

See also the networking commands in the System and Administrative

Commands chapter and the communications commands in the External

Filters, Programs and Commands chapter.

________________________________________________________________

Chapter 31. Of Zeros and Nulls

Faultily faultless, icily regular, splendidly null

Dead perfection; no more.

--Alfred Lord Tennyson

/dev/zero ... /dev/null

Uses of /dev/null

Think of /dev/null as a black hole. It is essentially the

equivalent of a write-only file. Everything written to it

disappears. Attempts to read or output from it result in

nothing. All the same, /dev/null can be quite useful from both

the command-line and in scripts.

Suppressing stdout.

cat $filename >/dev/null

Contents of the file will not list to stdout.

Suppressing stderr (from Example 16-3).

rm $badname 2>/dev/null

So error messages [stderr] deep-sixed.

Suppressing output from both stdout and stderr.

cat $filename 2>/dev/null >/dev/null

If "$filename" does not exist, there will be no error message output.

If "$filename" does exist, the contents of the file will not list to stdout.

Therefore, no output at all will result from the above line of code.

This can be useful in situations where the return code from a command

+ needs to be tested, but no output is desired.

cat $filename &>/dev/null

also works, as Baris Cicek points out.

Deleting contents of a file, but preserving the file itself,

with all attendant permissions (from Example 2-1 and Example

2-3):

cat /dev/null > /var/log/messages

: > /var/log/messages has same effect, but does not spawn a new process.

cat /dev/null > /var/log/wtmp

Automatically emptying the contents of a logfile (especially

good for dealing with those nasty "cookies" sent by commercial

Web sites):

Example 31-1. Hiding the cookie jar

Obsolete Netscape browser.

Same principle applies to newer browsers.

if [ -f ~/.netscape/cookies ] # Remove, if exists.

then

rm -f ~/.netscape/cookies

ln -s /dev/null ~/.netscape/cookies

All cookies now get sent to a black hole, rather than saved to disk.

Uses of /dev/zero

Like /dev/null, /dev/zero is a pseudo-device file, but it

actually produces a stream of nulls (binary zeros, not the

ASCII kind). Output written to /dev/zero disappears, and it is

fairly difficult to actually read the nulls emitted there,

though it can be done with od or a hex editor. The chief use

of /dev/zero is creating an initialized dummy file of

predetermined length intended as a temporary swap file.

Example 31-2. Setting up a swapfile using /dev/zero

!/bin/bash

Creating a swap file.

A swap file provides a temporary storage cache

+ which helps speed up certain filesystem operations.

ROOT_UID=0 # Root has $UID 0.

E_WRONG_USER=85 # Not root?

FILE=/swap

BLOCKSIZE=1024

MINBLOCKS=40

SUCCESS=0

This script must be run as root.

if [ "$UID" -ne "$ROOT_UID" ]

then

echo; echo "You must be root to run this script."; echo

exit $E_WRONG_USER

blocks=${1:-$MINBLOCKS} # Set to default of 40 blocks,

#+ if nothing specified on command-line.

This is the equivalent of the command block below.

--------------------------------------------------

if [ -n "$1" ]

then

blocks=$1

else

blocks=$MINBLOCKS

fi

--------------------------------------------------

if [ "$blocks" -lt $MINBLOCKS ]

then

blocks=$MINBLOCKS # Must be at least 40 blocks long.

echo "Creating swap file of size $blocks blocks (KB)."

dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file.

mkswap $FILE $blocks # Designate it a swap file.

swapon $FILE # Activate swap file.

retcode=$? # Everything worked?

Note that if one or more of these commands fails,

+ then it could cause nasty problems.

Exercise:

Rewrite the above block of code so that if it does not execute

+ successfully, then:

1) an error message is echoed to stderr,

2) all temporary files are cleaned up, and

3) the script exits in an orderly fashion with an

+ appropriate error code.

echo "Swap file created and activated."

exit $retcode

Another application of /dev/zero is to "zero out" a file of a

designated size for a special purpose, such as mounting a

filesystem on a loopback device (see Example 17-8) or

"securely" deleting a file (see Example 16-61).

Example 31-3. Creating a ramdisk

!/bin/bash

ramdisk.sh

A "ramdisk" is a segment of system RAM memory

+ which acts as if it were a filesystem.

Its advantage is very fast access (read/write time).

Disadvantages: volatility, loss of data on reboot or powerdown,

+ less RAM available to system.

Of what use is a ramdisk?

Keeping a large dataset, such as a table or dictionary on ramdisk,

+ speeds up data lookup, since memory access is much faster than disk access.

E_NON_ROOT_USER=70 # Must run as root.

ROOTUSER_NAME=root

MOUNTPT=/mnt/ramdisk # Create with mkdir /mnt/ramdisk.

SIZE=2000 # 2K blocks (change as appropriate)

BLOCKSIZE=1024 # 1K (1024 byte) block size

DEVICE=/dev/ram0 # First ram device

username=`id -nu`

if [ "$username" != "$ROOTUSER_NAME" ]

then

echo "Must be root to run \"`basename $0`\"."

exit $E_NON_ROOT_USER

if [ ! -d "$MOUNTPT" ] # Test whether mount point already there,

then #+ so no error if this script is run

mkdir $MOUNTPT #+ multiple times.

dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device.

# Why is this necessary?

mke2fs $DEVICE # Create an ext2 filesystem on it.

mount $DEVICE $MOUNTPT # Mount it.

chmod 777 $MOUNTPT # Enables ordinary user to access ramdisk.

# However, must be root to unmount it.

Need to test whether above commands succeed. Could cause problems otherwise.

Exercise: modify this script to make it safer.

echo "\"$MOUNTPT\" now available for use."

The ramdisk is now accessible for storing files, even by an ordinary user.

Caution, the ramdisk is volatile, and its contents will disappear

+ on reboot or power loss.

Copy anything you want saved to a regular directory.

After reboot, run this script to again set up ramdisk.

Remounting /mnt/ramdisk without the other steps will not work.

Suitably modified, this script can by invoked in /etc/rc.d/rc.local,

+ to set up ramdisk automatically at bootup.

That may be appropriate on, for example, a database server.

exit 0

In addition to all the above, /dev/zero is needed by ELF

(Executable and Linking Format) UNIX/Linux binaries.

________________________________________________________________

Chapter 32. Debugging

Debugging is twice as hard as writing the code in the first place.

Therefore, if you write the code as cleverly as possible, you are, by

definition, not smart enough to debug it.

--Brian Kernighan

The Bash shell contains no built-in debugger, and only bare-bones

debugging-specific commands and constructs. Syntax errors or outright

typos in the script generate cryptic error messages that are often of

no help in debugging a non-functional script.

Example 32-1. A buggy script

!/bin/bash

ex74.sh

This is a buggy script.

Where, oh where is the error?

a=37

if [$a -gt 27 ]

then

echo $a

exit $? # 0! Why?

Output from script:

./ex74.sh: [37: command not found

What's wrong with the above script? Hint: after the if.

Example 32-2. Missing keyword

!/bin/bash

missing-keyword.sh

What error message will this script generate? And why?

for a in 1 2 3

echo "$a"

done # Required keyword 'done' commented out in line 8.

exit 0 # Will not exit here!

=== #

From command line, after script terminates:

echo $? # 2

Output from script:

missing-keyword.sh: line 10: syntax error: unexpected end of file

Note that the error message does not necessarily reference the line

in which the error occurs, but the line where the Bash interpreter

finally becomes aware of the error.

Error messages may disregard comment lines in a script when reporting

the line number of a syntax error.

What if the script executes, but does not work as expected? This is

the all too familiar logic error.

Example 32-3. test24: another buggy script

!/bin/bash

This script is supposed to delete all filenames in current directory

+ containing embedded spaces.

It doesn't work.

Why not?

badname=`ls | grep ' '`

Try this:

echo "$badname"

rm "$badname"

exit 0

Try to find out what's wrong with Example 32-3 by uncommenting the

echo "$badname" line. Echo statements are useful for seeing whether

what you expect is actually what you get.

In this particular case, rm "$badname" will not give the desired

results because $badname should not be quoted. Placing it in quotes

ensures that rm has only one argument (it will match only one

filename). A partial fix is to remove to quotes from $badname and to

reset $IFS to contain only a newline, IFS= \n'. However, there are

simpler ways of going about it.

Correct methods of deleting filenames containing spaces.

rm *\ *

rm *" "*

rm *' '*

Thank you. S.C.

Summarizing the symptoms of a buggy script,

1. It bombs with a "syntax error" message, or

2. It runs, but does not work as expected (logic error).

3. It runs, works as expected, but has nasty side effects (logic

bomb).

Tools for debugging non-working scripts include

1. Inserting echo statements at critical points in the script to

trace the variables, and otherwise give a snapshot of what is

going on.

Tip

Even better is an echo that echoes only when debug is on.

debecho (debug-echo), by Stefano Falsetto ###

Will echo passed parameters only if DEBUG is set to a value. ###

debecho () {

if [ ! -z "$DEBUG" ]; then

echo "$1" >&2

# ^^^ to stderr

}

DEBUG=on

Whatever=whatnot

debecho $Whatever # whatnot

DEBUG=

Whatever=notwhat

debecho $Whatever # (Will not echo.)

2. Using the tee filter to check processes or data flows at critical

points.

3. Setting option flags -n -v -x

sh -n scriptname checks for syntax errors without actually

running the script. This is the equivalent of inserting set -n or

set -o noexec into the script. Note that certain types of syntax

errors can slip past this check.

sh -v scriptname echoes each command before executing it. This is

the equivalent of inserting set -v or set -o verbose in the

script.

The -n and -v flags work well together. sh -nv scriptname gives a

verbose syntax check.

sh -x scriptname echoes the result each command, but in an

abbreviated manner. This is the equivalent of inserting set -x or

set -o xtrace in the script.

Inserting set -u or set -o nounset in the script runs it, but

gives an unbound variable error message and aborts the script.

set -u # Or set -o nounset

Setting a variable to null will not trigger the error/abort.

unset_var=

echo $unset_var # Unset (and undeclared) variable.

echo "Should not echo!"

sh t2.sh

t2.sh: line 6: unset_var: unbound variable

4. Using an "assert" function to test a variable or condition at

critical points in a script. (This is an idea borrowed from C.)

Example 32-4. Testing a condition with an assert

!/bin/bash

assert.sh

assert () # If condition false,

{ #+ exit from script

#+ with appropriate error message.

E_PARAM_ERR=98

E_ASSERT_FAILED=99

if [ -z "$2" ] # Not enough parameters passed

then #+ to assert() function.

return $E_PARAM_ERR # No damage done.

lineno=$2

if [ ! $1 ]

then

echo "Assertion failed: \"$1\""

echo "File \"$0\", line $lineno" # Give name of file and line number.

exit $E_ASSERT_FAILED

# else

# return

# and continue executing the script.

} # Insert a similar assert() function into a script you need to debug.

a=5

b=4

condition="$a -lt $b" # Error message and exit from script.

# Try setting "condition" to something else

#+ and see what happens.

assert "$condition" $LINENO

The remainder of the script executes only if the "assert" does not fail.

Some commands.

Some more commands . . .

echo "This statement echoes only if the \"assert\" does not fail."

. . .

More commands . . .

exit $?

5. Using the $LINENO variable and the caller builtin.

6. Trapping at exit.

The exit command in a script triggers a signal 0, terminating the

process, that is, the script itself. [122] It is often useful to

trap the exit, forcing a "printout" of variables, for example.

The trap must be the first command in the script.

Trapping signals

trap

Specifies an action on receipt of a signal; also useful for

debugging.

A signal is a message sent to a process, either by the kernel or

another process, telling it to take some specified action (usually to

terminate). For example, hitting a Control-C sends a user interrupt,

an INT signal, to a running program.

A simple instance:

trap '' 2

Ignore interrupt 2 (Control-C), with no action specified.

trap 'echo "Control-C disabled."' 2

Message when Control-C pressed.

Example 32-5. Trapping at exit

!/bin/bash

Hunting variables with a trap.

trap 'echo Variable Listing --- a = $a b = $b' EXIT

EXIT is the name of the signal generated upon exit from a script.

The command specified by the "trap" doesn't execute until

+ the appropriate signal is sent.

echo "This prints before the \"trap\" --"

echo "even though the script sees the \"trap\" first."

echo

a=39

b=36

exit 0

Note that commenting out the 'exit' command makes no difference,

+ since the script exits in any case after running out of commands.

Example 32-6. Cleaning up after Control-C

!/bin/bash

logon.sh: A quick 'n dirty script to check whether you are on-line yet.

umask 177 # Make sure temp files are not world readable.

TRUE=1

LOGFILE=/var/log/messages

Note that $LOGFILE must be readable

+ (as root, chmod 644 /var/log/messages).

TEMPFILE=temp.$

Create a "unique" temp file name, using process id of the script.

Using 'mktemp' is an alternative.

For example:

TEMPFILE=`mktemp temp.XXXXXX`

KEYWORD=address

At logon, the line "remote IP address xxx.xxx.xxx.xxx"

appended to /var/log/messages.

ONLINE=22

USER_INTERRUPT=13

CHECK_LINES=100

How many lines in log file to check.

trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT

Cleans up the temp file if script interrupted by control-c.

echo

while [ $TRUE ] #Endless loop.

tail -n $CHECK_LINES $LOGFILE> $TEMPFILE

# Saves last 100 lines of system log file as temp file.

# Necessary, since newer kernels generate many log messages at log on.

search=`grep $KEYWORD $TEMPFILE`

# Checks for presence of the "IP address" phrase,

#+ indicating a successful logon.

if [ ! -z "$search" ] # Quotes necessary because of possible spaces.

then

echo "On-line"

rm -f $TEMPFILE # Clean up temp file.

exit $ONLINE

else

echo -n "." # The -n option to echo suppresses newline,

#+ so you get continuous rows of dots.

sleep 1

done

Note: if you change the KEYWORD variable to "Exit",

+ this script can be used while on-line

+ to check for an unexpected logoff.

Exercise: Change the script, per the above note,

and prettify it.

exit 0

Nick Drage suggests an alternate method:

while true

do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0

echo -n "." # Prints dots (.....) until connected.

sleep 2

done

Problem: Hitting Control-C to terminate this process may be insufficient.

+ (Dots may keep on echoing.)

Exercise: Fix this.

Stephane Chazelas has yet another alternative:

CHECK_INTERVAL=1

while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD"

do echo -n .

sleep $CHECK_INTERVAL

done

echo "On-line"

Exercise: Discuss the relative strengths and weaknesses

of each of these various approaches.

Example 32-7. A Simple Implementation of a Progress Bar

! /bin/bash

progress-bar2.sh

Author: Graham Ewart (with reformatting by ABS Guide author).

Used in ABS Guide with permission (thanks!).

Invoke this script with bash. It doesn't work with sh.

interval=1

long_interval=10

{

trap "exit" SIGUSR1

sleep $interval; sleep $interval

while true

echo -n '.' # Use dots.

sleep $interval

done; } & # Start a progress bar as a background process.

pid=$!

trap "echo !; kill -USR1 $pid; wait $pid" EXIT # To handle ^C.

echo -n 'Long-running process '

sleep $long_interval

echo ' Finished!'

kill -USR1 $pid

wait $pid # Stop the progress bar.

trap EXIT

exit $?

Note

The DEBUG argument to trap causes a specified action to execute after

every command in a script. This permits tracing variables, for

example.

Example 32-8. Tracing a variable

!/bin/bash

trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG

Echoes the value of $variable after every command.

variable=29; line=$LINENO

echo " Just initialized \$variable to $variable in line number $line."

let "variable *= 3"; line=$LINENO

echo " Just multiplied \$variable by 3 in line number $line."

exit 0

The "trap 'command1 . . . command2 . . .' DEBUG" construct is

+ more appropriate in the context of a complex script,

+ where inserting multiple "echo $variable" statements might be

+ awkward and time-consuming.

Thanks, Stephane Chazelas for the pointer.

Output of script:

VARIABLE-TRACE> $variable = ""

VARIABLE-TRACE> $variable = "29"

Just initialized $variable to 29.

VARIABLE-TRACE> $variable = "29"

VARIABLE-TRACE> $variable = "87"

Just multiplied $variable by 3.

VARIABLE-TRACE> $variable = "87"

Of course, the trap command has other uses aside from debugging, such

as disabling certain keystrokes within a script (see Example A-43).

Example 32-9. Running multiple processes (on an SMP box)

!/bin/bash

parent.sh

Running multiple processes on an SMP box.

Author: Tedman Eng

This is the first of two scripts,

+ both of which must be present in the current working directory.

LIMIT=$1 # Total number of process to start

NUMPROC=4 # Number of concurrent threads (forks?)

PROCID=1 # Starting Process ID

echo "My PID is $"

function start_thread() {

if [ $PROCID -le $LIMIT ] ; then

./child.sh $PROCID&

let "PROCID++"

else

echo "Limit reached."

wait

exit

}

while [ "$NUMPROC" -gt 0 ]; do

start_thread;

let "NUMPROC--"

done

while true

trap "start_thread" SIGRTMIN

done

exit 0

======== Second script follows ========

!/bin/bash

child.sh

Running multiple processes on an SMP box.

This script is called by parent.sh.

Author: Tedman Eng

temp=$RANDOM

index=$1

shift

let "temp %= 5"

let "temp += 4"

echo "Starting $index Time:$temp" "$@"

sleep ${temp}

echo "Ending $index"

kill -s SIGRTMIN $PPID

exit 0

======================= SCRIPT AUTHOR'S NOTES ======================= #

It's not completely bug free.

I ran it with limit = 500 and after the first few hundred iterations,

+ one of the concurrent threads disappeared!

Not sure if this is collisions from trap signals or something else.

Once the trap is received, there's a brief moment while executing the

+ trap handler but before the next trap is set. During this time, it may

+ be possible to miss a trap signal, thus miss spawning a child process.

No doubt someone may spot the bug and will be writing

+ . . . in the future.

===================================================================== #

----------------------------------------------------------------------#

The following is the original script written by Vernia Damiano.

Unfortunately, it doesn't work properly.

!/bin/bash

Must call script with at least one integer parameter

+ (number of concurrent processes).

All other parameters are passed through to the processes started.

INDICE=8 # Total number of process to start

TEMPO=5 # Maximum sleep time per process

E_BADARGS=65 # No arg(s) passed to script.

if [ $# -eq 0 ] # Check for at least one argument passed to script.

then

echo "Usage: `basename $0` number_of_processes [passed params]"

exit $E_BADARGS

NUMPROC=$1 # Number of concurrent process

shift

PARAMETRI=( "$@" ) # Parameters of each process

function avvia() {

local temp

local index

temp=$RANDOM

index=$1

shift

let "temp %= $TEMPO"

let "temp += 1"

echo "Starting $index Time:$temp" "$@"

sleep ${temp}

echo "Ending $index"

kill -s SIGRTMIN $

}

function parti() {

if [ $INDICE -gt 0 ] ; then

avvia $INDICE "${PARAMETRI[@]}" &

let "INDICE--"

else

trap : SIGRTMIN

}

trap parti SIGRTMIN

while [ "$NUMPROC" -gt 0 ]; do

parti;

let "NUMPROC--"

done

wait

trap - SIGRTMIN

exit $?

: <<SCRIPT_AUTHOR_COMMENTS

I had the need to run a program, with specified options, on a number of

different files, using a SMP machine. So I thought [I'd] keep running

a specified number of processes and start a new one each time . . . one

of these terminates.

The "wait" instruction does not help, since it waits for a given process

or *all* process started in background. So I wrote [this] bash script

that can do the job, using the "trap" instruction.

--Vernia Damiano

SCRIPT_AUTHOR_COMMENTS

Note

trap '' SIGNAL (two adjacent apostrophes) disables SIGNAL for the

remainder of the script. trap SIGNAL restores the functioning of

SIGNAL once more. This is useful to protect a critical portion of a

script from an undesirable interrupt.

trap '' 2 # Signal 2 is Control-C, now disabled.

command

trap 2 # Reenables Control-C

Version 3 of Bash adds the following internal variables for use by

the debugger.

1. $BASH_ARGC

Number of command-line arguments passed to script, similar to $#.

2. $BASH_ARGV

Final command-line parameter passed to script, equivalent ${!#}.

3. $BASH_COMMAND

Command currently executing.

4. $BASH_EXECUTION_STRING

The option string following the -c option to Bash.

5. $BASH_LINENO

In a function, indicates the line number of the function call.

6. $BASH_REMATCH

Array variable associated with =~ conditional regex matching.

7. $BASH_SOURCE

This is the name of the script, usually the same as $0.

8. $BASH_SUBSHELL

________________________________________________________________

Chapter 33. Options

Options are settings that change shell and/or script behavior.

The set command enables options within a script. At the point in the

script where you want the options to take effect, use set -o

option-name or, in short form, set -option-abbrev. These two forms

are equivalent.

#!/bin/bash

set -o verbose

# Echoes all commands before executing.

#!/bin/bash

set -v

# Exact same effect as above.

Note

To disable an option within a script, use set +o option-name or set

+option-abbrev.

#!/bin/bash

set -o verbose

# Command echoing on.

command

...

command

set +o verbose

# Command echoing off.

command

# Not echoed.

set -v

# Command echoing on.

command

...

command

set +v

# Command echoing off.

command

exit 0

An alternate method of enabling options in a script is to specify

them immediately following the #! script header.

#!/bin/bash -x

# Body of script follows.

It is also possible to enable script options from the command line.

Some options that will not work with set are available this way.

Among these are -i, force script to run interactive.

bash -v script-name

bash -o verbose script-name

The following is a listing of some useful options. They may be

specified in either abbreviated form (preceded by a single dash) or

by complete name (preceded by a double dash or by -o).

Table 33-1. Bash options

Abbreviation Name Effect

-B brace expansion Enable brace expansion (default setting = on)

+B brace expansion Disable brace expansion

-C noclobber Prevent overwriting of files by redirection (may be

overridden by >|)

-D (none) List double-quoted strings prefixed by $, but do not

execute commands in script

-a allexport Export all defined variables

-b notify Notify when jobs running in background terminate (not of

much use in a script)

-c ... (none) Read commands from ...

checkjobs Informs user of any open jobs upon shell exit. Introduced

in version 4 of Bash, and still "experimental." Usage: shopt -s

checkjobs (Caution: may hang!)

-e errexit Abort script at first error, when a command exits with

non-zero status (except in until or while loops, if-tests, list

constructs)

-f noglob Filename expansion (globbing) disabled

globstar globbing star-match Enables the ** globbing operator

(version 4+ of Bash). Usage: shopt -s globstar

-i interactive Script runs in interactive mode

-n noexec Read commands in script, but do not execute them (syntax

check)

-o Option-Name (none) Invoke the Option-Name option

-o posix POSIX Change the behavior of Bash, or invoked script, to

conform to POSIX standard.

-o pipefail pipe failure Causes a pipeline to return the exit status

of the last command in the pipe that returned a non-zero return

value.

-p privileged Script runs as "suid" (caution!)

-r restricted Script runs in restricted mode (see Chapter 22).

-s stdin Read commands from stdin

-t (none) Exit after first command

-u nounset Attempt to use undefined variable outputs error message,

and forces an exit

-v verbose Print each command to stdout before executing it

-x xtrace Similar to -v, but expands commands

- (none) End of options flag. All other arguments are positional

parameters.

-- (none) Unset positional parameters. If arguments given (-- arg1

arg2), positional parameters set to arguments.

________________________________________________________________

Chapter 34. Gotchas

Turandot: Gli enigmi sono tre, la morte una!

Caleph: No, no! Gli enigmi sono tre, una la vita!

--Puccini

Here are some (non-recommended!) scripting practices that will bring

excitement into an otherwise dull life.

* Assigning reserved words or characters to variable names.

case=value0 # Causes problems.

23skidoo=value1 # Also problems.

Variable names starting with a digit are reserved by the shell.

Try _23skidoo=value1. Starting variables with an underscore is okay.

However . . . using just an underscore will not work.

_=25

echo $_ # $_ is a special variable set to last arg of last command.

But . . . _ is a valid function name!

xyz((!*=value2 # Causes severe problems.

As of version 3 of Bash, periods are not allowed within variable names.

* Using a hyphen or other reserved characters in a variable name

(or function name).

var-1=23

Use 'var_1' instead.

function-whatever () # Error

Use 'function_whatever ()' instead.

As of version 3 of Bash, periods are not allowed within function names.

function.whatever () # Error

Use 'functionWhatever ()' instead.

* Using the same name for a variable and a function. This can make

a script difficult to understand.

do_something ()

{

echo "This function does something with \"$1\"."

}

do_something=do_something

do_something do_something

All this is legal, but highly confusing.

* Using whitespace inappropriately. In contrast to other

programming languages, Bash can be quite finicky about

whitespace.

var1 = 23 # 'var1=23' is correct.

On line above, Bash attempts to execute command "var1"

with the arguments "=" and "23".

let c = $a - $b # Instead: let c=$a-$b or let "c = $a - $b"

if [ $a -le 5] # if [ $a -le 5 ] is correct.

^^ if [ "$a" -le 5 ] is even better.

# [[ $a -le 5 ]] also works.

* Not terminating with a semicolon the final command in a code

block within curly brackets.

{ ls -l; df; echo "Done." }

bash: syntax error: unexpected end of file

{ ls -l; df; echo "Done."; }

^ ### Final command needs semicolon.

* Assuming uninitialized variables (variables before a value is

assigned to them) are "zeroed out". An uninitialized variable has

a value of null, not zero.

!/bin/bash

echo "uninitialized_var = $uninitialized_var"

uninitialized_var =

However . . .

if $BASH_VERSION >= 4.2; then

if [[ ! -v uninitialized_var ]]

then

uninitialized_var=0 # Initialize it to zero!

* Mixing up = and -eq in a test. Remember, = is for comparing

literal variables and -eq for integers.

if [ "$a" = 273 ] # Is $a an integer or string?

if [ "$a" -eq 273 ] # If $a is an integer.

Sometimes you can interchange -eq and = without adverse consequences.

However . . .

a=273.0 # Not an integer.

if [ "$a" = 273 ]

then

echo "Comparison works."

else

echo "Comparison does not work."

fi # Comparison does not work.

Same with a=" 273" and a="0273".

Likewise, problems trying to use "-eq" with non-integer values.

if [ "$a" -eq 273.0 ]

then

echo "a = $a"

fi # Aborts with an error message.

test.sh: [: 273.0: integer expression expected

* Misusing string comparison operators.

Example 34-1. Numerical and string comparison are not equivalent

!/bin/bash

bad-op.sh: Trying to use a string comparison on integers.

echo

number=1

The following while-loop has two errors:

+ one blatant, and the other subtle.

while [ "$number" < 5 ] # Wrong! Should be: while [ "$number" -lt 5 ]

echo -n "$number "

let "number += 1"

done

Attempt to run this bombs with the error message:

+ bad-op.sh: line 10: 5: No such file or directory

Within single brackets, "<" must be escaped,

+ and even then, it's still wrong for comparing integers.

echo "---------------------"

while [ "$number" \< 5 ] # 1 2 3 4

do #

echo -n "$number " # It *seems* to work, but . . .

let "number += 1" #+ it actually does an ASCII comparison,

done #+ rather than a numerical one.

echo; echo "---------------------"

This can cause problems. For example:

lesser=5

greater=105

if [ "$greater" \< "$lesser" ]

then

echo "$greater is less than $lesser"

fi # 105 is less than 5

In fact, "105" actually is less than "5"

+ in a string comparison (ASCII sort order).

echo

exit 0

* Attempting to use let to set string variables.

let "a = hello, you"

echo "$a" # 0

* Sometimes variables within "test" brackets ([ ]) need to be

quoted (double quotes). Failure to do so may cause unexpected

behavior. See Example 7-6, Example 20-5, and Example 9-6.

* Quoting a variable containing whitespace prevents splitting.

Sometimes this produces unintended consequences.

* Commands issued from a script may fail to execute because the

script owner lacks execute permission for them. If a user cannot

invoke a command from the command-line, then putting it into a

script will likewise fail. Try changing the attributes of the

command in question, perhaps even setting the suid bit (as root,

of course).

* Attempting to use - as a redirection operator (which it is not)

will usually result in an unpleasant surprise.

command1 2> - | command2

Trying to redirect error output of command1 into a pipe . . .

. . . will not work.

command1 2>& - | command2 # Also futile.

Thanks, S.C.

* Using Bash version 2+ functionality may cause a bailout with

error messages. Older Linux machines may have version 1.XX of

Bash as the default installation.

!/bin/bash

minimum_version=2

Since Chet Ramey is constantly adding features to Bash,

you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate.

E_BAD_VERSION=80

if [ "$BASH_VERSION" \< "$minimum_version" ]

then

echo "This script works only with Bash, version $minimum or greater."

echo "Upgrade strongly recommended."

exit $E_BAD_VERSION

...

* Using Bash-specific functionality in a Bourne shell script

(#!/bin/sh) on a non-Linux machine may cause unexpected behavior.

A Linux system usually aliases sh to bash, but this does not

necessarily hold true for a generic UNIX machine.

* Using undocumented features in Bash turns out to be a dangerous

practice. In previous releases of this book there were several

scripts that depended on the "feature" that, although the maximum

value of an exit or return value was 255, that limit did not

apply to negative integers. Unfortunately, in version 2.05b and

later, that loophole disappeared. See Example 24-9.

* In certain contexts, a misleading exit status may be returned.

This may occur when setting a local variable within a function or

when assigning an arithmetic value to a variable.

* The exit status of an arithmetic expression is not equivalent to

an error code.

var=1 && ((--var)) && echo $var

^^^^^^^^^ Here the and-list terminates with exit status 1.

$var doesn't echo!

echo $? # 1

* A script with DOS-type newlines (\r\n) will fail to execute,

since #!/bin/bash\r\n is not recognized, not the same as the

expected #!/bin/bash\n. The fix is to convert the script to

UNIX-style newlines.

!/bin/bash

echo "Here"

unix2dos $0 # Script changes itself to DOS format.

chmod 755 $0 # Change back to execute permission.

# The 'unix2dos' command removes execute permission.

./$0 # Script tries to run itself again.

# But it won't work as a DOS file.

echo "There"

exit 0

* A shell script headed by #!/bin/sh will not run in full

Bash-compatibility mode. Some Bash-specific functions might be

disabled. Scripts that need complete access to all the

Bash-specific extensions should start with #!/bin/bash.

* Putting whitespace in front of the terminating limit string of a

here document will cause unexpected behavior in a script.

* Putting more than one echo statement in a function whose output

is captured.

add2 ()

{

echo "Whatever ... " # Delete this line!

let "retval = $1 + $2"

echo $retval

}

num1=12

num2=43

echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)"

Sum of 12 and 43 = Whatever ...

55 The "echoes" concatenate.

This will not work.

* A script may not export variables back to its parent process, the

shell, or to the environment. Just as we learned in biology, a

child process can inherit from a parent, but not vice versa.

WHATEVER=/home/bozo

export WHATEVER

exit 0

bash$ echo $WHATEVER

bash$

Sure enough, back at the command prompt, $WHATEVER remains unset.

* Setting and manipulating variables in a subshell, then attempting

to use those same variables outside the scope of the subshell

will result an unpleasant surprise.

Example 34-2. Subshell Pitfalls

!/bin/bash

Pitfalls of variables in a subshell.

outer_variable=outer

echo

echo "outer_variable = $outer_variable"

echo

(

Begin subshell

echo "outer_variable inside subshell = $outer_variable"

inner_variable=inner # Set

echo "inner_variable inside subshell = $inner_variable"

outer_variable=inner # Will value change globally?

echo "outer_variable inside subshell = $outer_variable"

Will 'exporting' make a difference?

export inner_variable

export outer_variable

Try it and see.

End subshell

)

echo

echo "inner_variable outside subshell = $inner_variable" # Unset.

echo "outer_variable outside subshell = $outer_variable" # Unchanged.

echo

exit 0

What happens if you uncomment lines 19 and 20?

Does it make a difference?

* Piping echo output to a read may produce unexpected results. In

this scenario, the read acts as if it were running in a subshell.

Instead, use the set command (as in Example 15-18).

Example 34-3. Piping the output of echo to a read

!/bin/bash

badread.sh:

Attempting to use 'echo and 'read'

+ to assign variables non-interactively.

shopt -s lastpipe

a=aaa

b=bbb

c=ccc

echo "one two three" | read a b c

Try to reassign a, b, and c.

echo

echo "a = $a" # a = aaa

echo "b = $b" # b = bbb

echo "c = $c" # c = ccc

Reassignment failed.

However . . .

Uncommenting line 6:

shopt -s lastpipe

+ fixes the problem!

This is a new feature in Bash, version 4.2.

------------------------------

Try the following alternative.

var=`echo "one two three"`

set -- $var

a=$1; b=$2; c=$3

echo "-------"

echo "a = $a" # a = one

echo "b = $b" # b = two

echo "c = $c" # c = three

Reassignment succeeded.

------------------------------

Note also that an echo to a 'read' works within a subshell.

However, the value of the variable changes only within the subshell.

a=aaa # Starting all over again.

b=bbb

c=ccc

echo; echo

echo "one two three" | ( read a b c;

echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" )

a = one

b = two

c = three

echo "-----------------"

echo "Outside subshell: "

echo "a = $a" # a = aaa

echo "b = $b" # b = bbb

echo "c = $c" # c = ccc

echo

exit 0

In fact, as Anthony Richardson points out, piping to any loop can

cause a similar problem.

Loop piping troubles.

This example by Anthony Richardson,

+ with addendum by Wilbert Berendsen.

foundone=false

find $HOME -type f -atime +30 -size 100k |

while true

read f

echo "$f is over 100KB and has not been accessed in over 30 days"

echo "Consider moving the file to archives."

foundone=true

# ------------------------------------

echo "Subshell level = $BASH_SUBSHELL"

# Subshell level = 1

# Yes, we're inside a subshell.

# ------------------------------------

done

foundone will always be false here since it is

+ set to true inside a subshell

if [ $foundone = false ]

then

echo "No files need archiving."

=====================Now, here is the correct way:=================

foundone=false

for f in $(find $HOME -type f -atime +30 -size 100k) # No pipe here.

echo "$f is over 100KB and has not been accessed in over 30 days"

echo "Consider moving the file to archives."

foundone=true

done

if [ $foundone = false ]

then

echo "No files need archiving."

==================And here is another alternative==================

Places the part of the script that reads the variables

+ within a code block, so they share the same subshell.

Thank you, W.B.

find $HOME -type f -atime +30 -size 100k | {

foundone=false

while read f

echo "$f is over 100KB and has not been accessed in over 30 days"

echo "Consider moving the file to archives."

foundone=true

done

if ! $foundone

then

echo "No files need archiving."

}

A lookalike problem occurs when trying to write the stdout of a

tail -f piped to grep.

tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log

The "error.log" file will not have anything written to it.

As Samuli Kaipiainen points out, this results from grep

+ buffering its output.

The fix is to add the "--line-buffered" parameter to grep.

* Using "suid" commands within scripts is risky, as it may

compromise system security. [123]

* Using shell scripts for CGI programming may be problematic. Shell

script variables are not "typesafe," and this can cause

undesirable behavior as far as CGI is concerned. Moreover, it is

difficult to "cracker-proof" shell scripts.

* Bash does not handle the double slash (//) string correctly.

* Bash scripts written for Linux or BSD systems may need fixups to

run on a commercial UNIX machine. Such scripts often employ the

GNU set of commands and filters, which have greater functionality

than their generic UNIX counterparts. This is particularly true

of such text processing utilites as tr.

* Sadly, updates to Bash itself have broken older scripts that used

to work perfectly fine. Let us recall how risky it is to use

undocumented Bash features.

Danger is near thee --

Beware, beware, beware, beware.

Many brave hearts are asleep in the deep.

So beware --

Beware.

--A.J. Lamb and H.W. Petrie

________________________________________________________________

Chapter 35. Scripting With Style

Get into the habit of writing shell scripts in a structured and

systematic manner. Even on-the-fly and "written on the back of an

envelope" scripts will benefit if you take a few minutes to plan and

organize your thoughts before sitting down and coding.

Herewith are a few stylistic guidelines. This is not (necessarily)

intended as an Official Shell Scripting Stylesheet.

________________________________________________________________

35.1. Unofficial Shell Scripting Stylesheet

* Comment your code. This makes it easier for others to understand

(and appreciate), and easier for you to maintain.

PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"

It made perfect sense when you wrote it last year,

+ but now it's a complete mystery.

(From Antek Sawicki's "pw.sh" script.)

Add descriptive headers to your scripts and functions.

!/bin/bash

************************************************#

xyz.sh #

written by Bozo Bozeman #

July 05, 2001 #

#

Clean up project files. #

************************************************#

E_BADDIR=85 # No such directory.

projectdir=/home/bozo/projects # Directory to clean up.

--------------------------------------------------------- #

cleanup_pfiles () #

Removes all files in designated directory. #

Parameter: $target_directory #

Returns: 0 on success, $E_BADDIR if something went wrong. #

--------------------------------------------------------- #

cleanup_pfiles ()

{

if [ ! -d "$1" ] # Test if target directory exists.

then

echo "$1 is not a directory."

return $E_BADDIR

rm -f "$1"/*

return 0 # Success.

}

cleanup_pfiles $projectdir

exit $?

* Avoid using "magic numbers," [124] that is, "hard-wired" literal

constants. Use meaningful variable names instead. This makes the

script easier to understand and permits making changes and

updates without breaking the application.

if [ -f /var/log/messages ]

then

...

A year later, you decide to change the script to check /var/log/syslog.

It is now necessary to manually change the script, instance by instance,

+ and hope nothing breaks.

A better way:

LOGFILE=/var/log/messages # Only line that needs to be changed.

if [ -f "$LOGFILE" ]

then

...

* Choose descriptive names for variables and functions.

fl=`ls -al $dirname` # Cryptic.

file_listing=`ls -al $dirname` # Better.

MAXVAL=10 # All caps used for a script constant.

while [ "$index" -le "$MAXVAL" ]

...

E_NOTFOUND=95 # Uppercase for an errorcode,

#+ and name prefixed with E_.

if [ ! -e "$filename" ]

then

echo "File $filename not found."

exit $E_NOTFOUND

MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental

export MAIL_DIRECTORY #+ variable.

GetAnswer () # Mixed case works well for a

{ #+ function name, especially

prompt=$1 #+ when it improves legibility.

echo -n $prompt

read answer

return $answer

}

GetAnswer "What is your favorite number? "

favorite_number=$?

echo $favorite_number

_uservariable=23 # Permissible, but not recommended.

It's better for user-defined variables not to start with an underscore.

Leave that for system variables.

* Use exit codes in a systematic and meaningful way.

E_WRONG_ARGS=95

...

exit $E_WRONG_ARGS

Redundant and non-intuitive.

if COMMAND

...

More concise (if perhaps not quite as legible).

... reading the UNIX source code to the Bourne shell (/bin/sh). I was

shocked at how much simple algorithms could be made cryptic, and

therefore useless, by a poor choice of code style. I asked myself,

"Could someone be proud of this code?"

--Landon Noll

________________________________________________________________

Chapter 36. Miscellany

Nobody really knows what the Bourne shell's grammar is. Even

examination of the source code is little help.

--Tom Duff

________________________________________________________________

36.1. Interactive and non-interactive shells and scripts

An interactive shell reads commands from user input on a tty. Among

other things, such a shell reads startup files on activation,

displays a prompt, and enables job control by default. The user can

interact with the shell.

A shell running a script is always a non-interactive shell. All the

same, the script can still access its tty. It is even possible to

emulate an interactive shell in a script.

!/bin/bash

MY_PROMPT='$ '

while :

echo -n "$MY_PROMPT"

read line

eval "$line"

done

exit 0

This example script, and much of the above explanation supplied by

Stéphane Chazelas (thanks again).

Let us consider an interactive script to be one that requires input

from the user, usually with read statements (see Example 15-3). "Real

life" is actually a bit messier than that. For now, assume an

interactive script is bound to a tty, a script that a user has

invoked from the console or an xterm.

Init and startup scripts are necessarily non-interactive, since they

must run without human intervention. Many administrative and system

maintenance scripts are likewise non-interactive. Unvarying

repetitive tasks cry out for automation by non-interactive scripts.

Non-interactive scripts can run in the background, but interactive

ones hang, waiting for input that never comes. Handle that difficulty

by having an expect script or embedded here document feed input to an

interactive script running as a background job. In the simplest case,

redirect a file to supply input to a read statement (read variable

<file). These particular workarounds make possible general purpose

scripts that run in either interactive or non-interactive modes.

If a script needs to test whether it is running in an interactive

shell, it is simply a matter of finding whether the prompt variable,

$PS1 is set. (If the user is being prompted for input, then the

script needs to display a prompt.)

if [ -z $PS1 ] # no prompt?

if [ -v PS1 ] # On Bash 4.2+ ...

then

# non-interactive

...

else

# interactive

...

Alternatively, the script can test for the presence of option "i" in

the $- flag.

case $- in

i*) # interactive shell

;;

) # non-interactive shell

;;

(Courtesy of "UNIX F.A.Q.," 1993)

However, John Lange describes an alternative method, using the -t

test operator.

Test for a terminal!

fd=0 # stdin

As we recall, the -t test option checks whether the stdin, [ -t 0 ],

+ or stdout, [ -t 1 ], in a given script is running in a terminal.

if [ -t "$fd" ]

then

echo interactive

else

echo non-interactive

But, as John points out:

if [ -t 0 ] works ... when you're logged in locally

but fails when you invoke the command remotely via ssh.

So for a true test you also have to test for a socket.

if [[ -t "$fd" || -p /dev/stdin ]]

then

echo interactive

else

echo non-interactive

Note

Scripts may be forced to run in interactive mode with the -i option

or with a #!/bin/bash -i header. Be aware that this can cause erratic

script behavior or show error messages even when no error is present.

________________________________________________________________

36.2. Shell Wrappers

A wrapper is a shell script that embeds a system command or utility,

that accepts and passes a set of parameters to that command. [125]

Wrapping a script around a complex command-line simplifies invoking

it. This is expecially useful with sed and awk.

A sed or awk script would normally be invoked from the command-line

by a sed -e 'commands' or awk 'commands'. Embedding such a script in

a Bash script permits calling it more simply, and makes it reusable.

This also enables combining the functionality of sed and awk, for

example piping the output of a set of sed commands to awk. As a saved

executable file, you can then repeatedly invoke it in its original

form or modified, without the inconvenience of retyping it on the

command-line.

Example 36-1. shell wrapper

!/bin/bash

This simple script removes blank lines from a file.

No argument checking.

You might wish to add something like:

E_NOARGS=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` target-file"

exit $E_NOARGS

fi

sed -e /^$/d "$1"

Same as

sed -e '/^$/d' filename

invoked from the command-line.

The '-e' means an "editing" command follows (optional here).

'^' indicates the beginning of line, ' the end.

This matches lines with nothing between the beginning and the end --

+ blank lines.

The 'd' is the delete command.

Quoting the command-line arg permits

+ whitespace and special characters in the filename.

Note that this script doesn't actually change the target file.

If you need to do that, redirect its output.

exit

Example 36-2. A slightly more complex shell wrapper

!/bin/bash

subst.sh: a script that substitutes one pattern for

+ another in a file,

+ i.e., "sh subst.sh Smith Jones letter.txt".

Jones replaces Smith.

ARGS=3 # Script requires 3 arguments.

E_BADARGS=85 # Wrong number of arguments passed to script.

if [ $# -ne "$ARGS" ]

then

echo "Usage: `basename $0` old-pattern new-pattern filename"

exit $E_BADARGS

old_pattern=$1

new_pattern=$2

if [ -f "$3" ]

then

file_name=$3

else

echo "File \"$3\" does not exist."

exit $E_BADARGS

-----------------------------------------------

Here is where the heavy work gets done.

sed -e "s/$old_pattern/$new_pattern/g" $file_name

-----------------------------------------------

's' is, of course, the substitute command in sed,

+ and /pattern/ invokes address matching.

The 'g,' or global flag causes substitution for EVERY

+ occurence of $old_pattern on each line, not just the first.

Read the 'sed' docs for an in-depth explanation.

exit $? # Redirect the output of this script to write to a file.

Example 36-3. A generic shell wrapper that writes to a logfile

!/bin/bash

logging-wrapper.sh

Generic shell wrapper that performs an operation

+ and logs it.

DEFAULT_LOGFILE=logfile.txt

Set the following two variables.

OPERATION=

Can be a complex chain of commands,

+ for example an awk script or a pipe . . .

LOGFILE=

if [ -z "$LOGFILE" ]

then # If not set, default to ...

LOGFILE="$DEFAULT_LOGFILE"

Command-line arguments, if any, for the operation.

OPTIONS="$@"

Log it.

echo "`date` + `whoami` + $OPERATION "$@"" >> $LOGFILE

Now, do it.

exec $OPERATION "$@"

It's necessary to do the logging before the operation.

Why?

Example 36-4. A shell wrapper around an awk script

!/bin/bash

pr-ascii.sh: Prints a table of ASCII characters.

START=33 # Range of printable ASCII characters (decimal).

END=127 # Will not work for unprintable characters (> 127).

echo " Decimal Hex Character" # Header.

echo " ------- --- ---------"

for ((i=START; i<=END; i++))

echo $i | awk '{printf(" %3d %2x %c\n", $1, $1, $1)}'

The Bash printf builtin will not work in this context:

printf "%c" "$i"

done

exit 0

Decimal Hex Character

------- --- ---------

33 21 !

34 22 "

35 23 #

36 24 $

. . .

122 7a z

123 7b {

124 7c |

125 7d }

Redirect the output of this script to a file

+ or pipe it to "more": sh pr-asc.sh | more

Example 36-5. A shell wrapper around another awk script

!/bin/bash

Adds up a specified column (of numbers) in the target file.

Floating-point (decimal) numbers okay, because awk can handle them.

ARGS=2

E_WRONGARGS=85

if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.

then

echo "Usage: `basename $0` filename column-number"

exit $E_WRONGARGS

filename=$1

column_number=$2

Passing shell variables to the awk part of the script is a bit tricky.

One method is to strong-quote the Bash-script variable

+ within the awk script.

$BASH_SCRIPT_VAR'

^ ^

This is done in the embedded awk script below.

See the awk documentation for more details.

A multi-line awk script is here invoked by

awk '

...

'

Begin awk script.

-----------------------------

awk '

{ total += "${column_number}"'

}

END {

print total

}

' "$filename"

-----------------------------

End awk script.

It may not be safe to pass shell variables to an embedded awk script,

+ so Stephane Chazelas proposes the following alternative:

---------------------------------------

awk -v column_number="$column_number" '

{ total += $column_number

}

END {

print total

}' "$filename"

---------------------------------------

exit 0

For those scripts needing a single do-it-all tool, a Swiss army

knife, there is Perl. Perl combines the capabilities of sed and awk,

and throws in a large subset of C, to boot. It is modular and

contains support for everything ranging from object-oriented

programming up to and including the kitchen sink. Short Perl scripts

lend themselves to embedding within shell scripts, and there may be

some substance to the claim that Perl can totally replace shell

scripting (though the author of the ABS Guide remains skeptical).

Example 36-6. Perl embedded in a Bash script

!/bin/bash

Shell commands may precede the Perl script.

echo "This precedes the embedded Perl script within \"$0\"."

echo "==============================================================="

perl -e 'print "This line prints from an embedded Perl script.\n";'

Like sed, Perl also uses the "-e" option.

echo "==============================================================="

echo "However, the script may also contain shell and system commands."

exit 0

It is even possible to combine a Bash script and Perl script within

the same file. Depending on how the script is invoked, either the

Bash part or the Perl part will execute.

Example 36-7. Bash and Perl scripts combined

!/bin/bash

bashandperl.sh

echo "Greetings from the Bash part of the script, $0."

More Bash commands may follow here.

exit

End of Bash part of the script.

=======================================================

!/usr/bin/perl

This part of the script must be invoked with

perl -x bashandperl.sh

print "Greetings from the Perl part of the script, $0.\n";

Perl doesn't seem to like "echo" ...

More Perl commands may follow here.

End of Perl part of the script.

bash$ bash bashandperl.sh

Greetings from the Bash part of the script.

bash$ perl -x bashandperl.sh

Greetings from the Perl part of the script.

It is, of course, possible to embed even more exotic scripting

languages within shell wrappers. Python, for example ...

Example 36-8. Python embedded in a Bash script

!/bin/bash

ex56py.sh

Shell commands may precede the Python script.

echo "This precedes the embedded Python script within \"$0.\""

echo "==============================================================="

python -c 'print "This line prints from an embedded Python script.\n";'

Unlike sed and perl, Python uses the "-c" option.

python -c 'k = raw_input( "Hit a key to exit to outer script. " )'

echo "==============================================================="

echo "However, the script may also contain shell and system commands."

exit 0

Wrapping a script around mplayer and the Google's translation server,

you can create something that talks back to you.

Example 36-9. A script that speaks

!/bin/bash

Courtesy of:

http://elinux.org/RPi_Text_to_Speech_(Speech_Synthesis)

You must be on-line for this script to work,

+ so you can access the Google translation server.

Of course, mplayer must be present on your computer.

speak()

{

local IFS=+

# Invoke mplayer, then connect to Google translation server.

/usr/bin/mplayer -ao alsa -really-quiet -noconsolecontrols \

"http://translate.google.com/translate_tts?tl=en&q="$*""

# Google translates, but can also speak.

}

LINES=4

spk=$(tail -$LINES $0) # Tail end of same script!

speak "$spk"

exit

Browns. Nice talking to you.

One interesting example of a complex shell wrapper is Martin

Matusiak's undvd script, which provides an easy-to-use command-line

interface to the complex

[http://www.mplayerhq.hu/DOCS/HTML/en/mencoder.html] mencoder

utility. Another example is Itzchak Rehberg's

[http://projects.izzysoft.de/trac/ext3undel] Ext3Undel, a set of

scripts to recover deleted file on an ext3 filesystem.

________________________________________________________________

36.3. Tests and Comparisons: Alternatives

For tests, the [[ ]] construct may be more appropriate than [ ].

Likewise, arithmetic comparisons might benefit from the (( ))

construct.

a=8

All of the comparisons below are equivalent.

test "$a" -lt 16 && echo "yes, $a < 16" # "and list"

/bin/test "$a" -lt 16 && echo "yes, $a < 16"

[ "$a" -lt 16 ] && echo "yes, $a < 16"

[[ $a -lt 16 ]] && echo "yes, $a < 16" # Quoting variables within

(( a < 16 )) && echo "yes, $a < 16" # [[ ]] and (( )) not necessar

city="New York"

Again, all of the comparisons below are equivalent.

test "$city" \< Paris && echo "Yes, Paris is greater than $city"

# Greater ASCII order.

/bin/test "$city" \< Paris && echo "Yes, Paris is greater than $city"

[ "$city" \< Paris ] && echo "Yes, Paris is greater than $city"

[[ $city < Paris ]] && echo "Yes, Paris is greater than $city"

# Need not quote $city.

Thank you, S.C.

________________________________________________________________

36.4. Recursion: a script calling itself

Can a script recursively call itself? Indeed.

Example 36-10. A (useless) script that recursively calls itself

!/bin/bash

recurse.sh

Can a script recursively call itself?

Yes, but is this of any practical use?

(See the following.)

RANGE=10

MAXVAL=9

i=$RANDOM

let "i %= $RANGE" # Generate a random number between 0 and $RANGE - 1.

if [ "$i" -lt "$MAXVAL" ]

then

echo "i = $i"

./$0 # Script recursively spawns a new instance of itself.

fi # Each child script does the same, until

#+ a generated $i equals $MAXVAL.

Using a "while" loop instead of an "if/then" test causes problems.

Explain why.

exit 0

Note:

----

This script must have execute permission for it to work properly.

This is the case even if it is invoked by an "sh" command.

Explain why.

Example 36-11. A (useful) script that recursively calls itself

!/bin/bash

pb.sh: phone book

Written by Rick Boivie, and used with permission.

Modifications by ABS Guide author.

MINARGS=1 # Script needs at least one argument.

DATAFILE=./phonebook

# A data file in current working directory

#+ named "phonebook" must exist.

PROGNAME=$0

E_NOARGS=70 # No arguments error.

if [ $# -lt $MINARGS ]; then

echo "Usage: "$PROGNAME" data-to-look-up"

exit $E_NOARGS

if [ $# -eq $MINARGS ]; then

grep $1 "$DATAFILE"

# 'grep' prints an error message if $DATAFILE not present.

else

( shift; "$PROGNAME" $* ) | grep $1

# Script recursively calls itself.

exit 0 # Script exits here.

# Therefore, it's o.k. to put

#+ non-hashmarked comments and data after this point.

------------------------------------------------------------------------

Sample "phonebook" datafile:

John Doe 1555 Main St., Baltimore, MD 21228 (410) 222-3333

Mary Moe 9899 Jones Blvd., Warren, NH 03787 (603) 898-3232

Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567

Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678

Zoe Zenobia 4481 N. Baker St., San Francisco, SF 94338 (415) 501-1631

------------------------------------------------------------------------

$bash pb.sh Roe

Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567

Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678

$bash pb.sh Roe Sam

Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678

When more than one argument is passed to this script,

+ it prints only the line(s) containing all the arguments.

Example 36-12. Another (useful) script that recursively calls itself

!/bin/bash

usrmnt.sh, written by Anthony Richardson

Used in ABS Guide with permission.

usage: usrmnt.sh

description: mount device, invoking user must be listed in the

MNTUSERS group in the /etc/sudoers file.

----------------------------------------------------------

This is a usermount script that reruns itself using sudo.

A user with the proper permissions only has to type

usermount /dev/fd0 /mnt/floppy

instead of

sudo usermount /dev/fd0 /mnt/floppy

I use this same technique for all of my

+ sudo scripts, because I find it convenient.

----------------------------------------------------------

If SUDO_COMMAND variable is not set we are not being run through

+ sudo, so rerun ourselves. Pass the user's real and group id . . .

if [ -z "$SUDO_COMMAND" ]

then

mntusr=$(id -u) grpusr=$(id -g) sudo $0 $*

exit 0

We will only get here if we are being run by sudo.

/bin/mount $* -o uid=$mntusr,gid=$grpusr

exit 0

Additional notes (from the author of this script):

-------------------------------------------------

1) Linux allows the "users" option in the /etc/fstab

file so that any user can mount removable media.

But, on a server, I like to allow only a few

individuals access to removable media.

I find using sudo gives me more control.

2) I also find sudo to be more convenient than

accomplishing this task through groups.

3) This method gives anyone with proper permissions

root access to the mount command, so be careful

about who you allow access.

You can get finer control over which access can be mounted

by using this same technique in separate mntfloppy, mntcdrom,

and mntsamba scripts.

Caution

Too many levels of recursion can exhaust the script's stack space,

causing a segfault.

________________________________________________________________

36.5. "Colorizing" Scripts

The ANSI [126] escape sequences set screen attributes, such as bold

text, and color of foreground and background. DOS batch files

commonly used ANSI escape codes for color output, and so can Bash

scripts.

Example 36-13. A "colorized" address database

!/bin/bash

ex30a.sh: "Colorized" version of ex30.sh.

Crude address database

clear # Clear the screen.

echo -n " "

echo -e '\E[37;44m'"\033[1mContact List\033[0m"

# White on blue background

echo; echo

echo -e "\033[1mChoose one of the following persons:\033[0m"

# Bold

tput sgr0 # Reset attributes.

echo "(Enter only the first letter of name.)"

echo

echo -en '\E[47;34m'"\033[1mE\033[0m" # Blue

tput sgr0 # Reset colors to "normal."

echo "vans, Roland" # "[E]vans, Roland"

echo -en '\E[47;35m'"\033[1mJ\033[0m" # Magenta

tput sgr0

echo "ambalaya, Mildred"

echo -en '\E[47;32m'"\033[1mS\033[0m" # Green

tput sgr0

echo "mith, Julie"

echo -en '\E[47;31m'"\033[1mZ\033[0m" # Red

tput sgr0

echo "ane, Morris"

echo

read person

case "$person" in

Note variable is quoted.

"E" | "e" )

# Accept upper or lowercase input.

echo

echo "Roland Evans"

echo "4321 Flash Dr."

echo "Hardscrabble, CO 80753"

echo "(303) 734-9874"

echo "(303) 734-9892 fax"

echo "revans@zzy.net"

echo "Business partner & old friend"

;;

"J" | "j" )

echo

echo "Mildred Jambalaya"

echo "249 E. 7th St., Apt. 19"

echo "New York, NY 10009"

echo "(212) 533-2814"

echo "(212) 533-9972 fax"

echo "milliej@loisaida.com"

echo "Girlfriend"

echo "Birthday: Feb. 11"

;;

Add info for Smith & Zane later.

* )

# Default option.

# Empty input (hitting RETURN) fits here, too.

echo

echo "Not yet in database."

;;

esac

tput sgr0 # Reset colors to "normal."

echo

exit 0

Example 36-14. Drawing a box

!/bin/bash

Draw-box.sh: Drawing a box using ASCII characters.

Script by Stefano Palmeri, with minor editing by document author.

Minor edits suggested by Jim Angstadt.

Used in the ABS Guide with permission.

draw_box function doc ###

The "draw_box" function lets the user

+ draw a box in a terminal.

Usage: draw_box ROW COLUMN HEIGHT WIDTH [COLOR]

ROW and COLUMN represent the position

+ of the upper left angle of the box you're going to draw.

ROW and COLUMN must be greater than 0

+ and less than current terminal dimension.

HEIGHT is the number of rows of the box, and must be > 0.

HEIGHT + ROW must be <= than current terminal height.

WIDTH is the number of columns of the box and must be > 0.

WIDTH + COLUMN must be <= than current terminal width.

E.g.: If your terminal dimension is 20x80,

draw_box 2 3 10 45 is good

draw_box 2 3 19 45 has bad HEIGHT value (19+2 > 20)

draw_box 2 3 18 78 has bad WIDTH value (78+3 > 80)

COLOR is the color of the box frame.

This is the 5th argument and is optional.

0=black 1=red 2=green 3=tan 4=blue 5=purple 6=cyan 7=white.

If you pass the function bad arguments,

+ it will just exit with code 65,

+ and no messages will be printed on stderr.

Clear the terminal before you start to draw a box.

The clear command is not contained within the function.

This allows the user to draw multiple boxes, even overlapping ones.

end of draw_box function doc ###

draw_box(){

=============#

HORZ="-"

VERT="|"

CORNER_CHAR="+"

MINARGS=4

E_BADARGS=65

=============#

if [ $# -lt "$MINARGS" ]; then # If args are less than 4, exit.

exit $E_BADARGS

Looking for non digit chars in arguments.

Probably it could be done better (exercise for the reader?).

if echo $@ | tr -d [:blank:] | tr -d [:digit:] | grep . &> /dev/null; then

exit $E_BADARGS

BOX_HEIGHT=`expr $3 - 1` # -1 correction needed because angle char "+"

BOX_WIDTH=`expr $4 - 1` #+ is a part of both box height and width.

T_ROWS=`tput lines` # Define current terminal dimension

T_COLS=`tput cols` #+ in rows and columns.

if [ $1 -lt 1 ] || [ $1 -gt $T_ROWS ]; then # Start checking if arguments

exit $E_BADARGS #+ are correct.

if [ $2 -lt 1 ] || [ $2 -gt $T_COLS ]; then

exit $E_BADARGS

if [ `expr $1 + $BOX_HEIGHT + 1` -gt $T_ROWS ]; then

exit $E_BADARGS

if [ `expr $2 + $BOX_WIDTH + 1` -gt $T_COLS ]; then

exit $E_BADARGS

if [ $3 -lt 1 ] || [ $4 -lt 1 ]; then

exit $E_BADARGS

fi # End checking arguments.

plot_char(){ # Function within a function.

echo -e "\E[${1};${2}H"$3

}

echo -ne "\E[3${5}m" # Set box frame color, if defined.

start drawing the box

count=1 # Draw vertical lines using

for (( r=$1; count<=$BOX_HEIGHT; r++)); do #+ plot_char function.

plot_char $r $2 $VERT

let count=count+1

done

count=1

c=`expr $2 + $BOX_WIDTH`

for (( r=$1; count<=$BOX_HEIGHT; r++)); do

plot_char $r $c $VERT

let count=count+1

done

count=1 # Draw horizontal lines using

for (( c=$2; count<=$BOX_WIDTH; c++)); do #+ plot_char function.

plot_char $1 $c $HORZ

let count=count+1

done

count=1

r=`expr $1 + $BOX_HEIGHT`

for (( c=$2; count<=$BOX_WIDTH; c++)); do

plot_char $r $c $HORZ

let count=count+1

done

plot_char $1 $2 $CORNER_CHAR # Draw box angles.

plot_char $1 `expr $2 + $BOX_WIDTH` $CORNER_CHAR

plot_char `expr $1 + $BOX_HEIGHT` $2 $CORNER_CHAR

plot_char `expr $1 + $BOX_HEIGHT` `expr $2 + $BOX_WIDTH` $CORNER_CHAR

echo -ne "\E[0m" # Restore old colors.

P_ROWS=`expr $T_ROWS - 1` # Put the prompt at bottom of the terminal.

echo -e "\E[${P_ROWS};1H"

}

Now, let's try drawing a box.

clear # Clear the terminal.

R=2 # Row

C=3 # Column

H=10 # Height

W=45 # Width

col=1 # Color (red)

draw_box $R $C $H $W $col # Draw the box.

exit 0

Exercise:

--------

Add the option of printing text within the drawn box.

The simplest, and perhaps most useful ANSI escape sequence is bold

text, \033[1m ... \033[0m. The \033 represents an escape, the "[1"

turns on the bold attribute, while the "[0" switches it off. The "m"

terminates each term of the escape sequence.

bash$ echo -e "\033[1mThis is bold text.\033[0m"

A similar escape sequence switches on the underline attribute (on an

rxvt and an aterm).

bash$ echo -e "\033[4mThis is underlined text.\033[0m"

Note

With an echo, the -e option enables the escape sequences.

Other escape sequences change the text and/or background color.

bash$ echo -e '\E[34;47mThis prints in blue.'; tput sgr0

bash$ echo -e '\E[33;44m'"yellow text on blue background"; tput sgr0

bash$ echo -e '\E[1;33;44m'"BOLD yellow text on blue background"; tput sgr0

Note

It's usually advisable to set the bold attribute for light-colored

foreground text.

The tput sgr0 restores the terminal settings to normal. Omitting this

lets all subsequent output from that particular terminal remain blue.

Note

Since tput sgr0 fails to restore terminal settings under certain

circumstances, echo -ne \E[0m may be a better choice.

Use the following template for writing colored text on a colored

background.

echo -e '\E[COLOR1;COLOR2mSome text goes here.'

The "\E[" begins the escape sequence. The semicolon-separated numbers

"COLOR1" and "COLOR2" specify a foreground and a background color,

according to the table below. (The order of the numbers does not

matter, since the foreground and background numbers fall in

non-overlapping ranges.) The "m" terminates the escape sequence, and

the text begins immediately after that.

Note also that single quotes enclose the remainder of the command

sequence following the echo -e.

The numbers in the following table work for an rxvt terminal. Results

may vary for other terminal emulators.

Table 36-1. Numbers representing colors in Escape Sequences

Color Foreground Background

black 30 40

red 31 41

green 32 42

yellow 33 43

blue 34 44

magenta 35 45

cyan 36 46

white 37 47

Example 36-15. Echoing colored text

!/bin/bash

color-echo.sh: Echoing text messages in color.

Modify this script for your own purposes.

It's easier than hand-coding color.

black='\E[30;47m'

red='\E[31;47m'

green='\E[32;47m'

yellow='\E[33;47m'

blue='\E[34;47m'

magenta='\E[35;47m'

cyan='\E[36;47m'

white='\E[37;47m'

alias Reset="tput sgr0" # Reset text attributes to normal

#+ without clearing screen.

cecho () # Color-echo.

# Argument $1 = message

# Argument $2 = color

{

local default_msg="No message passed."

# Doesn't really need to be a local variable.

message=${1:-$default_msg} # Defaults to default message.

color=${2:-$black} # Defaults to black, if not specified.

echo -e "$color"

echo "$message"

Reset # Reset to normal.

return

}

Now, let's try it out.

----------------------------------------------------

cecho "Feeling blue..." $blue

cecho "Magenta looks more like purple." $magenta

cecho "Green with envy." $green

cecho "Seeing red?" $red

cecho "Cyan, more familiarly known as aqua." $cyan

cecho "No color passed (defaults to black)."

# Missing $color argument.

cecho "\"Empty\" color passed (defaults to black)." ""

# Empty $color argument.

cecho

# Missing $message and $color arguments.

cecho "" ""

# Empty $message and $color arguments.

----------------------------------------------------

echo

exit 0

Exercises:

---------

1) Add the "bold" attribute to the 'cecho ()' function.

2) Add options for colored backgrounds.

Example 36-16. A "horserace" game

!/bin/bash

horserace.sh: Very simple horserace simulation.

Author: Stefano Palmeri

Used with permission.

Goals of the script:

playing with escape sequences and terminal colors.

Exercise:

Edit the script to make it run less randomly,

+ set up a fake betting shop . . .

Um . . . um . . . it's starting to remind me of a movie . . .

The script gives each horse a random handicap.

The odds are calculated upon horse handicap

+ and are expressed in European(?) style.

E.g., odds=3.75 means that if you bet $1 and win,

+ you receive $3.75.

The script has been tested with a GNU/Linux OS,

+ using xterm and rxvt, and konsole.

On a machine with an AMD 900 MHz processor,

+ the average race time is 75 seconds.

On faster computers the race time would be lower.

So, if you want more suspense, reset the USLEEP_ARG variable.

Script by Stefano Palmeri.

E_RUNERR=65

Check if md5sum and bc are installed.

if ! which bc &> /dev/null; then

echo bc is not installed.

echo "Can\'t run . . . "

exit $E_RUNERR

if ! which md5sum &> /dev/null; then

echo md5sum is not installed.

echo "Can\'t run . . . "

exit $E_RUNERR

Set the following variable to slow down script execution.

It will be passed as the argument for usleep (man usleep)

+ and is expressed in microseconds (500000 = half a second).

USLEEP_ARG=0

Clean up the temp directory, restore terminal cursor and

+ terminal colors -- if script interrupted by Ctl-C.

trap 'echo -en "\E[?25h"; echo -en "\E[0m"; stty echo;\

tput cup 20 0; rm -fr $HORSE_RACE_TMP_DIR' TERM EXIT

See the chapter on debugging for an explanation of 'trap.'

Set a unique (paranoid) name for the temp directory the script needs.

HORSE_RACE_TMP_DIR=$HOME/.horserace-`date +%s`-`head -c10 /dev/urandom \

| md5sum | head -c30`

Create the temp directory and move right in.

mkdir $HORSE_RACE_TMP_DIR

cd $HORSE_RACE_TMP_DIR

This function moves the cursor to line $1 column $2 and then prints $3.

E.g.: "move_and_echo 5 10 linux" is equivalent to

+ "tput cup 4 9; echo linux", but with one command instead of two.

Note: "tput cup" defines 0 0 the upper left angle of the terminal,

+ echo defines 1 1 the upper left angle of the terminal.

move_and_echo() {

echo -ne "\E[${1};${2}H""$3"

}

Function to generate a pseudo-random number between 1 and 9.

random_1_9 ()

{

head -c10 /dev/urandom | md5sum | tr -d [a-z] | tr -d 0 | cut -c1

}

Two functions that simulate "movement," when drawing the horses.

draw_horse_one() {

echo -n " "//$MOVE_HORSE//

}

draw_horse_two(){

echo -n " "\\\\$MOVE_HORSE\\\\

}

Define current terminal dimension.

N_COLS=`tput cols`

N_LINES=`tput lines`

Need at least a 20-LINES X 80-COLUMNS terminal. Check it.

if [ $N_COLS -lt 80 ] || [ $N_LINES -lt 20 ]; then

echo "`basename $0` needs a 80-cols X 20-lines terminal."

echo "Your terminal is ${N_COLS}-cols X ${N_LINES}-lines."

exit $E_RUNERR

Start drawing the race field.

Need a string of 80 chars. See below.

BLANK80=`seq -s "" 100 | head -c80`

clear

Set foreground and background colors to white.

echo -ne '\E[37;47m'

Move the cursor on the upper left angle of the terminal.

tput cup 0 0

Draw six white lines.

for n in `seq 5`; do

echo $BLANK80 # Use the 80 chars string to colorize the terminal.

done

Sets foreground color to black.

echo -ne '\E[30m'

move_and_echo 3 1 "START 1"

move_and_echo 3 75 FINISH

move_and_echo 1 5 "|"

move_and_echo 1 80 "|"

move_and_echo 2 5 "|"

move_and_echo 2 80 "|"

move_and_echo 4 5 "| 2"

move_and_echo 4 80 "|"

move_and_echo 5 5 "V 3"

move_and_echo 5 80 "V"

Set foreground color to red.

echo -ne '\E[31m'

Some ASCII art.

move_and_echo 1 8 "..@@@..@@@@@...@@@@@.@...@..@@@@..."

move_and_echo 2 8 ".@...@...@.......@...@...@.@......."

move_and_echo 3 8 ".@@@@@...@.......@...@@@@@.@@@@...."

move_and_echo 4 8 ".@...@...@.......@...@...@.@......."

move_and_echo 5 8 ".@...@...@.......@...@...@..@@@@..."

move_and_echo 1 43 "@@@@...@@@...@@@@..@@@@..@@@@."

move_and_echo 2 43 "@...@.@...@.@.....@.....@....."

move_and_echo 3 43 "@@@@..@@@@@.@.....@@@@...@@@.."

move_and_echo 4 43 "@..@..@...@.@.....@.........@."

move_and_echo 5 43 "@...@.@...@..@@@@..@@@@.@@@@.."

Set foreground and background colors to green.

echo -ne '\E[32;42m'

Draw eleven green lines.

tput cup 5 0

for n in `seq 11`; do

echo $BLANK80

done

Set foreground color to black.

echo -ne '\E[30m'

tput cup 5 0

Draw the fences.

echo "++++++++++++++++++++++++++++++++++++++\

++++++++++++++++++++++++++++++++++++++++++"

tput cup 15 0

echo "++++++++++++++++++++++++++++++++++++++\

++++++++++++++++++++++++++++++++++++++++++"

Set foreground and background colors to white.

echo -ne '\E[37;47m'

Draw three white lines.

for n in `seq 3`; do

echo $BLANK80

done

Set foreground color to black.

echo -ne '\E[30m'

Create 9 files to stores handicaps.

for n in `seq 10 7 68`; do

touch $n

done

Set the first type of "horse" the script will draw.

HORSE_TYPE=2

Create position-file and odds-file for every "horse".

+ In these files, store the current position of the horse,

+ the type and the odds.

for HN in `seq 9`; do

touch horse_${HN}_position

touch odds_${HN}

echo \-1 > horse_${HN}_position

echo $HORSE_TYPE >> horse_${HN}_position

# Define a random handicap for horse.

HANDICAP=`random_1_9`

# Check if the random_1_9 function returned a good value.

while ! echo $HANDICAP | grep [1-9] &> /dev/null; do

HANDICAP=`random_1_9`

done

# Define last handicap position for horse.

LHP=`expr $HANDICAP \* 7 + 3`

for FILE in `seq 10 7 $LHP`; do

echo $HN >> $FILE

done

# Calculate odds.

case $HANDICAP in

1) ODDS=`echo $HANDICAP \* 0.25 + 1.25 | bc`

echo $ODDS > odds_${HN}

;;

2 | 3) ODDS=`echo $HANDICAP \* 0.40 + 1.25 | bc`

echo $ODDS > odds_${HN}

;;

4 | 5 | 6) ODDS=`echo $HANDICAP \* 0.55 + 1.25 | bc`

echo $ODDS > odds_${HN}

;;

7 | 8) ODDS=`echo $HANDICAP \* 0.75 + 1.25 | bc`

echo $ODDS > odds_${HN}

;;

9) ODDS=`echo $HANDICAP \* 0.90 + 1.25 | bc`

echo $ODDS > odds_${HN}

esac

done

Print odds.

print_odds() {

tput cup 6 0

echo -ne '\E[30;42m'

for HN in `seq 9`; do

echo "#$HN odds->" `cat odds_${HN}`

done

}

Draw the horses at starting line.

draw_horses() {

tput cup 6 0

echo -ne '\E[30;42m'

for HN in `seq 9`; do

echo /\\$HN/\\" "

done

}

print_odds

echo -ne '\E[47m'

Wait for a enter key press to start the race.

The escape sequence '\E[?25l' disables the cursor.

tput cup 17 0

echo -e '\E[?25l'Press [enter] key to start the race...

read -s

Disable normal echoing in the terminal.

This avoids key presses that might "contaminate" the screen

+ during the race.

stty -echo

--------------------------------------------------------

Start the race.

draw_horses

echo -ne '\E[37;47m'

move_and_echo 18 1 $BLANK80

echo -ne '\E[30m'

move_and_echo 18 1 Starting...

sleep 1

Set the column of the finish line.

WINNING_POS=74

Define the time the race started.

START_TIME=`date +%s`

COL variable needed by following "while" construct.

COL=0

while [ $COL -lt $WINNING_POS ]; do

MOVE_HORSE=0

# Check if the random_1_9 function has returned a good value.

while ! echo $MOVE_HORSE | grep [1-9] &> /dev/null; do

MOVE_HORSE=`random_1_9`

done

# Define old type and position of the "randomized horse".

HORSE_TYPE=`cat horse_${MOVE_HORSE}_position | tail -n 1`

COL=$(expr `cat horse_${MOVE_HORSE}_position | head -n 1`)

ADD_POS=1

# Check if the current position is an handicap position.

if seq 10 7 68 | grep -w $COL &> /dev/null; then

if grep -w $MOVE_HORSE $COL &> /dev/null; then

ADD_POS=0

grep -v -w $MOVE_HORSE $COL > ${COL}_new

rm -f $COL

mv -f ${COL}_new $COL

else ADD_POS=1

COL=`expr $COL + $ADD_POS`

echo $COL > horse_${MOVE_HORSE}_position # Store new position.

# Choose the type of horse to draw.

case $HORSE_TYPE in

1) HORSE_TYPE=2; DRAW_HORSE=draw_horse_two

;;

2) HORSE_TYPE=1; DRAW_HORSE=draw_horse_one

esac

echo $HORSE_TYPE >> horse_${MOVE_HORSE}_position

# Store current type.

# Set foreground color to black and background to green.

echo -ne '\E[30;42m'

# Move the cursor to new horse position.

tput cup `expr $MOVE_HORSE + 5` \

`cat horse_${MOVE_HORSE}_position | head -n 1`

# Draw the horse.

$DRAW_HORSE

usleep $USLEEP_ARG

# When all horses have gone beyond field line 15, reprint odds.

touch fieldline15

if [ $COL = 15 ]; then

echo $MOVE_HORSE >> fieldline15

if [ `wc -l fieldline15 | cut -f1 -d " "` = 9 ]; then

print_odds

: > fieldline15

# Define the leading horse.

HIGHEST_POS=`cat *position | sort -n | tail -1`

# Set background color to white.

echo -ne '\E[47m'

tput cup 17 0

echo -n Current leader: `grep -w $HIGHEST_POS *position | cut -c7`\

" "

done

Define the time the race finished.

FINISH_TIME=`date +%s`

Set background color to green and enable blinking text.

echo -ne '\E[30;42m'

echo -en '\E[5m'

Make the winning horse blink.

tput cup `expr $MOVE_HORSE + 5` \

`cat horse_${MOVE_HORSE}_position | head -n 1`

$DRAW_HORSE

Disable blinking text.

echo -en '\E[25m'

Set foreground and background color to white.

echo -ne '\E[37;47m'

move_and_echo 18 1 $BLANK80

Set foreground color to black.

echo -ne '\E[30m'

Make winner blink.

tput cup 17 0

echo -e "\E[5mWINNER: $MOVE_HORSE\E[25m"" Odds: `cat odds_${MOVE_HORSE}`"\

" Race time: `expr $FINISH_TIME - $START_TIME` secs"

Restore cursor and old colors.

echo -en "\E[?25h"

echo -en "\E[0m"

Restore echoing.

stty echo

Remove race temp directory.

rm -rf $HORSE_RACE_TMP_DIR

tput cup 19 0

exit 0

See also Example A-21, Example A-44, Example A-52, and Example A-40.

Caution

There is, however, a major problem with all this. ANSI escape

sequences are emphatically non-portable. What works fine on some

terminal emulators (or the console) may work differently, or not at

all, on others. A "colorized" script that looks stunning on the

script author's machine may produce unreadable output on someone

else's. This somewhat compromises the usefulness of colorizing

scripts, and possibly relegates this technique to the status of a

gimmick. Colorized scripts are probably inappropriate in a commercial

setting, i.e., your supervisor might disapprove.

Alister's [http://code.google.com/p/ansi-color/] ansi-color utility

(based on Moshe Jacobson's color utility considerably simplifies

using ANSI escape sequences. It substitutes a clean and logical

syntax for the clumsy constructs just discussed.

Henry/teikedvl has likewise created a utility

([http://scriptechocolor.sourceforge.net/]

http://scriptechocolor.sourceforge.net/) to simplify creation of

colorized scripts.

________________________________________________________________

36.6. Optimizations

Most shell scripts are quick 'n dirty solutions to non-complex

problems. As such, optimizing them for speed is not much of an issue.

Consider the case, though, where a script carries out an important

task, does it well, but runs too slowly. Rewriting it in a compiled

language may not be a palatable option. The simplest fix would be to

rewrite the parts of the script that slow it down. Is it possible to

apply principles of code optimization even to a lowly shell script?

Check the loops in the script. Time consumed by repetitive operations

adds up quickly. If at all possible, remove time-consuming operations

from within loops.

Use builtin commands in preference to system commands. Builtins

execute faster and usually do not launch a subshell when invoked.

Avoid unnecessary commands, particularly in a pipe.

cat "$file" | grep "$word"

grep "$word" "$file"

The above command-lines have an identical effect,

+ but the second runs faster since it launches one fewer subprocess.

The cat command seems especially prone to overuse in scripts.

Disabling certain Bash options can speed up scripts.

As Erik Brandsberg points out:

If you don't need Unicode support, you can get potentially a 2x or

more improvement in speed by simply setting the LC_ALL variable.

export LC_ALL=C

[specifies the locale as ANSI C,

thereby disabling Unicode support]

[In an example script ...]

Without [Unicode support]:

erik@erik-desktop:~/capture$ time ./cap-ngrep.sh

live2.pcap > out.txt

real 0m20.483s

user 1m34.470s

sys 0m12.869s

With [Unicode support]:

erik@erik-desktop:~/capture$ time ./cap-ngrep.sh

live2.pcap > out.txt

real 0m50.232s

user 3m51.118s

sys 0m11.221s

A large part of the overhead that is optimized is, I believe,

regex match using [[ string =~ REGEX ]],

but it may help with other portions of the code as well.

I hadn't [seen it] mentioned that this optimization helped

with Bash, but I had seen it helped with "grep,"

so why not try?

Note

Certain operators, notably expr, are very inefficient and might be

replaced by double parentheses arithmetic expansion. See Example

A-59.

Math tests

math via $(( ))

real 0m0.294s

user 0m0.288s

sys 0m0.008s

math via expr:

real 1m17.879s # Much slower!

user 0m3.600s

sys 0m8.765s

math via let:

real 0m0.364s

user 0m0.372s

sys 0m0.000s

Condition testing constructs in scripts deserve close scrutiny.

Substitute case for if-then constructs and combine tests when

possible, to minimize script execution time. Again, refer to Example

A-59.

Test using "case" construct:

real 0m0.329s

user 0m0.320s

sys 0m0.000s

Test with if [], no quotes:

real 0m0.438s

user 0m0.432s

sys 0m0.008s

Test with if [], quotes:

real 0m0.476s

user 0m0.452s

sys 0m0.024s

Test with if [], using -eq:

real 0m0.457s

user 0m0.456s

sys 0m0.000s

Note

Erik Brandsberg recommends using associative arrays in preference to

conventional numeric-indexed arrays in most cases. When overwriting

values in a numeric array, there is a significant performance penalty

vs. associative arrays. Running a test script confirms this. See

Example A-60.

Assignment tests

Assigning a simple variable

real 0m0.418s

user 0m0.416s

sys 0m0.004s

Assigning a numeric index array entry

real 0m0.582s

user 0m0.564s

sys 0m0.016s

Overwriting a numeric index array entry

real 0m21.931s

user 0m21.913s

sys 0m0.016s

Linear reading of numeric index array

real 0m0.422s

user 0m0.416s

sys 0m0.004s

Assigning an associative array entry

real 0m1.800s

user 0m1.796s

sys 0m0.004s

Overwriting an associative array entry

real 0m1.798s

user 0m1.784s

sys 0m0.012s

Linear reading an associative array entry

real 0m0.420s

user 0m0.420s

sys 0m0.000s

Assigning a random number to a simple variable

real 0m0.402s

user 0m0.388s

sys 0m0.016s

Assigning a sparse numeric index array entry randomly into 64k cells

real 0m12.678s

user 0m12.649s

sys 0m0.028s

Reading sparse numeric index array entry

real 0m0.087s

user 0m0.084s

sys 0m0.000s

Assigning a sparse associative array entry randomly into 64k cells

real 0m0.698s

user 0m0.696s

sys 0m0.004s

Reading sparse associative index array entry

real 0m0.083s

user 0m0.084s

sys 0m0.000s

Use the time and times tools to profile computation-intensive

commands. Consider rewriting time-critical code sections in C, or

even in assembler.

Try to minimize file I/O. Bash is not particularly efficient at

handling files, so consider using more appropriate tools for this

within the script, such as awk or Perl.

Write your scripts in a modular and coherent form, [127] so they can

be reorganized and tightened up as necessary. Some of the

optimization techniques applicable to high-level languages may work

for scripts, but others, such as loop unrolling, are mostly

irrelevant. Above all, use common sense.

For an excellent demonstration of how optimization can dramatically

reduce the execution time of a script, see Example 16-47.

________________________________________________________________

36.7. Assorted Tips

36.7.1. Ideas for more powerful scripts

* You have a problem that you want to solve by writing a Bash

script. Unfortunately, you don't know quite where to start. One

method is to plunge right in and code those parts of the script

that come easily, and write the hard parts as pseudo-code.

!/bin/bash

ARGCOUNT=1 # Need name as argument.

E_WRONGARGS=65

if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]

^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^

Can't figure out how to code this . . .

+ . . . so write it in pseudo-code.

then

echo "Usage: name-of-script name"

# ^^^^^^^^^^^^^^ More pseudo-code.

exit $E_WRONGARGS

. . .

exit 0

Later on, substitute working code for the pseudo-code.

Line 6 becomes:

if [ $# -ne "$ARGCOUNT" ]

Line 12 becomes:

echo "Usage: `basename $0` name"

For an example of using pseudo-code, see the Square Root

exercise.

* To keep a record of which user scripts have run during a

particular session or over a number of sessions, add the

following lines to each script you want to keep track of. This

will keep a continuing file record of the script names and

invocation times.

Append (>>) following to end of each script tracked.

whoami>> $SAVE_FILE # User invoking the script.

echo $0>> $SAVE_FILE # Script name.

date>> $SAVE_FILE # Date and time.

echo>> $SAVE_FILE # Blank line as separator.

Of course, SAVE_FILE defined and exported as environmental variable in ~/.b

ashrc

+ (something like ~/.scripts-run)

* The >> operator appends lines to a file. What if you wish to

prepend a line to an existing file, that is, to paste it in at

the beginning?

file=data.txt

title="***This is the title line of data text file***"

echo $title | cat - $file >$file.new

"cat -" concatenates stdout to $file.

End result is

+ to write a new file with $title appended at beginning.

This is a simplified variant of the Example 19-13 script given

earlier. And, of course, sed can also do this.

* A shell script may act as an embedded command inside another

shell script, a Tcl or wish script, or even a Makefile. It can be

invoked as an external shell command in a C program using the

system() call, i.e., system("script_name");.

* Setting a variable to the contents of an embedded sed or awk

script increases the readability of the surrounding shell

wrapper. See Example A-1 and Example 15-20.

* Put together files containing your favorite and most useful

definitions and functions. As necessary, "include" one or more of

these "library files" in scripts with either the dot (.) or

source command.

SCRIPT LIBRARY

------ -------

Note:

No "#!" here.

No "live code" either.

Useful variable definitions

ROOT_UID=0 # Root has $UID 0.

E_NOTROOT=101 # Not root user error.

MAXRETVAL=255 # Maximum (positive) return value of a function.

SUCCESS=0

FAILURE=-1

Functions

Usage () # "Usage:" message.

{

if [ -z "$1" ] # No arg passed.

then

msg=filename

else

msg=$@

echo "Usage: `basename $0` "$msg""

}

Check_if_root () # Check if root running script.

{ # From "ex39.sh" example.

if [ "$UID" -ne "$ROOT_UID" ]

then

echo "Must be root to run this script."

exit $E_NOTROOT

}

CreateTempfileName () # Creates a "unique" temp filename.

{ # From "ex51.sh" example.

prefix=temp

suffix=`eval date +%s`

Tempfilename=$prefix.$suffix

}

isalpha2 () # Tests whether *entire string* is alphabetic.

{ # From "isalpha.sh" example.

[ $# -eq 1 ] || return $FAILURE

case $1 in

*[!a-zA-Z]*|"") return $FAILURE;;

*) return $SUCCESS;;

esac # Thanks, S.C.

}

abs () # Absolute value.

{ # Caution: Max return value = 255.

E_ARGERR=-999999

if [ -z "$1" ] # Need arg passed.

then

return $E_ARGERR # Obvious error value returned.

if [ "$1" -ge 0 ] # If non-negative,

then #

absval=$1 # stays as-is.

else # Otherwise,

let "absval = (( 0 - $1 ))" # change sign.

return $absval

}

tolower () # Converts string(s) passed as argument(s)

{ #+ to lowercase.

if [ -z "$1" ] # If no argument(s) passed,

then #+ send error message

echo "(null)" #+ (C-style void-pointer error message)

return #+ and return from function.

echo "$@" | tr A-Z a-z

# Translate all passed arguments ($@).

return

Use command substitution to set a variable to function output.

For example:

oldvar="A seT of miXed-caSe LEtTerS"

newvar=`tolower "$oldvar"`

echo "$newvar" # a set of mixed-case letters

Exercise: Rewrite this function to change lowercase passed argument(s)

to uppercase ... toupper() [easy].

}

* Use special-purpose comment headers to increase clarity and

legibility in scripts.

Caution.

rm -rf *.zzy ## The "-rf" options to "rm" are very dangerous,

##+ especially with wild cards.

+ Line continuation.

This is line 1

+ of a multi-line comment,

+ and this is the final line.

* Note.

o List item.

> Another point of view.

while [ "$var1" != "end" ] #> while test "$var1" != "end"

* Dotan Barak contributes template code for a progress bar in a

script.

Example 36-17. A Progress Bar

!/bin/bash

progress-bar.sh

Author: Dotan Barak (very minor revisions by ABS Guide author).

Used in ABS Guide with permission (thanks!).

BAR_WIDTH=50

BAR_CHAR_START="["

BAR_CHAR_END="]"

BAR_CHAR_EMPTY="."

BAR_CHAR_FULL="="

BRACKET_CHARS=2

LIMIT=100

print_progress_bar()

{

# Calculate how many characters will be full.

let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"

# Calculate how many characters will be empty.

let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"

# Prepare the bar.

bar_line="${BAR_CHAR_START}"

for ((j=0; j<full_limit; j++)); do

bar_line="${bar_line}${BAR_CHAR_FULL}"

done

for ((j=0; j<empty_limit; j++)); do

bar_line="${bar_line}${BAR_CHAR_EMPTY}"

done

bar_line="${bar_line}${BAR_CHAR_END}"

printf "%3d%% %s" $2 ${bar_line}

}

Here is a sample of code that uses it.

MAX_PERCENT=100

for ((i=0; i<=MAX_PERCENT; i++)); do

usleep 10000

# ... Or run some other commands ...

print_progress_bar ${BAR_WIDTH} ${i}

echo -en "\r"

done

echo ""

exit

* A particularly clever use of if-test constructs is for comment

blocks.

!/bin/bash

COMMENT_BLOCK=

Try setting the above variable to some value

+ for an unpleasant surprise.

if [ $COMMENT_BLOCK ]; then

Comment block --

=================================

This is a comment line.

This is another comment line.

This is yet another comment line.

=================================

echo "This will not echo."

Comment blocks are error-free! Whee!

echo "No more comments, please."

exit 0

Compare this with using here documents to comment out code

blocks.

* Using the $? exit status variable, a script may test if a

parameter contains only digits, so it can be treated as an

integer.

!/bin/bash

SUCCESS=0

E_BADINPUT=85

test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null

An integer is either equal to 0 or not equal to 0.

2>/dev/null suppresses error message.

if [ $? -ne "$SUCCESS" ]

then

echo "Usage: `basename $0` integer-input"

exit $E_BADINPUT

let "sum = $1 + 25" # Would give error if $1 not integer.

echo "Sum = $sum"

Any variable, not just a command-line parameter, can be tested this way.

exit 0

* The 0 - 255 range for function return values is a severe

limitation. Global variables and other workarounds are often

problematic. An alternative method for a function to communicate

a value back to the main body of the script is to have the

function write to stdout (usually with echo) the "return value,"

and assign this to a variable. This is actually a variant of

command substitution.

Example 36-18. Return value trickery

!/bin/bash

multiplication.sh

multiply () # Multiplies params passed.

{ # Will accept a variable number of args.

local product=1

until [ -z "$1" ] # Until uses up arguments passed...

let "product *= $1"

shift

done

echo $product # Will not echo to stdout,

} #+ since this will be assigned to a variable.

mult1=15383; mult2=25211

val1=`multiply $mult1 $mult2`

Assigns stdout (echo) of function to the variable val1.

echo "$mult1 X $mult2 = $val1" # 387820813

mult1=25; mult2=5; mult3=20

val2=`multiply $mult1 $mult2 $mult3`

echo "$mult1 X $mult2 X $mult3 = $val2" # 2500

mult1=188; mult2=37; mult3=25; mult4=47

val3=`multiply $mult1 $mult2 $mult3 $mult4`

echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3" # 8173300

exit 0

The same technique also works for alphanumeric strings. This

means that a function can "return" a non-numeric value.

capitalize_ichar () # Capitalizes initial character

{ #+ of argument string(s) passed.

string0="$@" # Accepts multiple arguments.

firstchar=${string0:0:1} # First character.

string1=${string0:1} # Rest of string(s).

FirstChar=`echo "$firstchar" | tr a-z A-Z`

# Capitalize first character.

echo "$FirstChar$string1" # Output to stdout.

}

newstring=`capitalize_ichar "every sentence should start with a capital letter

."`

echo "$newstring" # Every sentence should start with a capital letter

It is even possible for a function to "return" multiple values

with this method.

Example 36-19. Even more return value trickery

!/bin/bash

sum-product.sh

A function may "return" more than one value.

sum_and_product () # Calculates both sum and product of passed args.

{

echo $(( $1 + $2 )) $(( $1 * $2 ))

Echoes to stdout each calculated value, separated by space.

}

echo

echo "Enter first number "

read first

echo

echo "Enter second number "

read second

echo

retval=`sum_and_product $first $second` # Assigns output of function.

sum=`echo "$retval" | awk '{print $1}'` # Assigns first field.

product=`echo "$retval" | awk '{print $2}'` # Assigns second field.

echo "$first + $second = $sum"

echo "$first * $second = $product"

echo

exit 0

Caution

There can be only one echo statement in the function for this to

work. If you alter the previous example:

sum_and_product ()

{

echo "This is the sum_and_product function." # This messes things up!

echo $(( $1 + $2 )) $(( $1 * $2 ))

}

...

retval=`sum_and_product $first $second` # Assigns output of function.

Now, this will not work correctly.

* Next in our bag of tricks are techniques for passing an array to

a function, then "returning" an array back to the main body of

the script.

Passing an array involves loading the space-separated elements of

the array into a variable with command substitution. Getting an

array back as the "return value" from a function uses the

previously mentioned strategem of echoing the array in the

function, then invoking command substitution and the ( ... )

operator to assign it to an array.

Example 36-20. Passing and returning arrays

!/bin/bash

array-function.sh: Passing an array to a function and ...

"returning" an array from a function

Pass_Array ()

{

local passed_array # Local variable!

passed_array=( `echo "$1"` )

echo "${passed_array[@]}"

# List all the elements of the new array

#+ declared and set within the function.

}

original_array=( element1 element2 element3 element4 element5 )

echo

echo "original_array = ${original_array[@]}"

List all elements of original array.

This is the trick that permits passing an array to a function.

**********************************

argument=`echo ${original_array[@]}`

**********************************

Pack a variable

+ with all the space-separated elements of the original array.

Attempting to just pass the array itself will not work.

This is the trick that allows grabbing an array as a "return value".

*****************************************

returned_array=( `Pass_Array "$argument"` )

*****************************************

Assign 'echoed' output of function to array variable.

echo "returned_array = ${returned_array[@]}"

echo "============================================================="

Now, try it again,

+ attempting to access (list) the array from outside the function.

Pass_Array "$argument"

The function itself lists the array, but ...

+ accessing the array from outside the function is forbidden.

echo "Passed array (within function) = ${passed_array[@]}"

NULL VALUE since the array is a variable local to the function.

echo

And here is an even more explicit example:

ret_array ()

{

for element in {11..20}

echo "$element " # Echo individual elements

done #+ of what will be assembled into an array.

}

arr=( $(ret_array) ) # Assemble into array.

echo "Capturing array \"arr\" from function ret_array () ..."

echo "Third element of array \"arr\" is ${arr[2]}." # 13 (zero-indexed)

echo -n "Entire array is: "

echo ${arr[@]} # 11 12 13 14 15 16 17 18 19 20

echo

exit 0

Nathan Coulter points out that passing arrays with elements containing

+ whitespace breaks this example.

For a more elaborate example of passing arrays to functions, see

Example A-10.

* Using the double-parentheses construct, it is possible to use

C-style syntax for setting and incrementing/decrementing

variables and in for and while loops. See Example 11-13 and

Example 11-18.

* Setting the path and umask at the beginning of a script makes it

more portable -- more likely to run on a "foreign" machine whose

user may have bollixed up the $PATH and umask.

!/bin/bash

PATH=/bin:/usr/bin:/usr/local/bin ; export PATH

umask 022 # Files that the script creates will have 755 permission.

Thanks to Ian D. Allen, for this tip.

* A useful scripting technique is to repeatedly feed the output of

a filter (by piping) back to the same filter, but with a

different set of arguments and/or options. Especially suitable

for this are tr and grep.

From "wstrings.sh" example.

wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \

tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

Example 36-21. Fun with anagrams

!/bin/bash

agram.sh: Playing games with anagrams.

Find anagrams of...

LETTERSET=etaoinshrdlu

FILTER='.......' # How many letters minimum?

1234567

anagram "$LETTERSET" | # Find all anagrams of the letterset...

grep "$FILTER" | # With at least 7 letters,

grep '^is' | # starting with 'is'

grep -v 's | # no plurals

grep -v 'ed # no past tense verbs

Possible to add many combinations of conditions and filters.

Uses "anagram" utility

+ that is part of the author's "yawl" word list package.

http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz

http://bash.deta.in/yawl-0.3.2.tar.gz

exit 0 # End of code.

bash$ sh agram.sh

islander

isolate

isolead

isotheral

Exercises:

---------

Modify this script to take the LETTERSET as a command-line parameter.

Parameterize the filters in lines 11 - 13 (as with $FILTER),

+ so that they can be specified by passing arguments to a function.

For a slightly different approach to anagramming,

+ see the agram2.sh script.

See also Example 29-4, Example 16-25, and Example A-9.

* Use "anonymous here documents" to comment out blocks of code, to

save having to individually comment out each line with a #. See

Example 19-11.

* Running a script on a machine that relies on a command that might

not be installed is dangerous. Use whatis to avoid potential

problems with this.

CMD=command1 # First choice.

PlanB=command2 # Fallback option.

command_test=$(whatis "$CMD" | grep 'nothing appropriate')

If 'command1' not found on system , 'whatis' will return

+ "command1: nothing appropriate."

A safer alternative is:

command_test=$(whereis "$CMD" | grep \/)

But then the sense of the following test would have to be reversed,

+ since the $command_test variable holds content only if

+ the $CMD exists on the system.

(Thanks, bojster.)

if [[ -z "$command_test" ]] # Check whether command present.

then

$CMD option1 option2 # Run command1 with options.

else # Otherwise,

$PlanB #+ run command2.

* An if-grep test may not return expected results in an error case,

when text is output to stderr, rather that stdout.

if ls -l nonexistent_filename | grep -q 'No such file or directory'

then echo "File \"nonexistent_filename\" does not exist."

Redirecting stderr to stdout fixes this.

if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'

^^^^

then echo "File \"nonexistent_filename\" does not exist."

Thanks, Chris Martin, for pointing this out.

* If you absolutely must access a subshell variable outside the

subshell, here's a way to do it.

TMPFILE=tmpfile # Create a temp file to store the variable.

( # Inside the subshell ...

inner_variable=Inner

echo $inner_variable

echo $inner_variable >>$TMPFILE # Append to temp file.

)

# Outside the subshell ...

echo; echo "-----"; echo

echo $inner_variable # Null, as expected.

echo "-----"; echo

Now ...

read inner_variable <$TMPFILE # Read back shell variable.

rm -f "$TMPFILE" # Get rid of temp file.

echo "$inner_variable" # It's an ugly kludge, but it works.

* The run-parts command is handy for running a set of command

scripts in a particular sequence, especially in combination with

cron or at.

* For doing multiple revisions on a complex script, use the rcs

Revision Control System package.

Among other benefits of this is automatically updated ID header

tags. The co command in rcs does a parameter replacement of

certain reserved key words, for example, replacing # $Id$ in a

script with something like:

$Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $

________________________________________________________________

36.7.2. Widgets

It would be nice to be able to invoke X-Windows widgets from a shell

script. There happen to exist several packages that purport to do so,

namely Xscript, Xmenu, and widtools. The first two of these no longer

seem to be maintained. Fortunately, it is still possible to obtain

widtools

[http://www.batse.msfc.nasa.gov/~mallozzi/home/software/xforms/src/wi

dtools-2.0.tgz] here.

Caution

The widtools (widget tools) package requires the XForms library to be

installed. Additionally, the Makefile needs some judicious editing

before the package will build on a typical Linux system. Finally,

three of the six widgets offered do not work (and, in fact,

segfault).

The dialog family of tools offers a method of calling "dialog"

widgets from a shell script. The original dialog utility works in a

text console, but its successors, gdialog, Xdialog, and kdialog use

X-Windows-based widget sets.

Example 36-22. Widgets invoked from a shell script

!/bin/bash

dialog.sh: Using 'gdialog' widgets.

Must have 'gdialog' installed on your system to run this script.

Or, you can replace all instance of 'gdialog' below with 'kdialog' ...

Version 1.1 (corrected 04/05/05)

This script was inspired by the following article.

"Scripting for X Productivity," by Marco Fioretti,

LINUX JOURNAL, Issue 113, September 2003, pp. 86-9.

Thank you, all you good people at LJ.

Input error in dialog box.

E_INPUT=85

Dimensions of display, input widgets.

HEIGHT=50

WIDTH=60

Output file name (constructed out of script name).

OUTFILE=$0.output

Display this script in a text widget.

gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH

Now, we'll try saving input in a file.

echo -n "VARIABLE=" > $OUTFILE

gdialog --title "User Input" --inputbox "Enter variable, please:" \

$HEIGHT $WIDTH 2>> $OUTFILE

if [ "$?" -eq 0 ]

It's good practice to check exit status.

then

echo "Executed \"dialog box\" without errors."

else

echo "Error(s) in \"dialog box\" execution."

# Or, clicked on "Cancel", instead of "OK" button.

rm $OUTFILE

exit $E_INPUT

Now, we'll retrieve and display the saved variable.

. $OUTFILE # 'Source' the saved file.

echo "The variable input in the \"input box\" was: "$VARIABLE""

rm $OUTFILE # Clean up by removing the temp file.

# Some applications may need to retain this file.

exit $?

Exercise: Rewrite this script using the 'zenity' widget set.

The xmessage command is a simple method of popping up a message/query

window. For example:

xmessage Fatal error in script! -button exit

The latest entry in the widget sweepstakes is zenity. This utility

pops up GTK+ dialog widgets-and-windows, and it works very nicely

within a script.

get_info ()

{

zenity --entry # Pops up query window . . .

#+ and prints user entry to stdout.

# Also try the --calendar and --scale options.

}

answer=$( get_info ) # Capture stdout in $answer variable.

echo "User entered: "$answer""

For other methods of scripting with widgets, try Tk or wish (Tcl

derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk

extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl

(Perl with Gtk extensions), or PyQt (Python with Qt extensions).

________________________________________________________________

36.8. Security Issues

36.8.1. Infected Shell Scripts

A brief warning about script security is indicated. A shell script

may contain a worm, trojan, or even a virus. For that reason, never

run as root a script (or permit it to be inserted into the system

startup scripts in /etc/rc.d) unless you have obtained said script

from a trusted source or you have carefully analyzed it to make

certain it does nothing harmful.

Various researchers at Bell Labs and other sites, including M.

Douglas McIlroy, Tom Duff, and Fred Cohen have investigated the

implications of shell script viruses. They conclude that it is all

too easy for even a novice, a "script kiddie," to write one. [128]

Here is yet another reason to learn scripting. Being able to look at

and understand scripts may protect your system from being compromised

by a rogue script.

________________________________________________________________

36.8.2. Hiding Shell Script Source

For security purposes, it may be necessary to render a script

unreadable. If only there were a utility to create a stripped binary

executable from a script. Francisco Rosales' shc -- generic shell

script compiler does exactly that.

Unfortunately, according to

[http://www.linuxjournal.com/article/8256] an article in the October,

2005 Linux Journal, the binary can, in at least some cases, be

decrypted to recover the original script source. Still, this could be

a useful method of keeping scripts secure from all but the most

skilled hackers.

________________________________________________________________

36.8.3. Writing Secure Shell Scripts

Dan Stromberg suggests the following guidelines for writing

(relatively) secure shell scripts.

* Don't put secret data in environment variables.

* Don't pass secret data in an external command's arguments (pass

them in via a pipe or redirection instead).

* Set your $PATH carefully. Don't just trust whatever path you

inherit from the caller if your script is running as root. In

fact, whenever you use an environment variable inherited from the

caller, think about what could happen if the caller put something

misleading in the variable, e.g., if the caller set $HOME to

/etc.

________________________________________________________________

36.9. Portability Issues

It is easier to port a shell than a shell script.

--Larry Wall

This book deals specifically with Bash scripting on a GNU/Linux

system. All the same, users of sh and ksh will find much of value

here.

As it happens, many of the various shells and scripting languages

seem to be converging toward the POSIX 1003.2 standard. Invoking Bash

with the --posix option or inserting a set -o posix at the head of a

script causes Bash to conform very closely to this standard. Another

alternative is to use a #!/bin/sh sha-bang header in the script,

rather than #!/bin/bash. [129] Note that /bin/sh is a link to

/bin/bash in Linux and certain other flavors of UNIX, and a script

invoked this way disables extended Bash functionality.

Most Bash scripts will run as-is under ksh, and vice-versa, since

Chet Ramey has been busily porting ksh features to the latest

versions of Bash.

On a commercial UNIX machine, scripts using GNU-specific features of

standard commands may not work. This has become less of a problem in

the last few years, as the GNU utilities have pretty much displaced

their proprietary counterparts even on "big-iron" UNIX. Caldera's

release of the source to many of the original UNIX utilities has

accelerated the trend.

Bash has certain features that the traditional Bourne shell lacks.

Among these are:

* Certain extended invocation options

* Command substitution using $( ) notation

* Brace expansion

* Certain array operations, and associative arrays

* The double brackets extended test construct

* The double-parentheses arithmetic-evaluation construct

* Certain string manipulation operations

* Process substitution

* A Regular Expression matching operator

* Bash-specific builtins

* Coprocesses

See the Bash F.A.Q. for a complete listing.

________________________________________________________________

36.9.1. A Test Suite

Let us illustrate some of the incompatibilities between Bash and the

classic Bourne shell. Download and install the "Heirloom Bourne

Shell" and run the following script, first using Bash, then the

classic sh.

Example 36-23. Test Suite

!/bin/bash

test-suite.sh

A partial Bash compatibility test suite.

Run this on your version of Bash, or some other shell.

default_option=FAIL # Tests below will fail unless . . .

echo

echo -n "Testing "

sleep 1; echo -n ". "

sleep 1; echo ". "

echo

Double brackets

String="Double brackets supported?"

echo -n "Double brackets test: "

if [[ "$String" = "Double brackets supported?" ]]

then

echo "PASS"

else

echo "FAIL"

Double brackets and regex matching

String="Regex matching supported?"

echo -n "Regex matching: "

if [[ "$String" =~ R.....matching* ]]

then

echo "PASS"

else

echo "FAIL"

Arrays

test_arr=$default_option # FAIL

Array=( If supports arrays will print PASS )

test_arr=${Array[5]}

echo "Array test: $test_arr"

Command Substitution

csub_test ()

{

echo "PASS"

}

test_csub=$default_option # FAIL

test_csub=$(csub_test)

echo "Command substitution test: $test_csub"

echo

Completing this script is an exercise for the reader.

Add to the above similar tests for double parentheses,

+ brace expansion, process substitution, etc.

exit $?

________________________________________________________________

36.10. Shell Scripting Under Windows

Even users running that other OS can run UNIX-like shell scripts, and

therefore benefit from many of the lessons of this book. The

[http://sourceware.cygnus.com/cygwin/] Cygwin package from Cygnus and

the [http://www.mkssoftware.com/] MKS utilities from Mortice Kern

Associates add shell scripting capabilities to Windows.

Another alternative is

[http://www2.research.att.com/~gsf/download/uwin/uwin.html] UWIN,

written by David Korn of AT&T, of Korn Shell fame.

In 2006, Microsoft released the Windows Powershell®, which contains

limited Bash-like command-line scripting capabilities.

________________________________________________________________

Chapter 37. Bash, versions 2, 3, and 4

37.1. Bash, version 2

The current version of Bash, the one you have running on your

machine, is most likely version 2.xx.yy, 3.xx.yy, or 4.xx.yy.

bash$ echo $BASH_VERSION

3.2.25(1)-release

The version 2 update of the classic Bash scripting language added

array variables, string and parameter expansion, and a better method

of indirect variable references, among other features.

Example 37-1. String expansion

!/bin/bash

String expansion.

Introduced with version 2 of Bash.

Strings of the form xxx'

+ have the standard escaped characters interpreted.

echo Ringing bell 3 times \a \a \a'

# May only ring once with certain terminals.

# Or ...

# May not ring at all, depending on terminal settings.

echo Three form feeds \f \f \f'

echo 10 newlines \n\n\n\n\n\n\n\n\n\n'

echo \102\141\163\150'

# B a s h

# Octal equivalent of characters.

exit

Example 37-2. Indirect variable references - the new way

!/bin/bash

Indirect variable referencing.

This has a few of the attributes of references in C++.

a=letter_of_alphabet

letter_of_alphabet=z

echo "a = $a" # Direct reference.

echo "Now a = ${!a}" # Indirect reference.

The ${!variable} notation is more intuitive than the old

+ eval var1=\$var2

echo

t=table_cell_3

table_cell_3=24

echo "t = ${!t}" # t = 24

table_cell_3=387

echo "Value of t changed to ${!t}" # 387

No 'eval' necessary.

This is useful for referencing members of an array or table,

+ or for simulating a multi-dimensional array.

An indexing option (analogous to pointer arithmetic)

+ would have been nice. Sigh.

exit 0

!/bin/bash

resistor-inventory.sh

Simple database / table-lookup application.

============================================================== #

Data

B1723_value=470 # Ohms

B1723_powerdissip=.25 # Watts

B1723_colorcode="yellow-violet-brown" # Color bands

B1723_loc=173 # Where they are

B1723_inventory=78 # How many

B1724_value=1000

B1724_powerdissip=.25

B1724_colorcode="brown-black-red"

B1724_loc=24N

B1724_inventory=243

B1725_value=10000

B1725_powerdissip=.125

B1725_colorcode="brown-black-orange"

B1725_loc=24N

B1725_inventory=89

============================================================== #

echo

PS3='Enter catalog number: '

echo

select catalog_number in "B1723" "B1724" "B1725"

Inv=${catalog_number}_inventory

Val=${catalog_number}_value

Pdissip=${catalog_number}_powerdissip

Loc=${catalog_number}_loc

Ccode=${catalog_number}_colorcode

echo

echo "Catalog number $catalog_number:"

# Now, retrieve value, using indirect referencing.

echo "There are ${!Inv} of [${!Val} ohm / ${!Pdissip} watt]\

resistors in stock." # ^ ^

# As of Bash 4.2, you can replace "ohm" with \u2126 (using echo -e).

echo "These are located in bin # ${!Loc}."

echo "Their color code is \"${!Ccode}\"."

break

done

echo; echo

Exercises:

---------

1) Rewrite this script to read its data from an external file.

2) Rewrite this script to use arrays,

+ rather than indirect variable referencing.

Which method is more straightforward and intuitive?

Which method is easier to code?

Notes:

-----

Shell scripts are inappropriate for anything except the most simple

+ database applications, and even then it involves workarounds and kludges.

Much better is to use a language with native support for data structures,

+ such as C++ or Java (or even Perl).

exit 0

Example 37-4. Using arrays and other miscellaneous trickery to deal

four random hands from a deck of cards

!/bin/bash

cards.sh

Deals four random hands from a deck of cards.

UNPICKED=0

PICKED=1

DUPE_CARD=99

LOWER_LIMIT=0

UPPER_LIMIT=51

CARDS_IN_SUIT=13

CARDS=52

declare -a Deck

declare -a Suits

declare -a Cards

It would have been easier to implement and more intuitive

+ with a single, 3-dimensional array.

Perhaps a future version of Bash will support multidimensional arrays.

initialize_Deck ()

{

i=$LOWER_LIMIT

until [ "$i" -gt $UPPER_LIMIT ]

Deck[i]=$UNPICKED # Set each card of "Deck" as unpicked.

let "i += 1"

done

echo

}

initialize_Suits ()

{

Suits[0]=C #Clubs

Suits[1]=D #Diamonds

Suits[2]=H #Hearts

Suits[3]=S #Spades

}

initialize_Cards ()

{

Cards=(2 3 4 5 6 7 8 9 10 J Q K A)

Alternate method of initializing an array.

}

pick_a_card ()

{

card_number=$RANDOM

let "card_number %= $CARDS" # Restrict range to 0 - 51, i.e., 52 cards.

if [ "${Deck[card_number]}" -eq $UNPICKED ]

then

Deck[card_number]=$PICKED

return $card_number

else

return $DUPE_CARD

}

parse_card ()

{

number=$1

let "suit_number = number / CARDS_IN_SUIT"

suit=${Suits[suit_number]}

echo -n "$suit-"

let "card_no = number % CARDS_IN_SUIT"

Card=${Cards[card_no]}

printf %-4s $Card

Print cards in neat columns.

}

seed_random () # Seed random number generator.

{ # What happens if you don't do this?

seed=`eval date +%s`

let "seed %= 32766"

RANDOM=$seed

} # Consider other methods of seeding the random number generator.

deal_cards ()

{

echo

cards_picked=0

while [ "$cards_picked" -le $UPPER_LIMIT ]

pick_a_card

t=$?

if [ "$t" -ne $DUPE_CARD ]

then

parse_card $t

u=$cards_picked+1

# Change back to 1-based indexing, temporarily. Why?

let "u %= $CARDS_IN_SUIT"

if [ "$u" -eq 0 ] # Nested if/then condition test.

then

echo

fi # Each hand set apart with a blank line.

let "cards_picked += 1"

done

echo

return 0

}

Structured programming:

Entire program logic modularized in functions.

===============

seed_random

initialize_Deck

initialize_Suits

initialize_Cards

deal_cards

===============

exit

Exercise 1:

Add comments to thoroughly document this script.

Exercise 2:

Add a routine (function) to print out each hand sorted in suits.

You may add other bells and whistles if you like.

Exercise 3:

Simplify and streamline the logic of the script.

________________________________________________________________

37.2. Bash, version 3

On July 27, 2004, Chet Ramey released version 3 of Bash. This update

fixed quite a number of bugs and added new features.

Some of the more important added features:

* A new, more generalized {a..z} brace expansion operator.

!/bin/bash

for i in {1..10}

Simpler and more straightforward than

+ for i in $(seq 10)

echo -n "$i "

done

echo

1 2 3 4 5 6 7 8 9 10

Or just . . .

echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z

echo {e..m} # e f g h i j k l m

echo {z..a} # z y x w v u t s r q p o n m l k j i h g f e d c b a

# Works backwards, too.

echo {25..30} # 25 26 27 28 29 30

echo {3..-2} # 3 2 1 0 -1 -2

echo {X..d} # X Y Z [ ] ^ _ ` a b c d

# Shows (some of) the ASCII characters between Z and a,

#+ but don't rely on this type of behavior because . . .

echo {]..a} # {]..a}

# Why?

You can tack on prefixes and suffixes.

echo "Number #"{1..4}, "..."

# Number #1, Number #2, Number #3, Number #4, ...

You can concatenate brace-expansion sets.

echo {1..3}{x..z}" +" "..."

# 1x + 1y + 1z + 2x + 2y + 2z + 3x + 3y + 3z + ...

# Generates an algebraic expression.

# This could be used to find permutations.

You can nest brace-expansion sets.

echo {{a..c},{1..3}}

# a b c 1 2 3

# The "comma operator" splices together strings.

########## ######### ############ ########### ######### ###############

Unfortunately, brace expansion does not lend itself to parameterization.

var1=1

var2=5

echo {$var1..$var2} # {1..5}

Yet, as Emiliano G. points out, using "eval" overcomes this limitation.

start=0

end=10

for index in $(eval echo {$start..$end})

echo -n "$index " # 0 1 2 3 4 5 6 7 8 9 10

done

echo

* The ${!array[@]} operator, which expands to all the indices of a

given array.

!/bin/bash

Array=(element-zero element-one element-two element-three)

echo ${Array[0]} # element-zero

# First element of array.

echo ${!Array[@]} # 0 1 2 3

# All the indices of Array.

for i in ${!Array[@]}

echo ${Array[i]} # element-zero

# element-one

# element-two

# element-three

# All the elements in Array.

done

* The =~ Regular Expression matching operator within a double

brackets test expression. (Perl has a similar operator.)

!/bin/bash

variable="This is a fine mess."

echo "$variable"

Regex matching with =~ operator within [[ double brackets ]].

if [[ "$variable" =~ T.........fin*es* ]]

NOTE: As of version 3.2 of Bash, expression to match no longer quoted.

then

echo "match found"

# match found

Or, more usefully:

!/bin/bash

input=$1

if [[ "$input" =~ "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]]

^ NOTE: Quoting not necessary, as of version 3.2 of Bash.

NNN-NN-NNNN (where each N is a digit).

then

echo "Social Security number."

# Process SSN.

else

echo "Not a Social Security number!"

# Or, ask for corrected input.

For additional examples of using the =~ operator, see Example

A-29, Example 19-14, Example A-35, and Example A-24.

* The new set -o pipefail option is useful for debugging pipes. If

this option is set, then the exit status of a pipe is the exit

status of the last command in the pipe to fail (return a non-zero

value), rather than the actual final command in the pipe.

See Example 16-43.

Caution

The update to version 3 of Bash breaks a few scripts that worked

under earlier versions. Test critical legacy scripts to make sure

they still work!

As it happens, a couple of the scripts in the Advanced Bash Scripting

Guide had to be fixed up (see Example 9-4, for instance).

________________________________________________________________

37.2.1. Bash, version 3.1

The version 3.1 update of Bash introduces a number of bugfixes and a

few minor changes.

* The += operator is now permitted in in places where previously

only the = assignment operator was recognized.

a=1

echo $a # 1

a+=5 # Won't work under versions of Bash earlier than 3.1.

echo $a # 15

a+=Hello

echo $a # 15Hello

Here, += functions as a string concatenation operator. Note that

its behavior in this particular context is different than within

a let construct.

a=1

echo $a # 1

let a+=5 # Integer arithmetic, rather than string concatenation.

echo $a # 6

let a+=Hello # Doesn't "add" anything to a.

echo $a # 6

Jeffrey Haemer points out that this concatenation operator can be

quite useful. In this instance, we append a directory to the

$PATH.

bash$ echo $PATH

/usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games

bash$ PATH+=:/opt/bin

bash$ echo $PATH

/usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games:/opt/bin

________________________________________________________________

37.2.2. Bash, version 3.2

This is pretty much a bugfix update.

* In global parameter substitutions, the pattern no longer anchors

at the start of the string.

* The --wordexp option disables process substitution.

* The =~ Regular Expression match operator no longer requires

quoting of the pattern within [[ ... ]].

Caution

In fact, quoting in this context is not advisable as it may cause

regex evaluation to fail. Chet Ramey states in the Bash FAQ that

quoting explicitly disables regex evaluation. See also the

[https://bugs.launchpad.net/ubuntu-website/+bug/109931] Ubuntu Bug

List and

[http://en.wikinerds.org/index.php/Bash_syntax_and_semantics]

Wikinerds on Bash syntax.

Setting shopt -s compat31 in a script causes reversion to the

original behavior.

________________________________________________________________

37.3. Bash, version 4

Chet Ramey announced Version 4 of Bash on the 20th of February, 2009.

This release has a number of significant new features, as well as

some important bugfixes.

Among the new goodies:

* Associative arrays. [130]

An associative array can be thought of as a set of two linked arrays

-- one holding the data, and the other the keys that index the

individual elements of the data array.

Example 37-5. A simple address database

!/bin/bash4

fetch_address.sh

declare -A address

-A option declares associative array.

address[Charles]="414 W. 10th Ave., Baltimore, MD 21236"

address[John]="202 E. 3rd St., New York, NY 10009"

address[Wilma]="1854 Vermont Ave, Los Angeles, CA 90023"

echo "Charles's address is ${address[Charles]}."

Charles's address is 414 W. 10th Ave., Baltimore, MD 21236.

echo "Wilma's address is ${address[Wilma]}."

Wilma's address is 1854 Vermont Ave, Los Angeles, CA 90023.

echo "John's address is ${address[John]}."

John's address is 202 E. 3rd St., New York, NY 10009.

echo

echo "${!address[*]}" # The array indices ...

Charles John Wilma

Example 37-6. A somewhat more elaborate address database

!/bin/bash4

fetch_address-2.sh

A more elaborate version of fetch_address.sh.

SUCCESS=0

E_DB=99 # Error code for missing entry.

declare -A address

-A option declares associative array.

store_address ()

{

address[$1]="$2"

return $?

}

fetch_address ()

{

if [[ -z "${address[$1]}" ]]

then

echo "$1's address is not in database."

return $E_DB

echo "$1's address is ${address[$1]}."

return $?

}

store_address "Lucas Fayne" "414 W. 13th Ave., Baltimore, MD 21236"

store_address "Arvid Boyce" "202 E. 3rd St., New York, NY 10009"

store_address "Velma Winston" "1854 Vermont Ave, Los Angeles, CA 90023"

Exercise:

Rewrite the above store_address calls to read data from a file,

+ then assign field 1 to name, field 2 to address in the array.

Each line in the file would have a format corresponding to the above.

Use a while-read loop to read from file, sed or awk to parse the fields.

fetch_address "Lucas Fayne"

Lucas Fayne's address is 414 W. 13th Ave., Baltimore, MD 21236.

fetch_address "Velma Winston"

Velma Winston's address is 1854 Vermont Ave, Los Angeles, CA 90023.

fetch_address "Arvid Boyce"

Arvid Boyce's address is 202 E. 3rd St., New York, NY 10009.

fetch_address "Bozo Bozeman"

Bozo Bozeman's address is not in database.

exit $? # In this case, exit code = 99, since that is function return.

See Example A-53 for an interesting usage of an associative

array.

Caution

Elements of the index array may include embedded space characters, or

even leading and/or trailing space characters. However, index array

elements containing only whitespace are not permitted.

address[ ]="Blank" # Error!

* Enhancements to the case construct: the ;;& and ;& terminators.

Example 37-7. Testing characters

!/bin/bash4

test_char ()

{

case "$1" in

[[:print:]] ) echo "$1 is a printable character.";;& # |

# The ;;& terminator continues to the next pattern test. |

[[:alnum:]] ) echo "$1 is an alpha/numeric character.";;& # v

[[:alpha:]] ) echo "$1 is an alphabetic character.";;& # v

[[:lower:]] ) echo "$1 is a lowercase alphabetic character.";;&

[[:digit:]] ) echo "$1 is an numeric character.";& # |

# The ;& terminator executes the next statement ... # |

%%%@@@@@ ) echo "********************************";; # v

^^^^^^^^ ... even with a dummy pattern.

esac

}

echo

test_char 3

3 is a printable character.

3 is an alpha/numeric character.

3 is an numeric character.

********************************

echo

test_char m

m is a printable character.

m is an alpha/numeric character.

m is an alphabetic character.

m is a lowercase alphabetic character.

echo

test_char /

/ is a printable character.

echo

The ;;& terminator can save complex if/then conditions.

The ;& is somewhat less useful.

* The new coproc builtin enables two parallel processes to

communicate and interact. As Chet Ramey states in the Bash FAQ

[131] , ver. 4.01:

There is a new 'coproc' reserved word that specifies a coproce

ss:

an asynchronous command run with two pipes connected to the cr

eating

shell. Coprocs can be named. The input and output file descrip

tors

and the PID of the coprocess are available to the calling shel

l in

variables with coproc-specific names.

George Dimitriu explains,

"... coproc ... is a feature used in Bash process substitution

which now is made publicly available."

This means it can be explicitly invoked in a script, rather th

just being a behind-the-scenes mechanism used by Bash.

Coprocesses use file descriptors. File descriptors enable

processes and pipes to communicate.

!/bin/bash4

A coprocess communicates with a while-read loop.

coproc { cat mx_data.txt; sleep 2; }

^^^^^^^

Try running this without "sleep 2" and see what happens.

while read -u ${COPROC[0]} line # ${COPROC[0]} is the

do #+ file descriptor of the coprocess.

echo "$line" | sed -e 's/line/NOT-ORIGINAL-TEXT/'

done

kill $COPROC_PID # No longer need the coprocess,

#+ so kill its PID.

But, be careful!

!/bin/bash4

echo; echo

a=aaa

b=bbb

c=ccc

coproc echo "one two three"

while read -u ${COPROC[0]} a b c; # Note that this loop

do #+ runs in a subshell.

echo "Inside while-read loop: ";

echo "a = $a"; echo "b = $b"; echo "c = $c"

echo "coproc file descriptor: ${COPROC[0]}"

done

a = one

b = two

c = three

So far, so good, but ...

echo "-----------------"

echo "Outside while-read loop: "

echo "a = $a" # a =

echo "b = $b" # b =

echo "c = $c" # c =

echo "coproc file descriptor: ${COPROC[0]}"

echo

The coproc is still running, but ...

+ it still doesn't enable the parent process

+ to "inherit" variables from the child process, the while-read loop.

Compare this to the "badread.sh" script.

Caution

The coprocess is asynchronous, and this might cause a problem. It may

terminate before another process has finished communicating with it.

!/bin/bash4

coproc cpname { for i in {0..10}; do echo "index = $i"; done; }

^^^^^^ This is a named coprocess.

read -u ${cpname[0]}

echo $REPLY # index = 0

echo ${COPROC[0]} #+ No output ... the coprocess timed out

after the first loop iteration.

However, George Dimitriu has a partial fix.

coproc cpname { for i in {0..10}; do echo "index = $i"; done; sleep 1;

echo hi > myo; cat - >> myo; }

^^^^^ This is a named coprocess.

echo "I am main" \04' >&${cpname[1]}

myfd=${cpname[0]}

echo myfd=$myfd

while read -u $myfd

do

echo $REPLY;

done

echo $cpname_PID

Run this with and without the commented-out while-loop, and it is

+ apparent that each process, the executing shell and the coprocess,

+ waits for the other to finish writing in its own write-enabled pipe.

* The new mapfile builtin makes it possible to load an array with

the contents of a text file without using a loop or command

substitution.

!/bin/bash4

mapfile Arr1 < $0

Same result as Arr1=( $(cat $0) )

echo "${Arr1[@]}" # Copies this entire script out to stdout.

echo "--"; echo

But, not the same as read -a !!!

read -a Arr2 < $0

echo "${Arr2[@]}" # Reads only first line of script into the array.

exit

* The read builtin got a minor facelift. The -t timeout option now

accepts (decimal) fractional values [132] and the -i option

permits preloading the edit buffer. [133] Unfortunately, these

enhancements are still a work in progress and not (yet) usable in

scripts.

* Parameter substitution gets case-modification operators.

!/bin/bash4

var=veryMixedUpVariable

echo ${var} # veryMixedUpVariable

echo ${var^} # VeryMixedUpVariable

* First char --> uppercase.

echo ${var^^} # VERYMIXEDUPVARIABLE

** All chars --> uppercase.

echo ${var,} # veryMixedUpVariable

* First char --> lowercase.

echo ${var,,} # verymixedupvariable

** All chars --> lowercase.

* The declare builtin now accepts the -l lowercase and -c

capitalize options.

!/bin/bash4

declare -l var1 # Will change to lowercase

var1=MixedCaseVARIABLE

echo "$var1" # mixedcasevariable

Same effect as echo $var1 | tr A-Z a-z

declare -c var2 # Changes only initial char to uppercase.

var2=originally_lowercase

echo "$var2" # Originally_lowercase

NOT the same effect as echo $var2 | tr a-z A-Z

* Brace expansion has more options.

Increment/decrement, specified in the final term within braces.

!/bin/bash4

echo {40..60..2}

40 42 44 46 48 50 52 54 56 58 60

All the even numbers, between 40 and 60.

echo {60..40..2}

60 58 56 54 52 50 48 46 44 42 40

All the even numbers, between 40 and 60, counting backwards.

In effect, a decrement.

echo {60..40..-2}

The same output. The minus sign is not necessary.

But, what about letters and symbols?

echo {X..d}

X Y Z [ ] ^ _ ` a b c d

Does not echo the \ which escapes a space.

Zero-padding, specified in the first term within braces, prefixes

each term in the output with the same number of zeroes.

bash4$ echo {010..15}

010 011 012 013 014 015

bash4$ echo {000..10}

000 001 002 003 004 005 006 007 008 009 010

* Substring extraction on positional parameters now starts with $0

as the zero-index. (This corrects an inconsistency in the

treatment of positional parameters.)

!/bin/bash

show-params.bash

Requires version 4+ of Bash.

Invoke this scripts with at least one positional parameter.

E_BADPARAMS=99

if [ -z "$1" ]

then

echo "Usage $0 param1 ..."

exit $E_BADPARAMS

echo ${@:0}

bash3 show-params.bash4 one two three

one two three

bash4 show-params.bash4 one two three

show-params.bash4 one two three

$0 $1 $2 $3

* The new ** globbing operator matches filenames and directories

recursively.

!/bin/bash4

filelist.bash4

shopt -s globstar # Must enable globstar, otherwise ** doesn't work.

# The globstar shell option is new to version 4 of Bash.

echo "Using *"; echo

for filename in *

echo "$filename"

done # Lists only files in current directory ($PWD).

echo; echo "--------------"; echo

echo "Using **"

for filename in **

echo "$filename"

done # Lists complete file tree, recursively.

exit

Using *

allmyfiles

filelist.bash4

--------------

Using **

allmyfiles

allmyfiles/file.index.txt

allmyfiles/my_music

allmyfiles/my_music/me-singing-60s-folksongs.ogg

allmyfiles/my_music/me-singing-opera.ogg

allmyfiles/my_music/piano-lesson.1.ogg

allmyfiles/my_pictures

allmyfiles/my_pictures/at-beach-with-Jade.png

allmyfiles/my_pictures/picnic-with-Melissa.png

filelist.bash4

* The new $BASHPID internal variable.

* There is a new builtin error-handling function named

command_not_found_handle.

!/bin/bash4

command_not_found_handle ()

{ # Accepts implicit parameters.

echo "The following command is not valid: \""$1\"""

echo "With the following argument(s): \""$2\"" \""$3\""" # $4, $5 ...

} # $1, $2, etc. are not explicitly passed to the function.

bad_command arg1 arg2

The following command is not valid: "bad_command"

With the following argument(s): "arg1" "arg2"

Editorial comment

Associative arrays? Coprocesses? Whatever happened to the lean and

mean Bash we have come to know and love? Could it be suffering from

(horrors!) "feature creep"? Or perhaps even Korn shell envy?

Note to Chet Ramey: Please add only essential features in future Bash

releases -- perhaps for-each loops and support for multi-dimensional

arrays. [134] Most Bash users won't need, won't use, and likely won't

greatly appreciate complex "features" like built-in debuggers, Perl

interfaces, and bolt-on rocket boosters.

________________________________________________________________

37.3.1. Bash, version 4.1

Version 4.1 of Bash, released in May, 2010, was primarily a bugfix

update.

* The printf command now accepts a -v option for setting array

indices.

* Within double brackets, the > and < string comparison operators

now conform to the locale. Since the locale setting may affect

the sorting order of string expressions, this has side-effects on

comparison tests within [[ ... ]] expressions.

* The read builtin now takes a -N option (read -N chars), which

causes the read to terminate after chars characters.

Example 37-8. Reading N characters

!/bin/bash

Requires Bash version -ge 4.1 ...

num_chars=61

read -N $num_chars var < $0 # Read first 61 characters of script!

echo "$var"

exit

Output of Script #######

!/bin/bash

Requires Bash version -ge 4.1 ...

num_chars=61

* Here documents embedded in $( ... ) command substitution

constructs may terminate with a simple ).

Example 37-9. Using a here document to set a variable

!/bin/bash

here-commsub.sh

Requires Bash version -ge 4.1 ...

multi_line_var=$( cat <<ENDxxx

------------------------------

This is line 1 of the variable

This is line 2 of the variable

This is line 3 of the variable

------------------------------

ENDxxx)

Rather than what Bash 4.0 requires:

+ that the terminating limit string and

+ the terminating close-parenthesis be on separate lines.

ENDxxx

)

echo "$multi_line_var"

Bash still emits a warning, though.

warning: here-document at line 10 delimited

+ by end-of-file (wanted `ENDxxx')

________________________________________________________________

37.3.2. Bash, version 4.2

Version 4.2 of Bash, released in February, 2011, contains a number of

new features and enhancements, in addition to bugfixes.

* Bash now supports the the \u and \U Unicode escape.

Unicode is a cross-platform standard for encoding into numerical

values letters and graphic symbols. This permits representing and

displaying characters in foreign alphabets and unusual fonts.

echo -e '\u2630' # Horizontal triple bar character.

Equivalent to the more roundabout:

echo -e "\xE2\x98\xB0"

# Recognized by earlier Bash versions.

echo -e '\u220F' # PI (Greek letter and mathematical symbol)

echo -e '\u0416' # Capital "ZHE" (Cyrillic letter)

echo -e '\u2708' # Airplane (Dingbat font) symbol

echo -e '\u2622' # Radioactivity trefoil

echo -e "The amplifier circuit requires a 100 \u2126 pull-up resistor."

unicode_var='\u2640'

echo -e $unicode_var # Female symbol

printf "$unicode_var \n" # Female symbol, with newline

And for something a bit more elaborate . . .

We can store Unicode symbols in an associative array,

+ then retrieve them by name.

Run this in a gnome-terminal or a terminal with a large, bold font

+ for better legibility.

declare -A symbol # Associative array.

symbol[script_E]='\u2130'

symbol[script_F]='\u2131'

symbol[script_J]='\u2110'

symbol[script_M]='\u2133'

symbol[Rx]='\u211E'

symbol[TEL]='\u2121'

symbol[FAX]='\u213B'

symbol[care_of]='\u2105'

symbol[account]='\u2100'

symbol[trademark]='\u2122'

echo -ne "${symbol[script_E]} "

echo -ne "${symbol[script_F]} "

echo -ne "${symbol[script_J]} "

echo -ne "${symbol[script_M]} "

echo -ne "${symbol[Rx]} "

echo -ne "${symbol[TEL]} "

echo -ne "${symbol[FAX]} "

echo -ne "${symbol[care_of]} "

echo -ne "${symbol[account]} "

echo -ne "${symbol[trademark]} "

echo

Note

The above example uses the ... ' string-expansion construct.

* When the lastpipe shell option is set, the last command in a pipe

doesn't run in a subshell.

Example 37-10. Piping input to a read

!/bin/bash

lastpipe-option.sh

line='' # Null value.

echo "\$line = "$line"" # $line =

echo

shopt -s lastpipe # Error on Bash version -lt 4.2.

echo "Exit status of attempting to set \"lastpipe\" option is $?"

1 if Bash version -lt 4.2, 0 otherwise.

echo

head -1 $0 | read line # Pipe the first line of the script to read.

^^^^^^^^^ Not in a subshell!!!

echo "\$line = "$line""

Older Bash releases $line =

Bash version 4.2 $line = #!/bin/bash

This option offers possible "fixups" for these example scripts:

Example 34-3 and Example 15-8.

* Negative array indices permit counting backwards from the end of

an array.

Example 37-11. Negative array indices

!/bin/bash

neg-array.sh

Requires Bash, version -ge 4.2.

array=( zero one two three four five ) # Six-element array.

0 1 2 3 4 5

-6 -5 -4 -3 -2 -1

Negative array indices now permitted.

echo ${array[-1]} # five

echo ${array[-2]} # four

...

echo ${array[-6]} # zero

Negative array indices count backward from the last element+1.

But, you cannot index past the beginning of the array.

echo ${array[-7]} # array: bad array subscript

So, what is this new feature good for?

echo "The last element in the array is "${array[-1]}""

Which is quite a bit more straightforward than:

echo "The last element in the array is "${array[${#array[*]}-1]}""

echo

And ...

index=0

let "neg_element_count = 0 - ${#array[*]}"

Number of elements, converted to a negative number.

while [ $index -gt $neg_element_count ]; do

((index--)); echo -n "${array[index]} "

done # Lists the elements in the array, backwards.

# We have just simulated the "tac" command on this array.

echo

!/bin/bash

Bash, version -ge 4.2

Negative length-index in substring extraction.

Important: It changes the interpretation of this construct!

stringZ=abcABC123ABCabc

echo ${stringZ} # abcABC123ABCabc

Position within string: 0123456789.....

echo ${stringZ:2:3} # cAB

Count 2 chars forward from string beginning, and extract 3 chars.

${string:position:length}

So far, nothing new, but now ...

# abcABC123ABCabc

Position within string: 0123....6543210

echo ${stringZ:3:-6} # ABC123

^

Index 3 chars forward from beginning and 6 chars backward from end,

+ and extract everything in between.

${string:offset-from-front:offset-from-end}

When the "length" parameter is negative,

+ it serves as an offset-from-end parameter.

!/bin/bash

mail-format.sh (ver. 1.1): Format e-mail messages.

Gets rid of carets, tabs, and also folds excessively long lines.

=================================================================

Standard Check for Script Argument(s)

ARGS=1

E_BADARGS=85

E_NOFILE=86

if [ $# -ne $ARGS ] # Correct number of arguments passed to script?

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

if [ -f "$1" ] # Check if file exists.

then

file_name=$1

else

echo "File \"$1\" does not exist."

exit $E_NOFILE

-----------------------------------------------------------------

MAXWIDTH=70 # Width to fold excessively long lines to.

=================================

A variable can hold a sed script.

It's a useful technique.

sedscript='s/^>//

s/^ *>//

s/^ *//

s/ *//'

=================================

Delete carets and tabs at beginning of lines,

+ then fold lines to $MAXWIDTH characters.

sed "$sedscript" $1 | fold -s --width=$MAXWIDTH

# -s option to "fold"

#+ breaks lines at whitespace, if possible.

This script was inspired by an article in a well-known trade journal

+ extolling a 164K MS Windows utility with similar functionality.

An nice set of text processing utilities and an efficient

+ scripting language provide an alternative to the bloated executables

+ of a clunky operating system.

exit $?

Example A-2. rn: A simple-minded file renaming utility

This script is a modification of Example 16-22.

! /bin/bash

rn.sh

Very simpleminded filename "rename" utility (based on "lowercase.sh").

The "ren" utility, by Vladimir Lanin (lanin@csd2.nyu.edu),

+ does a much better job of this.

ARGS=2

E_BADARGS=85

ONE=1 # For getting singular/plural right (see below).

if [ $# -ne "$ARGS" ]

then

echo "Usage: `basename $0` old-pattern new-pattern"

# As in "rn gif jpg", which renames all gif files in working directory to jp

exit $E_BADARGS

number=0 # Keeps track of how many files actually renamed.

for filename in *$1* #Traverse all matching files in directory.

if [ -f "$filename" ] # If finds match...

then

fname=`basename $filename` # Strip off path.

n=`echo $fname | sed -e "s/$1/$2/"` # Substitute new for old in filenam

mv $fname $n # Rename.

let "number += 1"

done

if [ "$number" -eq "$ONE" ] # For correct grammar.

then

echo "$number file renamed."

else

echo "$number files renamed."

exit $?

Exercises:

---------

What types of files will this not work on?

How can this be fixed?

Example A-3. blank-rename: Renames filenames containing blanks

This is an even simpler-minded version of previous script.

! /bin/bash

blank-rename.sh

Substitutes underscores for blanks in all the filenames in a directory.

ONE=1 # For getting singular/plural right (see below).

number=0 # Keeps track of how many files actually renamed.

FOUND=0 # Successful return value.

for filename in * #Traverse all files in directory.

echo "$filename" | grep -q " " # Check whether filename

if [ $? -eq $FOUND ] #+ contains space(s).

then

fname=$filename # Yes, this filename needs work.

n=`echo $fname | sed -e "s/ /_/g"` # Substitute underscore for blank.

mv "$fname" "$n" # Do the actual renaming.

let "number += 1"

done

if [ "$number" -eq "$ONE" ] # For correct grammar.

then

echo "$number file renamed."

else

echo "$number files renamed."

exit 0

Example A-4. encryptedpw: Uploading to an ftp site, using a locally

encrypted password

!/bin/bash

Example "ex72.sh" modified to use encrypted password.

Note that this is still rather insecure,

+ since the decrypted password is sent in the clear.

Use something like "ssh" if this is a concern.

E_BADARGS=85

if [ -z "$1" ]

then

echo "Usage: `basename $0` filename"

exit $E_BADARGS

Username=bozo # Change to suit.

pword=/home/bozo/secret/password_encrypted.file

File containing encrypted password.

Filename=`basename $1` # Strips pathname out of file name.

Server="XXX"

Directory="YYY" # Change above to actual server name & directory.

Password=`cruft <$pword` # Decrypt password.

Uses the author's own "cruft" file encryption package,

+ based on the classic "onetime pad" algorithm,

+ and obtainable from:

+ Primary-site: ftp://ibiblio.org/pub/Linux/utils/file

+ cruft-0.2.tar.gz [16k]

ftp -n $Server <<End-Of-Session

user $Username $Password

binary

bell

cd $Directory

put $Filename

bye

End-Of-Session

-n option to "ftp" disables auto-logon.

Note that "bell" rings 'bell' after each file transfer.

exit 0

Example A-5. copy-cd: Copying a data CD

!/bin/bash

copy-cd.sh: copying a data CD

CDROM=/dev/cdrom # CD ROM device

OF=/home/bozo/projects/cdimage.iso # output file

/xxxx/xxxxxxxx/ Change to suit your system.

BLOCKSIZE=2048

SPEED=10 # If unspecified, uses max spd.

DEVICE=/dev/cdrom older version.

DEVICE="1,0,0"

echo; echo "Insert source CD, but do *not* mount it."

echo "Press ENTER when ready. "

read ready # Wait for input, $ready not used.

echo; echo "Copying the source CD to $OF."

echo "This may take a while. Please be patient."

dd if=$CDROM of=$OF bs=$BLOCKSIZE # Raw device copy.

echo; echo "Remove data CD."

echo "Insert blank CDR."

echo "Press ENTER when ready. "

read ready # Wait for input, $ready not used.

echo "Copying $OF to CDR."

cdrecord -v -isosize speed=$SPEED dev=$DEVICE $OF # Old version.

wodim -v -isosize dev=$DEVICE $OF

Uses Joerg Schilling's "cdrecord" package (see its docs).

http://www.fokus.gmd.de/nthp/employees/schilling/cdrecord.html

Newer Linux distros may use "wodim" rather than "cdrecord" ...

echo; echo "Done copying $OF to CDR on device $CDROM."

echo "Do you want to erase the image file (y/n)? " # Probably a huge file.

read answer

case "$answer" in

[yY]) rm -f $OF

echo "$OF erased."

;;

) echo "$OF not erased.";;

esac

echo

Exercise:

Change the above "case" statement to also accept "yes" and "Yes" as input.

exit 0

Example A-6. Collatz series

!/bin/bash

collatz.sh

The notorious "hailstone" or Collatz series.

-------------------------------------------

1) Get the integer "seed" from the command-line.

2) NUMBER <-- seed

3) Print NUMBER.

4) If NUMBER is even, divide by 2, or

5)+ if odd, multiply by 3 and add 1.

6) NUMBER <-- result

7) Loop back to step 3 (for specified number of iterations).

The theory is that every such sequence,

+ no matter how large the initial value,

+ eventually settles down to repeating "4,2,1..." cycles,

+ even after fluctuating through a wide range of values.

This is an instance of an "iterate,"

+ an operation that feeds its output back into its input.

Sometimes the result is a "chaotic" series.

MAX_ITERATIONS=200

For large seed numbers (>32000), try increasing MAX_ITERATIONS.

h=${1:-$} # Seed.

# Use $PID as seed,

#+ if not specified as command-line arg.

echo

echo "C($h) -*- $MAX_ITERATIONS Iterations"

echo

for ((i=1; i<=MAX_ITERATIONS; i++))

echo -n "$h "

^^^

tab

printf does it better ...

COLWIDTH=%7d

printf $COLWIDTH $h

let "remainder = h % 2"

if [ "$remainder" -eq 0 ] # Even?

then

let "h /= 2" # Divide by 2.

else

let "h = h*3 + 1" # Multiply by 3 and add 1.

COLUMNS=10 # Output 10 values per line.

let "line_break = i % $COLUMNS"

if [ "$line_break" -eq 0 ]

then

echo

done

echo

For more information on this strange mathematical function,

+ see _Computers, Pattern, Chaos, and Beauty_, by Pickover, p. 185 ff.,

+ as listed in the bibliography.

exit 0

Example A-7. days-between: Days between two dates

!/bin/bash

days-between.sh: Number of days between two dates.

Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY

Note: Script modified to account for changes in Bash, v. 2.05b +,

+ that closed the loophole permitting large negative

+ integer return values.

ARGS=2 # Two command-line parameters expected.

E_PARAM_ERR=85 # Param error.

REFYR=1600 # Reference year.

CENTURY=100

DIY=365

ADJ_DIY=367 # Adjusted for leap year + fraction.

MIY=12

DIM=31

LEAPCYCLE=4

MAXRETVAL=255 # Largest permissible

#+ positive return value from a function.

diff= # Declare global variable for date difference.

value= # Declare global variable for absolute value.

day= # Declare globals for day, month, year.

month=

year=

Param_Error () # Command-line parameters wrong.

{

echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"

echo " (date must be after 1/3/1600)"

exit $E_PARAM_ERR

}

Parse_Date () # Parse date from command-line params.

{

month=${1%%/**}

dm=${1%/**} # Day and month.

day=${dm#*/}

let "year = `basename $1`" # Not a filename, but works just the same.

}

check_date () # Checks for invalid date(s) passed.

{

[ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] ||

[ "$year" -lt "$REFYR" ] && Param_Error

# Exit script on bad value(s).

# Uses or-list / and-list.

# Exercise: Implement more rigorous date checking.

}

strip_leading_zero () # Better to strip possible leading zero(s)

{ #+ from day and/or month

return ${1#0} #+ since otherwise Bash will interpret them

} #+ as octal values (POSIX.2, sect 2.9.2.1).

day_index () # Gauss' Formula:

{ # Days from March 1, 1600 to date passed as param.

# ^^^^^^^^^^^^^

day=$1

month=$2

year=$3

let "month = $month - 2"

if [ "$month" -le 0 ]

then

let "month += 12"

let "year -= 1"

let "year -= $REFYR"

let "indexyr = $year / $CENTURY"

let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr \

+ $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"

# For an in-depth explanation of this algorithm, see

#+ http://weblogs.asp.net/pgreborio/archive/2005/01/06/347968.aspx

echo $Days

}

calculate_difference () # Difference between two day indices.

{

let "diff = $1 - $2" # Global variable.

}

abs () # Absolute value

{ # Uses global "value" variable.

if [ "$1" -lt 0 ] # If negative

then #+ then

let "value = 0 - $1" #+ change sign,

else #+ else

let "value = $1" #+ leave it alone.

}

if [ $# -ne "$ARGS" ] # Require two command-line params.

then

Param_Error

Parse_Date $1

check_date $day $month $year # See if valid date.

strip_leading_zero $day # Remove any leading zeroes

day=$? #+ on day and/or month.

strip_leading_zero $month

month=$?

let "date1 = `day_index $day $month $year`"

Parse_Date $2

check_date $day $month $year

strip_leading_zero $day

day=$?

strip_leading_zero $month

month=$?

date2=$(day_index $day $month $year) # Command substitution.

calculate_difference $date1 $date2

abs $diff # Make sure it's positive.

diff=$value

echo $diff

exit 0

Exercise:

--------

If given only one command-line parameter, have the script

+ use today's date as the second.

Compare this script with

+ the implementation of Gauss' Formula in a C program at

+ http://buschencrew.hypermart.net/software/datedif

Example A-8. Making a dictionary

!/bin/bash

makedict.sh [make dictionary]

Modification of /usr/sbin/mkdict (/usr/sbin/cracklib-forman) script.

Original script copyright 1993, by Alec Muffett.

This modified script included in this document in a manner

+ consistent with the "LICENSE" document of the "Crack" package

+ that the original script is a part of.

This script processes text files to produce a sorted list

+ of words found in the files.

This may be useful for compiling dictionaries

+ and for other lexicographic purposes.

E_BADARGS=85

if [ ! -r "$1" ] # Need at least one

then #+ valid file argument.

echo "Usage: $0 files-to-process"

exit $E_BADARGS

SORT="sort" # No longer necessary to define

#+ options to sort. Changed from

#+ original script.

cat $* | # Dump specified files to stdout.

tr A-Z a-z | # Convert to lowercase.

tr ' ' '\012' | # New: change spaces to newlines.

tr -cd '\012[a-z][0-9]' | # Get rid of everything

#+ non-alphanumeric (in orig. script).

tr -c '\012a-z' '\012' | # Rather than deleting non-alpha

#+ chars, change them to newlines.

sort | # $SORT options unnecessary now.

uniq | # Remove duplicates.

grep -v '^#' | # Delete lines starting with #.

grep -v '^ # Delete blank lines.

exit $?

Example A-9. Soundex conversion

!/bin/bash

soundex.sh: Calculate "soundex" code for names

=======================================================

Soundex script

by

Mendel Cooper

thegrendel.abs@gmail.com

reldate: 23 January, 2002

Placed in the Public Domain.

A slightly different version of this script appeared in

+ Ed Schaefer's July, 2002 "Shell Corner" column

+ in "Unix Review" on-line,

+ http://www.unixreview.com/documents/uni1026336632258/

=======================================================

ARGCOUNT=1 # Need name as argument.

E_WRONGARGS=90

if [ $# -ne "$ARGCOUNT" ]

then

echo "Usage: `basename $0` name"

exit $E_WRONGARGS

assign_value () # Assigns numerical value

{ #+ to letters of name.

val1=bfpv # 'b,f,p,v' = 1

val2=cgjkqsxz # 'c,g,j,k,q,s,x,z' = 2

val3=dt # etc.

val4=l

val5=mn

val6=r

Exceptionally clever use of 'tr' follows.

Try to figure out what is going on here.

value=$( echo "$1" \

| tr -d wh \

| tr $val1 1 | tr $val2 2 | tr $val3 3 \

| tr $val4 4 | tr $val5 5 | tr $val6 6 \

| tr -s 123456 \

| tr -d aeiouy )

Assign letter values.

Remove duplicate numbers, except when separated by vowels.

Ignore vowels, except as separators, so delete them last.

Ignore 'w' and 'h', even as separators, so delete them first.

The above command substitution lays more pipe than a plumber <g>.

}

input_name="$1"

echo

echo "Name = $input_name"

Change all characters of name input to lowercase.

------------------------------------------------

name=$( echo $input_name | tr A-Z a-z )

------------------------------------------------

Just in case argument to script is mixed case.

Prefix of soundex code: first letter of name.

--------------------------------------------

char_pos=0 # Initialize character position.

prefix0=${name:$char_pos:1}

prefix=`echo $prefix0 | tr a-z A-Z`

# Uppercase 1st letter of soundex.

let "char_pos += 1" # Bump character position to 2nd letter of name

name1=${name:$char_pos}

++++++++++++++++++++++++++ Exception Patch ++++++++++++++++++++++++++++++

Now, we run both the input name and the name shifted one char

+ to the right through the value-assigning function.

If we get the same value out, that means that the first two characters

+ of the name have the same value assigned, and that one should cancel.

However, we also need to test whether the first letter of the name

+ is a vowel or 'w' or 'h', because otherwise this would bollix things up.

char1=`echo $prefix | tr A-Z a-z` # First letter of name, lowercased.

assign_value $name

s1=$value

assign_value $name1

s2=$value

assign_value $char1

s3=$value

s3=9$s3 # If first letter of name is a vowel

#+ or 'w' or 'h',

#+ then its "value" will be null (unset).

#+ Therefore, set it to 9, an otherwise

#+ unused value, which can be tested for.

if [[ "$s1" -ne "$s2" || "$s3" -eq 9 ]]

then

suffix=$s2

else

suffix=${s2:$char_pos}

++++++++++++++++++++++ end Exception Patch ++++++++++++++++++++++++++++++

padding=000 # Use at most 3 zeroes to pad.

soun=$prefix$suffix$padding # Pad with zeroes.

MAXLEN=4 # Truncate to maximum of 4 chars.

soundex=${soun:0:$MAXLEN}

echo "Soundex = $soundex"

echo

The soundex code is a method of indexing and classifying names

+ by grouping together the ones that sound alike.

The soundex code for a given name is the first letter of the name,

+ followed by a calculated three-number code.

Similar sounding names should have almost the same soundex codes.

Examples:

Smith and Smythe both have a "S-530" soundex.

Harrison = H-625

Hargison = H-622

Harriman = H-655

This works out fairly well in practice, but there are numerous anomalies.

The U.S. Census and certain other governmental agencies use soundex,

as do genealogical researchers.

For more information,

+ see the "National Archives and Records Administration home page",

+ http://www.nara.gov/genealogy/soundex/soundex.html

Exercise:

--------

Simplify the "Exception Patch" section of this script.

exit 0

Example A-10. Game of Life

!/bin/bash

life.sh: "Life in the Slow Lane"

Author: Mendel Cooper

License: GPL3

Version 0.2: Patched by Daniel Albers

+ to allow non-square grids as input.

Version 0.2.1: Added 2-second delay between generations.

##################################################################### #

This is the Bash script version of John Conway's "Game of Life". #

"Life" is a simple implementation of cellular automata. #

--------------------------------------------------------------------- #

On a rectangular grid, let each "cell" be either "living" or "dead." #

Designate a living cell with a dot, and a dead one with a blank space.#

Begin with an arbitrarily drawn dot-and-blank grid, #

+ and let this be the starting generation: generation 0. #

Determine each successive generation by the following rules: #

1) Each cell has 8 neighbors, the adjoining cells #

+ left, right, top, bottom, and the 4 diagonals. #

#

123 #

45 The is the cell under consideration. #

678 #

#

2) A living cell with either 2 or 3 living neighbors remains alive. #

SURVIVE=2 #

3) A dead cell with 3 living neighbors comes alive, a "birth." #

BIRTH=3 #

4) All other cases result in a dead cell for the next generation. #

##################################################################### #

startfile=gen0 # Read the starting generation from the file "gen0" ...

# Default, if no other file specified when invoking script.

if [ -n "$1" ] # Specify another "generation 0" file.

then

startfile="$1"

Abort script if "startfile" not specified

+ and

+ default file "gen0" not present.

E_NOSTARTFILE=86

if [ ! -e "$startfile" ]

then

echo "Startfile \""$startfile"\" missing!"

exit $E_NOSTARTFILE

ALIVE1=.

DEAD1=_

# Represent living and dead cells in the start-up file.

-----------------------------------------------------#

This script uses a 10 x 10 grid (may be increased,

+ but a large grid will slow down execution).

ROWS=10

COLS=10

Change above two variables to match desired grid size.

-----------------------------------------------------#

GENERATIONS=10 # How many generations to cycle through.

# Adjust this upwards

#+ if you have time on your hands.

NONE_ALIVE=85 # Exit status on premature bailout,

#+ if no cells left alive.

DELAY=2 # Pause between generations.

TRUE=0

FALSE=1

ALIVE=0

DEAD=1

avar= # Global; holds current generation.

generation=0 # Initialize generation count.

=================================================================

let "cells = $ROWS * $COLS" # How many cells.

Arrays containing "cells."

declare -a initial

declare -a current

display ()

{

alive=0 # How many cells alive at any given time.

# Initially zero.

declare -a arr

arr=( `echo "$1"` ) # Convert passed arg to array.

element_count=${#arr[*]}

local i

local rowcheck

for ((i=0; i<$element_count; i++))

# Insert newline at end of each row.

let "rowcheck = $i % COLS"

if [ "$rowcheck" -eq 0 ]

then

echo # Newline.

echo -n " " # Indent.

cell=${arr[i]}

if [ "$cell" = . ]

then

let "alive += 1"

echo -n "$cell" | sed -e 's/_/ /g'

# Print out array, changing underscores to spaces.

done

return

}

IsValid () # Test if cell coordinate valid.

{

if [ -z "$1" -o -z "$2" ] # Mandatory arguments missing?

then

return $FALSE

local row

local lower_limit=0 # Disallow negative coordinate.

local upper_limit

local left

local right

let "upper_limit = $ROWS * $COLS - 1" # Total number of cells.

if [ "$1" -lt "$lower_limit" -o "$1" -gt "$upper_limit" ]

then

return $FALSE # Out of array bounds.

row=$2

let "left = $row * $COLS" # Left limit.

let "right = $left + $COLS - 1" # Right limit.

if [ "$1" -lt "$left" -o "$1" -gt "$right" ]

then

return $FALSE # Beyond row boundary.

return $TRUE # Valid coordinate.

}

IsAlive () # Test whether cell is alive.

# Takes array, cell number, and

{ #+ state of cell as arguments.

GetCount "$1" $2 # Get alive cell count in neighborhood.

local nhbd=$?

if [ "$nhbd" -eq "$BIRTH" ] # Alive in any case.

then

return $ALIVE

if [ "$3" = "." -a "$nhbd" -eq "$SURVIVE" ]

then # Alive only if previously alive.

return $ALIVE

return $DEAD # Defaults to dead.

}

GetCount () # Count live cells in passed cell's neighborhood.

# Two arguments needed:

# $1) variable holding array

# $2) cell number

{

local cell_number=$2

local array

local top

local center

local bottom

local r

local row

local i

local t_top

local t_cen

local t_bot

local count=0

local ROW_NHBD=3

array=( `echo "$1"` )

let "top = $cell_number - $COLS - 1" # Set up cell neighborhood.

let "center = $cell_number - 1"

let "bottom = $cell_number + $COLS - 1"

let "r = $cell_number / $COLS"

for ((i=0; i<$ROW_NHBD; i++)) # Traverse from left to right.

let "t_top = $top + $i"

let "t_cen = $center + $i"

let "t_bot = $bottom + $i"

let "row = $r" # Count center row.

IsValid $t_cen $row # Valid cell position?

if [ $? -eq "$TRUE" ]

then

if [ ${array[$t_cen]} = "$ALIVE1" ] # Is it alive?

then # If yes, then ...

let "count += 1" # Increment count.

let "row = $r - 1" # Count top row.

IsValid $t_top $row

if [ $? -eq "$TRUE" ]

then

if [ ${array[$t_top]} = "$ALIVE1" ] # Redundancy here.

then # Can it be optimized?

let "count += 1"

let "row = $r + 1" # Count bottom row.

IsValid $t_bot $row

if [ $? -eq "$TRUE" ]

then

if [ ${array[$t_bot]} = "$ALIVE1" ]

then

let "count += 1"

done

if [ ${array[$cell_number]} = "$ALIVE1" ]

then

let "count -= 1" # Make sure value of tested cell itself

fi #+ is not counted.

return $count

}

next_gen () # Update generation array.

{

local array

local i=0

array=( `echo "$1"` ) # Convert passed arg to array.

while [ "$i" -lt "$cells" ]

IsAlive "$1" $i ${array[$i]} # Is the cell alive?

if [ $? -eq "$ALIVE" ]

then # If alive, then

array[$i]=. #+ represent the cell as a period.

else

array[$i]="_" # Otherwise underscore

fi #+ (will later be converted to space).

let "i += 1"

done

let "generation += 1" # Increment generation count.

Why was the above line commented out?

Set variable to pass as parameter to "display" function.

avar=`echo ${array[@]}` # Convert array back to string variable.

display "$avar" # Display it.

echo; echo

echo "Generation $generation - $alive alive"

if [ "$alive" -eq 0 ]

then

echo

echo "Premature exit: no more cells alive!"

exit $NONE_ALIVE # No point in continuing

fi #+ if no live cells.

}

=========================================================

main ()

{

Load initial array with contents of startup file.

initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\

Delete lines containing '#' comment character.

sed -e 's/\./\. /g' -e 's/_/_ /g'` )

Remove linefeeds and insert space between elements.

clear # Clear screen.

echo # Title

setterm -reverse on

echo "======================="

setterm -reverse off

echo " $GENERATIONS generations"

echo " of"

echo "\"Life in the Slow Lane\""

setterm -reverse on

echo "======================="

setterm -reverse off

sleep $DELAY # Display "splash screen" for 2 seconds.

-------- Display first generation. --------

Gen0=`echo ${initial[@]}`

display "$Gen0" # Display only.

echo; echo

echo "Generation $generation - $alive alive"

sleep $DELAY

-------------------------------------------

let "generation += 1" # Bump generation count.

echo

------- Display second generation. -------

Cur=`echo ${initial[@]}`

next_gen "$Cur" # Update & display.

sleep $DELAY

------------------------------------------

let "generation += 1" # Increment generation count.

------ Main loop for displaying subsequent generations ------

while [ "$generation" -le "$GENERATIONS" ]

Cur="$avar"

next_gen "$Cur"

let "generation += 1"

sleep $DELAY

done

==============================================================

echo

}

exit 0 # CEOF:EOF

The grid in this script has a "boundary problem."

The the top, bottom, and sides border on a void of dead cells.

Exercise: Change the script to have the grid wrap around,

+ so that the left and right sides will "touch,"

+ as will the top and bottom.

Exercise: Create a new "gen0" file to seed this script.

Use a 12 x 16 grid, instead of the original 10 x 10 one.

Make the necessary changes to the script,

+ so it will run with the altered file.

Exercise: Modify this script so that it can determine the grid size

+ from the "gen0" file, and set any variables necessary

+ for the script to run.

This would make unnecessary any changes to variables

+ in the script for an altered grid size.

Exercise: Optimize this script.

It has redundant code.

Example A-11. Data file for Game of Life

gen0

This is an example "generation 0" start-up file for "life.sh".

--------------------------------------------------------------

The "gen0" file is a 10 x 10 grid using a period (.) for live cells,

+ and an underscore (_) for dead ones. We cannot simply use spaces

+ for dead cells in this file because of a peculiarity in Bash arrays.

[Exercise for the reader: explain this.]

Lines beginning with a '#' are comments, and the script ignores them.

__.__..___

__.._.____

____.___..

_._______.

____._____

..__...___

____._____

___...____

__.._..___

_..___..__

+++

The following script is by Mark Moraes of the University of Toronto.

See the file Moraes-COPYRIGHT for permissions and restrictions. This

file is included in the combined HTML/source tarball of the ABS

Guide.

Example A-12. behead: Removing mail and news message headers

! /bin/sh

Strips off the header from a mail/News message i.e. till the first

+ empty line.

Author: Mark Moraes, University of Toronto

==> These comments added by author of this document.

if [ $# -eq 0 ]; then

==> If no command-line args present, then works on file redirected to stdin.

sed -e '1,/^$/d' -e '/^[ ]*$/d'

# --> Delete empty lines and all lines until

# --> first one beginning with white space.

else

==> If command-line args present, then work on files named.

for i do

sed -e '1,/^$/d' -e '/^[ ]*$/d' $i

# --> Ditto, as above.

done

exit

==> Exercise: Add error checking and other options.

==>

==> Note that the small sed script repeats, except for the arg passed.

==> Does it make sense to embed it in a function? Why or why not?

* Copyright University of Toronto 1988, 1989.

* Written by Mark Moraes

* Permission is granted to anyone to use this software for any purpose on

* any computer system, and to alter it and redistribute it freely, subject

* to the following restrictions:

* 1. The author and the University of Toronto are not responsible

* for the consequences of use of this software, no matter how awful,

* even if they arise from flaws in it.

* 2. The origin of this software must not be misrepresented, either by

* explicit claim or by omission. Since few users ever read sources,

* credits must appear in the documentation.

* 3. Altered versions must be plainly marked as such, and must not be

* misrepresented as being the original software. Since few users

* ever read sources, credits must appear in the documentation.

* 4. This notice may not be removed or altered.

Antek Sawicki contributed the following script, which makes very

clever use of the parameter substitution operators discussed in

Section 10.2.

Example A-13. password: Generating random 8-character passwords

!/bin/bash

Random password generator for Bash 2.x +

+ by Antek Sawicki <tenox@tenox.tc>,

+ who generously gave usage permission to the ABS Guide author.

==> Comments added by document author ==>

MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

==> Password will consist of alphanumeric characters.

LENGTH="8"

==> May change 'LENGTH' for longer password.

while [ "${n:=1}" -le "$LENGTH" ]

==> Recall that := is "default substitution" operator.

==> So, if 'n' has not been initialized, set it to 1.

PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"

# ==> Very clever, but tricky.

# ==> Starting from the innermost nesting...

# ==> ${#MATRIX} returns length of array MATRIX.

# ==> $RANDOM%${#MATRIX} returns random number between 1

# ==> and [length of MATRIX] - 1.

# ==> ${MATRIX:$(($RANDOM%${#MATRIX})):1}

# ==> returns expansion of MATRIX at random position, by length 1.

# ==> See {var:pos:len} parameter substitution in Chapter 9.

# ==> and the associated examples.

# ==> PASS=... simply pastes this result onto previous PASS (concatena

tion).

# ==> To visualize this more clearly, uncomment the following line

# echo "$PASS"

# ==> to see PASS being built up,

# ==> one character at a time, each iteration of the loop.

let n+=1

# ==> Increment 'n' for next pass.

done

echo "$PASS" # ==> Or, redirect to a file, as desired.

exit 0

James R. Van Zandt contributed this script which uses named pipes

and, in his words, "really exercises quoting and escaping."

Example A-14. fifo: Making daily backups, using named pipes

!/bin/bash

==> Script by James R. Van Zandt, and used here with his permission.

==> Comments added by author of this document.

HERE=`uname -n` # ==> hostname

THERE=bilbo

echo "starting remote backup to $THERE at `date +%r`"

# ==> `date +%r` returns time in 12-hour format, i.e. "08:08:34 PM".

# make sure /pipe really is a pipe and not a plain file

rm -rf /pipe

mkfifo /pipe # ==> Create a "named pipe", named "/pipe" ...

# ==> 'su xyz' runs commands as user "xyz".

# ==> 'ssh' invokes secure shell (remote login client).

su xyz -c "ssh $THERE \"cat > /home/xyz/backup/${HERE}-daily.tar.gz\" < /pip

e"&

cd /

tar -czf - bin boot dev etc home info lib man root sbin share usr var > /pip

# ==> Uses named pipe, /pipe, to communicate between processes:

# ==> 'tar/gzip' writes to /pipe and 'ssh' reads from /pipe.

# ==> The end result is this backs up the main directories, from / on down.

# ==> What are the advantages of a "named pipe" in this situation,

# ==>+ as opposed to an "anonymous pipe", with |?

# ==> Will an anonymous pipe even work here?

# ==> Is it necessary to delete the pipe before exiting the script?

# ==> How could that be done?

exit 0

Stéphane Chazelas used the following script to demonstrate generating

prime numbers without arrays.

Example A-15. Generating prime numbers using the modulo operator

!/bin/bash

primes.sh: Generate prime numbers, without using arrays.

Script contributed by Stephane Chazelas.

This does not use the classic "Sieve of Eratosthenes" algorithm,

+ but instead the more intuitive method of testing each candidate number

+ for factors (divisors), using the "%" modulo operator.

LIMIT=1000 # Primes, 2 ... 1000.

Primes()

{

(( n = $1 + 1 )) # Bump to next integer.

shift # Next parameter in list.

echo "_n=$n i=$i_"

if (( n == LIMIT ))

then echo $*

return

for i; do # "i" set to "@", previous values of $n.

echo "-n=$n i=$i-"

(( i * i > n )) && break # Optimization.

(( n % i )) && continue # Sift out non-primes using modulo operator.

Primes $n $@ # Recursion inside loop.

return

done

Primes $n $@ $n # Recursion outside loop.

# Successively accumulate

#+ positional parameters.

# "$@" is the accumulating list of primes.

}

Primes 1

exit $?

Pipe output of the script to 'fmt' for prettier printing.

Uncomment lines 16 and 24 to help figure out what is going on.

Compare the speed of this algorithm for generating primes

+ with the Sieve of Eratosthenes (ex68.sh).

Exercise: Rewrite this script without recursion.

Rick Boivie's revision of Jordi Sanfeliu's tree script.

Example A-16. tree: Displaying a directory tree

!/bin/bash

tree.sh

Written by Rick Boivie.

Used with permission.

This is a revised and simplified version of a script

+ by Jordi Sanfeliu (the original author), and patched by Ian Kjos.

This script replaces the earlier version used in

+ previous releases of the Advanced Bash Scripting Guide.

Copyright (c) 2002, by Jordi Sanfeliu, Rick Boivie, and Ian Kjos.

==> Comments added by the author of this document.

search () {

for dir in `echo *`

==> `echo *` lists all the files in current working directory,

+ ==> without line breaks.

==> Similar effect to for dir in *

==> but "dir in `echo *`" will not handle filenames with blanks.

if [ -d "$dir" ] ; then # ==> If it is a directory (-d)...

zz=0 # ==> Temp variable, keeping track of

# directory level.

while [ $zz != $1 ] # Keep track of inner nested loop.

echo -n "| " # ==> Display vertical connector symbol,

# ==> with 2 spaces & no line feed

# in order to indent.

zz=`expr $zz + 1` # ==> Increment zz.

done

if [ -L "$dir" ] ; then # ==> If directory is a symbolic link...

echo "+---$dir" `ls -l $dir | sed 's/^.*'$dir' //'`

# ==> Display horiz. connector and list directory name, but...

# ==> delete date/time part of long listing.

else

echo "+---$dir" # ==> Display horizontal connector symbol...

# ==> and print directory name.

numdirs=`expr $numdirs + 1` # ==> Increment directory count.

if cd "$dir" ; then # ==> If can move to subdirectory...

search `expr $1 + 1` # with recursion ;-)

# ==> Function calls itself.

cd ..

done

}

if [ $# != 0 ] ; then

cd $1 # Move to indicated directory.

#else # stay in current directory

echo "Initial directory = `pwd`"

numdirs=0

search 0

echo "Total directories = $numdirs"

exit 0

Patsie's version of a directory tree script.

Example A-17. tree2: Alternate directory tree script

!/bin/bash

tree2.sh

Lightly modified/reformatted by ABS Guide author.

Included in ABS Guide with permission of script author (thanks!).

Recursive file/dirsize checking script, by Patsie

This script builds a list of files/directories and their size (du -akx)

and processes this list to a human readable tree shape

The 'du -akx' is only as good as the permissions the owner has.

So preferably run as root* to get the best results, or use only on

directories for which you have read permissions. Anything you can't

read is not in the list.

* ABS Guide author advises caution when running scripts as root!

THIS IS CONFIGURABLE ##########

TOP=5 # Top 5 biggest (sub)directories.

MAXRECURS=5 # Max 5 subdirectories/recursions deep.

E_BL=80 # Blank line already returned.

E_DIR=81 # Directory not specified.

DON'T CHANGE ANYTHING BELOW THIS LINE ##########

PID=$ # Our own process ID.

SELF=`basename $0` # Our own program name.

TMP="/tmp/${SELF}.${PID}.tmp" # Temporary 'du' result.

Convert number to dotted thousand.

function dot { echo " $*" |

sed -e :a -e 's/$.*[0-9]$$[0-9]\{3\}$/\1,\2/;ta' |

tail -c 12; }

Usage: tree <recursion> <indent prefix> <min size> <directory>

function tree {

recurs="$1" # How deep nested are we?

prefix="$2" # What do we display before file/dirname?

minsize="$3" # What is the minumum file/dirsize?

dirname="$4" # Which directory are we checking?

Get ($TOP) biggest subdirs/subfiles from TMP file.

LIST=`egrep "[[:space:]]${dirname}/[^/]*$" "$TMP" |

awk '{if($1>'$minsize') print;}' | sort -nr | head -$TOP`

[ -z "$LIST" ] && return # Empty list, then go back.

cnt=0

num=`echo "$LIST" | wc -l` # How many entries in the list.

## Main loop

echo "$LIST" | while read size name; do

((cnt+=1)) # Count entry number.

bname=`basename "$name"` # We only need a basename of the entry.

[ -d "$name" ] && bname="$bname/"

# If it's a directory, append a slash.

echo "`dot $size`$prefix +-$bname"

# Display the result.

# Call ourself recursively if it's a directory

#+ and we're not nested too deep ($MAXRECURS).

# The recursion goes up: $((recurs+1))

# The prefix gets a space if it's the last entry,

#+ or a pipe if there are more entries.

# The minimum file/dirsize becomes

#+ a tenth of his parent: $((size/10)).

# Last argument is the full directory name to check.

if [ -d "$name" -a $recurs -lt $MAXRECURS ]; then

[ $cnt -lt $num ] \

|| (tree $((recurs+1)) "$prefix " $((size/10)) "$name") \

&& (tree $((recurs+1)) "$prefix |" $((size/10)) "$name")

done

[ $? -eq 0 ] && echo " $prefix"

# Every time we jump back add a 'blank' line.

return $E_BL

# We return 80 to tell we added a blank line already.

}

###

main program ###

###

rootdir="$@"

[ -d "$rootdir" ] ||

{ echo "$SELF: Usage: $SELF <directory>" >&2; exit $E_DIR; }

# We should be called with a directory name.

echo "Building inventory list, please wait ..."

# Show "please wait" message.

du -akx "$rootdir" 1>"$TMP" 2>/dev/null

# Build a temporary list of all files/dirs and their size.

size=`tail -1 "$TMP" | awk '{print $1}'`

# What is our rootdirectory's size?

echo "`dot $size` $rootdir"

# Display rootdirectory's entry.

tree 0 "" 0 "$rootdir"

# Display the tree below our rootdirectory.

rm "$TMP" 2>/dev/null

# Clean up TMP file.

exit $?

Noah Friedman permitted use of his string function script. It

essentially reproduces some of the C-library string manipulation

functions.

Example A-18. string functions: C-style string functions

!/bin/bash

string.bash --- bash emulation of string(3) library routines

Author: Noah Friedman <friedman@prep.ai.mit.edu>

==> Used with his kind permission in this document.

Created: 1992-07-01

Last modified: 1993-09-29

Public domain

Conversion to bash v2 syntax done by Chet Ramey

Commentary:

Code:

:docstring strcat:

Usage: strcat s1 s2

Strcat appends the value of variable s2 to variable s1.

Example:

a="foo"

b="bar"

strcat a b

echo $a

=> foobar

:end docstring:

;;;autoload ==> Autoloading of function commented out.

function strcat ()

{

local s1_val s2_val

s1_val=${!1} # indirect variable expansion

s2_val=${!2}

eval "$1"=\'"${s1_val}${s2_val}"\'

# ==> eval $1='${s1_val}${s2_val}' avoids problems,

# ==> if one of the variables contains a single quote.

}

:docstring strncat:

Usage: strncat s1 s2 $n

Line strcat, but strncat appends a maximum of n characters from the value

of variable s2. It copies fewer if the value of variabl s2 is shorter

than n characters. Echoes result on stdout.

Example:

a=foo

b=barbaz

strncat a b 3

echo $a

=> foobar

:end docstring:

;;;autoload

function strncat ()

{

local s1="$1"

local s2="$2"

local -i n="$3"

local s1_val s2_val

s1_val=${!s1} # ==> indirect variable expansion

s2_val=${!s2}

if [ ${#s2_val} -gt ${n} ]; then

s2_val=${s2_val:0:$n} # ==> substring extraction

eval "$s1"=\'"${s1_val}${s2_val}"\'

# ==> eval $1='${s1_val}${s2_val}' avoids problems,

# ==> if one of the variables contains a single quote.

}

:docstring strcmp:

Usage: strcmp $s1 $s2

Strcmp compares its arguments and returns an integer less than, equal to,

or greater than zero, depending on whether string s1 is lexicographically

less than, equal to, or greater than string s2.

:end docstring:

;;;autoload

function strcmp ()

{

[ "$1" = "$2" ] && return 0

[ "${1}" '<' "${2}" ] > /dev/null && return -1

return 1

}

:docstring strncmp:

Usage: strncmp $s1 $s2 $n

Like strcmp, but makes the comparison by examining a maximum of n

characters (n less than or equal to zero yields equality).

:end docstring:

;;;autoload

function strncmp ()

{

if [ -z "${3}" -o "${3}" -le "0" ]; then

return 0

if [ ${3} -ge ${#1} -a ${3} -ge ${#2} ]; then

strcmp "$1" "$2"

return $?

else

s1=${1:0:$3}

s2=${2:0:$3}

strcmp $s1 $s2

return $?

}

:docstring strlen:

Usage: strlen s

Strlen returns the number of characters in string literal s.

:end docstring:

;;;autoload

function strlen ()

{

eval echo "\${#${1}}"

# ==> Returns the length of the value of the variable

# ==> whose name is passed as an argument.

}

:docstring strspn:

Usage: strspn $s1 $s2

Strspn returns the length of the maximum initial segment of string s1,

which consists entirely of characters from string s2.

:end docstring:

;;;autoload

function strspn ()

{

# Unsetting IFS allows whitespace to be handled as normal chars.

local IFS=

local result="${1%%[!${2}]*}"

echo ${#result}

}

:docstring strcspn:

Usage: strcspn $s1 $s2

Strcspn returns the length of the maximum initial segment of string s1,

which consists entirely of characters not from string s2.

:end docstring:

;;;autoload

function strcspn ()

{

# Unsetting IFS allows whitspace to be handled as normal chars.

local IFS=

local result="${1%%[${2}]*}"

echo ${#result}

}

:docstring strstr:

Usage: strstr s1 s2

Strstr echoes a substring starting at the first occurrence of string s2 in

string s1, or nothing if s2 does not occur in the string. If s2 points to

a string of zero length, strstr echoes s1.

:end docstring:

;;;autoload

function strstr ()

{

# if s2 points to a string of zero length, strstr echoes s1

[ ${#2} -eq 0 ] && { echo "$1" ; return 0; }

# strstr echoes nothing if s2 does not occur in s1

case "$1" in

*$2*) ;;

*) return 1;;

esac

# use the pattern matching code to strip off the match and everything

# following it

first=${1/$2*/}

# then strip off the first unmatched portion of the string

echo "${1##$first}"

}

:docstring strtok:

Usage: strtok s1 s2

Strtok considers the string s1 to consist of a sequence of zero or more

text tokens separated by spans of one or more characters from the

separator string s2. The first call (with a non-empty string s1

specified) echoes a string consisting of the first token on stdout. The

function keeps track of its position in the string s1 between separate

calls, so that subsequent calls made with the first argument an empty

string will work through the string immediately following that token. In

this way subsequent calls will work through the string s1 until no tokens

remain. The separator string s2 may be different from call to call.

When no token remains in s1, an empty value is echoed on stdout.

:end docstring:

;;;autoload

function strtok ()

{

}

:docstring strtrunc:

Usage: strtrunc $n $s1 {$s2} {$...}

Used by many functions like strncmp to truncate arguments for comparison.

Echoes the first n characters of each string s1 s2 ... on stdout.

:end docstring:

;;;autoload

function strtrunc ()

{

n=$1 ; shift

for z; do

echo "${z:0:$n}"

done

}

provide string

string.bash ends here

========================================================================== #

==> Everything below here added by the document author.

==> Suggested use of this script is to delete everything below here,

==> and "source" this file into your own scripts.

strcat

string0=one

string1=two

echo

echo "Testing \"strcat\" function:"

echo "Original \"string0\" = $string0"

echo "\"string1\" = $string1"

strcat string0 string1

echo "New \"string0\" = $string0"

echo

strlen

echo

echo "Testing \"strlen\" function:"

str=123456789

echo "\"str\" = $str"

echo -n "Length of \"str\" = "

strlen str

echo

Exercise:

--------

Add code to test all the other string functions above.

exit 0

Michael Zick's complex array example uses the md5sum check sum

command to encode directory information.

Example A-19. Directory information

! /bin/bash

directory-info.sh

Parses and lists directory information.

NOTE: Change lines 273 and 353 per "README" file.

Michael Zick is the author of this script.

Used here with his permission.

Controls

If overridden by command arguments, they must be in the order:

Arg1: "Descriptor Directory"

Arg2: "Exclude Paths"

Arg3: "Exclude Directories"

Environment Settings override Defaults.

Command arguments override Environment Settings.

Default location for content addressed file descriptors.

MD5UCFS=${1:-${MD5UCFS:-'/tmpfs/ucfs'}}

Directory paths never to list or enter

declare -a \

EXCLUDE_PATHS=${2:-${EXCLUDE_PATHS:-'(/proc /dev /devfs /tmpfs)'}}

Directories never to list or enter

declare -a \

EXCLUDE_DIRS=${3:-${EXCLUDE_DIRS:-'(ucfs lost+found tmp wtmp)'}}

Files never to list or enter

declare -a \

EXCLUDE_FILES=${3:-${EXCLUDE_FILES:-'(core "Name with Spaces")'}}

Here document used as a comment block.

: <<LSfieldsDoc

# # # # List Filesystem Directory Information # # # # #

ListDirectory "FileGlob" "Field-Array-Name"

or

ListDirectory -of "FileGlob" "Field-Array-Filename"

'-of' meaning 'output to filename'

# # # #

String format description based on: ls (GNU fileutils) version 4.0.36

Produces a line (or more) formatted:

inode permissions hard-links owner group ...

32736 -rw------- 1 mszick mszick

size day month date hh:mm:ss year path

2756608 Sun Apr 20 08:53:06 2003 /home/mszick/core

Unless it is formatted:

inode permissions hard-links owner group ...

266705 crw-rw---- 1 root uucp

major minor day month date hh:mm:ss year path

4, 68 Sun Apr 20 09:27:33 2003 /dev/ttyS4

NOTE: that pesky comma after the major number

NOTE: the 'path' may be multiple fields:

/home/mszick/core

/proc/982/fd/0 -> /dev/null

/proc/982/fd/1 -> /home/mszick/.xsession-errors

/proc/982/fd/13 -> /tmp/tmpfZVVOCs (deleted)

/proc/982/fd/7 -> /tmp/kde-mszick/ksycoca

/proc/982/fd/8 -> socket:[11586]

/proc/982/fd/9 -> pipe:[11588]

If that isn't enough to keep your parser guessing,

either or both of the path components may be relative:

../Built-Shared -> Built-Static

../linux-2.4.20.tar.bz2 -> ../../../SRCS/linux-2.4.20.tar.bz2

The first character of the 11 (10?) character permissions field:

's' Socket

'd' Directory

'b' Block device

'c' Character device

'l' Symbolic link

NOTE: Hard links not marked - test for identical inode numbers

on identical filesystems.

All information about hard linked files are shared, except

for the names and the name's location in the directory system.

NOTE: A "Hard link" is known as a "File Alias" on some systems.

'-' An undistingushed file

Followed by three groups of letters for: User, Group, Others

Character 1: '-' Not readable; 'r' Readable

Character 2: '-' Not writable; 'w' Writable

Character 3, User and Group: Combined execute and special

'-' Not Executable, Not Special

'x' Executable, Not Special

's' Executable, Special

'S' Not Executable, Special

Character 3, Others: Combined execute and sticky (tacky?)

'-' Not Executable, Not Tacky

'x' Executable, Not Tacky

't' Executable, Tacky

'T' Not Executable, Tacky

Followed by an access indicator

Haven't tested this one, it may be the eleventh character

or it may generate another field

' ' No alternate access

'+' Alternate access

LSfieldsDoc

ListDirectory()

{

local -a T

local -i of=0 # Default return in variable

OLD_IFS=$IFS # Using BASH default ' \t\n'

case "$#" in

3) case "$1" in

-of) of=1 ; shift ;;

* ) return 1 ;;

esac ;;

2) : ;; # Poor man's "continue"

*) return 1 ;;

esac

# NOTE: the (ls) command is NOT quoted (")

T=( $(ls --inode --ignore-backups --almost-all --directory \

--full-time --color=none --time=status --sort=none \

--format=long $1) )

case $of in

# Assign T back to the array whose name was passed as $2

0) eval $2=$ \"\$\{T\[@\]\}\" $ ;;

# Write T into filename passed as $2

1) echo "${T[@]}" > "$2" ;;

esac

return 0

}

# # # # Is that string a legal number? # # # # #

IsNumber "Var"

# # # # There has to be a better way, sigh...

IsNumber()

{

local -i int

if [ $# -eq 0 ]

then

return 1

else

(let int=$1) 2>/dev/null

return $? # Exit status of the let thread

}

# # # # Index Filesystem Directory Information # # # # #

IndexList "Field-Array-Name" "Index-Array-Name"

or

IndexList -if Field-Array-Filename Index-Array-Name

IndexList -of Field-Array-Name Index-Array-Filename

IndexList -if -of Field-Array-Filename Index-Array-Filename

# # # #

: <<IndexListDoc

Walk an array of directory fields produced by ListDirectory

Having suppressed the line breaks in an otherwise line oriented

report, build an index to the array element which starts each line.

Each line gets two index entries, the first element of each line

(inode) and the element that holds the pathname of the file.

The first index entry pair (Line-Number==0) are informational:

Index-Array-Name[0] : Number of "Lines" indexed

Index-Array-Name[1] : "Current Line" pointer into Index-Array-Name

The following index pairs (if any) hold element indexes into

the Field-Array-Name per:

Index-Array-Name[Line-Number * 2] : The "inode" field element.

NOTE: This distance may be either +11 or +12 elements.

Index-Array-Name[(Line-Number * 2) + 1] : The "pathname" element.

NOTE: This distance may be a variable number of elements.

Next line index pair for Line-Number+1.

IndexListDoc

IndexList()

{

local -a LIST # Local of listname passed

local -a -i INDEX=( 0 0 ) # Local of index to return

local -i Lidx Lcnt

local -i if=0 of=0 # Default to variable names

case "$#" in # Simplistic option testing

0) return 1 ;;

1) return 1 ;;

2) : ;; # Poor man's continue

3) case "$1" in

-if) if=1 ;;

-of) of=1 ;;

* ) return 1 ;;

esac ; shift ;;

4) if=1 ; of=1 ; shift ; shift ;;

*) return 1

esac

# Make local copy of list

case "$if" in

0) eval LIST=$ \"\$\{$1\[@\]\}\" $ ;;

1) LIST=( $(cat $1) ) ;;

esac

# Grok (grope?) the array

Lcnt=${#LIST[@]}

Lidx=0

until (( Lidx >= Lcnt ))

if IsNumber ${LIST[$Lidx]}

then

local -i inode name

local ft

inode=Lidx

local m=${LIST[$Lidx+2]} # Hard Links field

ft=${LIST[$Lidx+1]:0:1} # Fast-Stat

case $ft in

b) ((Lidx+=12)) ;; # Block device

c) ((Lidx+=12)) ;; # Character device

*) ((Lidx+=11)) ;; # Anything else

esac

name=Lidx

case $ft in

-) ((Lidx+=1)) ;; # The easy one

b) ((Lidx+=1)) ;; # Block device

c) ((Lidx+=1)) ;; # Character device

d) ((Lidx+=1)) ;; # The other easy one

l) ((Lidx+=3)) ;; # At LEAST two more fields

A little more elegance here would handle pipes,

+ sockets, deleted files - later.

*) until IsNumber ${LIST[$Lidx]} || ((Lidx >= Lcnt))

((Lidx+=1))

done

;; # Not required

esac

INDEX[${#INDEX[*]}]=$inode

INDEX[${#INDEX[*]}]=$name

INDEX[0]=${INDEX[0]}+1 # One more "line" found

echo "Line: ${INDEX[0]} Type: $ft Links: $m Inode: \

${LIST[$inode]} Name: ${LIST[$name]}"

else

((Lidx+=1))

done

case "$of" in

0) eval $2=$ \"\$\{INDEX\[@\]\}\" $ ;;

1) echo "${INDEX[@]}" > "$2" ;;

esac

return 0 # What could go wrong?

}

# # # # Content Identify File # # # # #

DigestFile Input-Array-Name Digest-Array-Name

or

DigestFile -if Input-FileName Digest-Array-Name

# # # #

Here document used as a comment block.

: <<DigestFilesDoc

The key (no pun intended) to a Unified Content File System (UCFS)

is to distinguish the files in the system based on their content.

Distinguishing files by their name is just so 20th Century.

The content is distinguished by computing a checksum of that content.

This version uses the md5sum program to generate a 128 bit checksum

representative of the file's contents.

There is a chance that two files having different content might

generate the same checksum using md5sum (or any checksum). Should

that become a problem, then the use of md5sum can be replace by a

cyrptographic signature. But until then...

The md5sum program is documented as outputting three fields (and it

does), but when read it appears as two fields (array elements). This

is caused by the lack of whitespace between the second and third field.

So this function gropes the md5sum output and returns:

[0] 32 character checksum in hexidecimal (UCFS filename)

[1] Single character: ' ' text file, '*' binary file

[2] Filesystem (20th Century Style) name

Note: That name may be the character '-' indicating STDIN read.

DigestFilesDoc

DigestFile()

{

local if=0 # Default, variable name

local -a T1 T2

case "$#" in

3) case "$1" in

-if) if=1 ; shift ;;

* ) return 1 ;;

esac ;;

2) : ;; # Poor man's "continue"

*) return 1 ;;

esac

case $if in

0) eval T1=$ \"\$\{$1\[@\]\}\" $

T2=( $(echo ${T1[@]} | md5sum -) )

;;

1) T2=( $(md5sum $1) )

;;

esac

case ${#T2[@]} in

0) return 1 ;;

1) return 1 ;;

2) case ${T2[1]:0:1} in # SanScrit-2.0.5

\*) T2[${#T2[@]}]=${T2[1]:1}

T2[1]=\*

;;

*) T2[${#T2[@]}]=${T2[1]}

T2[1]=" "

;;

esac

;;

3) : ;; # Assume it worked

*) return 1 ;;

esac

local -i len=${#T2[0]}

if [ $len -ne 32 ] ; then return 1 ; fi

eval $2=$ \"\$\{T2\[@\]\}\" $

}

# # # # Locate File # # # # #

LocateFile [-l] FileName Location-Array-Name

or

LocateFile [-l] -of FileName Location-Array-FileName

# # # #

A file location is Filesystem-id and inode-number

Here document used as a comment block.

: <<StatFieldsDoc

Based on stat, version 2.2

stat -t and stat -lt fields

[0] name

[1] Total size

File - number of bytes

Symbolic link - string length of pathname

[2] Number of (512 byte) blocks allocated

[3] File type and Access rights (hex)

[4] User ID of owner

[5] Group ID of owner

[6] Device number

[7] Inode number

[8] Number of hard links

[9] Device type (if inode device) Major

[10] Device type (if inode device) Minor

[11] Time of last access

May be disabled in 'mount' with noatime

atime of files changed by exec, read, pipe, utime, mknod (mmap

atime of directories changed by addition/deletion of files

[12] Time of last modification

mtime of files changed by write, truncate, utime, mknod

mtime of directories changed by addtition/deletion of files

[13] Time of last change

ctime reflects time of changed inode information (owner, group

permissions, link count

-*-*- Per:

Return code: 0

Size of array: 14

Contents of array

Element 0: /home/mszick

Element 1: 4096

Element 2: 8

Element 3: 41e8

Element 4: 500

Element 5: 500

Element 6: 303

Element 7: 32385

Element 8: 22

Element 9: 0

Element 10: 0

Element 11: 1051221030

Element 12: 1051214068

Element 13: 1051214068

For a link in the form of linkname -> realname

stat -t linkname returns the linkname (link) information

stat -lt linkname returns the realname information

stat -tf and stat -ltf fields

[0] name

[1] ID-0? # Maybe someday, but Linux stat structure

[2] ID-0? # does not have either LABEL nor UUID

# fields, currently information must come

# from file-system specific utilities

These will be munged into:

[1] UUID if possible

[2] Volume Label if possible

Note: 'mount -l' does return the label and could return the UUID

[3] Maximum length of filenames

[4] Filesystem type

[5] Total blocks in the filesystem

[6] Free blocks

[7] Free blocks for non-root user(s)

[8] Block size of the filesystem

[9] Total inodes

[10] Free inodes

-*-*- Per:

Return code: 0

Size of array: 11

Contents of array

Element 0: /home/mszick

Element 1: 0

Element 2: 0

Element 3: 255

Element 4: ef53

Element 5: 2581445

Element 6: 2277180

Element 7: 2146050

Element 8: 4096

Element 9: 1311552

Element 10: 1276425

StatFieldsDoc

LocateFile [-l] FileName Location-Array-Name

LocateFile [-l] -of FileName Location-Array-FileName

LocateFile()

{

local -a LOC LOC1 LOC2

local lk="" of=0

case "$#" in

0) return 1 ;;

1) return 1 ;;

2) : ;;

*) while (( "$#" > 2 ))

case "$1" in

-l) lk=-1 ;;

-of) of=1 ;;

*) return 1 ;;

esac

shift

done ;;

esac

More Sanscrit-2.0.5

# LOC1=( $(stat -t $lk $1) )

# LOC2=( $(stat -tf $lk $1) )

# Uncomment above two lines if system has "stat" command installed.

LOC=( ${LOC1[@]:0:1} ${LOC1[@]:3:11}

${LOC2[@]:1:2} ${LOC2[@]:4:1} )

case "$of" in

0) eval $2=$ \"\$\{LOC\[@\]\}\" $ ;;

1) echo "${LOC[@]}" > "$2" ;;

esac

return 0

Which yields (if you are lucky, and have "stat" installed)

--- Location Discriptor ---

Return code: 0

Size of array: 15

Contents of array

Element 0: /home/mszick 20th Century name

Element 1: 41e8 Type and Permissions

Element 2: 500 User

Element 3: 500 Group

Element 4: 303 Device

Element 5: 32385 inode

Element 6: 22 Link count

Element 7: 0 Device Major

Element 8: 0 Device Minor

Element 9: 1051224608 Last Access

Element 10: 1051214068 Last Modify

Element 11: 1051214068 Last Status

Element 12: 0 UUID (to be)

Element 13: 0 Volume Label (to be)

Element 14: ef53 Filesystem type

}

And then there was some test code

ListArray() # ListArray Name

{

local -a Ta

eval Ta=$ \"\$\{$1\[@\]\}\" $

echo

echo "-*-*- List of Array -*-*-"

echo "Size of array $1: ${#Ta[*]}"

echo "Contents of array $1:"

for (( i=0 ; i<${#Ta[*]} ; i++ ))

echo -e "\tElement $i: ${Ta[$i]}"

done

return 0

}

declare -a CUR_DIR

For small arrays

ListDirectory "${PWD}" CUR_DIR

ListArray CUR_DIR

declare -a DIR_DIG

DigestFile CUR_DIR DIR_DIG

echo "The new \"name\" (checksum) for ${CUR_DIR[9]} is ${DIR_DIG[0]}"

declare -a DIR_ENT

BIG_DIR # For really big arrays - use a temporary file in ramdisk

BIG-DIR # ListDirectory -of "${CUR_DIR[11]}/*" "/tmpfs/junk2"

ListDirectory "${CUR_DIR[11]}/*" DIR_ENT

declare -a DIR_IDX

BIG-DIR # IndexList -if "/tmpfs/junk2" DIR_IDX

IndexList DIR_ENT DIR_IDX

declare -a IDX_DIG

BIG-DIR # DIR_ENT=( $(cat /tmpfs/junk2) )

BIG-DIR # DigestFile -if /tmpfs/junk2 IDX_DIG

DigestFile DIR_ENT IDX_DIG

Small (should) be able to parallize IndexList & DigestFile

Large (should) be able to parallize IndexList & DigestFile & the assignment

echo "The \"name\" (checksum) for the contents of ${PWD} is ${IDX_DIG[0]}"

declare -a FILE_LOC

LocateFile ${PWD} FILE_LOC

ListArray FILE_LOC

exit 0

Stéphane Chazelas demonstrates object-oriented programming in a Bash

script.

Mariusz Gniazdowski contributed a hash library for use in scripts.

Example A-20. Library of hash functions

Hash:

Hash function library

Author: Mariusz Gniazdowski <mariusz.gn-at-gmail.com>

Date: 2005-04-07

Functions making emulating hashes in Bash a little less painful.

Limitations:

* Only global variables are supported.

* Each hash instance generates one global variable per value.

* Variable names collisions are possible

+ if you define variable like hashhashname_key

* Keys must use chars that can be part of a Bash variable name

+ (no dashes, periods, etc.).

* The hash is created as a variable:

... hashname_keyname

So if somone will create hashes like:

myhash_ + mykey = myhash__mykey

myhash + _mykey = myhash__mykey

Then there will be a collision.

(This should not pose a major problem.)

Hash_config_varname_prefix=__hash__

Emulates: hash[key]=value

Params:

1 - hash

2 - key

3 - value

function hash_set {

eval "${Hash_config_varname_prefix}${1}_${2}=\"${3}\""

}

Emulates: value=hash[key]

Params:

1 - hash

2 - key

3 - value (name of global variable to set)

function hash_get_into {

eval "$3=\"\${Hash_config_varname_prefix}${1}_${2}\""

}

Emulates: echo hash[key]

Params:

1 - hash

2 - key

3 - echo params (like -n, for example)

function hash_echo {

eval "echo $3 \"\${Hash_config_varname_prefix}${1}_${2}\""

}

Emulates: hash1[key1]=hash2[key2]

Params:

1 - hash1

2 - key1

3 - hash2

4 - key2

function hash_copy {

eval "${Hash_config_varname_prefix}${1}_${2}\

=\"\${Hash_config_varname_prefix}${3}_${4}\""

}

Emulates: hash[keyN-1]=hash[key2]=...hash[key1]

Copies first key to rest of keys.

Params:

1 - hash1

2 - key1

3 - key2

. . .

N - keyN

function hash_dup {

local hashName="$1" keyName="$2"

shift 2

until [ ${#} -le 0 ]; do

eval "${Hash_config_varname_prefix}${hashName}_${1}\

=\"\${Hash_config_varname_prefix}${hashName}_${keyName}\""

shift;

done;

}

Emulates: unset hash[key]

Params:

1 - hash

2 - key

function hash_unset {

eval "unset ${Hash_config_varname_prefix}${1}_${2}"

}

Emulates something similar to: ref=&hash[key]

The reference is name of the variable in which value is held.

Params:

1 - hash

2 - key

3 - ref - Name of global variable to set.

function hash_get_ref_into {

eval "$3=\"${Hash_config_varname_prefix}${1}_${2}\""

}

Emulates something similar to: echo &hash[key]

That reference is name of variable in which value is held.

Params:

1 - hash

2 - key

3 - echo params (like -n for example)

function hash_echo_ref {

eval "echo $3 \"${Hash_config_varname_prefix}${1}_${2}\""

}

Emulates something similar to: $hash[key](param1, param2, ...)

Params:

1 - hash

2 - key

3,4, ... - Function parameters

function hash_call {

local hash key

hash=$1

key=$2

shift 2

eval "eval \"\${Hash_config_varname_prefix}${hash}_${key} \\\"\\\$@\\\"\""

}

Emulates something similar to: isset(hash[key]) or hash[key]==NULL

Params:

1 - hash

2 - key

Returns:

0 - there is such key

1 - there is no such key

function hash_is_set {

eval "if [[ \"\${${Hash_config_varname_prefix}${1}_${2}-a}\" = \"a\" &&

\"\${${Hash_config_varname_prefix}${1}_${2}-b}\" = \"b\" ]]

then return 1; else return 0; fi"

}

Emulates something similar to:

foreach($hash as $key => $value) { fun($key,$value); }

It is possible to write different variations of this function.

Here we use a function call to make it as "generic" as possible.

Params:

1 - hash

2 - function name

function hash_foreach {

local keyname oldIFS="$IFS"

IFS=' '

for i in $(eval "echo \${!${Hash_config_varname_prefix}${1}_*}"); do

keyname=$(eval "echo \${i##${Hash_config_varname_prefix}${1}_}")

eval "$2 $keyname \"\$i\""

done

IFS="$oldIFS"

}

NOTE: In lines 103 and 116, ampersand changed.

But, it doesn't matter, because these are comment lines anyhow.

Here is an example script using the foregoing hash library.

Example A-21. Colorizing text using hash functions

!/bin/bash

hash-example.sh: Colorizing text.

Author: Mariusz Gniazdowski <mariusz.gn-at-gmail.com>

. Hash.lib # Load the library of functions.

hash_set colors red "\033[0;31m"

hash_set colors blue "\033[0;34m"

hash_set colors light_blue "\033[1;34m"

hash_set colors light_red "\033[1;31m"

hash_set colors cyan "\033[0;36m"

hash_set colors light_green "\033[1;32m"

hash_set colors light_gray "\033[0;37m"

hash_set colors green "\033[0;32m"

hash_set colors yellow "\033[1;33m"

hash_set colors light_purple "\033[1;35m"

hash_set colors purple "\033[0;35m"

hash_set colors reset_color "\033[0;00m"

$1 - keyname

$2 - value

try_colors() {

echo -en "$2"

echo "This line is $1."

}

hash_foreach colors try_colors

hash_echo colors reset_color -en

echo -e '\nLet us overwrite some colors with yellow.\n'

It's hard to read yellow text on some terminals.

hash_dup colors yellow red light_green blue green light_gray cyan

hash_foreach colors try_colors

hash_echo colors reset_color -en

echo -e '\nLet us delete them and try colors once more . . .\n'

for i in red light_green blue green light_gray cyan; do

hash_unset colors $i

done

hash_foreach colors try_colors

hash_echo colors reset_color -en

hash_set other txt "Other examples . . ."

hash_echo other txt

hash_get_into other txt text

echo $text

hash_set other my_fun try_colors

hash_call other my_fun purple "`hash_echo colors purple`"

hash_echo colors reset_color -en

echo; echo "Back to normal?"; echo

exit $?

On some terminals, the "light" colors print in bold,

and end up looking darker than the normal ones.

Why is this?

An example illustrating the mechanics of hashing, but from a

different point of view.

Example A-22. More on hash functions

!/bin/bash

$Id: ha.sh,v 1.2 2005/04/21 23:24:26 oliver Exp $

Copyright 2005 Oliver Beckstein

Released under the GNU Public License

Author of script granted permission for inclusion in ABS Guide.

(Thank you!)

----------------------------------------------------------------

pseudo hash based on indirect parameter expansion

API: access through functions:

create the hash:

newhash Lovers

add entries (note single quotes for spaces)

addhash Lovers Tristan Isolde

addhash Lovers 'Romeo Montague' 'Juliet Capulet'

access value by key

gethash Lovers Tristan ----> Isolde

show all keys

keyshash Lovers ----> 'Tristan' 'Romeo Montague'

Convention: instead of perls' foo{bar} = boing' syntax,

use

'_foo_bar=boing' (two underscores, no spaces)

1) store key in _NAME_keys[]

2) store value in _NAME_values[] using the same integer index

The integer index for the last entry is _NAME_ptr

NOTE: No error or sanity checks, just bare bones.

function _inihash () {

# private function

# call at the beginning of each procedure

# defines: _keys _values _ptr

# Usage: _inihash NAME

local name=$1

_keys=_${name}_keys

_values=_${name}_values

_ptr=_${name}_ptr

}

function newhash () {

# Usage: newhash NAME

# NAME should not contain spaces or dots.

# Actually: it must be a legal name for a Bash variable.

# We rely on Bash automatically recognising arrays.

local name=$1

local _keys _values _ptr

_inihash ${name}

eval ${_ptr}=0

}

function addhash () {

# Usage: addhash NAME KEY 'VALUE with spaces'

# arguments with spaces need to be quoted with single quotes ''

local name=$1 k="$2" v="$3"

local _keys _values _ptr

_inihash ${name}

#echo "DEBUG(addhash): ${_ptr}=${!_ptr}"

eval let ${_ptr}=${_ptr}+1

eval "$_keys[${!_ptr}]=\"${k}\""

eval "$_values[${!_ptr}]=\"${v}\""

}

function gethash () {

# Usage: gethash NAME KEY

# Returns boing

# ERR=0 if entry found, 1 otherwise

# That's not a proper hash --

#+ we simply linearly search through the keys.

local name=$1 key="$2"

local _keys _values _ptr

local k v i found h

_inihash ${name}

# _ptr holds the highest index in the hash

found=0

for i in $(seq 1 ${!_ptr}); do

h="\${${_keys}[${i}]}" # Safer to do it in two steps,

eval k=${h} #+ especially when quoting for spaces.

if [ "${k}" = "${key}" ]; then found=1; break; fi

done;

[ ${found} = 0 ] && return 1;

# else: i is the index that matches the key

h="\${${_values}[${i}]}"

eval echo "${h}"

return 0;

}

function keyshash () {

# Usage: keyshash NAME

# Returns list of all keys defined for hash name.

local name=$1 key="$2"

local _keys _values _ptr

local k i h

_inihash ${name}

# _ptr holds the highest index in the hash

for i in $(seq 1 ${!_ptr}); do

h="\${${_keys}[${i}]}" # Safer to do it in two steps,

eval k=${h} #+ especially when quoting for spaces.

echo -n "'${k}' "

done;

}

-----------------------------------------------------------------------

Now, let's test it.

(Per comments at the beginning of the script.)

newhash Lovers

addhash Lovers Tristan Isolde

addhash Lovers 'Romeo Montague' 'Juliet Capulet'

Output results.

echo

gethash Lovers Tristan # Isolde

echo

keyshash Lovers # 'Tristan' 'Romeo Montague'

echo; echo

exit 0

Exercise:

--------

Add error checks to the functions.

Now for a script that installs and mounts those cute USB keychain

solid-state "hard drives."

Example A-23. Mounting USB keychain storage devices

!/bin/bash

==> usb.sh

==> Script for mounting and installing pen/keychain USB storage devices.

==> Runs as root at system startup (see below).

==>

==> Newer Linux distros (2004 or later) autodetect

==> and install USB pen drives, and therefore don't need this script.

==> But, it's still instructive.

This code is free software covered by GNU GPL license version 2 or above.

Please refer to http://www.gnu.org/ for the full license text.

Some code lifted from usb-mount by Michael Hamilton's usb-mount (LGPL)

+ see http://users.actrix.co.nz/michael/usbmount.html

INSTALL

-------

Put this in /etc/hotplug/usb/diskonkey.

Then look in /etc/hotplug/usb.distmap, and copy all usb-storage entries

+ into /etc/hotplug/usb.usermap, substituting "usb-storage" for "diskonkey".

Otherwise this code is only run during the kernel module invocation/removal

+ (at least in my tests), which defeats the purpose.

TODO

----

Handle more than one diskonkey device at one time (e.g. /dev/diskonkey1

+ and /mnt/diskonkey1), etc. The biggest problem here is the handling in

+ devlabel, which I haven't yet tried.

AUTHOR and SUPPORT

------------------

Konstantin Riabitsev, <icon linux duke edu>.

Send any problem reports to my email address at the moment.

==> Comments added by ABS Guide author.

SYMLINKDEV=/dev/diskonkey

MOUNTPOINT=/mnt/diskonkey

DEVLABEL=/sbin/devlabel

DEVLABELCONFIG=/etc/sysconfig/devlabel

IAM=$0

Functions lifted near-verbatim from usb-mount code.

function allAttachedScsiUsb {

find /proc/scsi/ -path '/proc/scsi/usb-storage*' -type f |

xargs grep -l 'Attached: Yes'

}

function scsiDevFromScsiUsb {

echo $1 | awk -F"[-/]" '{ n=$(NF-1);

print "/dev/sd" substr("abcdefghijklmnopqrstuvwxyz", n+1, 1) }'

}

if [ "${ACTION}" = "add" ] && [ -f "${DEVICE}" ]; then

# lifted from usbcam code.

if [ -f /var/run/console.lock ]; then

CONSOLEOWNER=`cat /var/run/console.lock`

elif [ -f /var/lock/console.lock ]; then

CONSOLEOWNER=`cat /var/lock/console.lock`

else

CONSOLEOWNER=

for procEntry in $(allAttachedScsiUsb); do

scsiDev=$(scsiDevFromScsiUsb $procEntry)

# Some bug with usb-storage?

# Partitions are not in /proc/partitions until they are accessed

#+ somehow.

/sbin/fdisk -l $scsiDev >/dev/null

# Most devices have partitioning info, so the data would be on

#+ /dev/sd?1. However, some stupider ones don't have any partitioning

#+ and use the entire device for data storage. This tries to

#+ guess semi-intelligently if we have a /dev/sd?1 and if not, then

#+ it uses the entire device and hopes for the better.

if grep -q `basename $scsiDev`1 /proc/partitions; then

part="$scsiDev""1"

else

part=$scsiDev

# Change ownership of the partition to the console user so they can

#+ mount it.

if [ ! -z "$CONSOLEOWNER" ]; then

chown $CONSOLEOWNER:disk $part

# This checks if we already have this UUID defined with devlabel.

# If not, it then adds the device to the list.

prodid=`$DEVLABEL printid -d $part`

if ! grep -q $prodid $DEVLABELCONFIG; then

# cross our fingers and hope it works

$DEVLABEL add -d $part -s $SYMLINKDEV 2>/dev/null

# Check if the mount point exists and create if it doesn't.

if [ ! -e $MOUNTPOINT ]; then

mkdir -p $MOUNTPOINT

# Take care of /etc/fstab so mounting is easy.

if ! grep -q "^$SYMLINKDEV" /etc/fstab; then

# Add an fstab entry

echo -e \

"$SYMLINKDEV\t\t$MOUNTPOINT\t\tauto\tnoauto,owner,kudzu 0 0" \

>> /etc/fstab

done

if [ ! -z "$REMOVER" ]; then

# Make sure this script is triggered on device removal.

mkdir -p `dirname $REMOVER`

ln -s $IAM $REMOVER

elif [ "${ACTION}" = "remove" ]; then

# If the device is mounted, unmount it cleanly.

if grep -q "$MOUNTPOINT" /etc/mtab; then

# unmount cleanly

umount -l $MOUNTPOINT

# Remove it from /etc/fstab if it's there.

if grep -q "^$SYMLINKDEV" /etc/fstab; then

grep -v "^$SYMLINKDEV" /etc/fstab > /etc/.fstab.new

mv -f /etc/.fstab.new /etc/fstab

exit 0

Converting a text file to HTML format.

Example A-24. Converting to HTML

!/bin/bash

tohtml.sh [v. 0.2.01, reldate: 04/13/12, a teeny bit less buggy]

Convert a text file to HTML format.

Author: Mendel Cooper

License: GPL3

Usage: sh tohtml.sh < textfile > htmlfile

Script can easily be modified to accept source and target filenames.

Assumptions:

1) Paragraphs in (target) text file are separated by a blank line.

2) Jpeg images (*.jpg) are located in "images" subdirectory.

In the target file, the image names are enclosed in square brackets,

for example, [image01.jpg].

3) Emphasized (italic) phrases begin with a space+underscore

+ or the first character on the line is an underscore,

+ and end with an underscore+space or underscore+end-of-line.

Settings

FNTSIZE=2 # Small-medium font size

IMGDIR="images" # Image directory

Headers

HDR01='<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">'

HDR02=''

HDR03=''

HDR10='<html>'

HDR11='<head>'

HDR11a='</head>'

HDR12a='<title>'

HDR12b='</title>'

HDR121='<META NAME="GENERATOR" CONTENT="tohtml.sh script">'

HDR13='<body bgcolor="#dddddd">' # Change background color to suit.

HDR14a='<font size='

HDR14b='>'

Footers

FTR10='</body>'

FTR11='</html>'

echo

echo "$line"

echo

Don't want extra blank lines in generated text!

} # End while

done

} # End process_text ()

write_footers () # Termination tags.

{

echo "$FTR10"

echo "$FTR11"

}

main () {

=========

write_headers

process_text

write_footers

=========

}

exit $?

Exercises:

---------

1) Fixup: Check for closing underscore before a comma or period.

2) Add a test for the presence of a closing underscore

+ in phrases to be italicized.

Here is something to warm the hearts of webmasters and mistresses: a

script that saves weblogs.

Example A-25. Preserving weblogs

!/bin/bash

archiveweblogs.sh v1.0

Troy Engel <tengel@fluid.com>

Slightly modified by document author.

Used with permission.

This script will preserve the normally rotated and

+ thrown away weblogs from a default RedHat/Apache installation.

It will save the files with a date/time stamp in the filename,

+ bzipped, to a given directory.

Run this from crontab nightly at an off hour,

+ as bzip2 can suck up some serious CPU on huge logs:

0 2 * * * /opt/sbin/archiveweblogs.sh

PROBLEM=66

Set this to your backup dir.

BKP_DIR=/opt/backups/weblogs

Default Apache/RedHat stuff

LOG_DAYS="4 3 2 1"

LOG_DIR=/var/log/httpd

LOG_FILES="access_log error_log"

Default RedHat program locations

LS=/bin/ls

MV=/bin/mv

ID=/usr/bin/id

CUT=/bin/cut

COL=/usr/bin/column

BZ2=/usr/bin/bzip2

Are we root?

USER=`$ID -u`

if [ "X$USER" != "X0" ]; then

echo "PANIC: Only root can run this script!"

exit $PROBLEM

Backup dir exists/writable?

if [ ! -x $BKP_DIR ]; then

echo "PANIC: $BKP_DIR doesn't exist or isn't writable!"

exit $PROBLEM

Move, rename and bzip2 the logs

for logday in $LOG_DAYS; do

for logfile in $LOG_FILES; do

MYFILE="$LOG_DIR/$logfile.$logday"

if [ -w $MYFILE ]; then

DTS=`$LS -lgo --time-style=+%Y%m%d $MYFILE | $COL -t | $CUT -d ' ' -f7`

$MV $MYFILE $BKP_DIR/$logfile.$DTS

$BZ2 $BKP_DIR/$logfile.$DTS

else

# Only spew an error if the file exits (ergo non-writable).

if [ -f $MYFILE ]; then

echo "ERROR: $MYFILE not writable. Skipping."

done

exit 0

How to keep the shell from expanding and reinterpreting text strings.

Example A-26. Protecting literal strings

! /bin/bash

protect_literal.sh

set -vx

:<<-'_Protect_Literal_String_Doc'

License: Unrestricted reuse in any form, for any purpose.

Warranty: None

Revision: $ID$

Documentation redirected to the Bash no-operation.

Bash will '/dev/null' this block when the script is first read.

(Uncomment the above set command to see this action.)

Remove the first (Sha-Bang) line when sourcing this as a library

procedure. Also comment out the example use code in the two

places where shown.

Usage:

_protect_literal_str 'Whatever string meets your ${fancy}'

Just echos the argument to standard out, hard quotes

restored.

$(_protect_literal_str 'Whatever string meets your ${fancy}')

as the right-hand-side of an assignment statement.

Does:

As the right-hand-side of an assignment, preserves the

hard quotes protecting the contents of the literal during

assignment.

Notes:

The strange names (_*) are used to avoid trampling on

the user's chosen names when this is sourced as a

library.

_Protect_Literal_String_Doc

The 'for illustration' function form

_protect_literal_str() {

Pick an un-used, non-printing character as local IFS.

Not required, but shows that we are ignoring it.

local IFS= \x1B' # \ESC character

Enclose the All-Elements-Of in hard quotes during assignment.

local tmp= \x27'$@ \x27'

local tmp= \''$@ \'' # Even uglier.

local len=${#tmp} # Info only.

echo $tmp is $len long. # Output AND information.

}

This is the short-named version.

_pls() {

local IFS= x1B' # \ESC character (not required)

echo \x27'$@ \x27' # Hard quoted parameter glob

}

:<<-'_Protect_Literal_String_Test'

# # Remove the above "# " to disable this code. # # #

See how that looks when printed.

echo

echo "- - Test One - -"

_protect_literal_str 'Hello $user'

_protect_literal_str 'Hello "${username}"'

echo

Which yields:

- - Test One - -

'Hello $user' is 13 long.

'Hello "${username}"' is 21 long.

Looks as expected, but why all of the trouble?

The difference is hidden inside the Bash internal order

+ of operations.

Which shows when you use it on the RHS of an assignment.

Declare an array for test values.

declare -a arrayZ

Assign elements with various types of quotes and escapes.

arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )

Now list that array and see what is there.

echo "- - Test Two - -"

for (( i=0 ; i<${#arrayZ[*]} ; i++ ))

echo Element $i: ${arrayZ[$i]} is: ${#arrayZ[$i]} long.

done

echo

Which yields:

- - Test Two - -

Element 0: zero is: 4 long. # Our marker element

Element 1: 'Hello ${Me}' is: 13 long. # Our "$(_pls '...' )"

Element 2: Hello ${You} is: 12 long. # Quotes are missing

Element 3: \'Pass: \' is: 10 long. # ${pw} expanded to nothing

Now make an assignment with that result.

declare -a array2=( ${arrayZ[@]} )

And print what happened.

echo "- - Test Three - -"

for (( i=0 ; i<${#array2[*]} ; i++ ))

echo Element $i: ${array2[$i]} is: ${#array2[$i]} long.

done

echo

Which yields:

- - Test Three - -

Element 0: zero is: 4 long. # Our marker element.

Element 1: Hello ${Me} is: 11 long. # Intended result.

Element 2: Hello is: 5 long. # ${You} expanded to nothing.

Element 3: 'Pass: is: 6 long. # Split on the whitespace.

Element 4: ' is: 1 long. # The end quote is here now.

Our Element 1 has had its leading and trailing hard quotes stripped.

Although not shown, leading and trailing whitespace is also stripped.

Now that the string contents are set, Bash will always, internally,

+ hard quote the contents as required during its operations.

Why?

Considering our "$(_pls 'Hello ${Me}')" construction:

" ... " -> Expansion required, strip the quotes.

$( ... ) -> Replace with the result of..., strip this.

_pls ' ... ' -> called with literal arguments, strip the quotes.

The result returned includes hard quotes; BUT the above processing

+ has already been done, so they become part of the value assigned.

Similarly, during further usage of the string variable, the ${Me}

+ is part of the contents (result) and survives any operations

(Until explicitly told to evaluate the string).

Hint: See what happens when the hard quotes ( \x27') are replaced

+ with soft quotes ( \x22') in the above procedures.

Interesting also is to remove the addition of any quoting.

_Protect_Literal_String_Test

# # Remove the above "# " to disable this code. # # #

exit 0

But, what if you want the shell to expand and reinterpret strings?

Example A-27. Unprotecting literal strings

! /bin/bash

unprotect_literal.sh

set -vx

:<<-'_UnProtect_Literal_String_Doc'

License: Unrestricted reuse in any form, for any purpose.

Warranty: None

Revision: $ID$

Documentation redirected to the Bash no-operation. Bash will

'/dev/null' this block when the script is first read.

(Uncomment the above set command to see this action.)

Remove the first (Sha-Bang) line when sourcing this as a library

procedure. Also comment out the example use code in the two

places where shown.

Usage:

Complement of the "$(_pls 'Literal String')" function.

(See the protect_literal.sh example.)

StringVar=$(_upls ProtectedSringVariable)

Does:

When used on the right-hand-side of an assignment statement;

makes the substitions embedded in the protected string.

Notes:

The strange names (_*) are used to avoid trampling on

the user's chosen names when this is sourced as a

library.

_UnProtect_Literal_String_Doc

_upls() {

local IFS= x1B' # \ESC character (not required)

eval echo $@ # Substitution on the glob.

}

:<<-'_UnProtect_Literal_String_Test'

# # Remove the above "# " to disable this code. # # #

_pls() {

local IFS= x1B' # \ESC character (not required)

echo \x27'$@ \x27' # Hard quoted parameter glob

}

Declare an array for test values.

declare -a arrayZ

Assign elements with various types of quotes and escapes.

arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )

Now make an assignment with that result.

declare -a array2=( ${arrayZ[@]} )

Which yielded:

- - Test Three - -

Element 0: zero is: 4 long # Our marker element.

Element 1: Hello ${Me} is: 11 long # Intended result.

Element 2: Hello is: 5 long # ${You} expanded to nothing.

Element 3: 'Pass: is: 6 long # Split on the whitespace.

Element 4: ' is: 1 long # The end quote is here now.

set -vx

Initialize 'Me' to something for the embedded ${Me} substitution.

This needs to be done ONLY just prior to evaluating the

+ protected string.

(This is why it was protected to begin with.)

Me="to the array guy."

Set a string variable destination to the result.

newVar=$(_upls ${array2[1]})

Show what the contents are.

echo $newVar

Do we really need a function to do this?

newerVar=$(eval echo ${array2[1]})

echo $newerVar

I guess not, but the _upls function gives us a place to hang

+ the documentation on.

This helps when we forget what a # construction like:

+ $(eval echo ... ) means.

What if Me isn't set when the protected string is evaluated?

unset Me

newestVar=$(_upls ${array2[1]})

echo $newestVar

Just gone, no hints, no runs, no errors.

Why in the world?

Setting the contents of a string variable containing character

+ sequences that have a meaning in Bash is a general problem in

+ script programming.

This problem is now solved in eight lines of code

+ (and four pages of description).

Where is all this going?

Dynamic content Web pages as an array of Bash strings.

Content set per request by a Bash 'eval' command

+ on the stored page template.

Not intended to replace PHP, just an interesting thing to do.

Don't have a webserver application?

No problem, check the example directory of the Bash source;

+ there is a Bash script for that also.

_UnProtect_Literal_String_Test

# # Remove the above "# " to disable this code. # # #

exit 0

This interesting script helps hunt down spammers.

Example A-28. Spammer Identification

!/bin/bash

$Id: is_spammer.bash,v 1.12.2.11 2004/10/01 21:42:33 mszick Exp $

Above line is RCS info.

The latest version of this script is available from http://www.morethan.org.

Spammer-identification

by Michael S. Zick

Used in the ABS Guide with permission.

Documentation

Whitespace == :Space:Tab:Line Feed:Carriage Return:

WSP_IFS= \x20' \x09' \x0A' \x0D'

No Whitespace == Line Feed:Carriage Return

NO_WSP= \x0A' \x0D'

Field separator for dotted decimal IP addresses

ADR_IFS=${NO_WSP}'.'

Array to dotted string conversions

DOT_IFS='.'${WSP_IFS}

# # Pending operations stack machine # # #

This set of functions described in func_stack.bash.

(See "Additional documentation" above.)

# #

Global stack of pending operations.

declare -f -a _pending_

Global sentinel for stack runners

declare -i _p_ctrl_

Global holder for currently executing function

declare -f _pend_current_

# # Debug version only - remove for regular use # # #

The function stored in _pend_hook_ is called

immediately before each pending function is

evaluated. Stack clean, _pend_current_ set.

This thingy demonstrated in pend_hook.bash.

declare -f _pend_hook_

# #

The do nothing function

pend_dummy() { : ; }

Clear and initialize the function stack.

pend_init() {

unset _pending_[@]

pend_func pend_stop_mark

_pend_hook_='pend_dummy' # Debug only.

}

Discard the top function on the stack.

pend_pop() {

if [ ${#_pending_[@]} -gt 0 ]

then

local -i _top_

_top_=${#_pending_[@]}-1

unset _pending_[$_top_]

}

pend_func function_name [$(printf '%q\n' arguments)]

pend_func() {

local IFS=${NO_WSP}

set -f

_pending_[${#_pending_[@]}]=$@

set +f

}

The function which stops the release:

pend_stop_mark() {

_p_ctrl_=0

}

pend_mark() {

pend_func pend_stop_mark

}

Execute functions until 'pend_stop_mark' . . .

pend_release() {

local -i _top_ # Declare _top_ as integer.

_p_ctrl_=${#_pending_[@]}

while [ ${_p_ctrl_} -gt 0 ]

_top_=${#_pending_[@]}-1

_pend_current_=${_pending_[$_top_]}

unset _pending_[$_top_]

$_pend_hook_ # Debug only.

eval $_pend_current_

done

}

Drop functions until 'pend_stop_mark' . . .

pend_drop() {

local -i _top_

local _pd_ctrl_=${#_pending_[@]}

while [ ${_pd_ctrl_} -gt 0 ]

_top_=$_pd_ctrl_-1

if [ "${_pending_[$_top_]}" == 'pend_stop_mark' ]

then

unset _pending_[$_top_]

break

else

unset _pending_[$_top_]

_pd_ctrl_=$_top_

done

if [ ${#_pending_[@]} -eq 0 ]

then

pend_func pend_stop_mark

}

Array editors ####

This function described in edit_exact.bash.

(See "Additional documentation," above.)

edit_exact <excludes_array_name> <target_array_name>

edit_exact() {

[ $# -eq 2 ] ||

[ $# -eq 3 ] || return 1

local -a _ee_Excludes

local -a _ee_Target

local _ee_x

local _ee_t

local IFS=${NO_WSP}

set -f

eval _ee_Excludes=$ \$\{$1\[@\]\} $

eval _ee_Target=$ \$\{$2\[@\]\} $

local _ee_len=${#_ee_Target[@]} # Original length.

local _ee_cnt=${#_ee_Excludes[@]} # Exclude list length.

[ ${_ee_len} -ne 0 ] || return 0 # Can't edit zero length.

[ ${_ee_cnt} -ne 0 ] || return 0 # Can't edit zero length.

for (( x = 0; x < ${_ee_cnt} ; x++ ))

_ee_x=${_ee_Excludes[$x]}

for (( n = 0 ; n < ${_ee_len} ; n++ ))

_ee_t=${_ee_Target[$n]}

if [ x"${_ee_t}" == x"${_ee_x}" ]

then

unset _ee_Target[$n] # Discard match.

[ $# -eq 2 ] && break # If 2 arguments, then done.

done

eval $2=$ \$\{_ee_Target\[@\]\} $

set +f

return 0

}

This function described in edit_by_glob.bash.

edit_by_glob <excludes_array_name> <target_array_name>

edit_by_glob() {

[ $# -eq 2 ] ||

[ $# -eq 3 ] || return 1

local -a _ebg_Excludes

local -a _ebg_Target

local _ebg_x

local _ebg_t

local IFS=${NO_WSP}

set -f

eval _ebg_Excludes=$ \$\{$1\[@\]\} $

eval _ebg_Target=$ \$\{$2\[@\]\} $

local _ebg_len=${#_ebg_Target[@]}

local _ebg_cnt=${#_ebg_Excludes[@]}

[ ${_ebg_len} -ne 0 ] || return 0

[ ${_ebg_cnt} -ne 0 ] || return 0

for (( x = 0; x < ${_ebg_cnt} ; x++ ))

_ebg_x=${_ebg_Excludes[$x]}

for (( n = 0 ; n < ${_ebg_len} ; n++ ))

[ $# -eq 3 ] && _ebg_x=${_ebg_x}'*' # Do prefix edit

if [ ${_ebg_Target[$n]:=} ] #+ if defined & set.

then

_ebg_t=${_ebg_Target[$n]/#${_ebg_x}/}

[ ${#_ebg_t} -eq 0 ] && unset _ebg_Target[$n]

done

eval $2=$ \$\{_ebg_Target\[@\]\} $

set +f

return 0

}

This function described in unique_lines.bash.

unique_lines <in_name> <out_name>

unique_lines() {

[ $# -eq 2 ] || return 1

local -a _ul_in

local -a _ul_out

local -i _ul_cnt

local -i _ul_pos

local _ul_tmp

local IFS=${NO_WSP}

set -f

eval _ul_in=$ \$\{$1\[@\]\} $

_ul_cnt=${#_ul_in[@]}

for (( _ul_pos = 0 ; _ul_pos < ${_ul_cnt} ; _ul_pos++ ))

if [ ${_ul_in[${_ul_pos}]:=} ] # If defined & not empty

then

_ul_tmp=${_ul_in[${_ul_pos}]}

_ul_out[${#_ul_out[@]}]=${_ul_tmp}

for (( zap = _ul_pos ; zap < ${_ul_cnt} ; zap++ ))

[ ${_ul_in[${zap}]:=} ] &&

[ 'x'${_ul_in[${zap}]} == 'x'${_ul_tmp} ] &&

unset _ul_in[${zap}]

done

eval $2=$ \$\{_ul_out\[@\]\} $

set +f

return 0

}

This function described in char_convert.bash.

to_lower <string>

to_lower() {

[ $# -eq 1 ] || return 1

local _tl_out

_tl_out=${1//A/a}

_tl_out=${_tl_out//B/b}

_tl_out=${_tl_out//C/c}

_tl_out=${_tl_out//D/d}

_tl_out=${_tl_out//E/e}

_tl_out=${_tl_out//F/f}

_tl_out=${_tl_out//G/g}

_tl_out=${_tl_out//H/h}

_tl_out=${_tl_out//I/i}

_tl_out=${_tl_out//J/j}

_tl_out=${_tl_out//K/k}

_tl_out=${_tl_out//L/l}

_tl_out=${_tl_out//M/m}

_tl_out=${_tl_out//N/n}

_tl_out=${_tl_out//O/o}

_tl_out=${_tl_out//P/p}

_tl_out=${_tl_out//Q/q}

_tl_out=${_tl_out//R/r}

_tl_out=${_tl_out//S/s}

_tl_out=${_tl_out//T/t}

_tl_out=${_tl_out//U/u}

_tl_out=${_tl_out//V/v}

_tl_out=${_tl_out//W/w}

_tl_out=${_tl_out//X/x}

_tl_out=${_tl_out//Y/y}

_tl_out=${_tl_out//Z/z}

echo ${_tl_out}

return 0

}

Application helper functions ####

Not everybody uses dots as separators (APNIC, for example).

This function described in to_dot.bash

to_dot <string>

to_dot() {

[ $# -eq 1 ] || return 1

echo ${1//[#|@|%]/.}

return 0

}

This function described in is_number.bash.

is_number <input>

is_number() {

[ "$#" -eq 1 ] || return 1 # is blank?

[ x"$1" == 'x0' ] && return 0 # is zero?

local -i tst

let tst=$1 2>/dev/null # else is numeric!

return $?

}

This function described in is_address.bash.

is_address <input>

is_address() {

[ $# -eq 1 ] || return 1 # Blank ==> false

local -a _ia_input

local IFS=${ADR_IFS}

_ia_input=( $1 )

if [ ${#_ia_input[@]} -eq 4 ] &&

is_number ${_ia_input[0]} &&

is_number ${_ia_input[1]} &&

is_number ${_ia_input[2]} &&

is_number ${_ia_input[3]} &&

[ ${_ia_input[0]} -lt 256 ] &&

[ ${_ia_input[1]} -lt 256 ] &&

[ ${_ia_input[2]} -lt 256 ] &&

[ ${_ia_input[3]} -lt 256 ]

then

return 0

else

return 1

}

This function described in split_ip.bash.

split_ip <IP_address>

+ <array_name_norm> [<array_name_rev>]

split_ip() {

[ $# -eq 3 ] || # Either three

[ $# -eq 2 ] || return 1 #+ or two arguments

local -a _si_input

local IFS=${ADR_IFS}

_si_input=( $1 )

IFS=${WSP_IFS}

eval $2=$\ \$\{_si_input\[@\]\}\ $

if [ $# -eq 3 ]

then

# Build query order array.

local -a _dns_ip

_dns_ip[0]=${_si_input[3]}

_dns_ip[1]=${_si_input[2]}

_dns_ip[2]=${_si_input[1]}

_dns_ip[3]=${_si_input[0]}

eval $3=$\ \$\{_dns_ip\[@\]\}\ $

return 0

}

This function described in dot_array.bash.

dot_array <array_name>

dot_array() {

[ $# -eq 1 ] || return 1 # Single argument required.

local -a _da_input

eval _da_input=$\ \$\{$1\[@\]\}\ $

local IFS=${DOT_IFS}

local _da_output=${_da_input[@]}

IFS=${WSP_IFS}

echo ${_da_output}

return 0

}

This function described in file_to_array.bash

file_to_array <file_name> <line_array_name>

file_to_array() {

[ $# -eq 2 ] || return 1 # Two arguments required.

local IFS=${NO_WSP}

local -a _fta_tmp_

_fta_tmp_=( $(cat $1) )

eval $2=$ \$\{_fta_tmp_\[@\]\} $

return 0

}

Columnized print of an array of multi-field strings.

col_print <array_name> <min_space> <

+ tab_stop [tab_stops]>

col_print() {

[ $# -gt 2 ] || return 0

local -a _cp_inp

local -a _cp_spc

local -a _cp_line

local _cp_min

local _cp_mcnt

local _cp_pos

local _cp_cnt

local _cp_tab

local -i _cp

local -i _cpf

local _cp_fld

# WARNING: FOLLOWING LINE NOT BLANK -- IT IS QUOTED SPACES.

local _cp_max='

set -f

local IFS=${NO_WSP}

eval _cp_inp=$\ \$\{$1\[@\]\}\ $

[ ${#_cp_inp[@]} -gt 0 ] || return 0 # Empty is easy.

_cp_mcnt=$2

_cp_min=${_cp_max:1:${_cp_mcnt}}

shift

_cp_cnt=$#

for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))

_cp_spc[${#_cp_spc[@]}]="${_cp_max:2:$1}" #"

shift

done

_cp_cnt=${#_cp_inp[@]}

for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))

_cp_pos=1

IFS=${NO_WSP} \x20'

_cp_line=( ${_cp_inp[${_cp}]} )

IFS=${NO_WSP}

for (( _cpf = 0 ; _cpf < ${#_cp_line[@]} ; _cpf++ ))

_cp_tab=${_cp_spc[${_cpf}]:${_cp_pos}}

if [ ${#_cp_tab} -lt ${_cp_mcnt} ]

then

_cp_tab="${_cp_min}"

echo -n "${_cp_tab}"

(( _cp_pos = ${_cp_pos} + ${#_cp_tab} ))

_cp_fld="${_cp_line[${_cpf}]}"

echo -n ${_cp_fld}

(( _cp_pos = ${_cp_pos} + ${#_cp_fld} ))

done

echo

done

set +f

return 0

}

# # # 'Hunt the Spammer' data flow # # # #

Application return code

declare -i _hs_RC

Original input, from which IP addresses are removed

After which, domain names to check

declare -a uc_name

Original input IP addresses are moved here

After which, IP addresses to check

declare -a uc_address

Names against which address expansion run

Ready for name detail lookup

declare -a chk_name

Addresses against which name expansion run

Ready for address detail lookup

declare -a chk_address

Recursion is depth-first-by-name.

The expand_input_address maintains this list

+ to prohibit looking up addresses twice during

+ domain name recursion.

declare -a been_there_addr

been_there_addr=( '127.0.0.1' ) # Whitelist localhost

Names which we have checked (or given up on)

declare -a known_name

Addresses which we have checked (or given up on)

declare -a known_address

List of zero or more Blacklist servers to check.

Each 'known_address' will be checked against each server,

+ with negative replies and failures suppressed.

declare -a list_server

Indirection limit - set to zero == no limit

indirect=${SPAMMER_LIMIT:=2}

# # # 'Hunt the Spammer' information output data # # # #

Any domain name may have multiple IP addresses.

Any IP address may have multiple domain names.

Therefore, track unique address-name pairs.

declare -a known_pair

declare -a reverse_pair

In addition to the data flow variables; known_address

+ known_name and list_server, the following are output to the

+ external graphics interface file.

Authority chain, parent -> SOA fields.

declare -a auth_chain

Reference chain, parent name -> child name

declare -a ref_chain

DNS chain - domain name -> address

declare -a name_address

Name and service pairs - domain name -> service

declare -a name_srvc

Name and resource pairs - domain name -> Resource Record

declare -a name_resource

Parent and Child pairs - parent name -> child name

This MAY NOT be the same as the ref_chain followed!

declare -a parent_child

Address and Blacklist hit pairs - address->server

declare -a address_hits

Dump interface file data

declare -f _dot_dump

_dot_dump=pend_dummy # Initially a no-op

Data dump is enabled by setting the environment variable SPAMMER_DATA

+ to the name of a writable file.

declare _dot_file

Helper function for the dump-to-dot-file function

dump_to_dot <array_name> <prefix>

dump_to_dot() {

local -a _dda_tmp

local -i _dda_cnt

local _dda_form=' '${2}'%04u %s\n'

local IFS=${NO_WSP}

eval _dda_tmp=$\ \$\{$1\[@\]\}\ $

_dda_cnt=${#_dda_tmp[@]}

if [ ${_dda_cnt} -gt 0 ]

then

for (( _dda = 0 ; _dda < _dda_cnt ; _dda++ ))

printf "${_dda_form}" \

"${_dda}" "${_dda_tmp[${_dda}]}" >>${_dot_file}

done

}

Which will also set _dot_dump to this function . . .

dump_dot() {

local -i _dd_cnt

echo '# Data vintage: '$(date -R) >${_dot_file}

echo '# ABS Guide: is_spammer.bash; v2, 2004-msz' >>${_dot_file}

echo >>${_dot_file}

echo 'digraph G {' >>${_dot_file}

if [ ${#known_name[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known domain name nodes' >>${_dot_file}

_dd_cnt=${#known_name[@]}

for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))

printf ' N%04u [label="%s"] ;\n' \

"${_dd}" "${known_name[${_dd}]}" >>${_dot_file}

done

if [ ${#known_address[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known address nodes' >>${_dot_file}

_dd_cnt=${#known_address[@]}

for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))

printf ' A%04u [label="%s"] ;\n' \

"${_dd}" "${known_address[${_dd}]}" >>${_dot_file}

done

echo >>${_dot_file}

echo '/*' >>${_dot_file}

echo ' * Known relationships :: User conversion to' >>${_dot_file}

echo ' * graphic form by hand or program required.' >>${_dot_file}

echo ' *' >>${_dot_file}

if [ ${#auth_chain[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Authority ref. edges followed & field source.' >>${_dot_file}

dump_to_dot auth_chain AC

if [ ${#ref_chain[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Name ref. edges followed and field source.' >>${_dot_file}

dump_to_dot ref_chain RC

if [ ${#name_address[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known name->address edges' >>${_dot_file}

dump_to_dot name_address NA

if [ ${#name_srvc[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known name->service edges' >>${_dot_file}

dump_to_dot name_srvc NS

if [ ${#name_resource[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known name->resource edges' >>${_dot_file}

dump_to_dot name_resource NR

if [ ${#parent_child[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known parent->child edges' >>${_dot_file}

dump_to_dot parent_child PC

if [ ${#list_server[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known Blacklist nodes' >>${_dot_file}

_dd_cnt=${#list_server[@]}

for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))

printf ' LS%04u [label="%s"] ;\n' \

"${_dd}" "${list_server[${_dd}]}" >>${_dot_file}

done

unique_lines address_hits address_hits

if [ ${#address_hits[@]} -gt 0 ]

then

echo >>${_dot_file}

echo '# Known address->Blacklist_hit edges' >>${_dot_file}

echo '# CAUTION: dig warnings can trigger false hits.' >>${_dot_file}

dump_to_dot address_hits AH

echo >>${_dot_file}

echo ' *' >>${_dot_file}

echo ' * That is a lot of relationships. Happy graphing.' >>${_dot_file}

echo ' */' >>${_dot_file}

echo '}' >>${_dot_file}

return 0

}

# # # 'Hunt the Spammer' execution flow # # # #

Execution trace is enabled by setting the

+ environment variable SPAMMER_TRACE to the name of a writable file.

declare -a _trace_log

declare _log_file

Function to fill the trace log

trace_logger() {

_trace_log[${#_trace_log[@]}]=${_pend_current_}

}

Dump trace log to file function variable.

declare -f _log_dump

_log_dump=pend_dummy # Initially a no-op.

Dump the trace log to a file.

dump_log() {

local -i _dl_cnt

_dl_cnt=${#_trace_log[@]}

for (( _dl = 0 ; _dl < _dl_cnt ; _dl++ ))

echo ${_trace_log[${_dl}]} >> ${_log_file}

done

_dl_cnt=${#_pending_[@]}

if [ ${_dl_cnt} -gt 0 ]

then

_dl_cnt=${_dl_cnt}-1

echo '# # # Operations stack not empty # # #' >> ${_log_file}

for (( _dl = ${_dl_cnt} ; _dl >= 0 ; _dl-- ))

echo ${_pending_[${_dl}]} >> ${_log_file}

done

}

# # Utility program 'dig' wrappers # # #

These wrappers are derived from the

+ examples shown in dig_wrappers.bash.

The major difference is these return

+ their results as a list in an array.

See dig_wrappers.bash for details and

+ use that script to develop any changes.

# #

Short form answer: 'dig' parses answer.

Forward lookup :: Name -> Address

short_fwd <domain_name> <array_name>

short_fwd() {

local -a _sf_reply

local -i _sf_rc

local -i _sf_cnt

IFS=${NO_WSP}

echo -n '.'

echo 'sfwd: '${1}

_sf_reply=( $(dig +short ${1} -c in -t a 2>/dev/null) )

_sf_rc=$?

if [ ${_sf_rc} -ne 0 ]

then

_trace_log[${#_trace_log[@]}]='## Lookup error '${_sf_rc}' on '${1}' ##'

[ ${_sf_rc} -ne 9 ] && pend_drop

return ${_sf_rc}

else

# Some versions of 'dig' return warnings on stdout.

_sf_cnt=${#_sf_reply[@]}

for (( _sf = 0 ; _sf < ${_sf_cnt} ; _sf++ ))

[ 'x'${_sf_reply[${_sf}]:0:2} == 'x;;' ] &&

unset _sf_reply[${_sf}]

done

eval $2=$ \$\{_sf_reply\[@\]\} $

return 0

}

Reverse lookup :: Address -> Name

short_rev <ip_address> <array_name>

short_rev() {

local -a _sr_reply

local -i _sr_rc

local -i _sr_cnt

IFS=${NO_WSP}

echo -n '.'

echo 'srev: '${1}

_sr_reply=( $(dig +short -x ${1} 2>/dev/null) )

_sr_rc=$?

if [ ${_sr_rc} -ne 0 ]

then

_trace_log[${#_trace_log[@]}]='## Lookup error '${_sr_rc}' on '${1}' ##'

[ ${_sr_rc} -ne 9 ] && pend_drop

return ${_sr_rc}

else

# Some versions of 'dig' return warnings on stdout.

_sr_cnt=${#_sr_reply[@]}

for (( _sr = 0 ; _sr < ${_sr_cnt} ; _sr++ ))

[ 'x'${_sr_reply[${_sr}]:0:2} == 'x;;' ] &&

unset _sr_reply[${_sr}]

done

eval $2=$ \$\{_sr_reply\[@\]\} $

return 0

}

Special format lookup used to query blacklist servers.

short_text <ip_address> <array_name>

short_text() {

local -a _st_reply

local -i _st_rc

local -i _st_cnt

IFS=${NO_WSP}

echo 'stxt: '${1}

_st_reply=( $(dig +short ${1} -c in -t txt 2>/dev/null) )

_st_rc=$?

if [ ${_st_rc} -ne 0 ]

then

_trace_log[${#_trace_log[@]}]='##Text lookup error '${_st_rc}' on '${1}'##

[ ${_st_rc} -ne 9 ] && pend_drop

return ${_st_rc}

else

# Some versions of 'dig' return warnings on stdout.

_st_cnt=${#_st_reply[@]}

for (( _st = 0 ; _st < ${#_st_cnt} ; _st++ ))

[ 'x'${_st_reply[${_st}]:0:2} == 'x;;' ] &&

unset _st_reply[${_st}]

done

eval $2=$ \$\{_st_reply\[@\]\} $

return 0

}

The long forms, a.k.a., the parse it yourself versions

RFC 2782 Service lookups

dig +noall +nofail +answer _ldap._tcp.openldap.org -t srv

_<service>._<protocol>.<domain_name>

_ldap._tcp.openldap.org. 3600 IN SRV 0 0 389 ldap.openldap.org.

domain TTL Class SRV Priority Weight Port Target

Forward lookup :: Name -> poor man's zone transfer

long_fwd <domain_name> <array_name>

long_fwd() {

local -a _lf_reply

local -i _lf_rc

local -i _lf_cnt

IFS=${NO_WSP}

echo -n ':'

echo 'lfwd: '${1}

_lf_reply=( $(

dig +noall +nofail +answer +authority +additional \

${1} -t soa ${1} -t mx ${1} -t any 2>/dev/null) )

_lf_rc=$?

if [ ${_lf_rc} -ne 0 ]

then

_trace_log[${#_trace_log[@]}]='# Zone lookup err '${_lf_rc}' on '${1}' #'

[ ${_lf_rc} -ne 9 ] && pend_drop

return ${_lf_rc}

else

# Some versions of 'dig' return warnings on stdout.

_lf_cnt=${#_lf_reply[@]}

for (( _lf = 0 ; _lf < ${_lf_cnt} ; _lf++ ))

[ 'x'${_lf_reply[${_lf}]:0:2} == 'x;;' ] &&

unset _lf_reply[${_lf}]

done

eval $2=$ \$\{_lf_reply\[@\]\} $

return 0

}

The reverse lookup domain name corresponding to the IPv6 address:

4321:0:1:2:3:4:567:89ab

would be (nibble, I.E: Hexdigit) reversed:

b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.ARPA.

Reverse lookup :: Address -> poor man's delegation chain

long_rev <rev_ip_address> <array_name>

long_rev() {

local -a _lr_reply

local -i _lr_rc

local -i _lr_cnt

local _lr_dns

_lr_dns=${1}'.in-addr.arpa.'

IFS=${NO_WSP}

echo -n ':'

echo 'lrev: '${1}

_lr_reply=( $(

dig +noall +nofail +answer +authority +additional \

${_lr_dns} -t soa ${_lr_dns} -t any 2>/dev/null) )

_lr_rc=$?

if [ ${_lr_rc} -ne 0 ]

then

_trace_log[${#_trace_log[@]}]='# Deleg lkp error '${_lr_rc}' on '${1}' #'

[ ${_lr_rc} -ne 9 ] && pend_drop

return ${_lr_rc}

else

# Some versions of 'dig' return warnings on stdout.

_lr_cnt=${#_lr_reply[@]}

for (( _lr = 0 ; _lr < ${_lr_cnt} ; _lr++ ))

[ 'x'${_lr_reply[${_lr}]:0:2} == 'x;;' ] &&

unset _lr_reply[${_lr}]

done

eval $2=$ \$\{_lr_reply\[@\]\} $

return 0

}

# # Application specific functions # # #

Mung a possible name; suppresses root and TLDs.

name_fixup <string>

name_fixup(){

local -a _nf_tmp

local -i _nf_end

local _nf_str

local IFS

_nf_str=$(to_lower ${1})

_nf_str=$(to_dot ${_nf_str})

_nf_end=${#_nf_str}-1

[ ${_nf_str:${_nf_end}} != '.' ] &&

_nf_str=${_nf_str}'.'

IFS=${ADR_IFS}

_nf_tmp=( ${_nf_str} )

IFS=${WSP_IFS}

_nf_end=${#_nf_tmp[@]}

case ${_nf_end} in

0) # No dots, only dots.

echo

return 1

;;

1) # Only a TLD.

echo

return 1

;;

2) # Maybe okay.

echo ${_nf_str}

return 0

# Needs a lookup table?

if [ ${#_nf_tmp[1]} -eq 2 ]

then # Country coded TLD.

echo

return 1

else

echo ${_nf_str}

return 0

;;

esac

echo ${_nf_str}

return 0

}

Grope and mung original input(s).

split_input() {

[ ${#uc_name[@]} -gt 0 ] || return 0

local -i _si_cnt

local -i _si_len

local _si_str

unique_lines uc_name uc_name

_si_cnt=${#uc_name[@]}

for (( _si = 0 ; _si < _si_cnt ; _si++ ))

_si_str=${uc_name[$_si]}

if is_address ${_si_str}

then

uc_address[${#uc_address[@]}]=${_si_str}

unset uc_name[$_si]

else

if ! uc_name[$_si]=$(name_fixup ${_si_str})

then

unset ucname[$_si]

done

uc_name=( ${uc_name[@]} )

_si_cnt=${#uc_name[@]}

_trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd name input(s).#'

_si_cnt=${#uc_address[@]}

_trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd addr input(s).#'

return 0

}

# # Discovery functions -- recursively interlocked by external data # # #

# # The leading 'if list is empty; return 0' in each is required. # # #

Recursion limiter

limit_chk() <next_level>

limit_chk() {

local -i _lc_lmt

# Check indirection limit.

if [ ${indirect} -eq 0 ] || [ $# -eq 0 ]

then

# The 'do-forever' choice

echo 1 # Any value will do.

return 0 # OK to continue.

else

# Limiting is in effect.

if [ ${indirect} -lt ${1} ]

then

echo ${1} # Whatever.

return 1 # Stop here.

else

_lc_lmt=${1}+1 # Bump the given limit.

echo ${_lc_lmt} # Echo it.

return 0 # OK to continue.

}

For each name in uc_name:

Move name to chk_name.

Add addresses to uc_address.

Pend expand_input_address.

Repeat until nothing new found.

expand_input_name <indirection_limit>

expand_input_name() {

[ ${#uc_name[@]} -gt 0 ] || return 0

local -a _ein_addr

local -a _ein_new

local -i _ucn_cnt

local -i _ein_cnt

local _ein_tst

_ucn_cnt=${#uc_name[@]}

if ! _ein_cnt=$(limit_chk ${1})

then

return 0

for (( _ein = 0 ; _ein < _ucn_cnt ; _ein++ ))

if short_fwd ${uc_name[${_ein}]} _ein_new

then

for (( _ein_cnt = 0 ; _ein_cnt < ${#_ein_new[@]}; _ein_cnt++ ))

_ein_tst=${_ein_new[${_ein_cnt}]}

if is_address ${_ein_tst}

then

_ein_addr[${#_ein_addr[@]}]=${_ein_tst}

done

unique_lines _ein_addr _ein_addr # Scrub duplicates.

edit_exact chk_address _ein_addr # Scrub pending detail.

edit_exact known_address _ein_addr # Scrub already detailed.

if [ ${#_ein_addr[@]} -gt 0 ] # Anything new?

then

uc_address=( ${uc_address[@]} ${_ein_addr[@]} )

pend_func expand_input_address ${1}

_trace_log[${#_trace_log[@]}]='#Add '${#_ein_addr[@]}' unchkd addr inp.#'

edit_exact chk_name uc_name # Scrub pending detail.

edit_exact known_name uc_name # Scrub already detailed.

if [ ${#uc_name[@]} -gt 0 ]

then

chk_name=( ${chk_name[@]} ${uc_name[@]} )

pend_func detail_each_name ${1}

unset uc_name[@]

return 0

}

For each address in uc_address:

Move address to chk_address.

Add names to uc_name.

Pend expand_input_name.

Repeat until nothing new found.

expand_input_address <indirection_limit>

expand_input_address() {

[ ${#uc_address[@]} -gt 0 ] || return 0

local -a _eia_addr

local -a _eia_name

local -a _eia_new

local -i _uca_cnt

local -i _eia_cnt

local _eia_tst

unique_lines uc_address _eia_addr

unset uc_address[@]

edit_exact been_there_addr _eia_addr

_uca_cnt=${#_eia_addr[@]}

[ ${_uca_cnt} -gt 0 ] &&

been_there_addr=( ${been_there_addr[@]} ${_eia_addr[@]} )

for (( _eia = 0 ; _eia < _uca_cnt ; _eia++ ))

if short_rev ${_eia_addr[${_eia}]} _eia_new

then

for (( _eia_cnt = 0 ; _eia_cnt < ${#_eia_new[@]} ; _eia_cnt++ ))

_eia_tst=${_eia_new[${_eia_cnt}]}

if _eia_tst=$(name_fixup ${_eia_tst})

then

_eia_name[${#_eia_name[@]}]=${_eia_tst}

done

unique_lines _eia_name _eia_name # Scrub duplicates.

edit_exact chk_name _eia_name # Scrub pending detail.

edit_exact known_name _eia_name # Scrub already detailed.

if [ ${#_eia_name[@]} -gt 0 ] # Anything new?

then

uc_name=( ${uc_name[@]} ${_eia_name[@]} )

pend_func expand_input_name ${1}

_trace_log[${#_trace_log[@]}]='#Add '${#_eia_name[@]}' unchkd name inp.#'

edit_exact chk_address _eia_addr # Scrub pending detail.

edit_exact known_address _eia_addr # Scrub already detailed.

if [ ${#_eia_addr[@]} -gt 0 ] # Anything new?

then

chk_address=( ${chk_address[@]} ${_eia_addr[@]} )

pend_func detail_each_address ${1}

return 0

}

The parse-it-yourself zone reply.

The input is the chk_name list.

detail_each_name <indirection_limit>

detail_each_name() {

[ ${#chk_name[@]} -gt 0 ] || return 0

local -a _den_chk # Names to check

local -a _den_name # Names found here

local -a _den_address # Addresses found here

local -a _den_pair # Pairs found here

local -a _den_rev # Reverse pairs found here

local -a _den_tmp # Line being parsed

local -a _den_auth # SOA contact being parsed

local -a _den_new # The zone reply

local -a _den_pc # Parent-Child gets big fast

local -a _den_ref # So does reference chain

local -a _den_nr # Name-Resource can be big

local -a _den_na # Name-Address

local -a _den_ns # Name-Service

local -a _den_achn # Chain of Authority

local -i _den_cnt # Count of names to detail

local -i _den_lmt # Indirection limit

local _den_who # Named being processed

local _den_rec # Record type being processed

local _den_cont # Contact domain

local _den_str # Fixed up name string

local _den_str2 # Fixed up reverse

local IFS=${WSP_IFS}

# Local, unique copy of names to check

unique_lines chk_name _den_chk

unset chk_name[@] # Done with globals.

# Less any names already known

edit_exact known_name _den_chk

_den_cnt=${#_den_chk[@]}

# If anything left, add to known_name.

[ ${_den_cnt} -gt 0 ] &&

known_name=( ${known_name[@]} ${_den_chk[@]} )

# for the list of (previously) unknown names . . .

for (( _den = 0 ; _den < _den_cnt ; _den++ ))

_den_who=${_den_chk[${_den}]}

if long_fwd ${_den_who} _den_new

then

unique_lines _den_new _den_new

if [ ${#_den_new[@]} -eq 0 ]

then

_den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}

# Parse each line in the reply.

for (( _line = 0 ; _line < ${#_den_new[@]} ; _line++ ))

IFS=${NO_WSP} \x09' \x20'

_den_tmp=( ${_den_new[${_line}]} )

IFS=${WSP_IFS}

# If usable record and not a warning message . . .

if [ ${#_den_tmp[@]} -gt 4 ] && [ 'x'${_den_tmp[0]} != 'x;;' ]

then

_den_rec=${_den_tmp[3]}

_den_nr[${#_den_nr[@]}]=${_den_who}' '${_den_rec}

# Begin at RFC1033 (+++)

case ${_den_rec} in

<name> [<ttl>] [<class>] SOA <origin> <person>

SOA) # Start Of Authority

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str}' SOA'

# SOA origin -- domain name of master zone record

if _den_str2=$(name_fixup ${_den_tmp[4]})

then

_den_name[${#_den_name[@]}]=${_den_str2}

_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str2}' SOA.O'

# Responsible party e-mail address (possibly bogus).

# Possibility of first.last@domain.name ignored.

set -f

if _den_str2=$(name_fixup ${_den_tmp[5]})

then

IFS=${ADR_IFS}

_den_auth=( ${_den_str2} )

IFS=${WSP_IFS}

if [ ${#_den_auth[@]} -gt 2 ]

then

_den_cont=${_den_auth[1]}

for (( _auth = 2 ; _auth < ${#_den_auth[@]} ; _auth++ ))

_den_cont=${_den_cont}'.'${_den_auth[${_auth}]}

done

_den_name[${#_den_name[@]}]=${_den_cont}'.'

_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_cont}'. SOA.C'

set +f

;;

A) # IP(v4) Address Record

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}

_den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' A'

else

_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'

_den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}

_den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain A'

_den_address[${#_den_address[@]}]=${_den_tmp[4]}

_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}

;;

NS) # Name Server Record

# Domain name being serviced (may be other than current)

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' NS'

# Domain name of service provider

if _den_str2=$(name_fixup ${_den_tmp[4]})

then

_den_name[${#_den_name[@]}]=${_den_str2}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' NSH'

_den_ns[${#_den_ns[@]}]=${_den_str2}' NS'

_den_pc[${#_den_pc[@]}]=${_den_str}' '${_den_str2}

;;

MX) # Mail Server Record

# Domain name being serviced (wildcards not handled here)

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MX'

# Domain name of service provider

if _den_str=$(name_fixup ${_den_tmp[5]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MXH'

_den_ns[${#_den_ns[@]}]=${_den_str}' MX'

_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}

;;

PTR) # Reverse address record

# Special name

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' PTR'

# Host name (not a CNAME)

if _den_str2=$(name_fixup ${_den_tmp[4]})

then

_den_rev[${#_den_rev[@]}]=${_den_str}' '${_den_str2}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' PTRH'

_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}

;;

AAAA) # IP(v6) Address Record

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}

_den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' AAAA'

else

_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'

_den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}

_den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain'

# No processing for IPv6 addresses

_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}

;;

CNAME) # Alias name record

# Nickname

if _den_str=$(name_fixup ${_den_tmp[0]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CNAME'

_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}

# Hostname

if _den_str=$(name_fixup ${_den_tmp[4]})

then

_den_name[${#_den_name[@]}]=${_den_str}

_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CHOST'

_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}

;;

TXT)

;;

esac

done

else # Lookup error == 'A' record 'unknown address'

_den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}

done

# Control dot array growth.

unique_lines _den_achn _den_achn # Works best, all the same.

edit_exact auth_chain _den_achn # Works best, unique items.

if [ ${#_den_achn[@]} -gt 0 ]

then

IFS=${NO_WSP}

auth_chain=( ${auth_chain[@]} ${_den_achn[@]} )

IFS=${WSP_IFS}

unique_lines _den_ref _den_ref # Works best, all the same.

edit_exact ref_chain _den_ref # Works best, unique items.

if [ ${#_den_ref[@]} -gt 0 ]

then

IFS=${NO_WSP}

ref_chain=( ${ref_chain[@]} ${_den_ref[@]} )

IFS=${WSP_IFS}

unique_lines _den_na _den_na

edit_exact name_address _den_na

if [ ${#_den_na[@]} -gt 0 ]

then

IFS=${NO_WSP}

name_address=( ${name_address[@]} ${_den_na[@]} )

IFS=${WSP_IFS}

unique_lines _den_ns _den_ns

edit_exact name_srvc _den_ns

if [ ${#_den_ns[@]} -gt 0 ]

then

IFS=${NO_WSP}

name_srvc=( ${name_srvc[@]} ${_den_ns[@]} )

IFS=${WSP_IFS}

unique_lines _den_nr _den_nr

edit_exact name_resource _den_nr

if [ ${#_den_nr[@]} -gt 0 ]

then

IFS=${NO_WSP}

name_resource=( ${name_resource[@]} ${_den_nr[@]} )

IFS=${WSP_IFS}

unique_lines _den_pc _den_pc

edit_exact parent_child _den_pc

if [ ${#_den_pc[@]} -gt 0 ]

then

IFS=${NO_WSP}

parent_child=( ${parent_child[@]} ${_den_pc[@]} )

IFS=${WSP_IFS}

# Update list known_pair (Address and Name).

unique_lines _den_pair _den_pair

edit_exact known_pair _den_pair

if [ ${#_den_pair[@]} -gt 0 ] # Anything new?

then

IFS=${NO_WSP}

known_pair=( ${known_pair[@]} ${_den_pair[@]} )

IFS=${WSP_IFS}

# Update list of reverse pairs.

unique_lines _den_rev _den_rev

edit_exact reverse_pair _den_rev

if [ ${#_den_rev[@]} -gt 0 ] # Anything new?

then

IFS=${NO_WSP}

reverse_pair=( ${reverse_pair[@]} ${_den_rev[@]} )

IFS=${WSP_IFS}

# Check indirection limit -- give up if reached.

if ! _den_lmt=$(limit_chk ${1})

then

return 0

Execution engine is LIFO. Order of pend operations is important.

Did we define any new addresses?

unique_lines _den_address _den_address # Scrub duplicates.

edit_exact known_address _den_address # Scrub already processed.

edit_exact un_address _den_address # Scrub already waiting.

if [ ${#_den_address[@]} -gt 0 ] # Anything new?

then

uc_address=( ${uc_address[@]} ${_den_address[@]} )

pend_func expand_input_address ${_den_lmt}

_trace_log[${#_trace_log[@]}]='# Add '${#_den_address[@]}' unchkd addr. #'

Did we find any new names?

unique_lines _den_name _den_name # Scrub duplicates.

edit_exact known_name _den_name # Scrub already processed.

edit_exact uc_name _den_name # Scrub already waiting.

if [ ${#_den_name[@]} -gt 0 ] # Anything new?

then

uc_name=( ${uc_name[@]} ${_den_name[@]} )

pend_func expand_input_name ${_den_lmt}

_trace_log[${#_trace_log[@]}]='#Added '${#_den_name[@]}' unchkd name#'

return 0

}

The parse-it-yourself delegation reply

Input is the chk_address list.

detail_each_address <indirection_limit>

detail_each_address() {

[ ${#chk_address[@]} -gt 0 ] || return 0

unique_lines chk_address chk_address

edit_exact known_address chk_address

if [ ${#chk_address[@]} -gt 0 ]

then

known_address=( ${known_address[@]} ${chk_address[@]} )

unset chk_address[@]

return 0

}

# # Application specific output functions # # #

Pretty print the known pairs.

report_pairs() {

echo

echo 'Known network pairs.'

col_print known_pair 2 5 30

if [ ${#auth_chain[@]} -gt 0 ]

then

echo

echo 'Known chain of authority.'

col_print auth_chain 2 5 30 55

if [ ${#reverse_pair[@]} -gt 0 ]

then

echo

echo 'Known reverse pairs.'

col_print reverse_pair 2 5 55

return 0

}

Check an address against the list of blacklist servers.

A good place to capture for GraphViz: address->status(server(reports))

check_lists <ip_address>

check_lists() {

[ $# -eq 1 ] || return 1

local -a _cl_fwd_addr

local -a _cl_rev_addr

local -a _cl_reply

local -i _cl_rc

local -i _ls_cnt

local _cl_dns_addr

local _cl_lkup

split_ip ${1} _cl_fwd_addr _cl_rev_addr

_cl_dns_addr=$(dot_array _cl_rev_addr)'.'

_ls_cnt=${#list_server[@]}

echo ' Checking address '${1}

for (( _cl = 0 ; _cl < _ls_cnt ; _cl++ ))

_cl_lkup=${_cl_dns_addr}${list_server[${_cl}]}

if short_text ${_cl_lkup} _cl_reply

then

if [ ${#_cl_reply[@]} -gt 0 ]

then

echo ' Records from '${list_server[${_cl}]}

address_hits[${#address_hits[@]}]=${1}' '${list_server[${_cl}]}

_hs_RC=2

for (( _clr = 0 ; _clr < ${#_cl_reply[@]} ; _clr++ ))

echo ' '${_cl_reply[${_clr}]}

done

return 0

}

# # The usual application glue # # #

Who did it?

credits() {

echo

echo 'Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz'

}

How to use it?

(See also, "Quickstart" at end of script.)

usage() {

cat <<-'_usage_statement_'

The script is_spammer.bash requires either one or two arguments.

arg 1) May be one of:

a) A domain name

b) An IPv4 address

c) The name of a file with any mix of names

and addresses, one per line.

arg 2) May be one of:

a) A Blacklist server domain name

b) The name of a file with Blacklist server

domain names, one per line.

c) If not present, a default list of (free)

Blacklist servers is used.

d) If a filename of an empty, readable, file

is given,

Blacklist server lookup is disabled.

All script output is written to stdout.

Return codes: 0 -> All OK, 1 -> Script failure,

2 -> Something is Blacklisted.

Requires the external program 'dig' from the 'bind-9'

set of DNS programs. See: http://www.isc.org

The domain name lookup depth limit defaults to 2 levels.

Set the environment variable SPAMMER_LIMIT to change.

SPAMMER_LIMIT=0 means 'unlimited'

Limit may also be set on the command-line.

If arg#1 is an integer, the limit is set to that value

and then the above argument rules are applied.

Setting the environment variable 'SPAMMER_DATA' to a filename

will cause the script to write a GraphViz graphic file.

For the development version;

Setting the environment variable 'SPAMMER_TRACE' to a filename

will cause the execution engine to log a function call trace.

_usage_statement_

}

The default list of Blacklist servers:

Many choices, see: http://www.spews.org/lists.html

declare -a default_servers

See: http://www.spamhaus.org (Conservative, well maintained)

default_servers[0]='sbl-xbl.spamhaus.org'

See: http://ordb.org (Open mail relays)

default_servers[1]='relays.ordb.org'

See: http://www.spamcop.net/ (You can report spammers here)

default_servers[2]='bl.spamcop.net'

See: http://www.spews.org (An 'early detect' system)

default_servers[3]='l2.spews.dnsbl.sorbs.net'

See: http://www.dnsbl.us.sorbs.net/using.shtml

default_servers[4]='dnsbl.sorbs.net'

See: http://dsbl.org/usage (Various mail relay lists)

default_servers[5]='list.dsbl.org'

default_servers[6]='multihop.dsbl.org'

default_servers[7]='unconfirmed.dsbl.org'

User input argument #1

setup_input() {

if [ -e ${1} ] && [ -r ${1} ] # Name of readable file

then

file_to_array ${1} uc_name

echo 'Using filename >'${1}'< as input.'

else

if is_address ${1} # IP address?

then

uc_address=( ${1} )

echo 'Starting with address >'${1}'<'

else # Must be a name.

uc_name=( ${1} )

echo 'Starting with domain name >'${1}'<'

return 0

}

User input argument #2

setup_servers() {

if [ -e ${1} ] && [ -r ${1} ] # Name of a readable file

then

file_to_array ${1} list_server

echo 'Using filename >'${1}'< as blacklist server list.'

else

list_server=( ${1} )

echo 'Using blacklist server >'${1}'<'

return 0

}

User environment variable SPAMMER_TRACE

live_log_die() {

if [ ${SPAMMER_TRACE:=} ] # Wants trace log?

then

if [ ! -e ${SPAMMER_TRACE} ]

then

if ! touch ${SPAMMER_TRACE} 2>/dev/null

then

pend_func echo $(printf '%q\n' \

'Unable to create log file >'${SPAMMER_TRACE}'<')

pend_release

exit 1

_log_file=${SPAMMER_TRACE}

_pend_hook_=trace_logger

_log_dump=dump_log

else

if [ ! -w ${SPAMMER_TRACE} ]

then

pend_func echo $(printf '%q\n' \

'Unable to write log file >'${SPAMMER_TRACE}'<')

pend_release

exit 1

_log_file=${SPAMMER_TRACE}

echo '' > ${_log_file}

_pend_hook_=trace_logger

_log_dump=dump_log

return 0

}

User environment variable SPAMMER_DATA

data_capture() {

if [ ${SPAMMER_DATA:=} ] # Wants a data dump?

then

if [ ! -e ${SPAMMER_DATA} ]

then

if ! touch ${SPAMMER_DATA} 2>/dev/null

then

pend_func echo $(printf '%q]n' \

'Unable to create data output file >'${SPAMMER_DATA}'<')

pend_release

exit 1

_dot_file=${SPAMMER_DATA}

_dot_dump=dump_dot

else

if [ ! -w ${SPAMMER_DATA} ]

then

pend_func echo $(printf '%q\n' \

'Unable to write data output file >'${SPAMMER_DATA}'<')

pend_release

exit 1

_dot_file=${SPAMMER_DATA}

_dot_dump=dump_dot

return 0

}

Grope user specified arguments.

do_user_args() {

if [ $# -gt 0 ] && is_number $1

then

indirect=$1

shift

case $# in # Did user treat us well?

if ! setup_input $1 # Needs error checking.

then

pend_release

$_log_dump

exit 1

list_server=( ${default_servers[@]} )

_list_cnt=${#list_server[@]}

echo 'Using default blacklist server list.'

echo 'Search depth limit: '${indirect}

;;

if ! setup_input $1 # Needs error checking.

then

pend_release

$_log_dump

exit 1

if ! setup_servers $2 # Needs error checking.

then

pend_release

$_log_dump

exit 1

echo 'Search depth limit: '${indirect}

;;

pend_func usage

pend_release

$_log_dump

exit 1

;;

esac

return 0

}

A general purpose debug tool.

list_array <array_name>

list_array() {

[ $# -eq 1 ] || return 1 # One argument required.

local -a _la_lines

set -f

local IFS=${NO_WSP}

eval _la_lines=$\ \$\{$1\[@\]\}\ $

echo

echo "Element count "${#_la_lines[@]}" array "${1}

local _ln_cnt=${#_la_lines[@]}

for (( _i = 0; _i < ${_ln_cnt}; _i++ ))

echo 'Element '$_i' >'${_la_lines[$_i]}'<'

done

set +f

return 0

}

# # 'Hunt the Spammer' program code # # #

pend_init # Ready stack engine.

pend_func credits # Last thing to print.

# # Deal with user # # #

live_log_die # Setup debug trace log.

data_capture # Setup data capture file.

echo

do_user_args $@

# # Haven't exited yet - There is some hope # # #

Discovery group - Execution engine is LIFO - pend

in reverse order of execution.

_hs_RC=0 # Hunt the Spammer return code

pend_mark

pend_func report_pairs # Report name-address pairs.

# The two detail_* are mutually recursive functions.

# They also pend expand_* functions as required.

# These two (the last of ???) exit the recursion.

pend_func detail_each_address # Get all resources of addresses.

pend_func detail_each_name # Get all resources of names.

# The two expand_* are mutually recursive functions,

#+ which pend additional detail_* functions as required.

pend_func expand_input_address 1 # Expand input names by address.

pend_func expand_input_name 1 # #xpand input addresses by name.

# Start with a unique set of names and addresses.

pend_func unique_lines uc_address uc_address

pend_func unique_lines uc_name uc_name

# Separate mixed input of names and addresses.

pend_func split_input

pend_release

# # Pairs reported -- Unique list of IP addresses found

echo

_ip_cnt=${#known_address[@]}

if [ ${#list_server[@]} -eq 0 ]

then

echo 'Blacklist server list empty, none checked.'

else

if [ ${_ip_cnt} -eq 0 ]

then

echo 'Known address list empty, none checked.'

else

_ip_cnt=${_ip_cnt}-1 # Start at top.

echo 'Checking Blacklist servers.'

for (( _ip = _ip_cnt ; _ip >= 0 ; _ip-- ))

pend_func check_lists $( printf '%q\n' ${known_address[$_ip]} )

done

pend_release

$_dot_dump # Graphics file dump

$_log_dump # Execution trace

echo

Example output from script #

:<<-'_is_spammer_outputs_'

./is_spammer.bash 0 web4.alojamentos7.com

Starting with domain name >web4.alojamentos7.com<

Using default blacklist server list.

Search depth limit: 0

.:....::::...:::...:::.......::..::...:::.......::

Known network pairs.

66.98.208.97 web4.alojamentos7.com.

66.98.208.97 ns1.alojamentos7.com.

69.56.202.147 ns2.alojamentos.ws.

66.98.208.97 alojamentos7.com.

66.98.208.97 web.alojamentos7.com.

69.56.202.146 ns1.alojamentos.ws.

69.56.202.146 alojamentos.ws.

66.235.180.113 ns1.alojamentos.org.

66.235.181.192 ns2.alojamentos.org.

66.235.180.113 alojamentos.org.

66.235.180.113 web6.alojamentos.org.

216.234.234.30 ns1.theplanet.com.

12.96.160.115 ns2.theplanet.com.

216.185.111.52 mail1.theplanet.com.

69.56.141.4 spooling.theplanet.com.

216.185.111.40 theplanet.com.

216.185.111.40 www.theplanet.com.

216.185.111.52 mail.theplanet.com.

Checking Blacklist servers.

Checking address 66.98.208.97

Records from dnsbl.sorbs.net

"Spam Received See: http://www.dnsbl.sorbs.net/lookup.shtml?66.98.208.97"

Checking address 69.56.202.147

Checking address 69.56.202.146

Checking address 66.235.180.113

Checking address 66.235.181.192

Checking address 216.185.111.40

Checking address 216.234.234.30

Checking address 12.96.160.115

Checking address 216.185.111.52

Checking address 69.56.141.4

Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz

_is_spammer_outputs_

exit ${_hs_RC}

The script ignores everything from here on down #

+ because of the 'exit' command, just above. #

Quickstart

==========

Prerequisites

Bash version 2.05b or 3.00 (bash --version)

A version of Bash which supports arrays. Array

support is included by default Bash configurations.

'dig,' version 9.x.x (dig $HOSTNAME, see first line of output)

A version of dig which supports the +short options.

See: dig_wrappers.bash for details.

Optional Prerequisites

'named,' a local DNS caching program. Any flavor will do.

Do twice: dig $HOSTNAME

Check near bottom of output for: SERVER: 127.0.0.1#53

That means you have one running.

Optional Graphics Support

'date,' a standard *nix thing. (date -R)

dot Program to convert graphic description file to a

diagram. (dot -V)

A part of the Graph-Viz set of programs.

See: [http://www.research.att.com/sw/tools/graphviz||GraphViz]

'dotty,' a visual editor for graphic description files.

Also a part of the Graph-Viz set of programs.

Quick Start

In the same directory as the is_spammer.bash script;

Do: ./is_spammer.bash

Usage Details

1. Blacklist server choices.

(a) To use default, built-in list: Do nothing.

(b) To use your own list:

i. Create a file with a single Blacklist server

domain name per line.

ii. Provide that filename as the last argument to

the script.

to the script.

(d) To disable Blacklist lookups:

i. Create an empty file (touch spammer.nul)

Your choice of filename.

ii. Provide the filename of that empty file as the

last argument to the script.

2. Search depth limit.

(a) To use the default value of 2: Do nothing.

(b) To set a different limit:

A limit of 0 means: no limit.

i. export SPAMMER_LIMIT=1

or whatever limit you want.

ii. OR provide the desired limit as the first

argument to the script.

3. Optional execution trace log.

(a) To use the default setting of no log output: Do nothing.

(b) To write an execution trace log:

export SPAMMER_TRACE=spammer.log

or whatever filename you want.

4. Optional graphic description file.

(a) To use the default setting of no graphic file: Do nothing.

(b) To write a Graph-Viz graphic description file:

export SPAMMER_DATA=spammer.dot

or whatever filename you want.

5. Where to start the search.

(a) Starting with a single domain name:

i. Without a command-line search limit: First

argument to script.

ii. With a command-line search limit: Second

argument to script.

(b) Starting with a single IP address:

i. Without a command-line search limit: First

argument to script.

ii. With a command-line search limit: Second

argument to script.

Create a file with one name or address per line.

Your choice of filename.

i. Without a command-line search limit: Filename as

first argument to script.

ii. With a command-line search limit: Filename as

second argument to script.

6. What to do with the display output.

(a) To view display output on screen: Do nothing.

(b) To save display output to a file: Redirect stdout to a filename.

7. Temporary end of decision making.

press RETURN

wait (optionally, watch the dots and colons).

8. Optionally check the return code.

(a) Return code 0: All OK

(b) Return code 1: Script setup failure

9. Where is my graph (diagram)?

The script does not directly produce a graph (diagram).

It only produces a graphic description file. You can

process the graphic descriptor file that was output

with the 'dot' program.

Until you edit that descriptor file, to describe the

relationships you want shown, all that you will get is

a bunch of labeled name and address nodes.

All of the script's discovered relationships are within

a comment block in the graphic descriptor file, each

with a descriptive heading.

The editing required to draw a line between a pair of

nodes from the information in the descriptor file may

be done with a text editor.

Given these lines somewhere in the descriptor file:

Known domain name nodes

N0000 [label="guardproof.info."] ;

N0002 [label="third.guardproof.info."] ;

Known address nodes

A0000 [label="61.141.32.197"] ;

Known name->address edges

NA0000 third.guardproof.info. 61.141.32.197

Known parent->child edges

PC0000 guardproof.info. third.guardproof.info.

Turn that into the following lines by substituting node

identifiers into the relationships:

Known domain name nodes

N0000 [label="guardproof.info."] ;

N0002 [label="third.guardproof.info."] ;

Known address nodes

A0000 [label="61.141.32.197"] ;

PC0000 guardproof.info. third.guardproof.info.

N0000->N0002 ;

NA0000 third.guardproof.info. 61.141.32.197

N0002->A0000 ;

Known name->address edges

NA0000 third.guardproof.info. 61.141.32.197

Known parent->child edges

PC0000 guardproof.info. third.guardproof.info.

Process that with the 'dot' program, and you have your

first network diagram.

In addition to the conventional graphic edges, the

descriptor file includes similar format pair-data that

describes services, zone records (sub-graphs?),

blacklisted addresses, and other things which might be

interesting to include in your graph. This additional

information could be displayed as different node

shapes, colors, line sizes, etc.

The descriptor file can also be read and edited by a

Bash script (of course). You should be able to find

most of the functions required within the

"is_spammer.bash" script.

End Quickstart.

Additional Note

========== ====

Michael Zick points out that there is a "makeviz.bash" interactive

Web site at rediris.es. Can't give the full URL, since this is not

a publically accessible site.

Another anti-spam script.

Example A-29. Spammer Hunt

!/bin/bash

whx.sh: "whois" spammer lookup

Author: Walter Dnes

Slight revisions (first section) by ABS Guide author.

Used in ABS Guide with permission.

Needs version 3.x or greater of Bash to run (because of =~ operator).

Commented by script author and ABS Guide author.

E_BADARGS=85 # Missing command-line arg.

E_NOHOST=86 # Host not found.

E_TIMEOUT=87 # Host lookup timed out.

E_UNDEF=88 # Some other (undefined) error.

HOSTWAIT=10 # Specify up to 10 seconds for host query reply.

# The actual wait may be a bit longer.

OUTFILE=whois.txt # Output file.

PORT=4321

if [ -z "$1" ] # Check for (required) command-line arg.

then

echo "Usage: $0 domain name or IP address"

exit $E_BADARGS

if [[ "$1" =~ [a-zA-Z][a-zA-Z]$ ]] # Ends in two alpha chars?

then # It's a domain name &&

#+ must do host lookup.

IPADDR=$(host -W $HOSTWAIT $1 | awk '{print $4}')

# Doing host lookup

#+ to get IP address.

# Extract final field.

else

IPADDR="$1" # Command-line arg was IP address.

echo; echo "IP Address is: "$IPADDR""; echo

if [ -e "$OUTFILE" ]

then

rm -f "$OUTFILE"

echo "Stale output file \"$OUTFILE\" removed."; echo

Sanity checks.

(This section needs more work.)

===============================

if [ -z "$IPADDR" ]

No response.

then

echo "Host not found!"

exit $E_NOHOST # Bail out.

if [[ "$IPADDR" =~ ^[;;] ]]

;; Connection timed out; no servers could be reached.

then

echo "Host lookup timed out!"

exit $E_TIMEOUT # Bail out.

if [[ "$IPADDR" =~ [(NXDOMAIN)]$ ]]

Host xxxxxxxxx.xxx not found: 3(NXDOMAIN)

then

echo "Host not found!"

exit $E_NOHOST # Bail out.

if [[ "$IPADDR" =~ [(SERVFAIL)]$ ]]

Host xxxxxxxxx.xxx not found: 2(SERVFAIL)

then

echo "Host not found!"

exit $E_NOHOST # Bail out.

======================== Main body of script ========================

AFRINICquery() {

Define the function that queries AFRINIC. Echo a notification to the

+ screen, and then run the actual query, redirecting output to $OUTFILE.

echo "Searching for $IPADDR in whois.afrinic.net"

whois -h whois.afrinic.net "$IPADDR" > $OUTFILE

Check for presence of reference to an rwhois.

Warn about non-functional rwhois.infosat.net server

+ and attempt rwhois query.

if grep -e "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE"

then

echo " " >> $OUTFILE

echo "***" >> $OUTFILE

echo "Warning: rwhois.infosat.net was not working \

as of 2005/02/02" >> $OUTFILE

echo " when this script was written." >> $OUTFILE

echo "***" >> $OUTFILE

echo " " >> $OUTFILE

RWHOIS=`grep "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\

sed "s/$^.*$$rwhois\..*$$:4.*$/\2/"`

whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE

}

APNICquery() {

echo "Searching for $IPADDR in whois.apnic.net"

whois -h whois.apnic.net "$IPADDR" > $OUTFILE

Just about every country has its own internet registrar.

I don't normally bother consulting them, because the regional registry

+ usually supplies sufficient information.

There are a few exceptions, where the regional registry simply

+ refers to the national registry for direct data.

These are Japan and South Korea in APNIC, and Brasil in LACNIC.

The following if statement checks $OUTFILE (whois.txt) for the presence

+ of "KR" (South Korea) or "JP" (Japan) in the country field.

If either is found, the query is re-run against the appropriate

+ national registry.

if grep -E "^country:[ ]+KR$" "$OUTFILE"

then

echo "Searching for $IPADDR in whois.krnic.net"

whois -h whois.krnic.net "$IPADDR" >> $OUTFILE

elif grep -E "^country:[ ]+JP$" "$OUTFILE"

then

echo "Searching for $IPADDR in whois.nic.ad.jp"

whois -h whois.nic.ad.jp "$IPADDR"/e >> $OUTFILE

}

ARINquery() {

echo "Searching for $IPADDR in whois.arin.net"

whois -h whois.arin.net "$IPADDR" > $OUTFILE

Several large internet providers listed by ARIN have their own

+ internal whois service, referred to as "rwhois".

A large block of IP addresses is listed with the provider

+ under the ARIN registry.

To get the IP addresses of 2nd-level ISPs or other large customers,

+ one has to refer to the rwhois server on port 4321.

I originally started with a bunch of "if" statements checking for

+ the larger providers.

This approach is unwieldy, and there's always another rwhois server

+ that I didn't know about.

A more elegant approach is to check $OUTFILE for a reference

+ to a whois server, parse that server name out of the comment section,

+ and re-run the query against the appropriate rwhois server.

The parsing looks a bit ugly, with a long continued line inside

+ backticks.

But it only has to be done once, and will work as new servers are added.

@ ABS Guide author comment: it isn't all that ugly, and is, in fact,

@+ an instructive use of Regular Expressions.

if grep -E "^Comment: .*rwhois.[^ ]+" "$OUTFILE"

then

RWHOIS=`grep -e "^Comment:.*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\

sed "s/^$.*$$rwhois\.[^ ]\+$$.*$$/\2/"`

echo "Searching for $IPADDR in ${RWHOIS}"

whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE

}

LACNICquery() {

echo "Searching for $IPADDR in whois.lacnic.net"

whois -h whois.lacnic.net "$IPADDR" > $OUTFILE

The following if statement checks $OUTFILE (whois.txt) for

+ the presence of "BR" (Brasil) in the country field.

If it is found, the query is re-run against whois.registro.br.

if grep -E "^country:[ ]+BR$" "$OUTFILE"

then

echo "Searching for $IPADDR in whois.registro.br"

whois -h whois.registro.br "$IPADDR" >> $OUTFILE

}

RIPEquery() {

echo "Searching for $IPADDR in whois.ripe.net"

whois -h whois.ripe.net "$IPADDR" > $OUTFILE

}

Initialize a few variables.

* slash8 is the most significant octet

* slash16 consists of the two most significant octets

* octet2 is the second most significant octet

slash8=`echo $IPADDR | cut -d. -f 1`

if [ -z "$slash8" ] # Yet another sanity check.

then

echo "Undefined error!"

exit $E_UNDEF

slash16=`echo $IPADDR | cut -d. -f 1-2`

^ Period specified as 'cut" delimiter.

if [ -z "$slash16" ]

then

echo "Undefined error!"

exit $E_UNDEF

octet2=`echo $slash16 | cut -d. -f 2`

if [ -z "$octet2" ]

then

echo "Undefined error!"

exit $E_UNDEF

Check for various odds and ends of reserved space.

There is no point in querying for those addresses.

if [ $slash8 == 0 ]; then

echo $IPADDR is '"This Network"' space\; Not querying

elif [ $slash8 == 10 ]; then

echo $IPADDR is RFC1918 space\; Not querying

elif [ $slash8 == 14 ]; then

echo $IPADDR is '"Public Data Network"' space\; Not querying

elif [ $slash8 == 127 ]; then

echo $IPADDR is loopback space\; Not querying

elif [ $slash16 == 169.254 ]; then

echo $IPADDR is link-local space\; Not querying

elif [ $slash8 == 172 ] && [ $octet2 -ge 16 ] && [ $octet2 -le 31 ];then

echo $IPADDR is RFC1918 space\; Not querying

elif [ $slash16 == 192.168 ]; then

echo $IPADDR is RFC1918 space\; Not querying

elif [ $slash8 -ge 224 ]; then

echo $IPADDR is either Multicast or reserved space\; Not querying

elif [ $slash8 -ge 200 ] && [ $slash8 -le 201 ]; then LACNICquery "$IPADDR"

elif [ $slash8 -ge 202 ] && [ $slash8 -le 203 ]; then APNICquery "$IPADDR"

elif [ $slash8 -ge 210 ] && [ $slash8 -le 211 ]; then APNICquery "$IPADDR"

elif [ $slash8 -ge 218 ] && [ $slash8 -le 223 ]; then APNICquery "$IPADDR"

If we got this far without making a decision, query ARIN.

If a reference is found in $OUTFILE to APNIC, AFRINIC, LACNIC, or RIPE,

+ query the appropriate whois server.

else

ARINquery "$IPADDR"

if grep "whois.afrinic.net" "$OUTFILE"; then

AFRINICquery "$IPADDR"

elif grep -E "^OrgID:[ ]+RIPE$" "$OUTFILE"; then

RIPEquery "$IPADDR"

elif grep -E "^OrgID:[ ]+APNIC$" "$OUTFILE"; then

APNICquery "$IPADDR"

elif grep -E "^OrgID:[ ]+LACNIC$" "$OUTFILE"; then

LACNICquery "$IPADDR"

@ ---------------------------------------------------------------

Try also:

wget http://logi.cc/nw/whois.php3?ACTION=doQuery&DOMAIN=$IPADDR

@ ---------------------------------------------------------------

We've now finished the querying.

Echo a copy of the final result to the screen.

cat $OUTFILE

Or "less $OUTFILE" . . .

exit 0

@ ABS Guide author comments:

@ Nothing fancy here, but still a very useful tool for hunting spammers.

@ Sure, the script can be cleaned up some, and it's still a bit buggy,

@+ (exercise for reader), but all the same, it's a nice piece of coding

@+ by Walter Dnes.

@ Thank you!

"Little Monster's" front end to wget.

Example A-30. Making wget easier to use

!/bin/bash

wgetter2.bash

Author: Little Monster [monster@monstruum.co.uk]

==> Used in ABS Guide with permission of script author.

==> This script still needs debugging and fixups (exercise for reader).

==> It could also use some additional editing in the comments.

This is wgetter2 --

+ a Bash script to make wget a bit more friendly, and save typing.

Carefully crafted by Little Monster.

More or less complete on 02/02/2005.

If you think this script can be improved,

+ email me at: monster@monstruum.co.uk

==> and cc: to the author of the ABS Guide, please.

This script is licenced under the GPL.

You are free to copy, alter and re-use it,

+ but please don't try to claim you wrote it.

Log your changes here instead.

=======================================================================

changelog:

07/02/2005. Fixups by Little Monster.

02/02/2005. Minor additions by Little Monster.

(See after # +++++++++++ )

29/01/2005. Minor stylistic edits and cleanups by author of ABS Guide.

Added exit error codes.

22/11/2004. Finished initial version of second version of wgetter:

wgetter2 is born.

01/12/2004. Changed 'runn' function so it can be run 2 ways --

either ask for a file name or have one input on the CL.

01/12/2004. Made sensible handling of no URL's given.

01/12/2004. Made loop of main options, so you don't

have to keep calling wgetter 2 all the time.

Runs as a session instead.

01/12/2004. Added looping to 'runn' function.

Simplified and improved.

01/12/2004. Added state to recursion setting.

Enables re-use of previous value.

05/12/2004. Modified the file detection routine in the 'runn' function

so it's not fooled by empty values, and is cleaner.

01/02/2004. Added cookie finding routine from later version (which

isn't ready yet), so as not to have hard-coded paths.

=======================================================================

Error codes for abnormal exit.

E_USAGE=67 # Usage message, then quit.

E_NO_OPTS=68 # No command-line args entered.

E_NO_URLS=69 # No URLs passed to script.

E_NO_SAVEFILE=70 # No save filename passed to script.

E_USER_EXIT=71 # User decides to quit.

Basic default wget command we want to use.

This is the place to change it, if required.

NB: if using a proxy, set http_proxy = yourproxy in .wgetrc.

Otherwise delete --proxy=on, below.

====================================================================

CommandA="wget -nc -c -t 5 --progress=bar --random-wait --proxy=on -r"

====================================================================

--------------------------------------------------------------------

Set some other variables and explain them.

pattern=" -A .jpg,.JPG,.jpeg,.JPEG,.gif,.GIF,.htm,.html,.shtml,.php"

# wget's option to only get certain types of file.

# comment out if not using

today=`date +%F` # Used for a filename.

home=$HOME # Set HOME to an internal variable.

# In case some other path is used, change it here.

depthDefault=3 # Set a sensible default recursion.

Depth=$depthDefault # Otherwise user feedback doesn't tie in properly.

RefA="" # Set blank referring page.

Flag="" # Default to not saving anything,

#+ or whatever else might be wanted in future.

lister="" # Used for passing a list of urls directly to wget.

Woptions="" # Used for passing wget some options for itself.

inFile="" # Used for the run function.

newFile="" # Used for the run function.

savePath="$home/w-save"

Config="$home/.wgetter2rc"

# This is where some variables can be stored,

#+ if permanently changed from within the script.

Cookie_List="$home/.cookielist"

# So we know where the cookies are kept . . .

cFlag="" # Part of the cookie file selection routine.

Define the options available. Easy to change letters here if needed.

These are the optional options; you don't just wait to be asked.

save=s # Save command instead of executing it.

cook=c # Change cookie file for this session.

help=h # Usage guide.

list=l # Pass wget the -i option and URL list.

runn=r # Run saved commands as an argument to the option.

inpu=i # Run saved commands interactively.

wopt=w # Allow to enter options to pass directly to wget.

--------------------------------------------------------------------

if [ -z "$1" ]; then # Make sure we get something for wget to eat.

echo "You must at least enter a URL or option!"

echo "-$help for usage."

exit $E_NO_OPTS

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

added added added added added added added added added added added added

if [ ! -e "$Config" ]; then # See if configuration file exists.

echo "Creating configuration file, $Config"

echo "# This is the configuration file for wgetter2" > "$Config"

echo "# Your customised settings will be saved in this file" >> "$Config"

else

source $Config # Import variables we set outside the script.

if [ ! -e "$Cookie_List" ]; then

# Set up a list of cookie files, if there isn't one.

echo "Hunting for cookies . . ."

find -name cookies.txt >> $Cookie_List # Create the list of cookie files.

fi # Isolate this in its own 'if' statement,

#+ in case we got interrupted while searching.

if [ -z "$cFlag" ]; then # If we haven't already done this . . .

echo # Make a nice space after the command prompt.

echo "Looks like you haven't set up your source of cookies yet."

n=0 # Make sure the counter

#+ doesn't contain random values.

while read; do

Cookies[$n]=$REPLY # Put the cookie files we found into an array.

echo "$n) ${Cookies[$n]}" # Create a menu.

n=$(( n + 1 )) # Increment the counter.

done < $Cookie_List # Feed the read statement.

echo "Enter the number of the cookie file you want to use."

echo "If you won't be using cookies, just press RETURN."

echo

echo "I won't be asking this again. Edit $Config"

echo "If you decide to change at a later date"

echo "or use the -${cook} option for per session changes."

read

if [ ! -z $REPLY ]; then # User didn't just press return.

Cookie=" --load-cookies ${Cookies[$REPLY]}"

# Set the variable here as well as in the config file.

echo "Cookie=\" --load-cookies ${Cookies[$REPLY]}\"" >> $Config

echo "cFlag=1" >> $Config # So we know not to ask again.

end added section end added section end added section end added section

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Another variable.

This one may or may not be subject to variation.

A bit like the small print.

CookiesON=$Cookie

echo "cookie file is $CookiesON" # For debugging.

echo "home is ${home}" # For debugging.

# Got caught with this one!

wopts()

{

echo "Enter options to pass to wget."

echo "It is assumed you know what you're doing."

echo

echo "You can pass their arguments here too."

That is to say, everything passed here is passed to wget.

read Wopts

Read in the options to be passed to wget.

Woptions=" $Wopts"

^ Why the leading space?

Assign to another variable.

Just for fun, or something . . .

echo "passing options ${Wopts} to wget"

Mainly for debugging.

Is cute.

return

}

save_func()

{

echo "Settings will be saved."

if [ ! -d $savePath ]; then # See if directory exists.

mkdir $savePath # Create the directory to save things in

#+ if it isn't already there.

Flag=S

Tell the final bit of code what to do.

Set a flag since stuff is done in main.

return

}

usage() # Tell them how it works.

{

echo "Welcome to wgetter. This is a front end to wget."

echo "It will always run wget with these options:"

echo "$CommandA"

echo "and the pattern to match: $pattern \

(which you can change at the top of this script)."

echo "It will also ask you for recursion depth, \

and if you want to use a referring page."

echo "Wgetter accepts the following options:"

echo ""

echo "-$help : Display this help."

echo "-$save : Save the command to a file $savePath/wget-($today) \

instead of running it."

echo "-$runn : Run saved wget commands instead of starting a new one -"

echo "Enter filename as argument to this option."

echo "-$inpu : Run saved wget commands interactively --"

echo "The script will ask you for the filename."

echo "-$cook : Change the cookies file for this session."

echo "-$list : Tell wget to use URL's from a list instead of \

from the command-line."

echo "-$wopt : Pass any other options direct to wget."

echo ""

echo "See the wget man page for additional options \

you can pass to wget."

echo ""

exit $E_USAGE # End here. Don't process anything else.

}

list_func() # Gives the user the option to use the -i option to wget,

#+ and a list of URLs.

{

while [ 1 ]; do

echo "Enter the name of the file containing URL's (press q to change

your mind)."

read urlfile

if [ ! -e "$urlfile" ] && [ "$urlfile" != q ]; then

# Look for a file, or the quit option.

echo "That file does not exist!"

elif [ "$urlfile" = q ]; then # Check quit option.

echo "Not using a url list."

return

else

echo "using $urlfile."

echo "If you gave url's on the command-line, I'll use those first."

# Report wget standard behaviour to the user.

lister=" -i $urlfile" # This is what we want to pass to wget.

return

done

}

cookie_func() # Give the user the option to use a different cookie file.

{

while [ 1 ]; do

echo "Change the cookies file. Press return if you don't want to change

it."

read Cookies

# NB: this is not the same as Cookie, earlier.

# There is an 's' on the end.

# Bit like chocolate chips.

if [ -z "$Cookies" ]; then # Escape clause for wusses.

return

elif [ ! -e "$Cookies" ]; then

echo "File does not exist. Try again." # Keep em going . . .

else

CookiesON=" --load-cookies $Cookies" # File is good -- use it!

return

done

}

run_func()

{

if [ -z "$OPTARG" ]; then

Test to see if we used the in-line option or the query one.

if [ ! -d "$savePath" ]; then # If directory doesn't exist . . .

echo "$savePath does not appear to exist."

echo "Please supply path and filename of saved wget commands:"

read newFile

until [ -f "$newFile" ]; do # Keep going till we get something.

echo "Sorry, that file does not exist. Please try again."

# Try really hard to get something.

read newFile

done

-----------------------------------------------------------------------

if [ -z ( grep wget ${newfile} ) ]; then

# Assume they haven't got the right file and bail out.

echo "Sorry, that file does not contain wget commands. Aborting."

exit

fi

This is bogus code.

It doesn't actually work.

If anyone wants to fix it, feel free!

-----------------------------------------------------------------------

filePath="${newFile}"

else

echo "Save path is $savePath"

echo "Please enter name of the file which you want to use."

echo "You have a choice of:"

ls $savePath # Give them a choice.

read inFile

until [ -f "$savePath/$inFile" ]; do # Keep going till

#+ we get something.

if [ ! -f "${savePath}/${inFile}" ]; then # If file doesn't exist.

echo "Sorry, that file does not exist. Please choose from:"

ls $savePath # If a mistake is made.

read inFile

done

filePath="${savePath}/${inFile}" # Make one variable . . .

else filePath="${savePath}/${OPTARG}" # Which can be many things . . .

if [ ! -f "$filePath" ]; then # If a bogus file got through.

echo "You did not specify a suitable file."

echo "Run this script with the -${save} option first."

echo "Aborting."

exit $E_NO_SAVEFILE

echo "Using: $filePath"

while read; do

eval $REPLY

echo "Completed: $REPLY"

done < $filePath # Feed the actual file we are using into a 'while' loop.

exit

}

Fish out any options we are using for the script.

This is based on the demo in "Learning The Bash Shell" (O'Reilly).

while getopts ":$save$cook$help$list$runn:$inpu$wopt" opt

case $opt in

$save) save_func;; # Save some wgetter sessions for later.

$cook) cookie_func;; # Change cookie file.

$help) usage;; # Get help.

$list) list_func;; # Allow wget to use a list of URLs.

$runn) run_func;; # Useful if you are calling wgetter from,

#+ for example, a cron script.

$inpu) run_func;; # When you don't know what your files are named.

$wopt) wopts;; # Pass options directly to wget.

\?) echo "Not a valid option."

echo "Use -${wopt} to pass options directly to wget,"

echo "or -${help} for help";; # Catch anything else.

esac

done

shift $((OPTIND - 1)) # Do funky magic stuff with $#.

if [ -z "$1" ] && [ -z "$lister" ]; then

# We should be left with at least one URL

#+ on the command-line, unless a list is

#+ being used -- catch empty CL's.

echo "No URL's given! You must enter them on the same line as wgetter2."

echo "E.g., wgetter2 http://somesite http://anothersite."

echo "Use $help option for more information."

exit $E_NO_URLS # Bail out, with appropriate error code.

URLS=" $@"

Use this so that URL list can be changed if we stay in the option loop.

while [ 1 ]; do

# This is where we ask for the most used options.

# (Mostly unchanged from version 1 of wgetter)

if [ -z $curDepth ]; then

Current=""

else Current=" Current value is $curDepth"

echo "How deep should I go? \

(integer: Default is $depthDefault.$Current)"

read Depth # Recursion -- how far should we go?

inputB="" # Reset this to blank on each pass of the loop.

echo "Enter the name of the referring page (default is none)."

read inputB # Need this for some sites.

echo "Do you want to have the output logged to the terminal"

echo "(y/n, default is yes)?"

read noHide # Otherwise wget will just log it to a file.

case $noHide in # Now you see me, now you don't.

y|Y ) hide="";;

n|N ) hide=" -b";;

* ) hide="";;

esac

if [ -z ${Depth} ]; then

# User accepted either default or current depth,

#+ in which case Depth is now empty.

if [ -z ${curDepth} ]; then

# See if a depth was set on a previous iteration.

Depth="$depthDefault"

# Set the default recursion depth if nothing

#+ else to use.

else Depth="$curDepth" # Otherwise, set the one we used before.

Recurse=" -l $Depth" # Set how deep we want to go.

curDepth=$Depth # Remember setting for next time.

if [ ! -z $inputB ]; then

RefA=" --referer=$inputB" # Option to use referring page.

WGETTER="${CommandA}${pattern}${hide}${RefA}${Recurse}\

${CookiesON}${lister}${Woptions}${URLS}"

# Just string the whole lot together . . .

# NB: no embedded spaces.

# They are in the individual elements so that if any are empty,

#+ we don't get an extra space.

if [ -z "${CookiesON}" ] && [ "$cFlag" = "1" ] ; then

echo "Warning -- can't find cookie file"

# This should be changed,

#+ in case the user has opted to not use cookies.

if [ "$Flag" = "S" ]; then

echo "$WGETTER" >> $savePath/wget-${today}

# Create a unique filename for today, or append to it if it exists.

echo "$inputB" >> $savePath/site-list-${today}

# Make a list, so it's easy to refer back to,

#+ since the whole command is a bit confusing to look at.

echo "Command saved to the file $savePath/wget-${today}"

# Tell the user.

echo "Referring page URL saved to the file$ \

savePath/site-list-${today}"

# Tell the user.

Saver=" with save option"

# Stick this somewhere, so it appears in the loop if set.

else

echo "*****************"

echo "*****Getting*****"

echo "*****************"

echo ""

echo "$WGETTER"

echo ""

echo "*****************"

eval "$WGETTER"

echo ""

echo "Starting over$Saver."

echo "If you want to stop, press q."

echo "Otherwise, enter some URL's:"

# Let them go again. Tell about save option being set.

read

case $REPLY in

# Need to change this to a 'trap' clause.

q|Q ) exit $E_USER_EXIT;; # Exercise for the reader?

* ) URLS=" $REPLY";;

esac

echo ""

done

exit 0

Example A-31. A podcasting script

!/bin/bash

bashpodder.sh:

By Linc 10/1/2004

Find the latest script at

+ http://linc.homeunix.org:8080/scripts/bashpodder

Last revision 12/14/2004 - Many Contributors!

If you use this and have made improvements or have comments

+ drop me an email at linc dot fessenden at gmail dot com

I'd appreciate it!

==> ABS Guide extra comments.

==> Author of this script has kindly granted permission

==>+ for inclusion in ABS Guide.

==> ################################################################

==> What is "podcasting"?

==> It's broadcasting "radio shows" over the Internet.

==> These shows can be played on iPods and other music file players.

==> This script makes it possible.

==> See documentation at the script author's site, above.

==> ################################################################

Make script crontab friendly:

cd $(dirname $0)

==> Change to directory where this script lives.

datadir is the directory you want podcasts saved to:

datadir=$(date +%Y-%m-%d)

==> Will create a date-labeled directory, named: YYYY-MM-DD

Check for and create datadir if necessary:

if test ! -d $datadir

then

mkdir $datadir

Delete any temp file:

rm -f temp.log

Read the bp.conf file and wget any url not already

+ in the podcast.log file:

while read podcast

do # ==> Main action follows.

file=$(wget -q $podcast -O - | tr '\r' '\n' | tr \' \" | \

sed -n 's/.*url="$[^"]*$".*/\1/p')

for url in $file

echo $url >> temp.log

if ! grep "$url" podcast.log > /dev/null

then

wget -q -P $datadir "$url"

done

done < bp.conf

Move dynamically created log file to permanent log file:

cat podcast.log >> temp.log

sort temp.log | uniq > podcast.log

rm temp.log

Create an m3u playlist:

ls $datadir | grep -v m3u > $datadir/podcast.m3u

exit 0

For a different scripting approach to Podcasting,

see Phil Salkie's article,

"Internet Radio to Podcast with Shell Tools"

in the September, 2005 issue of LINUX JOURNAL,

http://www.linuxjournal.com/article/8171

Example A-32. Nightly backup to a firewire HD

!/bin/bash

nightly-backup.sh

http://www.richardneill.org/source.php#nightly-backup-rsync

This is Free Software licensed under the GNU GPL.

==> Included in ABS Guide with script author's kind permission.

==> (Thanks!)

This does a backup from the host computer to a locally connected

+ firewire HDD using rsync and ssh.

(Script should work with USB-connected device (see lines 40-43).

It then rotates the backups.

Run it via cron every night at 5am.

This only backs up the home directory.

If ownerships (other than the user's) should be preserved,

+ then run the rsync process as root (and re-instate the -o).

We save every day for 7 days, then every week for 4 weeks,

+ then every month for 3 months.

See: http://www.mikerubel.org/computers/rsync_snapshots/

+ for more explanation of the theory.

Save as: $HOME/bin/nightly-backup_firewire-hdd.sh

Known bugs:

----------

i) Ideally, we want to exclude ~/.tmp and the browser caches.

ii) If the user is sitting at the computer at 5am,

+ and files are modified while the rsync is occurring,

+ then the BACKUP_JUSTINCASE branch gets triggered.

To some extent, this is a

+ feature, but it also causes a "disk-space leak".

BEGIN CONFIGURATION SECTION ############################################

LOCAL_USER=rjn # User whose home directory should be backed up.

MOUNT_POINT=/backup # Mountpoint of backup drive.

# NO trailing slash!

# This must be unique (eg using a udev symlink)

MOUNT_POINT=/media/disk # For USB-connected device.

SOURCE_DIR=/home/$LOCAL_USER # NO trailing slash - it DOES matter to rsync.

BACKUP_DEST_DIR=$MOUNT_POINT/backup/`hostname -s`.${LOCAL_USER}.nightly_backup

DRY_RUN=false #If true, invoke rsync with -n, to do a dry run.

# Comment out or set to false for normal use.

VERBOSE=false # If true, make rsync verbose.

# Comment out or set to false otherwise.

COMPRESS=false # If true, compress.

# Good for internet, bad on LAN.

# Comment out or set to false otherwise.

Exit Codes ###

E_VARS_NOT_SET=64

E_COMMANDLINE=65

E_MOUNT_FAIL=70

E_NOSOURCEDIR=71

E_UNMOUNTED=72

E_BACKUP=73

END CONFIGURATION SECTION ##############################################

Check that all the important variables have been set:

if [ -z "$LOCAL_USER" ] ||

[ -z "$SOURCE_DIR" ] ||

[ -z "$MOUNT_POINT" ] ||

[ -z "$BACKUP_DEST_DIR" ]

then

echo 'One of the variables is not set! Edit the file: $0. BACKUP FAILED.'

exit $E_VARS_NOT_SET

if [ "$#" != 0 ] # If command-line param(s) . . .

then # Here document(ation).

cat <<-ENDOFTEXT

Automatic Nightly backup run from cron.

Read the source for more details: $0

The backup directory is $BACKUP_DEST_DIR .

It will be created if necessary; initialisation is no longer required.

WARNING: Contents of $BACKUP_DEST_DIR are rotated.

Directories named 'backup.\$i' will eventually be DELETED.

We keep backups from every day for 7 days (1-8),

then every week for 4 weeks (9-12),

then every month for 3 months (13-15).

You may wish to add this to your crontab using 'crontab -e'

# Back up files: $SOURCE_DIR to $BACKUP_DEST_DIR

#+ every night at 3:15 am

15 03 * * * /home/$LOCAL_USER/bin/nightly-backup_firewire-hdd.sh

Don't forget to verify the backups are working,

especially if you don't read cron's mail!"

ENDOFTEXT

exit $E_COMMANDLINE

Parse the options.

==================

if [ "$DRY_RUN" == "true" ]; then

DRY_RUN="-n"

echo "WARNING:"

echo "THIS IS A 'DRY RUN'!"

echo "No data will actually be transferred!"

else

DRY_RUN=""

if [ "$VERBOSE" == "true" ]; then

VERBOSE="-v"

else

VERBOSE=""

if [ "$COMPRESS" == "true" ]; then

COMPRESS="-z"

else

COMPRESS=""

Every week (actually of 8 days) and every month,

+ extra backups are preserved.

DAY_OF_MONTH=`date +%d` # Day of month (01..31).

if [ $DAY_OF_MONTH = 01 ]; then # First of month.

MONTHSTART=true

elif [ $DAY_OF_MONTH = 08 \

-o $DAY_OF_MONTH = 16 \

-o $DAY_OF_MONTH = 24 ]; then

# Day 8,16,24 (use 8, not 7 to better handle 31-day months)

WEEKSTART=true

Check that the HDD is mounted.

At least, check that something is mounted here!

We can use something unique to the device, rather than just guessing

+ the scsi-id by having an appropriate udev rule in

+ /etc/udev/rules.d/10-rules.local

+ and by putting a relevant entry in /etc/fstab.

Eg: this udev rule:

BUS="scsi", KERNEL="sd*", SYSFS{vendor}="WDC WD16",

SYSFS{model}="00JB-00GVA0 ", NAME="%k", SYMLINK="lacie_1394d%n"

if mount | grep $MOUNT_POINT >/dev/null; then

echo "Mount point $MOUNT_POINT is indeed mounted. OK"

else

echo -n "Attempting to mount $MOUNT_POINT..."

# If it isn't mounted, try to mount it.

sudo mount $MOUNT_POINT 2>/dev/null

if mount | grep $MOUNT_POINT >/dev/null; then

UNMOUNT_LATER=TRUE

echo "OK"

# Note: Ensure that this is also unmounted

#+ if we exit prematurely with failure.

else

echo "FAILED"

echo -e "Nothing is mounted at $MOUNT_POINT. BACKUP FAILED!"

exit $E_MOUNT_FAIL

Check that source dir exists and is readable.

if [ ! -r $SOURCE_DIR ] ; then

echo "$SOURCE_DIR does not exist, or cannot be read. BACKUP FAILED."

exit $E_NOSOURCEDIR

Check that the backup directory structure is as it should be.

If not, create it.

Create the subdirectories.

Note that backup.0 will be created as needed by rsync.

for ((i=1;i<=15;i++)); do

if [ ! -d $BACKUP_DEST_DIR/backup.$i ]; then

if /bin/mkdir -p $BACKUP_DEST_DIR/backup.$i ; then

# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ No [ ] test brackets. Why?

echo "Warning: directory $BACKUP_DEST_DIR/backup.$i is missing,"

echo "or was not initialised. (Re-)creating it."

else

echo "ERROR: directory $BACKUP_DEST_DIR/backup.$i"

echo "is missing and could not be created."

if [ "$UNMOUNT_LATER" == "TRUE" ]; then

# Before we exit, unmount the mount point if necessary.

sudo umount $MOUNT_POINT &&

echo "Unmounted $MOUNT_POINT again. Giving up."

exit $E_UNMOUNTED

done

Set the permission to 700 for security

+ on an otherwise permissive multi-user system.

if ! /bin/chmod 700 $BACKUP_DEST_DIR ; then

echo "ERROR: Could not set permissions on $BACKUP_DEST_DIR to 700."

if [ "$UNMOUNT_LATER" == "TRUE" ]; then

# Before we exit, unmount the mount point if necessary.

cd ; sudo umount $MOUNT_POINT \

&& echo "Unmounted $MOUNT_POINT again. Giving up."

exit $E_UNMOUNTED

Create the symlink: current -> backup.1 if required.

A failure here is not critical.

cd $BACKUP_DEST_DIR

if [ ! -h current ] ; then

if ! /bin/ln -s backup.1 current ; then

echo "WARNING: could not create symlink current -> backup.1"

Now, do the rsync.

echo "Now doing backup with rsync..."

echo "Source dir: $SOURCE_DIR"

echo -e "Backup destination dir: $BACKUP_DEST_DIR\n"

/usr/bin/rsync $DRY_RUN $VERBOSE -a -S --delete --modify-window=60 \

--link-dest=../backup.1 $SOURCE_DIR $BACKUP_DEST_DIR/backup.0/

Only warn, rather than exit if the rsync failed,

+ since it may only be a minor problem.

E.g., if one file is not readable, rsync will fail.

This shouldn't prevent the rotation.

Not using, e.g., `date +%a` since these directories

+ are just full of links and don't consume that much space.

if [ $? != 0 ]; then

BACKUP_JUSTINCASE=backup.`date +%F_%T`.justincase

echo "WARNING: the rsync process did not entirely succeed."

echo "Something might be wrong."

echo "Saving an extra copy at: $BACKUP_JUSTINCASE"

echo "WARNING: if this occurs regularly, a LOT of space will be consumed,"

echo "even though these are just hard-links!"

Save a readme in the backup parent directory.

Save another one in the recent subdirectory.

echo "Backup of $SOURCE_DIR on `hostname` was last run on \

`date`" > $BACKUP_DEST_DIR/README.txt

echo "This backup of $SOURCE_DIR on `hostname` was created on \

`date`" > $BACKUP_DEST_DIR/backup.0/README.txt

If we are not in a dry run, rotate the backups.

[ -z "$DRY_RUN" ] &&

# Check how full the backup disk is.

# Warn if 90%. if 98% or more, we'll probably fail, so give up.

# (Note: df can output to more than one line.)

# We test this here, rather than before

#+ so that rsync may possibly have a chance.

DISK_FULL_PERCENT=`/bin/df $BACKUP_DEST_DIR |

tr "\n" ' ' | awk '{print $12}' | grep -oE [0-9]+ `

echo "Disk space check on backup partition \

$MOUNT_POINT $DISK_FULL_PERCENT% full."

if [ $DISK_FULL_PERCENT -gt 90 ]; then

echo "Warning: Disk is greater than 90% full."

if [ $DISK_FULL_PERCENT -gt 98 ]; then

echo "Error: Disk is full! Giving up."

if [ "$UNMOUNT_LATER" == "TRUE" ]; then

# Before we exit, unmount the mount point if necessary.

cd; sudo umount $MOUNT_POINT &&

echo "Unmounted $MOUNT_POINT again. Giving up."

exit $E_UNMOUNTED

# Create an extra backup.

# If this copy fails, give up.

if [ -n "$BACKUP_JUSTINCASE" ]; then

if ! /bin/cp -al $BACKUP_DEST_DIR/backup.0 \

$BACKUP_DEST_DIR/$BACKUP_JUSTINCASE

then

echo "ERROR: Failed to create extra copy \

$BACKUP_DEST_DIR/$BACKUP_JUSTINCASE"

if [ "$UNMOUNT_LATER" == "TRUE" ]; then

# Before we exit, unmount the mount point if necessary.

cd ;sudo umount $MOUNT_POINT &&

echo "Unmounted $MOUNT_POINT again. Giving up."

exit $E_UNMOUNTED

# At start of month, rotate the oldest 8.

if [ "$MONTHSTART" == "true" ]; then

echo -e "\nStart of month. \

Removing oldest backup: $BACKUP_DEST_DIR/backup.15" &&

/bin/rm -rf $BACKUP_DEST_DIR/backup.15 &&

echo "Rotating monthly,weekly backups: \

$BACKUP_DEST_DIR/backup.[8-14] -> $BACKUP_DEST_DIR/backup.[9-15]" &&

/bin/mv $BACKUP_DEST_DIR/backup.14 $BACKUP_DEST_DIR/backup.15 &&

/bin/mv $BACKUP_DEST_DIR/backup.13 $BACKUP_DEST_DIR/backup.14 &&

/bin/mv $BACKUP_DEST_DIR/backup.12 $BACKUP_DEST_DIR/backup.13 &&

/bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12 &&

/bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11 &&

/bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10 &&

/bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9

# At start of week, rotate the second-oldest 4.

elif [ "$WEEKSTART" == "true" ]; then

echo -e "\nStart of week. \

Removing oldest weekly backup: $BACKUP_DEST_DIR/backup.12" &&

/bin/rm -rf $BACKUP_DEST_DIR/backup.12 &&

echo "Rotating weekly backups: \

$BACKUP_DEST_DIR/backup.[8-11] -> $BACKUP_DEST_DIR/backup.[9-12]" &&

/bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12 &&

/bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11 &&

/bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10 &&

/bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9

else

echo -e "\nRemoving oldest daily backup: $BACKUP_DEST_DIR/backup.8" &&

/bin/rm -rf $BACKUP_DEST_DIR/backup.8

fi &&

# Every day, rotate the newest 8.

echo "Rotating daily backups: \

$BACKUP_DEST_DIR/backup.[1-7] -> $BACKUP_DEST_DIR/backup.[2-8]" &&

/bin/mv $BACKUP_DEST_DIR/backup.7 $BACKUP_DEST_DIR/backup.8 &&

/bin/mv $BACKUP_DEST_DIR/backup.6 $BACKUP_DEST_DIR/backup.7 &&

/bin/mv $BACKUP_DEST_DIR/backup.5 $BACKUP_DEST_DIR/backup.6 &&

/bin/mv $BACKUP_DEST_DIR/backup.4 $BACKUP_DEST_DIR/backup.5 &&

/bin/mv $BACKUP_DEST_DIR/backup.3 $BACKUP_DEST_DIR/backup.4 &&

/bin/mv $BACKUP_DEST_DIR/backup.2 $BACKUP_DEST_DIR/backup.3 &&

/bin/mv $BACKUP_DEST_DIR/backup.1 $BACKUP_DEST_DIR/backup.2 &&

/bin/mv $BACKUP_DEST_DIR/backup.0 $BACKUP_DEST_DIR/backup.1 &&

SUCCESS=true

if [ "$UNMOUNT_LATER" == "TRUE" ]; then

# Unmount the mount point if it wasn't mounted to begin with.

cd ; sudo umount $MOUNT_POINT && echo "Unmounted $MOUNT_POINT again."

if [ "$SUCCESS" == "true" ]; then

echo 'SUCCESS!'

exit 0

Should have already exited if backup worked.

echo 'BACKUP FAILED! Is this just a dry run? Is the disk full?) '

exit $E_BACKUP

Example A-33. An expanded cd command

cdll

by Phil Braham

############################################

Latest version of this script available from

http://freshmeat.net/projects/cd/

############################################

.cd_new

An enhancement of the Unix cd command

There are unlimited stack entries and special entries. The stack

entries keep the last cd_maxhistory

directories that have been used. The special entries can be

assigned to commonly used directories.

The special entries may be pre-assigned by setting the environment

variables CDSn or by using the -u or -U command.

The following is a suggestion for the .profile file:

. cdll # Set up the cd command

alias cd='cd_new' # Replace the cd command

cd -U # Upload pre-assigned entries for

#+ the stack and special entries

cd -D # Set non-default mode

alias @="cd_new @" # Allow @ to be used to get history

For help type:

cd -h or

cd -H

Version 1.2.1

Written by Phil Braham - Realtime Software Pty Ltd

(realtime@mpx.com.au)

Please send any suggestions or enhancements to the author (also at

phil@braham.net)

cd_hm ()

{

${PRINTF} "%s" "cd [dir] [0-9] [@[s|h] [-g [<dir>]] [-d] \

[-D] [-r<n>] [dir|0-9] [-R<n>] [<dir>|0-9]

[-s<n>] [-S<n>] [-u] [-U] [-f] [-F] [-h] [-H] [-v]

<dir> Go to directory

0-n Go to previous directory (0 is previous, 1 is last but 1 etc)

n is up to max history (default is 50)

@ List history and special entries

@h List history entries

@s List special entries

-g [<dir>] Go to literal name (bypass special names)

This is to allow access to dirs called '0','1','-h' etc

-d Change default action - verbose. (See note)

-D Change default action - silent. (See note)

-s<n> Go to the special entry <n>*

-S<n> Go to the special entry <n>

and replace it with the current dir*

-r<n> [<dir>] Go to directory <dir>

and then put it on special entry <n>*

-R<n> [<dir>] Go to directory <dir>

and put current dir on special entry <n>*

-a<n> Alternative suggested directory. See note below.

-f [<file>] File entries to <file>.

-u [<file>] Update entries from <file>.

If no filename supplied then default file

(${CDPath}${2:-"$CDFile"}) is used

-F and -U are silent versions

-v Print version number

-h Help

-H Detailed help

*The special entries (0 - 9) are held until log off, replaced by another

entry or updated with the -u command

Alternative suggested directories:

If a directory is not found then CD will suggest any

possibilities. These are directories starting with the same letters

and if any are found they are listed prefixed with -a<n>

where <n> is a number.

It's possible to go to the directory by entering cd -a<n>

on the command line.

The directory for -r<n> or -R<n> may be a number.

For example:

$ cd -r3 4 Go to history entry 4 and put it on special entry 3

$ cd -R3 4 Put current dir on the special entry 3

and go to history entry 4

$ cd -s3 Go to special entry 3

Note that commands R,r,S and s may be used without a number

and refer to 0:

$ cd -s Go to special entry 0

$ cd -S Go to special entry 0 and make special

entry 0 current dir

$ cd -r 1 Go to history entry 1 and put it on special entry 0

$ cd -r Go to history entry 0 and put it on special entry 0

if ${TEST} "$CD_MODE" = "PREV"

then

${PRINTF} "$cd_mnset"

else

${PRINTF} "$cd_mset"

}

cd_Hm ()

{

cd_hm

${PRINTF} "%s" "

The previous directories (0-$cd_maxhistory) are stored in the

environment variables CD[0] - CD[$cd_maxhistory]

Similarly the special directories S0 - $cd_maxspecial are in

the environment variable CDS[0] - CDS[$cd_maxspecial]

and may be accessed from the command line

The default pathname for the -f and -u commands is $CDPath

The default filename for the -f and -u commands is $CDFile

Set the following environment variables:

CDL_PROMPTLEN - Set to the length of prompt you require.

Prompt string is set to the right characters of the

current directory.

If not set then prompt is left unchanged

CDL_PROMPT_PRE - Set to the string to prefix the prompt.

Default is:

non-root: \"\\[\\e[01;34m\\]\" (sets colour to blue).

root: \"\\[\\e[01;31m\\]\" (sets colour to red).

CDL_PROMPT_POST - Set to the string to suffix the prompt.

Default is:

non-root: \"\\[\\e[00m\\]$\"

(resets colour and displays $).

root: \"\\[\\e[00m\\]#\"

(resets colour and displays #).

CDPath - Set the default path for the -f & -u options.

Default is home directory

CDFile - Set the default filename for the -f & -u options.

Default is cdfile

cd_version

}

cd_version ()

{

printf "Version: ${VERSION_MAJOR}.${VERSION_MINOR} Date: ${VERSION_DATE}\n"

}

Truncate right.

params:

p1 - string

p2 - length to truncate to

returns string in tcd

cd_right_trunc ()

{

local tlen=${2}

local plen=${#1}

local str="${1}"

local diff

local filler="<--"

if ${TEST} ${plen} -le ${tlen}

then

tcd="${str}"

else

let diff=${plen}-${tlen}

elen=3

if ${TEST} ${diff} -le 2

then

let elen=${diff}

tlen=-${tlen}

let tlen=${tlen}+${elen}

tcd=${filler:0:elen}${str:tlen}

}

Three versions of do history:

cd_dohistory - packs history and specials side by side

cd_dohistoryH - Shows only hstory

cd_dohistoryS - Shows only specials

cd_dohistory ()

{

cd_getrc

${PRINTF} "History:\n"

local -i count=${cd_histcount}

while ${TEST} ${count} -ge 0

cd_right_trunc "${CD[count]}" ${cd_lchar}

${PRINTF} "%2d %-${cd_lchar}.${cd_lchar}s " ${count} "${tcd}"

cd_right_trunc "${CDS[count]}" ${cd_rchar}

${PRINTF} "S%d %-${cd_rchar}.${cd_rchar}s\n" ${count} "${tcd}"

count=${count}-1

done

}

cd_dohistoryH ()

{

cd_getrc

${PRINTF} "History:\n"

local -i count=${cd_maxhistory}

while ${TEST} ${count} -ge 0

${PRINTF} "${count} %-${cd_flchar}.${cd_flchar}s\n" ${CD[$count]}

count=${count}-1

done

}

cd_dohistoryS ()

{

cd_getrc

${PRINTF} "Specials:\n"

local -i count=${cd_maxspecial}

while ${TEST} ${count} -ge 0

${PRINTF} "S${count} %-${cd_flchar}.${cd_flchar}s\n" ${CDS[$count]}

count=${count}-1

done

}

cd_getrc ()

{

cd_flchar=$(stty -a | awk -F \;

'/rows/ { print $2 $3 }' | awk -F \ '{ print $4 }')

if ${TEST} ${cd_flchar} -ne 0

then

cd_lchar=${cd_flchar}/2-5

cd_rchar=${cd_flchar}/2-5

cd_flchar=${cd_flchar}-5

else

cd_flchar=${FLCHAR:=75}

# cd_flchar is used for for the @s & @h history

cd_lchar=${LCHAR:=35}

cd_rchar=${RCHAR:=35}

}

cd_doselection ()

{

local -i nm=0

cd_doflag="TRUE"

if ${TEST} "${CD_MODE}" = "PREV"

then

if ${TEST} -z "$cd_npwd"

then

cd_npwd=0

tm=$(echo "${cd_npwd}" | cut -b 1)

if ${TEST} "${tm}" = "-"

then

pm=$(echo "${cd_npwd}" | cut -b 2)

nm=$(echo "${cd_npwd}" | cut -d $pm -f2)

case "${pm}" in

a) cd_npwd=${cd_sugg[$nm]} ;;

s) cd_npwd="${CDS[$nm]}" ;;

S) cd_npwd="${CDS[$nm]}" ; CDS[$nm]=`pwd` ;;

r) cd_npwd="$2" ; cd_specDir=$nm ; cd_doselection "$1" "$2";;

R) cd_npwd="$2" ; CDS[$nm]=`pwd` ; cd_doselection "$1" "$2";;

esac

if ${TEST} "${cd_npwd}" != "." -a "${cd_npwd}" \

!= ".." -a "${cd_npwd}" -le ${cd_maxhistory} >>/dev/null 2>&1

then

cd_npwd=${CD[$cd_npwd]}

else

case "$cd_npwd" in

@) cd_dohistory ; cd_doflag="FALSE" ;;

@h) cd_dohistoryH ; cd_doflag="FALSE" ;;

@s) cd_dohistoryS ; cd_doflag="FALSE" ;;

-h) cd_hm ; cd_doflag="FALSE" ;;

-H) cd_Hm ; cd_doflag="FALSE" ;;

-f) cd_fsave "SHOW" $2 ; cd_doflag="FALSE" ;;

-u) cd_upload "SHOW" $2 ; cd_doflag="FALSE" ;;

-F) cd_fsave "NOSHOW" $2 ; cd_doflag="FALSE" ;;

-U) cd_upload "NOSHOW" $2 ; cd_doflag="FALSE" ;;

-g) cd_npwd="$2" ;;

-d) cd_chdefm 1; cd_doflag="FALSE" ;;

-D) cd_chdefm 0; cd_doflag="FALSE" ;;

-r) cd_npwd="$2" ; cd_specDir=0 ; cd_doselection "$1" "$2";;

-R) cd_npwd="$2" ; CDS[0]=`pwd` ; cd_doselection "$1" "$2";;

-s) cd_npwd="${CDS[0]}" ;;

-S) cd_npwd="${CDS[0]}" ; CDS[0]=`pwd` ;;

-v) cd_version ; cd_doflag="FALSE";;

esac

}

cd_chdefm ()

{

if ${TEST} "${CD_MODE}" = "PREV"

then

CD_MODE=""

if ${TEST} $1 -eq 1

then

${PRINTF} "${cd_mset}"

else

CD_MODE="PREV"

if ${TEST} $1 -eq 1

then

${PRINTF} "${cd_mnset}"

}

cd_fsave ()

{

local sfile=${CDPath}${2:-"$CDFile"}

if ${TEST} "$1" = "SHOW"

then

${PRINTF} "Saved to %s\n" $sfile

${RM} -f ${sfile}

local -i count=0

while ${TEST} ${count} -le ${cd_maxhistory}

echo "CD[$count]=\"${CD[$count]}\"" >> ${sfile}

count=${count}+1

done

count=0

while ${TEST} ${count} -le ${cd_maxspecial}

echo "CDS[$count]=\"${CDS[$count]}\"" >> ${sfile}

count=${count}+1

done

}

cd_upload ()

{

local sfile=${CDPath}${2:-"$CDFile"}

if ${TEST} "${1}" = "SHOW"

then

${PRINTF} "Loading from %s\n" ${sfile}

. ${sfile}

}

cd_new ()

{

local -i count

local -i choose=0

cd_npwd="${1}"

cd_specDir=-1

cd_doselection "${1}" "${2}"

if ${TEST} ${cd_doflag} = "TRUE"

then

if ${TEST} "${CD[0]}" != "`pwd`"

then

count=$cd_maxhistory

while ${TEST} $count -gt 0

CD[$count]=${CD[$count-1]}

count=${count}-1

done

CD[0]=`pwd`

command cd "${cd_npwd}" 2>/dev/null

if ${TEST} $? -eq 1

then

${PRINTF} "Unknown dir: %s\n" "${cd_npwd}"

local -i ftflag=0

for i in "${cd_npwd}"*

if ${TEST} -d "${i}"

then

if ${TEST} ${ftflag} -eq 0

then

${PRINTF} "Suggest:\n"

ftflag=1

${PRINTF} "\t-a${choose} %s\n" "$i"

cd_sugg[$choose]="${i}"

choose=${choose}+1

done

if ${TEST} ${cd_specDir} -ne -1

then

CDS[${cd_specDir}]=`pwd`

if ${TEST} ! -z "${CDL_PROMPTLEN}"

then

cd_right_trunc "${PWD}" ${CDL_PROMPTLEN}

cd_rp=${CDL_PROMPT_PRE}${tcd}${CDL_PROMPT_POST}

export PS1="$(echo -ne ${cd_rp})"

}

#

Initialisation here #

#

VERSION_MAJOR="1"

VERSION_MINOR="2.1"

VERSION_DATE="24-MAY-2003"

alias cd=cd_new

Set up commands

RM=/bin/rm

TEST=test

PRINTF=printf # Use builtin printf

#

Change this to modify the default pre- and post prompt strings. #

These only come into effect if CDL_PROMPTLEN is set. #

#

if ${TEST} ${EUID} -eq 0

then

CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="$HOSTNAME@"}

CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;31m\\]"} # Root is in red

CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]#"}

else

CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;34m\\]"} # Users in blue

CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]$"}

cd_maxhistory defines the max number of history entries allowed.

typeset -i cd_maxhistory=50

cd_maxspecial defines the number of special entries.

typeset -i cd_maxspecial=9

cd_histcount defines the number of entries displayed in

+ the history command.

typeset -i cd_histcount=9

export CDPath=${HOME}/

Change these to use a different #

+ default path and filename #

export CDFile=${CDFILE:=cdfile} # for the -u and -f commands #

typeset -i cd_lchar cd_rchar cd_flchar

# This is the number of chars to allow for the #

cd_flchar=${FLCHAR:=75} #+ cd_flchar is used for for the @s & @h history#

typeset -ax CD CDS

cd_mset="\n\tDefault mode is now set - entering cd with no parameters \

has the default action\n\tUse cd -d or -D for cd to go to \

previous directory with no parameters\n"

cd_mnset="\n\tNon-default mode is now set - entering cd with no \

parameters is the same as entering cd 0\n\tUse cd -d or \

-D to change default cd action\n"

==================================================================== #

: <<DOCUMENTATION

Written by Phil Braham. Realtime Software Pty Ltd.

Released under GNU license. Free to use. Please pass any modifications

or comments to the author Phil Braham:

realtime@mpx.com.au

=======================================================================

cdll is a replacement for cd and incorporates similar functionality to

the bash pushd and popd commands but is independent of them.

This version of cdll has been tested on Linux using Bash. It will work

on most Linux versions but will probably not work on other shells without

modification.

Introduction

============

cdll allows easy moving about between directories. When changing to a new

directory the current one is automatically put onto a stack. By default

50 entries are kept, but this is configurable. Special directories can be

kept for easy access - by default up to 10, but this is configurable. The

most recent stack entries and the special entries can be easily viewed.

The directory stack and special entries can be saved to, and loaded from,

a file. This allows them to be set up on login, saved before logging out

or changed when moving project to project.

In addition, cdll provides a flexible command prompt facility that allows,

for example, a directory name in colour that is truncated from the left

if it gets too long.

Setting up cdll

===============

Copy cdll to either your local home directory or a central directory

such as /usr/bin (this will require root access).

Copy the file cdfile to your home directory. It will require read and

write access. This a default file that contains a directory stack and

special entries.

To replace the cd command you must add commands to your login script.

The login script is one or more of:

/etc/profile

~/.bash_profile

~/.bash_login

~/.profile

~/.bashrc

/etc/bash.bashrc.local

To setup your login, ~/.bashrc is recommended, for global (and root) setup

add the commands to /etc/bash.bashrc.local

To set up on login, add the command:

. <dir>/cdll

For example if cdll is in your local home directory:

. ~/cdll

If in /usr/bin then:

. /usr/bin/cdll

If you want to use this instead of the buitin cd command then add:

alias cd='cd_new'

We would also recommend the following commands:

alias @='cd_new @'

cd -U

cd -D

If you want to use cdll's prompt facilty then add the following:

CDL_PROMPTLEN=nn

Where nn is a number described below. Initially 99 would be suitable

number.

Thus the script looks something like this:

######################################################################

# CD Setup

######################################################################

CDL_PROMPTLEN=21 # Allow a prompt length of up to 21 characters

. /usr/bin/cdll # Initialise cdll

alias cd='cd_new' # Replace the built in cd command

alias @='cd_new @' # Allow @ at the prompt to display history

cd -U # Upload directories

cd -D # Set default action to non-posix

######################################################################

The full meaning of these commands will become clear later.

There are a couple of caveats. If another program changes the directory

without calling cdll, then the directory won't be put on the stack and

also if the prompt facility is used then this will not be updated. Two

programs that can do this are pushd and popd. To update the prompt and

stack simply enter:

cd .

Note that if the previous entry on the stack is the current directory

then the stack is not updated.

Usage

=====

cd [dir] [0-9] [@[s|h] [-g <dir>] [-d] [-D] [-r<n>]

[dir|0-9] [-R<n>] [<dir>|0-9] [-s<n>] [-S<n>]

[-u] [-U] [-f] [-F] [-h] [-H] [-v]

<dir> Go to directory

0-n Goto previous directory (0 is previous,

1 is last but 1, etc.)

n is up to max history (default is 50)

@ List history and special entries (Usually available as $ @)

@h List history entries

@s List special entries

-g [<dir>] Go to literal name (bypass special names)

This is to allow access to dirs called '0','1','-h' etc

-d Change default action - verbose. (See note)

-D Change default action - silent. (See note)

-s<n> Go to the special entry <n>

-S<n> Go to the special entry <n>

and replace it with the current dir

-r<n> [<dir>] Go to directory <dir>

and then put it on special entry <n>

-R<n> [<dir>] Go to directory <dir>

and put current dir on special entry <n>

-a<n> Alternative suggested directory. See note below.

-f [<file>] File entries to <file>.

-u [<file>] Update entries from <file>.

If no filename supplied then default file (~/cdfile) is used

-F and -U are silent versions

-v Print version number

-h Help

-H Detailed help

Examples

========

These examples assume non-default mode is set (that is, cd with no

parameters will go to the most recent stack directory), that aliases

have been set up for cd and @ as described above and that cd's prompt

facility is active and the prompt length is 21 characters.

/home/phil$ @

# List the entries with the @

History:

# Output of the @ command

.....

# Skipped these entries for brevity

1 /home/phil/ummdev S1 /home/phil/perl

# Most recent two history entries

0 /home/phil/perl/eg S0 /home/phil/umm/ummdev

# and two special entries are shown

/home/phil$ cd /home/phil/utils/Cdll

# Now change directories

/home/phil/utils/Cdll$ @

# Prompt reflects the directory.

History:

# New history

.....

1 /home/phil/perl/eg S1 /home/phil/perl

# History entry 0 has moved to 1

0 /home/phil S0 /home/phil/umm/ummdev

# and the most recent has entered

To go to a history entry:

/home/phil/utils/Cdll$ cd 1

# Go to history entry 1.

/home/phil/perl/eg$

# Current directory is now what was 1

To go to a special entry:

/home/phil/perl/eg$ cd -s1

# Go to special entry 1

/home/phil/umm/ummdev$

# Current directory is S1

To go to a directory called, for example, 1:

/home/phil$ cd -g 1

# -g ignores the special meaning of 1

/home/phil/1$

To put current directory on the special list as S1:

cd -r1 . # OR

cd -R1 . # These have the same effect if the directory is

#+ . (the current directory)

To go to a directory and add it as a special

The directory for -r<n> or -R<n> may be a number.

For example:

$ cd -r3 4 Go to history entry 4 and put it on special entry 3

$ cd -R3 4 Put current dir on the special entry 3 and go to

history entry 4

$ cd -s3 Go to special entry 3

Note that commands R,r,S and s may be used without a number and

refer to 0:

$ cd -s Go to special entry 0

$ cd -S Go to special entry 0 and make special entry 0

current dir

$ cd -r 1 Go to history entry 1 and put it on special entry 0

$ cd -r Go to history entry 0 and put it on special entry 0

Alternative suggested directories:

If a directory is not found, then CD will suggest any

possibilities. These are directories starting with the same letters

and if any are found they are listed prefixed with -a<n>

where <n> is a number. It's possible to go to the directory

by entering cd -a<n> on the command line.

Use cd -d or -D to change default cd action. cd -H will show

current action.

The history entries (0-n) are stored in the environment variables

CD[0] - CD[n]

Similarly the special directories S0 - 9 are in the environment

variable CDS[0] - CDS[9]

and may be accessed from the command line, for example:

ls -l ${CDS[3]}

cat ${CD[8]}/file.txt

The default pathname for the -f and -u commands is ~

The default filename for the -f and -u commands is cdfile

Configuration

=============

The following environment variables can be set:

CDL_PROMPTLEN - Set to the length of prompt you require.

Prompt string is set to the right characters of the current

directory. If not set, then prompt is left unchanged. Note

that this is the number of characters that the directory is

shortened to, not the total characters in the prompt.

CDL_PROMPT_PRE - Set to the string to prefix the prompt.

Default is:

non-root: "\\[\\e[01;34m\\]" (sets colour to blue).

root: "\\[\\e[01;31m\\]" (sets colour to red).

CDL_PROMPT_POST - Set to the string to suffix the prompt.

Default is:

non-root: "\\[\\e[00m\\]$"

(resets colour and displays $).

root: "\\[\\e[00m\\]#"

(resets colour and displays #).

Note:

CDL_PROMPT_PRE & _POST only t

CDPath - Set the default path for the -f & -u options.

Default is home directory

CDFile - Set the default filename for the -f & -u options.

Default is cdfile

There are three variables defined in the file cdll which control the

number of entries stored or displayed. They are in the section labeled

'Initialisation here' towards the end of the file.

cd_maxhistory - The number of history entries stored.

Default is 50.

cd_maxspecial - The number of special entries allowed.

Default is 9.

cd_histcount - The number of history and special entries

displayed. Default is 9.

Note that cd_maxspecial should be >= cd_histcount to avoid displaying

special entries that can't be set.

Version: 1.2.1 Date: 24-MAY-2003

DOCUMENTATION

Example A-34. A soundcard setup script

!/bin/bash

soundcard-on.sh

Script author: Mkarcher

http://www.thinkwiki.org/wiki ...

/Script_for_configuring_the_CS4239_sound_chip_in_PnP_mode

ABS Guide author made minor changes and added comments.

Couldn't contact script author to ask for permission to use, but ...

+ the script was released under the FDL,

+ so its use here should be both legal and ethical.

Sound-via-pnp-script for Thinkpad 600E

+ and possibly other computers with onboard CS4239/CS4610

+ that do not work with the PCI driver

+ and are not recognized by the PnP code of snd-cs4236.

Also for some 770-series Thinkpads, such as the 770x.

Run as root user, of course.

These are old and very obsolete laptop computers,

+ but this particular script is very instructive,

+ as it shows how to set up and hack device files.

Search for sound card pnp device:

for dev in /sys/bus/pnp/devices/*

grep CSC0100 $dev/id > /dev/null && WSSDEV=$dev

grep CSC0110 $dev/id > /dev/null && CTLDEV=$dev

done

On 770x:

WSSDEV = /sys/bus/pnp/devices/00:07

CTLDEV = /sys/bus/pnp/devices/00:06

These are symbolic links to /sys/devices/pnp0/ ...

Activate devices:

Thinkpad boots with devices disabled unless "fast boot" is turned off

+ (in BIOS).

echo activate > $WSSDEV/resources

echo activate > $CTLDEV/resources

Parse resource settings.

{ read # Discard "state = active" (see below).

read bla port1

read bla port2

read bla port3

read bla irq

read bla dma1

read bla dma2

# The "bla's" are labels in the first field: "io," "state," etc.

# These are discarded.

# Hack: with PnPBIOS: ports are: port1: WSS, port2:

#+ OPL, port3: sb (unneeded)

# with ACPI-PnP:ports are: port1: OPL, port2: sb, port3: WSS

# (ACPI bios seems to be wrong here, the PnP-card-code in snd-cs4236.c

#+ uses the PnPBIOS port order)

# Detect port order using the fixed OPL port as reference.

if [ ${port2%%-*} = 0x388 ]

# ^^^^ Strip out everything following hyphen in port address.

# So, if port1 is 0x530-0x537

#+ we're left with 0x530 -- the start address of the port.

then

# PnPBIOS: usual order

port=${port1%%-*}

oplport=${port2%%-*}

else

# ACPI: mixed-up order

port=${port3%%-*}

oplport=${port1%%-*}

} < $WSSDEV/resources

To see what's going on here:

---------------------------

cat /sys/devices/pnp0/00:07/resources

state = active

io 0x530-0x537

io 0x388-0x38b

io 0x220-0x233

irq 5

dma 1

dma 0

^^^ "bla" labels in first field (discarded).

{ read # Discard first line, as above.

read bla port1

cport=${port1%%-*}

# ^^^^

# Just want _start_ address of port.

} < $CTLDEV/resources

Load the module:

modprobe --ignore-install snd-cs4236 port=$port cport=$cport\

fm_port=$oplport irq=$irq dma1=$dma1 dma2=$dma2 isapnp=0 index=0

See the modprobe manpage.

exit $?

Example A-35. Locating split paragraphs in a text file

!/bin/bash

find-splitpara.sh

Finds split paragraphs in a text file,

+ and tags the line numbers.

ARGCOUNT=1 # Expect one arg.

OFF=0 # Flag states.

ON=1

E_WRONGARGS=85

file="$1" # Target filename.

lineno=1 # Line number. Start at 1.

Flag=$OFF # Blank line flag.

if [ $# -ne "$ARGCOUNT" ]

then

echo "Usage: `basename $0` FILENAME"

exit $E_WRONGARGS

file_read () # Scan file for pattern, then print line.

{

while read line

if [[ "$line" =~ ^[a-z] && $Flag -eq $ON ]]

then # Line begins with lowercase character, following blank line.

echo -n "$lineno:: "

echo "$line"

if [[ "$line" =~ ^$ ]]

then # If blank line,

Flag=$ON #+ set flag.

else

Flag=$OFF

((lineno++))

done

} < $file # Redirect file into function's stdin.

file_read

exit $?

----------------------------------------------------------------

This is line one of an example paragraph, bla, bla, bla.

This is line two, and line three should follow on next line, but

there is a blank line separating the two parts of the paragraph.

----------------------------------------------------------------

Running this script on a file containing the above paragraph

yields:

4:: there is a blank line separating the two parts of the paragraph.

There will be additional output for all the other split paragraphs

in the target file.

Example A-36. Insertion sort

!/bin/bash

insertion-sort.bash: Insertion sort implementation in Bash

Heavy use of Bash array features:

+ (string) slicing, merging, etc

URL: http://www.lugmen.org.ar/~jjo/jjotip/insertion-sort.bash.d

+ /insertion-sort.bash.sh

Author: JuanJo Ciarlante <jjo@irrigacion.gov.ar>

Lightly reformatted by ABS Guide author.

License: GPLv2

Used in ABS Guide with author's permission (thanks!).

Test with: ./insertion-sort.bash -t

Or: bash insertion-sort.bash -t

The following doesn't work:

sh insertion-sort.bash -t

Why not? Hint: which Bash-specific features are disabled

+ when running a script by 'sh script.sh'?

: ${DEBUG:=0} # Debug, override with: DEBUG=1 ./scriptname . . .

Parameter substitution -- set DEBUG to 0 if not previously set.

Global array: "list"

typeset -a list

Load whitespace-separated numbers from stdin.

if [ "$1" = "-t" ]; then

DEBUG=1

read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) # Random list.

^ ^ process substition

else

read -a list

numelem=${#list[*]}

Shows the list, marking the element whose index is $1

+ by surrounding it with the two chars passed as $2.

Whole line prefixed with $3.

showlist()

{

echo "$3"${list[@]:0:$1} ${2:0:1}${list[$1]}${2:1:1} ${list[@]:$1+1};

}

Loop _pivot_ -- from second element to end of list.

for(( i=1; i<numelem; i++ )) do

((DEBUG))&&showlist i "[]" " "

# From current _pivot_, back to first element.

for(( j=i; j; j-- )) do

# Search for the 1st elem. less than current "pivot" . . .

[[ "${list[j-1]}" -le "${list[i]}" ]] && break

done

(( i==j )) && continue ## No insertion was needed for this element.

# . . . Move list[i] (pivot) to the left of list[j]:

list=(${list[@]:0:j} ${list[i]} ${list[j]}\

# {0,j-1} {i} {j}

${list[@]:j+1:i-(j+1)} ${list[@]:i+1})

# {j+1,i-1} {i+1,last}

((DEBUG))&&showlist j "<>" "*"

done

echo

echo "------"

echo Result:\n'${list[@]}

exit $?

Example A-37. Standard Deviation

!/bin/bash

sd.sh: Standard Deviation

The Standard Deviation indicates how consistent a set of data is.

It shows to what extent the individual data points deviate from the

+ arithmetic mean, i.e., how much they "bounce around" (or cluster).

It is essentially the average deviation-distance of the

+ data points from the mean.

=========================================================== #

To calculate the Standard Deviation:

1 Find the arithmetic mean (average) of all the data points.

2 Subtract each data point from the arithmetic mean,

and square that difference.

3 Add all of the individual difference-squares in # 2.

4 Divide the sum in # 3 by the number of data points.

This is known as the "variance."

5 The square root of # 4 gives the Standard Deviation.

=========================================================== #

count=0 # Number of data points; global.

SC=9 # Scale to be used by bc. Nine decimal places.

E_DATAFILE=90 # Data file error.

----------------- Set data file ---------------------

if [ ! -z "$1" ] # Specify filename as cmd-line arg?

then

datafile="$1" # ASCII text file,

else #+ one (numerical) data point per line!

datafile=sample.dat

fi # See example data file, below.

if [ ! -e "$datafile" ]

then

echo "\""$datafile"\" does not exist!"

exit $E_DATAFILE

-----------------------------------------------------

arith_mean ()

{

local rt=0 # Running total.

local am=0 # Arithmetic mean.

local ct=0 # Number of data points.

while read value # Read one data point at a time.

rt=$(echo "scale=$SC; $rt + $value" | bc)

(( ct++ ))

done

am=$(echo "scale=$SC; $rt / $ct" | bc)

echo $am; return $ct # This function "returns" TWO values!

# Caution: This little trick will not work if $ct > 255!

# To handle a larger number of data points,

#+ simply comment out the "return $ct" above.

} <"$datafile" # Feed in data file.

sd ()

{

mean1=$1 # Arithmetic mean (passed to function).

n=$2 # How many data points.

sum2=0 # Sum of squared differences ("variance").

avg2=0 # Average of $sum2.

sdev=0 # Standard Deviation.

while read value # Read one line at a time.

diff=$(echo "scale=$SC; $mean1 - $value" | bc)

# Difference between arith. mean and data point.

dif2=$(echo "scale=$SC; $diff * $diff" | bc) # Squared.

sum2=$(echo "scale=$SC; $sum2 + $dif2" | bc) # Sum of squares.

done

avg2=$(echo "scale=$SC; $sum2 / $n" | bc) # Avg. of sum of squares.

sdev=$(echo "scale=$SC; sqrt($avg2)" | bc) # Square root =

echo $sdev # Standard Deviation.

} <"$datafile" # Rewinds data file.

======================================================= #

mean=$(arith_mean); count=$? # Two returns from function!

std_dev=$(sd $mean $count)

echo

echo "Number of data points in \""$datafile"\" = $count"

echo "Arithmetic mean (average) = $mean"

echo "Standard Deviation = $std_dev"

echo

======================================================= #

exit

This script could stand some drastic streamlining,

+ but not at the cost of reduced legibility, please.

++++++++++++++++++++++++++++++++++++++++ #

A sample data file (sample1.dat):

18.35

19.0

18.88

18.91

18.64 $ sh sd.sh sample1.dat

Number of data points in "sample1.dat" = 5

Arithmetic mean (average) = 18.756000000

Standard Deviation = .235338054

++++++++++++++++++++++++++++++++++++++++ #

Example A-38. A pad file generator for shareware authors

!/bin/bash

pad.sh

PAD (xml) file creator

+ Written by Mendel Cooper <thegrendel.abs@gmail.com>.

+ Released to the Public Domain.

Generates a "PAD" descriptor file for shareware

+ packages, according to the specifications

+ of the ASP.

http://www.asp-shareware.org/pad

Accepts (optional) save filename as a command-line argument.

if [ -n "$1" ]

then

savefile=$1

else

savefile=save_file.xml # Default save_file name.

===== PAD file headers =====

HDR1="<?xml version=\"1.0\" encoding=\"Windows-1252\" ?>"

HDR2="<XML_DIZ_INFO>"

HDR3="<MASTER_PAD_VERSION_INFO>"

HDR4="\t<MASTER_PAD_VERSION>1.15</MASTER_PAD_VERSION>"

HDR5="\t<MASTER_PAD_INFO>Portable Application Description, or PAD

for short, is a data set that is used by shareware authors to

disseminate information to anyone interested in their software products.

To find out more go to http://www.asp-shareware.org/pad</MASTER_PAD_INFO>"

HDR6="</MASTER_PAD_VERSION_INFO>"

============================

fill_in ()

{

if [ -z "$2" ]

then

echo -n "$1? " # Get user input.

else

echo -n "$1 $2? " # Additional query?

read var # May paste to fill in field.

# This shows how flexible "read" can be.

if [ -z "$var" ]

then

echo -e "\t\t<$1 />" >>$savefile # Indent with 2 tabs.

return

else

echo -e "\t\t<$1>$var</$1>" >>$savefile

return ${#var} # Return length of input string.

}

check_field_length () # Check length of program description fields.

{

# $1 = maximum field length

# $2 = actual field length

if [ "$2" -gt "$1" ]

then

echo "Warning: Maximum field length of $1 characters exceeded!"

}

clear # Clear screen.

echo "PAD File Creator"

echo "--- ---- -------"

echo

Write File Headers to file.

echo $HDR1 >$savefile

echo $HDR2 >>$savefile

echo $HDR3 >>$savefile

echo -e $HDR4 >>$savefile

echo -e $HDR5 >>$savefile

echo $HDR6 >>$savefile

Company_Info

echo "COMPANY INFO"

CO_HDR="Company_Info"

echo "<$CO_HDR>" >>$savefile

fill_in Company_Name

fill_in Address_1

fill_in Address_2

fill_in City_Town

fill_in State_Province

fill_in Zip_Postal_Code

fill_in Country

If applicable:

fill_in ASP_Member "[Y/N]"

fill_in ASP_Member_Number

fill_in ESC_Member "[Y/N]"

fill_in Company_WebSite_URL

clear # Clear screen between sections.

# Contact_Info

echo "CONTACT INFO"

CONTACT_HDR="Contact_Info"

echo "<$CONTACT_HDR>" >>$savefile

fill_in Author_First_Name

fill_in Author_Last_Name

fill_in Author_Email

fill_in Contact_First_Name

fill_in Contact_Last_Name

fill_in Contact_Email

echo -e "\t</$CONTACT_HDR>" >>$savefile

# END Contact_Info

clear

# Support_Info

echo "SUPPORT INFO"

SUPPORT_HDR="Support_Info"

echo "<$SUPPORT_HDR>" >>$savefile

fill_in Sales_Email

fill_in Support_Email

fill_in General_Email

fill_in Sales_Phone

fill_in Support_Phone

fill_in General_Phone

fill_in Fax_Phone

echo -e "\t</$SUPPORT_HDR>" >>$savefile

# END Support_Info

echo "</$CO_HDR>" >>$savefile

END Company_Info

clear

Program_Info

echo "PROGRAM INFO"

PROGRAM_HDR="Program_Info"

echo "<$PROGRAM_HDR>" >>$savefile

fill_in Program_Name

fill_in Program_Version

fill_in Program_Release_Month

fill_in Program_Release_Day

fill_in Program_Release_Year

fill_in Program_Cost_Dollars

fill_in Program_Cost_Other

fill_in Program_Type "[Shareware/Freeware/GPL]"

fill_in Program_Release_Status "[Beta, Major Upgrade, etc.]"

fill_in Program_Install_Support

fill_in Program_OS_Support "[Win9x/Win2k/Linux/etc.]"

fill_in Program_Language "[English/Spanish/etc.]"

echo; echo

# File_Info

echo "FILE INFO"

FILEINFO_HDR="File_Info"

echo "<$FILEINFO_HDR>" >>$savefile

fill_in Filename_Versioned

fill_in Filename_Previous

fill_in Filename_Generic

fill_in Filename_Long

fill_in File_Size_Bytes

fill_in File_Size_K

fill_in File_Size_MB

echo -e "\t</$FILEINFO_HDR>" >>$savefile

# END File_Info

clear

# Expire_Info

echo "EXPIRE INFO"

EXPIRE_HDR="Expire_Info"

echo "<$EXPIRE_HDR>" >>$savefile

fill_in Has_Expire_Info "Y/N"

fill_in Expire_Count

fill_in Expire_Based_On

fill_in Expire_Other_Info

fill_in Expire_Month

fill_in Expire_Day

fill_in Expire_Year

echo -e "\t</$EXPIRE_HDR>" >>$savefile

# END Expire_Info

clear

# More Program_Info

echo "ADDITIONAL PROGRAM INFO"

fill_in Program_Change_Info

fill_in Program_Specific_Category

fill_in Program_Categories

fill_in Includes_JAVA_VM "[Y/N]"

fill_in Includes_VB_Runtime "[Y/N]"

fill_in Includes_DirectX "[Y/N]"

# END More Program_Info

echo "</$PROGRAM_HDR>" >>$savefile

END Program_Info

clear

Program Description

echo "PROGRAM DESCRIPTIONS"

PROGDESC_HDR="Program_Descriptions"

echo "<$PROGDESC_HDR>" >>$savefile

LANG="English"

echo "<$LANG>" >>$savefile

fill_in Keywords "[comma + space separated]"

echo

echo "45, 80, 250, 450, 2000 word program descriptions"

echo "(may cut and paste into field)"

It would be highly appropriate to compose the following

+ "Char_Desc" fields with a text editor,

+ then cut-and-paste the text into the answer fields.

echo

echo " |---------------45 characters---------------|"

fill_in Char_Desc_45

check_field_length 45 "$?"

echo

fill_in Char_Desc_80

check_field_length 80 "$?"

fill_in Char_Desc_250

check_field_length 250 "$?"

fill_in Char_Desc_450

fill_in Char_Desc_2000

echo "</$LANG>" >>$savefile

echo "</$PROGDESC_HDR>" >>$savefile

END Program Description

clear

echo "Done."; echo; echo

echo "Save file is: \""$savefile"\""

exit 0

Example A-39. A man page editor

!/bin/bash

maned.sh

A rudimentary man page editor

Version: 0.1 (Alpha, probably buggy)

Author: Mendel Cooper <thegrendel.abs@gmail.com>

Reldate: 16 June 2008

License: GPL3

savefile= # Global, used in multiple functions.

E_NOINPUT=90 # User input missing (error). May or may not be critical.

=========== Markup Tags ============ #

TopHeader=".TH"

NameHeader=".SH NAME"

SyntaxHeader=".SH SYNTAX"

SynopsisHeader=".SH SYNOPSIS"

InstallationHeader=".SH INSTALLATION"

DescHeader=".SH DESCRIPTION"

OptHeader=".SH OPTIONS"

FilesHeader=".SH FILES"

EnvHeader=".SH ENVIRONMENT"

AuthHeader=".SH AUTHOR"

BugsHeader=".SH BUGS"

SeeAlsoHeader=".SH SEE ALSO"

BOLD=".B"

Add more tags, as needed.

See groff docs for markup meanings.

==================================== #

start ()

{

clear # Clear screen.

echo "ManEd"

echo "-----"

echo

echo "Simple man page creator"

echo "Author: Mendel Cooper"

echo "License: GPL3"

echo; echo; echo

}

progname ()

{

echo -n "Program name? "

read name

echo -n "Manpage section? [Hit RETURN for default (\"1\") ] "

read section

if [ -z "$section" ]

then

section=1 # Most man pages are in section 1.

if [ -n "$name" ]

then

savefile=""$name"."$section"" # Filename suffix = section.

echo -n "$1 " >>$savefile

name1=$(echo "$name" | tr a-z A-Z) # Change to uppercase,

#+ per man page convention.

echo -n "$name1" >>$savefile

else

echo "Error! No input." # Mandatory input.

exit $E_NOINPUT # Critical!

# Exercise: The script-abort if no filename input is a bit clumsy.

# Rewrite this section so a default filename is used

#+ if no input.

echo -n " \"$section\"">>$savefile # Append, always append.

echo -n "Version? "

read ver

echo -n " \"Version $ver \"">>$savefile

echo >>$savefile

echo -n "Short description [0 - 5 words]? "

read sdesc

echo "$NameHeader">>$savefile

echo ""$BOLD" "$name"">>$savefile

echo "\- "$sdesc"">>$savefile

}

fill_in ()

{ # This function more or less copied from "pad.sh" script.

echo -n "$2? " # Get user input.

read var # May paste (a single line only!) to fill in field.

if [ -n "$var" ]

then

echo "$1 " >>$savefile

echo -n "$var" >>$savefile

else # Don't append empty field to file.

return $E_NOINPUT # Not critical here.

echo >>$savefile

}

end ()

{

clear

echo -n "Would you like to view the saved man page (y/n)? "

read ans

if [ "$ans" = "n" -o "$ans" = "N" ]; then exit; fi

exec less "$savefile" # Exit script and hand off control to "less" ...

#+ ... which formats for viewing man page source.

}

---------------------------------------- #

start

progname "$TopHeader"

fill_in "$SynopsisHeader" "Synopsis"

fill_in "$DescHeader" "Long description"

May paste in single line of text.

fill_in "$OptHeader" "Options"

fill_in "$FilesHeader" "Files"

fill_in "$AuthHeader" "Author"

fill_in "$BugsHeader" "Bugs"

fill_in "$SeeAlsoHeader" "See also"

fill_in "$OtherHeader" ... as necessary.

end # ... exit not needed.

---------------------------------------- #

Note that the generated man page will usually

+ require manual fine-tuning with a text editor.

However, it's a distinct improvement upon

+ writing man source from scratch

+ or even editing a blank man page template.

The main deficiency of the script is that it permits

+ pasting only a single text line into the input fields.

This may be a long, cobbled-together line, which groff

will automatically wrap and hyphenate.

However, if you want multiple (newline-separated) paragraphs,

+ these must be inserted by manual text editing on the

+ script-generated man page.

Exercise (difficult): Fix this!

This script is not nearly as elaborate as the

+ full-featured "manedit" package

+ http://freshmeat.net/projects/manedit/

+ but it's much easier to use.

Example A-40. Petals Around the Rose

!/bin/bash -i

petals.sh

Petals Around the Rose #

#

Version 0.1 Created by Serghey Rodin #

Version 0.2 Modded by ABS Guide Author #

#

License: GPL3 #

Used in ABS Guide with permission. #

##################################################################### #

hits=0 # Correct guesses.

WIN=6 # Mastered the game.

ALMOST=5 # One short of mastery.

EXIT=exit # Give up early?

RANDOM=$ # Seeds the random number generator from PID of script.

Bones (ASCII graphics for dice)

bone1[1]="| |"

bone1[2]="| o |"

bone1[3]="| o |"

bone1[4]="| o o |"

bone1[5]="| o o |"

bone1[6]="| o o |"

bone2[1]="| o |"

bone2[2]="| |"

bone2[3]="| o |"

bone2[4]="| |"

bone2[5]="| o |"

bone2[6]="| o o |"

bone3[1]="| |"

bone3[2]="| o |"

bone3[3]="| o |"

bone3[4]="| o o |"

bone3[5]="| o o |"

bone3[6]="| o o |"

bone="+---------+"

Functions

instructions () {

clear

echo -n "Do you need instructions? (y/n) "; read ans

if [ "$ans" = "y" -o "$ans" = "Y" ]; then

clear

echo -e '\E[34;47m' # Blue type.

"cat document"

cat <<INSTRUCTIONSZZZ

The name of the game is Petals Around the Rose,

and that name is significant.

Five dice will roll and you must guess the "answer" for each roll.

It will be zero or an even number.

After your guess, you will be told the answer for the roll, but . . .

that's ALL the information you will get.

Six consecutive correct guesses admits you to the

Fellowship of the Rose.

INSTRUCTIONSZZZ

echo -e "\033[0m" # Turn off blue.

else clear

}

fortune ()

{

RANGE=7

FLOOR=0

number=0

while [ "$number" -le $FLOOR ]

number=$RANDOM

let "number %= $RANGE" # 1 - 6.

done

return $number

}

throw () { # Calculate each individual die.

fortune; B1=$?

fortune; B2=$?

fortune; B3=$?

fortune; B4=$?

fortune; B5=$?

calc () { # Function embedded within a function!

case "$1" in

3 ) rose=2;;

5 ) rose=4;;

* ) rose=0;;

esac # Simplified algorithm.

# Doesn't really get to the heart of the matter.

return $rose

}

answer=0

calc "$B1"; answer=$(expr $answer + $(echo $?))

calc "$B2"; answer=$(expr $answer + $(echo $?))

calc "$B3"; answer=$(expr $answer + $(echo $?))

calc "$B4"; answer=$(expr $answer + $(echo $?))

calc "$B5"; answer=$(expr $answer + $(echo $?))

}

game ()

{ # Generate graphic display of dice throw.

throw

echo -e "\033[1m" # Bold.

echo -e "\n"

echo -e "$bone\t$bone\t$bone\t$bone\t$bone"

echo -e \

"${bone1[$B1]}\t${bone1[$B2]}\t${bone1[$B3]}\t${bone1[$B4]}\t${bone1[$B5]}"

echo -e \

"${bone2[$B1]}\t${bone2[$B2]}\t${bone2[$B3]}\t${bone2[$B4]}\t${bone2[$B5]}"

echo -e \

"${bone3[$B1]}\t${bone3[$B2]}\t${bone3[$B3]}\t${bone3[$B4]}\t${bone3[$B5]}"

echo -e "$bone\t$bone\t$bone\t$bone\t$bone"

echo -e "\n\n\t\t"

echo -e "\033[0m" # Turn off bold.

echo -n "There are how many petals around the rose? "

}

============================================================== #

instructions

while [ "$petal" != "$EXIT" ] # Main loop.

game

read petal

echo "$petal" | grep [0-9] >/dev/null # Filter response for digit.

# Otherwise just roll dice again.

if [ "$?" -eq 0 ] # If-loop #1.

then

if [ "$petal" == "$answer" ]; then # If-loop #2.

echo -e "\nCorrect. There are $petal petals around the rose.\n"

(( hits++ ))

if [ "$hits" -eq "$WIN" ]; then # If-loop #3.

echo -e '\E[31;47m' # Red type.

echo -e "\033[1m" # Bold.

echo "You have unraveled the mystery of the Rose Petals!"

echo "Welcome to the Fellowship of the Rose!!!"

echo "(You are herewith sworn to secrecy.)"; echo

echo -e "\033[0m" # Turn off red & bold.

break # Exit!

else echo "You have $hits correct so far."; echo

if [ "$hits" -eq "$ALMOST" ]; then

echo "Just one more gets you to the heart of the mystery!"; echo

fi # Close if-loop #3.

else

echo -e "\nWrong. There are $answer petals around the rose.\n"

hits=0 # Reset number of correct guesses.

fi # Close if-loop #2.

echo -n "Hit ENTER for the next roll, or type \"exit\" to end. "

read

if [ "$REPLY" = "$EXIT" ]; then exit

fi # Close if-loop #1.

clear

done # End of main (while) loop.

exit $?

Resources:

---------

1) http://en.wikipedia.org/wiki/Petals_Around_the_Rose

(Wikipedia entry.)

2) http://www.borrett.id.au/computing/petals-bg.htm

(How Bill Gates coped with the Petals Around the Rose challenge.)

Example A-41. Quacky: a Perquackey-type word game

!/bin/bash

qky.sh

QUACKEY: a somewhat simplified version of Perquackey [TM]. #

#

Author: Mendel Cooper <thegrendel.abs@gmail.com> #

version 0.1.02 03 May, 2008 #

License: GPL3 #

WLIST=/usr/share/dict/word.lst

^^^^^^^^ Word list file found here.

ASCII word list, one word per line, UNIX format.

A suggested list is the script author's "yawl" word list package.

http://bash.deta.in/yawl-0.3.2.tar.gz

or

http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz

NONCONS=0 # Word not constructable from letter set.

CONS=1 # Constructable.

SUCCESS=0

NG=1

FAILURE=''

NULL=0 # Zero out value of letter (if found).

MINWLEN=3 # Minimum word length.

MAXCAT=5 # Maximum number of words in a given category.

PENALTY=200 # General-purpose penalty for unacceptable words.

total=

E_DUP=70 # Duplicate word error.

TIMEOUT=10 # Time for word input.

NVLET=10 # 10 letters for non-vulnerable.

VULET=13 # 13 letters for vulnerable (not yet implemented!).

declare -a Words

declare -a Status

declare -a Score=( 0 0 0 0 0 0 0 0 0 0 0 )

letters=( a n s r t m l k p r b c i d s i d z e w u e t f

e y e r e f e g t g h h i t r s c i t i d i j a t a o l a

m n a n o v n w o s e l n o s p a q e e r a b r s a o d s

t g t i t l u e u v n e o x y m r k )

Letter distribution table shamelessly borrowed from "Wordy" game,

+ ca. 1992, written by a certain fine fellow named Mendel Cooper.

declare -a LS

numelements=${#letters[@]}

randseed="$1"

instructions ()

{

clear

echo "Welcome to QUACKEY, the anagramming word construction game."; echo

echo -n "Do you need instructions? (y/n) "; read ans

if [ "$ans" = "y" -o "$ans" = "Y" ]; then

clear

echo -e '\E[31;47m' # Red foreground. '\E[34;47m' for blue.

cat <<INSTRUCTION1

QUACKEY is a variant of Perquackey [TM].

The rules are the same, but the scoring is simplified

and plurals of previously played words are allowed.

"Vulnerable" play is not yet implemented,

but it is otherwise feature-complete.

As the game begins, the player gets 10 letters.

The object is to construct valid dictionary words

of at least 3-letter length from the letterset.

Each word-length category

-- 3-letter, 4-letter, 5-letter, ... --

fills up with the fifth word entered,

and no further words in that category are accepted.

The penalty for too-short (two-letter), duplicate, unconstructable,

and invalid (not in dictionary) words is -200. The same penalty applies

to attempts to enter a word in a filled-up category.

INSTRUCTION1

echo -n "Hit ENTER for next page of instructions. "; read az1

cat <<INSTRUCTION2

The scoring mostly corresponds to classic Perquackey:

The first 3-letter word scores 60, plus 10 for each additional one.

The first 4-letter word scores 120, plus 20 for each additional one.

The first 5-letter word scores 200, plus 50 for each additional one.

The first 6-letter word scores 300, plus 100 for each additional one.

The first 7-letter word scores 500, plus 150 for each additional one.

The first 8-letter word scores 750, plus 250 for each additional one.

The first 9-letter word scores 1000, plus 500 for each additional one.

The first 10-letter word scores 2000, plus 2000 for each additional one.

Category completion bonuses are:

3-letter words 100

4-letter words 200

5-letter words 400

6-letter words 800

7-letter words 2000

8-letter words 10000

This is a simplification of the absurdly baroque Perquackey bonus

scoring system.

INSTRUCTION2

echo -n "Hit ENTER for final page of instructions. "; read az1

cat <<INSTRUCTION3

Hitting just ENTER for a word entry ends the game.

Individual word entry is timed to a maximum of 10 seconds.

** Timing out on an entry ends the game. ***

Aside from that, the game is untimed.

--------------------------------------------------

Game statistics are automatically saved to a file.

--------------------------------------------------

For competitive ("duplicate") play, a previous letterset

may be duplicated by repeating the script's random seed,

command-line parameter \$1.

For example, "qky 7633" specifies the letterset

c a d i f r h u s k ...

INSTRUCTION3

echo; echo -n "Hit ENTER to begin game. "; read az1

echo -e "\033[0m" # Turn off red.

else clear

clear

}

seed_random ()

{ # Seed random number generator.

if [ -n "$randseed" ] # Can specify random seed.

then #+ for play in competitive mode.

RANDOM="$randseed"

echo "RANDOM seed set to "$randseed""

else

randseed="$" # Or get random seed from process ID.

echo "RANDOM seed not specified, set to Process ID of script ($)."

RANDOM="$randseed"

echo

}

get_letset ()

{

element=0

echo -n "Letterset:"

for lset in $(seq $NVLET)

do # Pick random letters to fill out letterset.

LS[element]="${letters[$((RANDOM%numelements))]}"

((element++))

done

echo

echo "${LS[@]}"

}

add_word ()

{

wrd="$1"

local idx=0

Status[0]=""

Status[3]=""

Status[4]=""

while [ "${Words[idx]}" != '' ]

if [ "${Words[idx]}" = "$wrd" ]

then

Status[3]="Duplicate-word-PENALTY"

let "Score[0]= 0 - $PENALTY"

let "Score[1]-=$PENALTY"

return $E_DUP

((idx++))

done

Words[idx]="$wrd"

get_score

}

get_score()

{

local wlen=0

local score=0

local bonus=0

local first_word=0

local add_word=0

local numwords=0

wlen=${#wrd}

numwords=${Score[wlen]}

Score[2]=0

Status[4]="" # Initialize "bonus" to 0.

case "$wlen" in

3) first_word=60

add_word=10;;

4) first_word=120

add_word=20;;

5) first_word=200

add_word=50;;

6) first_word=300

add_word=100;;

7) first_word=500

add_word=150;;

8) first_word=750

add_word=250;;

9) first_word=1000

add_word=500;;

10) first_word=2000

add_word=2000;; # This category modified from original rules!

esac

((Score[wlen]++))

if [ ${Score[wlen]} -eq $MAXCAT ]

then # Category completion bonus scoring simplified!

case $wlen in

3 ) bonus=100;;

4 ) bonus=200;;

5 ) bonus=400;;

6 ) bonus=800;;

7 ) bonus=2000;;

8 ) bonus=10000;;

esac # Needn't worry about 9's and 10's.

Status[4]="Category-$wlen-completion***BONUS***"

Score[2]=$bonus

else

Status[4]="" # Erase it.

let "score = $first_word + $add_word * $numwords"

if [ "$numwords" -eq 0 ]

then

Score[0]=$score

else

Score[0]=$add_word

fi # All this to distinguish last-word score

#+ from total running score.

let "Score[1] += ${Score[0]}"

let "Score[1] += ${Score[2]}"

}

get_word ()

{

local wrd=''

read -t $TIMEOUT wrd # Timed read.

echo $wrd

}

is_constructable ()

{ # This is the most complex and difficult-to-write function.

local -a local_LS=( "${LS[@]}" ) # Local copy of letter set.

local is_found=0

local idx=0

local pos

local strlen

local local_word=( "$1" )

strlen=${#local_word}

while [ "$idx" -lt "$strlen" ]

is_found=$(expr index "${local_LS[*]}" "${local_word:idx:1}")

if [ "$is_found" -eq "$NONCONS" ] # Not constructable!

then

echo "$FAILURE"; return

else

((pos = ($is_found - 1) / 2)) # Compensate for spaces betw. letters!

local_LS[pos]=$NULL # Zero out used letters.

((idx++)) # Bump index.

done

echo "$SUCCESS"

return

}

is_valid ()

{ # Surprisingly easy to check if word in dictionary ...

fgrep -qw "$1" "$WLIST" # ... courtesy of 'grep' ...

echo $?

}

check_word ()

{

if [ -z "$1" ]

then

return

Status[1]=""

Status[2]=""

Status[3]=""

Status[4]=""

iscons=$(is_constructable "$1")

if [ "$iscons" ]

then

Status[1]="constructable"

v=$(is_valid "$1")

if [ "$v" -eq "$SUCCESS" ]

then

Status[2]="valid"

strlen=${#1}

if [ ${Score[strlen]} -eq "$MAXCAT" ] # Category full!

then

Status[3]="Category-$strlen-overflow-PENALTY"

return $NG

case "$strlen" in

1 | 2 )

Status[3]="Two-letter-word-PENALTY"

return $NG;;

* )

Status[3]=""

return $SUCCESS;;

esac

else

Status[3]="Not-valid-PENALTY"

return $NG

else

Status[3]="Not-constructable-PENALTY"

return $NG

### FIXME: Streamline the above code block.

}

display_words ()

{

local idx=0

local wlen0

clear

echo "Letterset: ${LS[@]}"

echo "Threes: Fours: Fives: Sixes: Sevens: Eights:"

echo "------------------------------------------------------------"

while [ "${Words[idx]}" != '' ]

wlen0=${#Words[idx]}

case "$wlen0" in

3) ;;

4) echo -n " " ;;

5) echo -n " " ;;

6) echo -n " " ;;

7) echo -n " " ;;

8) echo -n " " ;;

esac

echo "${Words[idx]}"

((idx++))

done

### FIXME: The word display is pretty crude.

}

play ()

{

word="Start game" # Dummy word, to start ...

while [ "$word" ] # If player just hits return (null word),

do #+ then game ends.

echo "$word: "${Status[@]}""

echo -n "Last score: [${Score[0]}] TOTAL score: [${Score[1]}]: Next

word: "

total=${Score[1]}

word=$(get_word)

check_word "$word"

if [ "$?" -eq "$SUCCESS" ]

then

add_word "$word"

else

let "Score[0]= 0 - $PENALTY"

let "Score[1]-=$PENALTY"

display_words

done # Exit game.

### FIXME: The play () function calls too many other functions.

### This verges on "spaghetti code" !!!

}

end_of_game ()

{ # Save and display stats.

#######################Autosave##########################

savefile=qky.save.$

# ^^ PID of script

echo `date` >> $savefile

echo "Letterset # $randseed (random seed) ">> $savefile

echo -n "Letterset: " >> $savefile

echo "${LS[@]}" >> $savefile

echo "---------" >> $savefile

echo "Words constructed:" >> $savefile

echo "${Words[@]}" >> $savefile

echo >> $savefile

echo "Score: $total" >> $savefile

echo "Statistics for this round saved in \""$savefile"\""

#########################################################

echo "Score for this round: $total"

echo "Words: ${Words[@]}"

}

---------#

instructions

seed_random

get_letset

play

end_of_game

---------#

exit $?

TODO:

1) Clean up code!

2) Prettify the display_words () function (maybe with widgets?).

3) Improve the time-out ... maybe change to untimed entry,

+ but with a time limit for the overall round.

4) An on-screen countdown timer would be nice.

5) Implement "vulnerable" mode of play for compatibility with classic

+ version of the game.

6) Improve save-to-file capability (and maybe make it optional).

7) Fix bugs!!!

For more info, reference:

http://bash.deta.in/qky.README.html

Example A-42. Nim

!/bin/bash

nim.sh: Game of Nim

Author: Mendel Cooper

Reldate: 15 July 2008

License: GPL3

ROWS=5 # Five rows of pegs (or matchsticks).

WON=91 # Exit codes to keep track of wins/losses.

LOST=92 # Possibly useful if running in batch mode.

QUIT=99

peg_msg= # Peg/Pegs?

Rows=( 0 5 4 3 2 1 ) # Array holding play info.

${Rows[0]} holds total number of pegs, updated after each turn.

Other array elements hold number of pegs in corresponding row.

instructions ()

{

clear

tput bold

echo "Welcome to the game of Nim."; echo

echo -n "Do you need instructions? (y/n) "; read ans

if [ "$ans" = "y" -o "$ans" = "Y" ]; then

clear

echo -e '\E[33;41m' # Yellow fg., over red bg.; bold.

cat <<INSTRUCTIONS

Nim is a game with roots in the distant past.

This particular variant starts with five rows of pegs.

1: | | | | |

2: | | | |

3: | | |

4: | |

5: |

The number at the left identifies the row.

The human player moves first, and alternates turns with the bot.

A turn consists of removing at least one peg from a single row.

It is permissable to remove ALL the pegs from a row.

For example, in row 2, above, the player can remove 1, 2, 3, or 4 pegs.

The player who removes the last peg loses.

The strategy consists of trying to be the one who removes

the next-to-last peg(s), leaving the loser with the final peg.

To exit the game early, hit ENTER during your turn.

INSTRUCTIONS

echo; echo -n "Hit ENTER to begin game. "; read azx

echo -e "\033[0m" # Restore display.

else tput sgr0; clear

clear

}

tally_up ()

{

let "Rows[0] = ${Rows[1]} + ${Rows[2]} + ${Rows[3]} + ${Rows[4]} + \

${Rows[5]}" # Add up how many pegs remaining.

}

display ()

{

index=1 # Start with top row.

echo

while [ "$index" -le "$ROWS" ]

p=${Rows[index]}

echo -n "$index: " # Show row number.

# ------------------------------------------------

# Two concurrent inner loops.

indent=$index

while [ "$indent" -gt 0 ]

echo -n " " # Staggered rows.

((indent--)) # Spacing between pegs.

done

while [ "$p" -gt 0 ]

echo -n "| "

((p--))

done

# -----------------------------------------------

echo

((index++))

done

tally_up

rp=${Rows[0]}

if [ "$rp" -eq 1 ]

then

peg_msg=peg

final_msg="Game over."

else # Game not yet over . . .

peg_msg=pegs

final_msg="" # . . . So "final message" is blank.

echo " $rp $peg_msg remaining."

echo " "$final_msg""

echo

}

player_move ()

{

echo "Your move:"

echo -n "Which row? "

while read idx

do # Validity check, etc.

if [ -z "$idx" ] # Hitting return quits.

then

echo "Premature exit."; echo

tput sgr0 # Restore display.

exit $QUIT

if [ "$idx" -gt "$ROWS" -o "$idx" -lt 1 ] # Bounds check.

then

echo "Invalid row number!"

echo -n "Which row? "

else

break

# TODO:

# Add check for non-numeric input.

# Also, script crashes on input outside of range of long double.

# Fix this.

done

echo -n "Remove how many? "

while read num

do # Validity check.

if [ -z "$num" ]

then

echo "Premature exit."; echo

tput sgr0 # Restore display.

exit $QUIT

if [ "$num" -gt ${Rows[idx]} -o "$num" -lt 1 ]

then

echo "Cannot remove $num!"

echo -n "Remove how many? "

else

break

done

# TODO:

# Add check for non-numeric input.

# Also, script crashes on input outside of range of long double.

# Fix this.

let "Rows[idx] -= $num"

display

tally_up

if [ ${Rows[0]} -eq 1 ]

then

echo " Human wins!"

echo " Congratulations!"

tput sgr0 # Restore display.

echo

exit $WON

if [ ${Rows[0]} -eq 0 ]

then # Snatching defeat from the jaws of victory . . .

echo " Fool!"

echo " You just removed the last peg!"

echo " Bot wins!"

tput sgr0 # Restore display.

echo

exit $LOST

}

bot_move ()

{

row_b=0

while [[ $row_b -eq 0 || ${Rows[row_b]} -eq 0 ]]

row_b=$RANDOM # Choose random row.

let "row_b %= $ROWS"

done

num_b=0

r0=${Rows[row_b]}

if [ "$r0" -eq 1 ]

then

num_b=1

else

let "num_b = $r0 - 1"

# Leave only a single peg in the row.

fi # Not a very strong strategy,

#+ but probably a bit better than totally random.

let "Rows[row_b] -= $num_b"

echo -n "Bot: "

echo "Removing from row $row_b ... "

if [ "$num_b" -eq 1 ]

then

peg_msg=peg

else

peg_msg=pegs

echo " $num_b $peg_msg."

display

tally_up

if [ ${Rows[0]} -eq 1 ]

then

echo " Bot wins!"

tput sgr0 # Restore display.

exit $WON

}

================================================== #

instructions # If human player needs them . . .

tput bold # Bold characters for easier viewing.

display # Show game board.

while [ true ] # Main loop.

do # Alternate human and bot turns.

player_move

bot_move

done

================================================== #

Exercise:

--------

Improve the bot's strategy.

There is, in fact, a Nim strategy that can force a win.

See the Wikipedia article on Nim: http://en.wikipedia.org/wiki/Nim

Recode the bot to use this strategy (rather difficult).

Curiosities:

-----------

Nim played a prominent role in Alain Resnais' 1961 New Wave film,

+ Last Year at Marienbad.

In 1978, Leo Christopherson wrote an animated version of Nim,

+ Android Nim, for the TRS-80 Model I.

Example A-43. A command-line stopwatch

!/bin/sh

sw.sh

A command-line Stopwatch

Author: Pádraig Brady

http://www.pixelbeat.org/scripts/sw

(Minor reformatting by ABS Guide author.)

Used in ABS Guide with script author's permission.

Notes:

This script starts a few processes per lap, in addition to

the shell loop processing, so the assumption is made that

this takes an insignificant amount of time compared to

the response time of humans (~.1s) (or the keyboard

interrupt rate (~.05s)).

'?' for splits must be entered twice if characters

(erroneously) entered before it (on the same line).

'?' since not generating a signal may be slightly delayed

on heavily loaded systems.

Lap timings on ubuntu may be slightly delayed due to:

https://bugs.launchpad.net/bugs/62511

Changes:

V1.0, 23 Aug 2005, Initial release

V1.1, 26 Jul 2007, Allow both splits and laps from single invocation.

Only start timer after a key is pressed.

Indicate lap number

Cache programs at startup so there is less error

due to startup delays.

V1.2, 01 Aug 2007, Work around `date` commands that don't have

nanoseconds.

Use stty to change interrupt keys to space for

laps etc.

Ignore other input as it causes problems.

V1.3, 01 Aug 2007, Testing release.

V1.4, 02 Aug 2007, Various tweaks to get working under ubuntu

and Mac OS X.

V1.5, 27 Jun 2008, set LANG=C as got vague bug report about it.

export LANG=C

ulimit -c 0 # No coredumps from SIGQUIT.

trap '' TSTP # Ignore Ctrl-Z just in case.

save_tty=`stty -g` && trap "stty $save_tty" EXIT # Restore tty on exit.

stty quit ' ' # Space for laps rather than Ctrl-\.

stty eof '?' # ? for splits rather than Ctrl-D.

stty -echo # Don't echo input.

cache_progs() {

stty > /dev/null

date > /dev/null

grep . < /dev/null

(echo "import time" | python) 2> /dev/null

bc < /dev/null

sed '' < /dev/null

printf '1' > /dev/null

/usr/bin/time false 2> /dev/null

cat < /dev/null

}

cache_progs # To minimise startup delay.

date +%s.%N | grep -qF 'N' && use_python=1 # If `date` lacks nanoseconds.

now() {

if [ "$use_python" ]; then

echo "import time; print time.time()" 2>/dev/null | python

else

printf "%.2f" `date +%s.%N`

}

fmt_seconds() {

seconds=$1

mins=`echo $seconds/60 | bc`

if [ "$mins" != "0" ]; then

seconds=`echo "$seconds - ($mins*60)" | bc`

echo "$mins:$seconds"

else

echo "$seconds"

}

total() {

end=`now`

total=`echo "$end - $start" | bc`

fmt_seconds $total

}

stop() {

[ "$lapped" ] && lap "$laptime" "display"

total

exit

}

lap() {

laptime=`echo "$1" | sed -n 's/.*real[^0-9.]*$.*$/\1/p'`

[ ! "$laptime" -o "$laptime" = "0.00" ] && return

# Signals too frequent.

laptotal=`echo $laptime+0$laptotal | bc`

if [ "$2" = "display" ]; then

lapcount=`echo 0$lapcount+1 | bc`

laptime=`fmt_seconds $laptotal`

echo $laptime "($lapcount)"

lapped="true"

laptotal="0"

}

echo -n "Space for lap | ? for split | Ctrl-C to stop | Space to start...">&2

while true; do

trap true INT QUIT # Set signal handlers.

laptime=`/usr/bin/time -p 2>&1 cat >/dev/null`

ret=$?

trap '' INT QUIT # Ignore signals within this script.

if [ $ret -eq 1 -o $ret -eq 2 -o $ret -eq 130 ]; then # SIGINT = stop

[ ! "$start" ] && { echo >&2; exit; }

stop

elif [ $ret -eq 3 -o $ret -eq 131 ]; then # SIGQUIT = lap

if [ ! "$start" ]; then

start=`now` || exit 1

echo >&2

continue

lap "$laptime" "display"

else # eof = split

[ ! "$start" ] && continue

total

lap "$laptime" # Update laptotal.

done

exit $?

Example A-44. An all-purpose shell scripting homework assignment

solution

!/bin/bash

homework.sh: All-purpose homework assignment solution.

Author: M. Leo Cooper

If you substitute your own name as author, then it is plagiarism,

+ possibly a lesser sin than cheating on your homework!

License: Public Domain

This script may be turned in to your instructor

+ in fulfillment of ALL shell scripting homework assignments.

It's sparsely commented, but you, the student, can easily remedy that.

The script author repudiates all responsibility!

DLA=1

P1=2

P2=4

P3=7

PP1=0

PP2=8

MAXL=9

E_LZY=99

declare -a L

L[0]="3 4 0 17 29 8 13 18 19 17 20 2 19 14 17 28"

L[1]="8 29 12 14 18 19 29 4 12 15 7 0 19 8 2 0 11 11 24 29 17 4 6 17 4 19"

L[2]="29 19 7 0 19 29 8 29 7 0 21 4 29 13 4 6 11 4 2 19 4 3"

L[3]="19 14 29 2 14 12 15 11 4 19 4 29 19 7 8 18 29"

L[4]="18 2 7 14 14 11 22 14 17 10 29 0 18 18 8 6 13 12 4 13 19 26"

L[5]="15 11 4 0 18 4 29 0 2 2 4 15 19 29 12 24 29 7 20 12 1 11 4 29"

L[6]="4 23 2 20 18 4 29 14 5 29 4 6 17 4 6 8 14 20 18 29"

L[7]="11 0 25 8 13 4 18 18 27"

L[8]="0 13 3 29 6 17 0 3 4 29 12 4 29 0 2 2 14 17 3 8 13 6 11 24 26"

L[9]="19 7 0 13 10 29 24 14 20 26"

declare -a \

alph=( A B C D E F G H I J K L M N O P Q R S T U V W X Y Z . , : ' ' )

pt_lt ()

{

echo -n "${alph[$1]}"

echo -n -e "\a"

sleep $DLA

}

b_r ()

{

echo -e '\E[31;48m\033[1m'

}

cr ()

{

echo -e "\a"

sleep $DLA

}

restore ()

{

echo -e '\033[0m' # Bold off.

tput sgr0 # Normal.

}

p_l ()

{

for ltr in $1

pt_lt "$ltr"

done

}

----------------------

b_r

for i in $(seq 0 $MAXL)

p_l "${L[i]}"

if [[ "$i" -eq "$P1" || "$i" -eq "$P2" || "$i" -eq "$P3" ]]

then

elif [[ "$i" -eq "$PP1" || "$i" -eq "$PP2" ]]

then

cr; cr

done

restore

----------------------

echo

exit $E_LZY

A typical example of an obfuscated script that is difficult

+ to understand, and frustrating to maintain.

In your career as a sysadmin, you'll run into these critters

+ all too often.

Example A-45. The Knight's Tour

!/bin/bash

ktour.sh

author: mendel cooper

reldate: 12 Jan 2009

license: public domain

(Not much sense GPLing something that's pretty much in the common

+ domain anyhow.)

The Knight's Tour, a classic problem. #

===================================== #

The knight must move onto every square of the chess board, #

but cannot revisit any square he has already visited. #

#

And just why is Sir Knight unwelcome for a return visit? #

Could it be that he has a habit of partying into the wee hours #

+ of the morning? #

Possibly he leaves pizza crusts in the bed, empty beer bottles #

+ all over the floor, and clogs the plumbing. . . . #

#

------------------------------------------------------------- #

#

Usage: ktour.sh [start-square] [stupid] #

#

Note that start-square can be a square number #

+ in the range 0 - 63 ... or #

a square designator in conventional chess notation, #

such as a1, f5, h3, etc. #

#

If start-square-number not supplied, #

+ then starts on a random square somewhere on the board. #

#

"stupid" as second parameter sets the stupid strategy. #

#

Examples: #

ktour.sh 23 starts on square #23 (h3) #

ktour.sh g6 stupid starts on square #46, #

using "stupid" (non-Warnsdorff) strategy. #

DEBUG= # Set this to echo debugging info to stdout.

SUCCESS=0

FAIL=99

BADMOVE=-999

FAILURE=1

LINELEN=21 # How many moves to display per line.

---------------------------------------- #

Board array params

ROWS=8 # 8 x 8 board.

COLS=8

let "SQUARES = $ROWS * $COLS"

let "MAX = $SQUARES - 1"

MIN=0

64 squares on board, indexed from 0 to 63.

VISITED=1

UNVISITED=-1

UNVSYM="##"

---------------------------------------- #

Global variables.

startpos= # Starting position (square #, 0 - 63).

currpos= # Current position.

movenum= # Move number.

CRITPOS=37 # Have to patch for f5 starting position!

declare -i board

Use a one-dimensional array to simulate a two-dimensional one.

This can make life difficult and result in ugly kludges; see below.

declare -i moves # Offsets from current knight position.

initialize_board ()

{

local idx

for idx in {0..63}

board[$idx]=$UNVISITED

done

}

print_board ()

{

local idx

echo " _____________________________________"

for row in {7..0} # Reverse order of rows ...

do #+ so it prints in chessboard order.

let "rownum = $row + 1" # Start numbering rows at 1.

echo -n "$rownum |" # Mark board edge with border and

for column in {0..7} #+ "algebraic notation."

let "idx = $ROWS*$row + $column"

if [ ${board[idx]} -eq $UNVISITED ]

then

echo -n "$UNVSYM " ##

else # Mark square with move number.

printf "%02d " "${board[idx]}"; echo -n " "

done

echo -e -n "\b\b\b|" # \b is a backspace.

echo # -e enables echoing escaped chars.

done

echo " -------------------------------------"

echo " a b c d e f g h"

}

failure()

{ # Whine, then bail out.

echo

print_board

echo

echo " Waah!!! Ran out of squares to move to!"

echo -n " Knight's Tour attempt ended"

echo " on $(to_algebraic $currpos) [square #$currpos]"

echo " after just $movenum moves!"

echo

exit $FAIL

}

xlat_coords () # Translate x/y coordinates to board position

{ #+ (board-array element #).

# For user input of starting board position as x/y coords.

# This function not used in initial release of ktour.sh.

# May be used in an updated version, for compatibility with

#+ standard implementation of the Knight's Tour in C, Python, etc.

if [ -z "$1" -o -z "$2" ]

then

return $FAIL

local xc=$1

local yc=$2

let "board_index = $xc * $ROWS + yc"

if [ $board_index -lt $MIN -o $board_index -gt $MAX ]

then

return $FAIL # Strayed off the board!

else

return $board_index

}

to_algebraic () # Translate board position (board-array element #)

{ #+ to standard algebraic notation used by chess players.

if [ -z "$1" ]

then

return $FAIL

local element_no=$1 # Numerical board position.

local col_arr=( a b c d e f g h )

local row_arr=( 1 2 3 4 5 6 7 8 )

let "row_no = $element_no / $ROWS"

let "col_no = $element_no % $ROWS"

t1=${col_arr[col_no]}; t2=${row_arr[row_no]}

local apos=$t1$t2 # Concatenate.

echo $apos

}

from_algebraic () # Translate standard algebraic chess notation

{ #+ to numerical board position (board-array element #).

# Or recognize numerical input & return it unchanged.

if [ -z "$1" ]

then

return $FAIL

fi # If no command-line arg, then will default to random start pos.

local ix

local ix_count=0

local b_index # Board index [0-63]

local alpos="$1"

arow=${alpos:0:1} # position = 0, length = 1

acol=${alpos:1:1}

if [[ $arow =~ [[:digit:]] ]] # Numerical input?

then # POSIX char class

if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!

then return $FAIL

else if [ $alpos -gt $MAX ] # Off board?

then return $FAIL

else return $alpos # Return digit(s) unchanged . . .

fi #+ if within range.

if [[ $acol -eq $MIN || $acol -gt $ROWS ]]

then # Outside of range 1 - 8?

return $FAIL

for ix in a b c d e f g h

do # Convert column letter to column number.

if [ "$arow" = "$ix" ]

then

break

((ix_count++)) # Find index count.

done

((acol--)) # Decrementing converts to zero-based array.

let "b_index = $ix_count + $acol * $ROWS"

if [ $b_index -gt $MAX ] # Off board?

then

return $FAIL

return $b_index

}

generate_moves () # Calculate all valid knight moves,

{ #+ relative to current position ($1),

#+ and store in ${moves} array.

local kt_hop=1 # One square :: short leg of knight move.

local kt_skip=2 # Two squares :: long leg of knight move.

local valmov=0 # Valid moves.

local row_pos; let "row_pos = $1 % $COLS"

let "move1 = -$kt_skip + $ROWS" # 2 sideways to-the-left, 1 up

if [[ `expr $row_pos - $kt_skip` -lt $MIN ]] # An ugly, ugly kludge!

then # Can't move off board.

move1=$BADMOVE # Not even temporarily.

else

((valmov++))

let "move2 = -$kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-left, 2 up

if [[ `expr $row_pos - $kt_hop` -lt $MIN ]] # Kludge continued ...

then

move2=$BADMOVE

else

((valmov++))

let "move3 = $kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-right, 2 up

if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]

then

move3=$BADMOVE

else

((valmov++))

let "move4 = $kt_skip + $ROWS" # 2 sideways to-the-right, 1 up

if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]

then

move4=$BADMOVE

else

((valmov++))

let "move5 = $kt_skip - $ROWS" # 2 sideways to-the-right, 1 dn

if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]

then

move5=$BADMOVE

else

((valmov++))

let "move6 = $kt_hop - $kt_skip * $ROWS" # 1 sideways to-the-right, 2 dn

if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]

then

move6=$BADMOVE

else

((valmov++))

let "move7 = -$kt_hop - $kt_skip * $ROWS" # 1 sideways to-the-left, 2 dn

if [[ `expr $row_pos - $kt_hop` -lt $MIN ]]

then

move7=$BADMOVE

else

((valmov++))

let "move8 = -$kt_skip - $ROWS" # 2 sideways to-the-left, 1 dn

if [[ `expr $row_pos - $kt_skip` -lt $MIN ]]

then

move8=$BADMOVE

else

((valmov++))

fi # There must be a better way to do this.

local m=( $valmov $move1 $move2 $move3 $move4 $move5 $move6 $move7 $move8 )

# ${moves[0]} = number of valid moves.

# ${moves[1]} ... ${moves[8]} = possible moves.

echo "${m[*]}" # Elements of array to stdout for capture in a var.

}

is_on_board () # Is position actually on the board?

{

if [[ "$1" -lt "$MIN" || "$1" -gt "$MAX" ]]

then

return $FAILURE

else

return $SUCCESS

}

do_move () # Move the knight!

{

local valid_moves=0

local aapos

currposl="$1"

lmin=$ROWS

iex=0

squarel=

mpm=

mov=

declare -a p_moves

########################## DECIDE-MOVE #############################

if [ $startpos -ne $CRITPOS ]

then # CRITPOS = square #37

decide_move

else # Needs a special patch for startpos=37 !!!

decide_move_patched # Why this particular move and no other ???

####################################################################

(( ++movenum )) # Increment move count.

let "square = $currposl + ${moves[iex]}"

################## DEBUG ###############

if [ "$DEBUG" ]

then debug # Echo debugging information.

##############################################

if [[ "$square" -gt $MAX || "$square" -lt $MIN ||

${board[square]} -ne $UNVISITED ]]

then

(( --movenum )) # Decrement move count,

echo "RAN OUT OF SQUARES!!!" #+ since previous one was invalid.

return $FAIL

board[square]=$movenum

currpos=$square # Update current position.

((valid_moves++)); # moves[0]=$valid_moves

aapos=$(to_algebraic $square)

echo -n "$aapos "

test $(( $Moves % $LINELEN )) -eq 0 && echo

# Print LINELEN=21 moves per line. A valid tour shows 3 complete lines.

return $valid_moves # Found a square to move to!

}

do_move_stupid() # Dingbat algorithm,

{ #+ courtesy of script author, *not* Warnsdorff.

local valid_moves=0

local movloc

local squareloc

local aapos

local cposloc="$1"

for movloc in {1..8}

do # Move to first-found unvisited square.

let "squareloc = $cposloc + ${moves[movloc]}"

is_on_board $squareloc

if [ $? -eq $SUCCESS ] && [ ${board[squareloc]} -eq $UNVISITED ]

then # Add conditions to above if-test to improve algorithm.

(( ++movenum ))

board[squareloc]=$movenum

currpos=$squareloc # Update current position.

((valid_moves++)); # moves[0]=$valid_moves

aapos=$(to_algebraic $squareloc)

echo -n "$aapos "

test $(( $Moves % $LINELEN )) -eq 0 && echo # Print 21 moves/line.

return $valid_moves # Found a square to move to!

done

return $FAIL

# If no square found in all 8 loop iterations,

#+ then Knight's Tour attempt ends in failure.

# Dingbat algorithm will typically fail after about 30 - 40 moves,

#+ but executes _much_ faster than Warnsdorff's in do_move() function.

}

decide_move () # Which move will we make?

{ # But, fails on startpos=37 !!!

for mov in {1..8}

let "squarel = $currposl + ${moves[mov]}"

is_on_board $squarel

if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]

then # Find accessible square with least possible future moves.

# This is Warnsdorff's algorithm.

# What happens is that the knight wanders toward the outer edge

#+ of the board, then pretty much spirals inward.

# Given two or more possible moves with same value of

#+ least-possible-future-moves, this implementation chooses

#+ the _first_ of those moves.

# This means that there is not necessarily a unique solution

#+ for any given starting position.

possible_moves $squarel

mpm=$?

p_moves[mov]=$mpm

if [ $mpm -lt $lmin ] # If less than previous minimum ...

then # ^^

lmin=$mpm # Update minimum.

iex=$mov # Save index.

done

}

decide_move_patched () # Decide which move to make,

{ # ^^^^^^^ #+ but only if startpos=37 !!!

for mov in {1..8}

let "squarel = $currposl + ${moves[mov]}"

is_on_board $squarel

if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]

then

possible_moves $squarel

mpm=$?

p_moves[mov]=$mpm

if [ $mpm -le $lmin ] # If less-than-or equal to prev. minimum!

then # ^^

lmin=$mpm

iex=$mov

done # There has to be a better way to do this.

}

possible_moves () # Calculate number of possible moves,

{ #+ given the current position.

if [ -z "$1" ]

then

return $FAIL

local curr_pos=$1

local valid_movl=0

local icx=0

local movl

local sq

declare -a movesloc

movesloc=( $(generate_moves $curr_pos) )

for movl in {1..8}

let "sq = $curr_pos + ${movesloc[movl]}"

is_on_board $sq

if [ $? -eq $SUCCESS ] && [ ${board[sq]} -eq $UNVISITED ]

then

((valid_movl++));

done

return $valid_movl # Found a square to move to!

}

strategy ()

{

echo

if [ -n "$STUPID" ]

then

for Moves in {1..63}

cposl=$1

moves=( $(generate_moves $currpos) )

do_move_stupid "$currpos"

if [ $? -eq $FAIL ]

then

failure

done

# Don't need an "else" clause here,

#+ because Stupid Strategy will always fail and exit!

for Moves in {1..63}

cposl=$1

moves=( $(generate_moves $currpos) )

do_move "$currpos"

if [ $? -eq $FAIL ]

then

failure

done

# Could have condensed above two do-loops into a single one,

echo #+ but this would have slowed execution.

print_board

echo

echo "Knight's Tour ends on $(to_algebraic $currpos) [square #$currpos]."

return $SUCCESS

}

debug ()

{ # Enable this by setting DEBUG=1 near beginning of script.

local n

echo "================================="

echo " At move number $movenum:"

echo " *** possible moves = $mpm ***"

echo "### square = $square ###"

echo "lmin = $lmin"

echo "${moves[@]}"

for n in {1..8}

echo -n "($n):${p_moves[n]} "

done

echo

echo "iex = $iex :: moves[iex] = ${moves[iex]}"

echo "square = $square"

echo "================================="

echo

} # Gives pretty complete status after ea. move.

=============================================================== #

int main () {

from_algebraic "$1"

startpos=$?

if [ "$startpos" -eq "$FAIL" ] # Okay even if no $1.

then # ^^^^^^^^^^^ Okay even if input -lt 0.

echo "No starting square specified (or illegal input)."

let "startpos = $RANDOM % $SQUARES" # 0 - 63 permissable range.

if [ "$2" = "stupid" ]

then

STUPID=1

echo -n " ### Stupid Strategy ###"

else

STUPID=''

echo -n " *** Warnsdorff's Algorithm ***"

initialize_board

movenum=0

board[startpos]=$movenum # Mark each board square with move number.

currpos=$startpos

algpos=$(to_algebraic $startpos)

echo; echo "Starting from $algpos [square #$startpos] ..."; echo

echo -n "Moves:"

strategy "$currpos"

echo

exit 0 # return 0;

} # End of main() pseudo-function.

=============================================================== #

Exercises:

---------

1) Extend this example to a 10 x 10 board or larger.

2) Improve the "stupid strategy" by modifying the

do_move_stupid function.

Hint: Prevent straying into corner squares in early moves

(the exact opposite of Warnsdorff's algorithm!).

3) This script could stand considerable improvement and

streamlining, especially in the poorly-written

generate_moves() function

and in the DECIDE-MOVE patch in the do_move() function.

Must figure out why standard algorithm fails for startpos=37 ...

+ but _not_ on any other, including symmetrical startpos=26.

Possibly, when calculating possible moves, counts the move back

+ to the originating square. If so, it might be a relatively easy fix.

Example A-46. Magic Squares

!/bin/bash

msquare.sh

Magic Square generator (odd-order squares only!)

Author: mendel cooper

reldate: 19 Jan. 2009

License: Public Domain

A C-program by the very talented Kwon Young Shin inspired this script.

http://user.chollian.net/~brainstm/MagicSquare.htm

Definition: A "magic square" is a two-dimensional array

of integers in which all the rows, columns,

and long diagonals add up to the same number.

Being "square," the array has the same number

of rows and columns. That number is the "order."

An example of a magic square of order 3 is:

8 1 6

3 5 7

4 9 2

All the rows, columns, and the two long diagonals add up to 15.

Globals

EVEN=2

MAXSIZE=31 # 31 rows x 31 cols.

E_usage=90 # Invocation error.

dimension=

declare -i square

usage_message ()

{

echo "Usage: $0 order"

echo " ... where \"order\" (square size) is an ODD integer"

echo " in the range 3 - 31."

# Actually works for squares up to order 159,

#+ but large squares will not display pretty-printed in a term window.

# Try increasing MAXSIZE, above.

exit $E_usage

}

calculate () # Here's where the actual work gets done.

{

local row col index dimadj j k cell_val=1

dimension=$1

let "dimadj = $dimension * 3"; let "dimadj /= 2" # x 1.5, then truncate.

for ((j=0; j < dimension; j++))

for ((k=0; k < dimension; k++))

do # Calculate indices, then convert to 1-dim. array index.

# Bash doesn't support multidimensional arrays. Pity.

let "col = $k - $j + $dimadj"; let "col %= $dimension"

let "row = $j * 2 - $k + $dimension"; let "row %= $dimension"

let "index = $row*($dimension) + $col"

square[$index]=cell_val; ((cell_val++))

done

} # Plain math, visualization not required.

print_square () # Output square, one row at a time.

{

local row col idx d1

let "d1 = $dimension - 1" # Adjust for zero-indexed array.

for row in $(seq 0 $d1)

for col in $(seq 0 $d1)

let "idx = $row * $dimension + $col"

printf "%3d " "${square[idx]}"; echo -n " "

done # Displays up to 13th order neatly in 80-column term window.

echo # Newline after each row.

done

}

if [[ -z "$1" ]] || [[ "$1" -gt $MAXSIZE ]]

then

usage_message

let "test_even = $1 % $EVEN"

if [ $test_even -eq 0 ]

then # Can't handle even-order squares.

usage_message

calculate $1

print_square # echo "${square[@]}" # DEBUG

exit $?

Exercises:

---------

1) Add a function to calculate the sum of each row, column,

and long diagonal. The sums must match.

This is the "magic constant" of that particular order square.

2) Have the print_square function auto-calculate how much space

to allot between square elements for optimized display.

This might require parameterizing the "printf" line.

3) Add appropriate functions for generating magic squares

with an even number of rows/columns.

This is non-trivial(!).

See the URL for Kwon Young Shin, above, for help.

Example A-47. Fifteen Puzzle

!/bin/bash

fifteen.sh

Classic "Fifteen Puzzle"

Author: Antonio Macchi

Lightly edited and commented by ABS Guide author.

Used in ABS Guide with permission. (Thanks!)

The invention of the Fifteen Puzzle is attributed to either

+ Sam Loyd or Noyes Palmer Chapman.

The puzzle was wildly popular in the late 19th-century.

Object: Rearrange the numbers so they read in order,

+ from 1 - 15: ________________

| 1 2 3 4 |

| 5 6 7 8 |

| 9 10 11 12 |

| 13 14 15 |

----------------

Constants #

SQUARES=16 #

FAIL=70 #

E_PREMATURE_EXIT=80 #

Data #

Puzzle=( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 " " )

Functions #

function swap

{

local tmp

tmp=${Puzzle[$1]}

Puzzle[$1]=${Puzzle[$2]}

Puzzle[$2]=$tmp

}

function Jumble

{ # Scramble the pieces at beginning of round.

local i pos1 pos2

for i in {1..100}

pos1=$(( $RANDOM % $SQUARES))

pos2=$(( $RANDOM % $SQUARES ))

swap $pos1 $pos2

done

}

function PrintPuzzle

{

local i1 i2 puzpos

puzpos=0

clear

echo "Enter quit to exit."; echo # Better that than Ctl-C.

echo ",----.----.----.----." # Top border.

for i1 in {1..4}

for i2 in {1..4}

printf "| %2s " "${Puzzle[$puzpos]}"

(( puzpos++ ))

done

echo "|" # Right-side border.

test $i1 = 4 || echo "+----+----+----+----+"

done

echo "'----'----'----'----'" # Bottom border.

}

function GetNum

{ # Test for valid input.

local puznum garbage

while true

echo "Moves: $moves" # Also counts invalid moves.

read -p "Number to move: " puznum garbage

if [ "$puznum" = "quit" ]; then echo; exit $E_PREMATURE_EXIT; fi

test -z "$puznum" -o -n "${puznum//[0-9]/}" && continue

test $puznum -gt 0 -a $puznum -lt $SQUARES && break

done

return $puznum

}

function GetPosFromNum

{ # $1 = puzzle-number

local puzpos

for puzpos in {0..15}

test "${Puzzle[$puzpos]}" = "$1" && break

done

return $puzpos

}

function Move

{ # $1=Puzzle-pos

test $1 -gt 3 && test "${Puzzle[$(( $1 - 4 ))]}" = " "\

&& swap $1 $(( $1 - 4 )) && return 0

test $(( $1%4 )) -ne 3 && test "${Puzzle[$(( $1 + 1 ))]}" = " "\

&& swap $1 $(( $1 + 1 )) && return 0

test $1 -lt 12 && test "${Puzzle[$(( $1 + 4 ))]}" = " "\

&& swap $1 $(( $1 + 4 )) && return 0

test $(( $1%4 )) -ne 0 && test "${Puzzle[$(( $1 - 1 ))]}" = " " &&\

swap $1 $(( $1 - 1 )) && return 0

return 1

}

function Solved

{

local pos

for pos in {0..14}

test "${Puzzle[$pos]}" = $(( $pos + 1 )) || return $FAIL

# Check whether number in each square = square number.

done

return 0 # Successful solution.

}

MAIN () #######################{

moves=0

Jumble

while true # Loop continuously until puzzle solved.

echo; echo

PrintPuzzle

echo

while true

GetNum

puznum=$?

GetPosFromNum $puznum

puzpos=$?

((moves++))

Move $puzpos && break

done

Solved && break

done

echo;echo

PrintPuzzle

echo; echo "BRAVO!"; echo

exit 0

}

Exercise:

--------

Rewrite the script to display the letters A - O,

+ rather than the numbers 1 - 15.

Example A-48. The Towers of Hanoi, graphic version

! /bin/bash

The Towers Of Hanoi

http://hanoi.kernelthread.com

hanoi2.bash

Version 2.00: modded for ASCII-graphic display.

Version 2.01: fixed no command-line param bug.

Uses code contributed by Antonio Macchi,

+ with heavy editing by ABS Guide author.

This variant falls under the original copyright, see above.

Used in ABS Guide with Amit Singh's permission (thanks!).

Variables && sanity check ###

E_NOPARAM=86

E_BADPARAM=87 # Illegal no. of disks passed to script.

E_NOEXIT=88

DISKS=${1:-$E_NOPARAM} # Must specify how many disks.

Moves=0

MWIDTH=7

MARGIN=2

Arbitrary "magic" constants; work okay for relatively small # of disks.

BASEWIDTH=51 # Original code.

let "basewidth = $MWIDTH * $DISKS + $MARGIN" # "Base" beneath rods.

Above "algorithm" could likely stand improvement.

Display variables ###

let "disks1 = $DISKS - 1"

let "spaces1 = $DISKS"

let "spaces2 = 2 * $DISKS"

let "lastmove_t = $DISKS - 1" # Final move?

declare -a Rod1 Rod2 Rod3

######################### ###

function repeat { # $1=char $2=number of repetitions

local n # Repeat-print a character.

for (( n=0; n<$2; n++ )); do

echo -n "$1"

done

}

function FromRod {

local rod summit weight sequence

while true; do

rod=$1

test ${rod/[^123]/} || continue

sequence=$(echo $(seq 0 $disks1 | tac))

for summit in $sequence; do

eval weight=\${Rod${rod}[$summit]}

test $weight -ne 0 &&

{ echo "$rod $summit $weight"; return; }

done

}

function ToRod { # $1=previous (FromRod) weight

local rod firstfree weight sequence

while true; do

rod=$2

test ${rod/[^123]} || continue

sequence=$(echo $(seq 0 $disks1 | tac))

for firstfree in $sequence; do

eval weight=\${Rod${rod}[$firstfree]}

test $weight -gt 0 && { (( firstfree++ )); break; }

done

test $weight -gt $1 -o $firstfree = 0 &&

{ echo "$rod $firstfree"; return; }

done

}

function PrintRods {

local disk rod empty fill sp sequence

repeat " " $spaces1

echo -n "|"

repeat " " $spaces2

echo -n "|"

repeat " " $spaces2

echo "|"

sequence=$(echo $(seq 0 $disks1 | tac))

for disk in $sequence; do

for rod in {1..3}; do

eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))

eval fill=\${Rod${rod}[$disk]}

repeat " " $empty

test $fill -gt 0 && repeat "*" $fill || echo -n "|"

repeat " " $empty

done

echo

done

repeat "=" $basewidth # Print "base" beneath rods.

echo

}

display ()

{

echo

PrintRods

# Get rod-number, summit and weight

first=( `FromRod $1` )

eval Rod${first[0]}[${first[1]}]=0

# Get rod-number and first-free position

second=( `ToRod ${first[2]} $2` )

eval Rod${second[0]}[${second[1]}]=${first[2]}

echo; echo; echo

if [ "${Rod3[lastmove_t]}" = 1 ]

then # Last move? If yes, then display final position.

echo "+ Final Position: $Moves moves"; echo

PrintRods

}

From here down, almost the same as original (hanoi.bash) script.

dohanoi() { # Recursive function.

case $1 in

;;

dohanoi "$(($1-1))" $2 $4 $3

if [ "$Moves" -ne 0 ]

then

echo "+ Position after move $Moves"

((Moves++))

echo -n " Next move will be: "

echo $2 "-->" $3

display $2 $3

dohanoi "$(($1-1))" $4 $3 $2

;;

esac

}

setup_arrays ()

{

local dim n elem

let "dim1 = $1 - 1"

elem=$dim1

for n in $(seq 0 $dim1)

let "Rod1[$elem] = 2 * $n + 1"

Rod2[$n]=0

Rod3[$n]=0

((elem--))

done

}

Main ###

setup_arrays $DISKS

echo; echo "+ Start Position"

case $# in

1) case $(($1>0)) in # Must have at least one disk.

disks=$1

dohanoi $1 1 3 2

Total moves = 2^n - 1, where n = number of disks.

echo

exit 0;

;;

echo "$0: Illegal value for number of disks";

exit $E_BADPARAM;

;;

esac

;;

clear

echo "usage: $0 N"

echo " Where \"N\" is the number of disks."

exit $E_NOPARAM;

;;

esac

exit $E_NOEXIT # Shouldn't exit here.

Note:

Redirect script output to a file, otherwise it scrolls off display.

Example A-49. The Towers of Hanoi, alternate graphic version

! /bin/bash

The Towers Of Hanoi

http://hanoi.kernelthread.com

hanoi2.bash

Version 2: modded for ASCII-graphic display.

Uses code contributed by Antonio Macchi,

+ with heavy editing by ABS Guide author.

This variant also falls under the original copyright, see above.

Used in ABS Guide with Amit Singh's permission (thanks!).

Variables #

E_NOPARAM=86

E_BADPARAM=87 # Illegal no. of disks passed to script.

E_NOEXIT=88

DELAY=2 # Interval, in seconds, between moves. Change, if desired.

DISKS=$1

Moves=0

MWIDTH=7

MARGIN=2

Arbitrary "magic" constants, work okay for relatively small # of disks.

BASEWIDTH=51 # Original code.

let "basewidth = $MWIDTH * $DISKS + $MARGIN" # "Base" beneath rods.

Above "algorithm" could likely stand improvement.

Display variables.

let "disks1 = $DISKS - 1"

let "spaces1 = $DISKS"

let "spaces2 = 2 * $DISKS"

let "lastmove_t = $DISKS - 1" # Final move?

declare -a Rod1 Rod2 Rod3

function repeat { # $1=char $2=number of repetitions

local n # Repeat-print a character.

for (( n=0; n<$2; n++ )); do

echo -n "$1"

done

}

function FromRod {

local rod summit weight sequence

while true; do

rod=$1

test ${rod/[^123]/} || continue

sequence=$(echo $(seq 0 $disks1 | tac))

for summit in $sequence; do

eval weight=\${Rod${rod}[$summit]}

test $weight -ne 0 &&

{ echo "$rod $summit $weight"; return; }

done

}

function ToRod { # $1=previous (FromRod) weight

local rod firstfree weight sequence

while true; do

rod=$2

test ${rod/[^123]} || continue

sequence=$(echo $(seq 0 $disks1 | tac))

for firstfree in $sequence; do

eval weight=\${Rod${rod}[$firstfree]}

test $weight -gt 0 && { (( firstfree++ )); break; }

done

test $weight -gt $1 -o $firstfree = 0 &&

{ echo "$rod $firstfree"; return; }

done

}

function PrintRods {

local disk rod empty fill sp sequence

tput cup 5 0

repeat " " $spaces1

echo -n "|"

repeat " " $spaces2

echo -n "|"

repeat " " $spaces2

echo "|"

sequence=$(echo $(seq 0 $disks1 | tac))

for disk in $sequence; do

for rod in {1..3}; do

eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))

eval fill=\${Rod${rod}[$disk]}

repeat " " $empty

test $fill -gt 0 && repeat "*" $fill || echo -n "|"

repeat " " $empty

done

echo

done

repeat "=" $basewidth # Print "base" beneath rods.

echo

}

display ()

{

echo

PrintRods

# Get rod-number, summit and weight

first=( `FromRod $1` )

eval Rod${first[0]}[${first[1]}]=0

# Get rod-number and first-free position

second=( `ToRod ${first[2]} $2` )

eval Rod${second[0]}[${second[1]}]=${first[2]}

if [ "${Rod3[lastmove_t]}" = 1 ]

then # Last move? If yes, then display final position.

tput cup 0 0

echo; echo "+ Final Position: $Moves moves"

PrintRods

sleep $DELAY

}

From here down, almost the same as original (hanoi.bash) script.

dohanoi() { # Recursive function.

case $1 in

;;

dohanoi "$(($1-1))" $2 $4 $3

if [ "$Moves" -ne 0 ]

then

tput cup 0 0

echo; echo "+ Position after move $Moves"

((Moves++))

echo -n " Next move will be: "

echo $2 "-->" $3

display $2 $3

dohanoi "$(($1-1))" $4 $3 $2

;;

esac

}

setup_arrays ()

{

local dim n elem

let "dim1 = $1 - 1"

elem=$dim1

for n in $(seq 0 $dim1)

let "Rod1[$elem] = 2 * $n + 1"

Rod2[$n]=0

Rod3[$n]=0

((elem--))

done

}

Main ###

trap "tput cnorm" 0

tput civis

clear

setup_arrays $DISKS

tput cup 0 0

echo; echo "+ Start Position"

case $# in

1) case $(($1>0)) in # Must have at least one disk.

disks=$1

dohanoi $1 1 3 2

Total moves = 2^n - 1, where n = # of disks.

echo

exit 0;

;;

echo "$0: Illegal value for number of disks";

exit $E_BADPARAM;

;;

esac

;;

echo "usage: $0 N"

echo " Where \"N\" is the number of disks."

exit $E_NOPARAM;

;;

esac

exit $E_NOEXIT # Shouldn't exit here.

Exercise:

--------

There is a minor bug in the script that causes the display of

+ the next-to-last move to be skipped.

+ Fix this.

Example A-50. An alternate version of the getopt-simple.sh script

!/bin/bash

UseGetOpt.sh

Author: Peggy Russell <prusselltechgroup@gmail.com>

UseGetOpt () {

declare inputOptions

declare -r E_OPTERR=85

declare -r ScriptName=${0##*/}

declare -r ShortOpts="adf:hlt"

declare -r LongOpts="aoption,debug,file:,help,log,test"

DoSomething () {

echo "The function name is '${FUNCNAME}'"

# Recall that $FUNCNAME is an internal variable

#+ holding the name of the function it is in.

}

inputOptions=$(getopt -o "${ShortOpts}" --long \

"${LongOpts}" --name "${ScriptName}" -- "${@}")

if [[ ($? -ne 0) || ($# -eq 0) ]]; then

echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"

exit $E_OPTERR

eval set -- "${inputOptions}"

# Only for educational purposes. Can be removed.

#-----------------------------------------------

echo "++ Test: Number of arguments: [$#]"

echo '++ Test: Looping through "$@"'

for a in "$@"; do

echo " ++ [$a]"

done

#-----------------------------------------------

while true; do

case "${1}" in

--aoption | -a) # Argument found.

echo "Option [$1]"

;;

--debug | -d) # Enable informational messages.

echo "Option [$1] Debugging enabled"

;;

--file | -f) # Check for optional argument.

case "$2" in #+ Double colon is optional argument.

"") # Not there.

echo "Option [$1] Use default"

shift

;;

*) # Got it

echo "Option [$1] Using input [$2]"

shift

;;

esac

DoSomething

;;

--log | -l) # Enable Logging.

echo "Option [$1] Logging enabled"

;;

--test | -t) # Enable testing.

echo "Option [$1] Testing enabled"

;;

--help | -h)

echo "Option [$1] Display help"

break

;;

--) # Done! $# is argument number for "--", $@ is "--"

echo "Option [$1] Dash Dash"

break

;;

echo "Major internal error!"

exit 8

;;

esac

echo "Number of arguments: [$#]"

shift

done

shift

# Only for educational purposes. Can be removed.

#----------------------------------------------------------------------

echo "++ Test: Number of arguments after \"--\" is [$#] They are: [$@]"

echo '++ Test: Looping through "$@"'

for a in "$@"; do

echo " ++ [$a]"

done

#----------------------------------------------------------------------

}

M A I N ########################

If you remove "function UseGetOpt () {" and corresponding "}",

+ you can uncomment the "exit 0" line below, and invoke this script

+ with the various options from the command-line.

-------------------------------------------------------------------

exit 0

echo "Test 1"

UseGetOpt -f myfile one "two three" four

echo;echo "Test 2"

UseGetOpt -h

echo;echo "Test 3 - Short Options"

UseGetOpt -adltf myfile anotherfile

echo;echo "Test 4 - Long Options"

UseGetOpt --aoption --debug --log --test --file myfile anotherfile

exit

Example A-51. The version of the UseGetOpt.sh example used in the Tab

Expansion appendix

!/bin/bash

UseGetOpt-2.sh

Modified version of the script for illustrating tab-expansion

+ of command-line options.

See the "Introduction to Tab Expansion" appendix.

Possible options: -a -d -f -l -t -h

+ --aoption, --debug --file --log --test -- help --

Author of original script: Peggy Russell <prusselltechgroup@gmail.com>

UseGetOpt () {

declare inputOptions

declare -r E_OPTERR=85

declare -r ScriptName=${0##*/}

declare -r ShortOpts="adf:hlt"

declare -r LongOpts="aoption,debug,file:,help,log,test"

DoSomething () {

echo "The function name is '${FUNCNAME}'"

}

inputOptions=$(getopt -o "${ShortOpts}" --long \

"${LongOpts}" --name "${ScriptName}" -- "${@}")

if [[ ($? -ne 0) || ($# -eq 0) ]]; then

echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"

exit $E_OPTERR

eval set -- "${inputOptions}"

while true; do

case "${1}" in

--aoption | -a) # Argument found.

echo "Option [$1]"

;;

--debug | -d) # Enable informational messages.

echo "Option [$1] Debugging enabled"

;;

--file | -f) # Check for optional argument.

case "$2" in #+ Double colon is optional argument.

"") # Not there.

echo "Option [$1] Use default"

shift

;;

*) # Got it

echo "Option [$1] Using input [$2]"

shift

;;

esac

DoSomething

;;

--log | -l) # Enable Logging.

echo "Option [$1] Logging enabled"

;;

--test | -t) # Enable testing.

echo "Option [$1] Testing enabled"

;;

--help | -h)

echo "Option [$1] Display help"

break

;;

--) # Done! $# is argument number for "--", $@ is "--"

echo "Option [$1] Dash Dash"

break

;;

echo "Major internal error!"

exit 8

;;

esac

echo "Number of arguments: [$#]"

shift

done

shift

}

exit

Example A-52. Cycling through all the possible color backgrounds

!/bin/bash

show-all-colors.sh

Displays all 256 possible background colors, using ANSI escape sequences.

Author: Chetankumar Phulpagare

Used in ABS Guide with permission.

T1=8

T2=6

T3=36

offset=0

for num1 in {0..7}

do {

for num2 in {0,1}

do {

shownum=`echo "$offset + $T1 * ${num2} + $num1" | bc`

echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"

}

done

echo

}

done

offset=16

for num1 in {0..5}

do {

for num2 in {0..5}

do {

for num3 in {0..5}

do {

shownum=`echo "$offset + $T2 * ${num3} \

+ $num2 + $T3 * ${num1}" | bc`

echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"

}

done

echo

}

done

}

done

offset=232

for num1 in {0..23}

do {

shownum=`expr $offset + $num1`

echo -en "\E[0;48;5;${shownum}m ${shownum}\E[0m"

}

done

echo

Example A-53. Morse Code Practice

!/bin/bash

sam.sh, v. .01a

Still Another Morse (code training script)

With profuse apologies to Sam (F.B.) Morse.

Author: Mendel Cooper

License: GPL3

Reldate: 05/25/11

Morse code training script.

Converts arguments to audible dots and dashes.

Note: lowercase input only at this time.

Get the wav files from the source tarball:

http://bash.deta.in/abs-guide-latest.tar.bz2

DOT='soundfiles/dot.wav'

DASH='soundfiles/dash.wav'

Maybe move soundfiles to /usr/local/sounds?

LETTERSPACE=300000 # Microseconds.

WORDSPACE=980000

Nice and slow, for beginners. Maybe 5 wpm?

EXIT_MSG="May the Morse be with you!"

E_NOARGS=75 # No command-line args?

declare -A morse # Associative array!

======================================= #

morse[a]="dot; dash"

morse[b]="dash; dot; dot; dot"

morse[c]="dash; dot; dash; dot"

morse[d]="dash; dot; dot"

morse[e]="dot"

morse[f]="dot; dot; dash; dot"

morse[g]="dash; dash; dot"

morse[h]="dot; dot; dot; dot"

morse[i]="dot; dot;"

morse[j]="dot; dash; dash; dash"

morse[k]="dash; dot; dash"

morse[l]="dot; dash; dot; dot"

morse[m]="dash; dash"

morse[n]="dash; dot"

morse[o]="dash; dash; dash"

morse[p]="dot; dash; dash; dot"

morse[q]="dash; dash; dot; dash"

morse[r]="dot; dash; dot"

morse[s]="dot; dot; dot"

morse[t]="dash"

morse[u]="dot; dot; dash"

morse[v]="dot; dot; dot; dash"

morse[w]="dot; dash; dash"

morse[x]="dash; dot; dot; dash"

morse[y]="dash; dot; dash; dash"

morse[z]="dash; dash; dot; dot"

morse[0]="dash; dash; dash; dash; dash"

morse[1]="dot; dash; dash; dash; dash"

morse[2]="dot; dot; dash; dash; dash"

morse[3]="dot; dot; dot; dash; dash"

morse[4]="dot; dot; dot; dot; dash"

morse[5]="dot; dot; dot; dot; dot"

morse[6]="dash; dot; dot; dot; dot"

morse[7]="dash; dash; dot; dot; dot"

morse[8]="dash; dash; dash; dot; dot"

morse[9]="dash; dash; dash; dash; dot"

The following must be escaped or quoted.

morse[?]="dot; dot; dash; dash; dot; dot"

morse[.]="dot; dash; dot; dash; dot; dash"

morse[,]="dash; dash; dot; dot; dash; dash"

morse[/]="dash; dot; dot; dash; dot"

morse[\@]="dot; dash; dash; dot; dash; dot"

======================================= #

play_letter ()

{

eval ${morse[$1]} # Play dots, dashes from appropriate sound files.

# Why is 'eval' necessary here?

usleep $LETTERSPACE # Pause in between letters.

}

extract_letters ()

{ # Slice string apart, letter by letter.

local pos=0 # Starting at left end of string.

local len=1 # One letter at a time.

strlen=${#1}

while [ $pos -lt $strlen ]

letter=${1:pos:len}

# ^^^^^^^^^^^^ See Chapter 10.1.

play_letter $letter

echo -n "*" # Mark letter just played.

((pos++))

done

}

Play the sounds ############

dot() { aplay "$DOT" 2&>/dev/null; }

dash() { aplay "$DASH" 2&>/dev/null; }

no_args ()

{

declare -a usage

usage=( $0 word1 word2 ... )

echo "Usage:"; echo

echo ${usage[*]}

for index in 0 1 2 3

extract_letters ${usage[index]}

usleep $WORDSPACE

echo -n " " # Print space between words.

done

echo "Usage: $0 word1 word2 ... "

echo; echo

}

int main()

{

clear # Clear the terminal screen.

echo " SAM"

echo "Still Another Morse code trainer"

echo " Author: Mendel Cooper"

echo; echo;

if [ -z "$1" ]

then

no_args

echo; echo; echo "$EXIT_MSG"; echo

exit $E_NOARGS

echo; echo "$*" # Print text that will be played.

until [ -z "$1" ]

extract_letters $1

shift # On to next word.

usleep $WORDSPACE

echo -n " " # Print space between words.

done

echo; echo; echo "$EXIT_MSG"; echo

exit 0

}

Exercises:

---------

1) Have the script accept either lowercase or uppercase words

+ as arguments. Hint: Use 'tr' . . .

2) Have the script optionally accept input from a text file.

Example A-54. Base64 encoding/decoding

!/bin/bash

base64.sh: Bash implementation of Base64 encoding and decoding.

Used in ABSG with permission (thanks!).

Encode or decode original Base64 (and also Base64url)

+ from STDIN to STDOUT.

Usage:

Encode

$ ./base64.sh < binary-file > binary-file.base64

Decode

$ ./base64.sh -d < binary-file.base64 > binary-file

Reference:

[1] RFC4648 - "The Base16, Base32, and Base64 Data Encodings"

http://tools.ietf.org/html/rfc4648#section-5

The base64_charset[] array contains entire base64 charset,

and additionally the character "=" ...

base64_charset=( {A..Z} {a..z} {0..9} + / = )

# Nice illustration of brace expansion.

Uncomment the ### line below to use base64url encoding instead of

+ original base64.

base64_charset=( {A..Z} {a..z} {0..9} - _ = )

Output text width when encoding

+ (64 characters, just like openssl output).

text_width=64

function display_base64_char {

Convert a 6-bit number (between 0 and 63) into its corresponding values

+ in Base64, then display the result with the specified text width.

printf "${base64_charset[$1]}"; (( width++ ))

(( width % text_width == 0 )) && printf "\n"

}

function encode_base64 {

Encode three 8-bit hexadecimal codes into four 6-bit numbers.

# We need two local int array variables:

# c8[]: to store the codes of the 8-bit characters to encode

# c6[]: to store the corresponding encoded values on 6-bit

declare -a -i c8 c6

# Convert hexadecimal to decimal.

c8=( $(printf "ibase=16; ${1:0:2}\n${1:2:2}\n${1:4:2}\n" | bc) )

# Let's play with bitwise operators

#+ (3x8-bit into 4x6-bits conversion).

(( c6[0] = c8[0] >> 2 ))

(( c6[1] = ((c8[0] & 3) << 4) | (c8[1] >> 4) ))

# The following operations depend on the c8 element number.

case ${#c8[*]} in

3) (( c6[2] = ((c8[1] & 15) << 2) | (c8[2] >> 6) ))

(( c6[3] = c8[2] & 63 )) ;;

2) (( c6[2] = (c8[1] & 15) << 2 ))

(( c6[3] = 64 )) ;;

1) (( c6[2] = c6[3] = 64 )) ;;

esac

for char in ${c6[@]}; do

display_base64_char ${char}

done

}

function decode_base64 {

Decode four base64 characters into three hexadecimal ASCII characters.

# c8[]: to store the codes of the 8-bit characters

# c6[]: to store the corresponding Base64 values on 6-bit

declare -a -i c8 c6

# Find decimal value corresponding to the current base64 character.

for current_char in ${1:0:1} ${1:1:1} ${1:2:1} ${1:3:1}; do

[ "${current_char}" = "=" ] && break

position=0

while [ "${current_char}" != "${base64_charset[${position}]}" ]; do

(( position++ ))

done

c6=( ${c6[*]} ${position} )

done

# Let's play with bitwise operators

#+ (4x8-bit into 3x6-bits conversion).

(( c8[0] = (c6[0] << 2) | (c6[1] >> 4) ))

# The next operations depends on the c6 elements number.

case ${#c6[*]} in

3) (( c8[1] = ( (c6[1] & 15) << 4) | (c6[2] >> 2) ))

(( c8[2] = (c6[2] & 3) << 6 )); unset c8[2] ;;

4) (( c8[1] = ( (c6[1] & 15) << 4) | (c6[2] >> 2) ))

(( c8[2] = ( (c6[2] & 3) << 6) | c6[3] )) ;;

esac

for char in ${c8[*]}; do

printf "\x$(printf "%x" ${char})"

done

}

main ()

if [ "$1" = "-d" ]; then # decode

# Reformat STDIN in pseudo 4x6-bit groups.

content=$(cat - | tr -d "\n" | sed -r "s/(.{4})/\1 /g")

for chars in ${content}; do decode_base64 ${chars}; done

else

# Make a hexdump of stdin and reformat in 3-byte groups.

content=$(cat - | xxd -ps -u | sed -r "s/(\w{6})/\1 /g" |

tr -d "\n")

for chars in ${content}; do encode_base64 ${chars}; done

echo

Example A-55. Inserting text in a file using sed

!/bin/bash

Prepends a string at a specified line

+ in files with names ending in "sample"

+ in the current working directory.

000000000000000000000000000000000000

This script overwrites files!

Be careful running it in a directory

+ where you have important files!!!

000000000000000000000000000000000000

Create a couple of files to operate on ...

01sample

02sample

... etc.

These files must not be empty, else the prepend will not work.

lineno=1 # Append at line 1 (prepend).

filespec="*sample" # Filename pattern to operate on.

string=$(whoami) # Will set your username as string to insert.

# It could just as easily be any other string.

for file in $filespec # Specify which files to alter.

do # ^^^^^^^^^

sed -i ""$lineno"i "$string"" $file

^^ -i option edits files in-place.

^ Insert (i) command.

echo ""$file" altered!"

done

echo "Warning: files possibly clobbered!"

exit 0

Exercise:

Add error checking to this script.

It needs it badly.

Example A-56. The Gronsfeld Cipher

!/bin/bash

gronsfeld.bash

License: GPL3

Reldate 06/23/11

This is an implementation of the Gronsfeld Cipher.

It's essentially a stripped-down variant of the

+ polyalphabetic Vigenère Tableau, but with only 10 alphabets.

The classic Gronsfeld has a numeric sequence as the key word,

+ but here we substitute a letter string, for ease of use.

Allegedly, this cipher was invented by the eponymous Count Gronsfeld

+ in the 17th Century. It was at one time considered to be unbreakable.

Note that this is ###not### a secure cipher by modern standards.

Global Variables #

Enc_suffix="29379" # Encrypted text output with this 5-digit suffix.

# This functions as a decryption flag,

#+ and when used to generate passwords adds security.

Default_key="gronsfeldk"

# The script uses this if key not entered below

# (at "Keychain").

# Change the above two values frequently

#+ for added security.

GROUPLEN=5 # Output in groups of 5 letters, per tradition.

alpha1=( abcdefghijklmnopqrstuvwxyz )

alpha2=( {A..Z} ) # Output in all caps, per tradition.

# Use alpha2=( {a..z} ) for password generator.

wraplen=26 # Wrap around if past end of alphabet.

dflag= # Decrypt flag (set if $Enc_suffix present).

E_NOARGS=76 # Missing command-line args?

DEBUG=77 # Debugging flag.

declare -a offsets # This array holds the numeric shift values for

#+ encryption/decryption.

Keychain#########

key= ### Put key here!!!

# 10 characters!

Function

: ()

{ # Encrypt or decrypt, depending on whether $dflag is set.

# Why ": ()" as a function name? Just to prove that it can be done.

local idx keydx mlen off1 shft

local plaintext="$1"

local mlen=${#plaintext}

for (( idx=0; idx<$mlen; idx++ ))

let "keydx = $idx % $keylen"

shft=${offsets[keydx]}

if [ -n "$dflag" ]

then # Decrypt!

let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) - $shft"

# Shift backward to decrypt.

else # Encrypt!

let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) + $shft"

# Shift forward to encrypt.

test $(( $idx % $GROUPLEN)) = 0 && echo -n " " # Groups of 5 letters.

# Comment out above line for output as a string without whitespace,

#+ for example, if using the script as a password generator.

((off1--)) # Normalize. Why is this necessary?

if [ $off1 -lt 0 ]

then # Catch negative indices.

let "off1 += $wraplen"

((off1 %= $wraplen)) # Wrap around if past end of alphabet.

echo -n "${alpha2[off1]}"

done

if [ -z "$dflag" ]

then

echo " $Enc_suffix"

echo "$Enc_suffix" # For password generator.

else

echo

} # End encrypt/decrypt function.

int main () {

Check for command-line args.

if [ -z "$1" ]

then

echo "Usage: $0 TEXT TO ENCODE/DECODE"

exit $E_NOARGS

if [ ${!#} == "$Enc_suffix" ]

^^^^^ Final command-line arg.

then

dflag=ON

echo -n "+" # Flag decrypted text with a "+" for easy ID.

if [ -z "$key" ]

then

key="$Default_key" # "gronsfeldk" per above.

keylen=${#key}

for (( idx=0; idx<$keylen; idx++ ))

do # Calculate shift values for encryption/decryption.

offsets[idx]=$(expr index "${alpha1[*]}" ${key:idx:1}) # Normalize.

((offsets[idx]--)) # Necessary because "expr index" starts at 1,

#+ whereas array count starts at 0.

# Generate array of numerical offsets corresponding to the key.

# There are simpler ways to accomplish this.

done

args=$(echo "$*" | sed -e 's/ //g' | tr A-Z a-z | sed -e 's/[0-9]//g')

Remove whitespace and digits from command-line args.

Can modify to also remove punctuation characters, if desired.

# Debug:

# echo "$args"; exit $DEBUG

: "$args" # Call the function named ":".

: is a null operator, except . . . when it's a function name!

exit $? # } End-of-script

************************************************************** #

This script can function as a password generator,

+ with several minor mods, see above.

That would allow an easy-to-remember password, even the word

+ "password" itself, which encrypts to vrgfotvo29379

+ a fairly secure password not susceptible to a dictionary attack.

Or, you could use your own name (surely that's easy to remember!).

For example, Bozo Bozeman encrypts to hfnbttdppkt29379.

************************************************************** #

Example A-57. Bingo Number Generator

!/bin/bash

bingo.sh

Bingo number generator

Reldate 20Aug12, License: Public Domain

This script generates bingo numbers.

Hitting a key generates a new number.

Hitting 'q' terminates the script.

In a given run of the script, there will be no duplicate numbers.

When the script terminates, it prints a log of the numbers generated.

MIN=1 # Lowest allowable bingo number.

MAX=75 # Highest allowable bingo number.

COLS=15 # Numbers in each column (B I N G O).

SINGLE_DIGIT_MAX=9

declare -a Numbers

Prefix=(B I N G O)

initialize_Numbers ()

{ # Zero them out to start.

# They'll be incremented if chosen.

local index=0

until [ "$index" -gt $MAX ]

Numbers[index]=0

((index++))

done

Numbers[0]=1 # Flag zero, so it won't be selected.

}

generate_number ()

{

local number

while [ 1 ]

let "number = $(expr $RANDOM % $MAX)"

if [ ${Numbers[number]} -eq 0 ] # Number not yet called.

then

let "Numbers[number]+=1" # Flag it in the array.

break # And terminate loop.

fi # Else if already called, loop and generate another number.

done

# Exercise: Rewrite this more elegantly as an until-loop.

return $number

}

print_numbers_called ()

{ # Print out the called number log in neat columns.

# echo ${Numbers[@]}

local pre2=0 # Prefix a zero, so columns will align

#+ on single-digit numbers.

echo "Number Stats"

for (( index=1; index<=MAX; index++))

count=${Numbers[index]}

let "t = $index - 1" # Normalize, since array begins with index 0.

let "column = $(expr $t / $COLS)"

pre=${Prefix[column]}

echo -n "${Prefix[column]} "

if [ $(expr $t % $COLS) -eq 0 ]

then

echo # Newline at end of row.

if [ "$index" -gt $SINGLE_DIGIT_MAX ] # Check for single-digit number.

then

echo -n "$pre$index#$count "

else # Prefix a zero.

echo -n "$pre$pre2$index#$count "

done

}

main () {

RANDOM=$ # Seed random number generator.

initialize_Numbers # Zero out the number tracking array.

clear

echo "Bingo Number Caller"; echo

while [[ "$key" != "q" ]] # Main loop.

read -s -n1 -p "Hit a key for the next number [q to exit] " key

# Usually 'q' exits, but not always.

# Can always hit Ctl-C if q fails.

echo

generate_number; new_number=$?

let "column = $(expr $new_number / $COLS)"

echo -n "${Prefix[column]} " # B-I-N-G-O

echo $new_number

done

echo; echo

Game over ...

print_numbers_called

echo; echo "[#0 = not called . . . #1 = called]"

echo

exit 0

}

Certainly, this script could stand some improvement.

www.instructables.com/id/Binguino-An-Arduino-based-Bingo-Number-Generato/

To end this section, a review of the basics . . . and more.

Example A-58. Basics Reviewed

!/bin/bash

basics-reviewed.bash

File extension == *.bash == specific to Bash

License: Use in any form, for any purpose.

Revision: $ID$

Edited for layout by M.C.

(author of the "Advanced Bash Scripting Guide")

Fixes and updates (04/08) by Cliff Bamford.

This script tested under Bash versions 2.04, 2.05a and 2.05b.

It may not work with earlier versions.

This demonstration script generates one --intentional--

+ "command not found" error message. See line 436.

The current Bash maintainer, Chet Ramey, has fixed the items noted

+ for later versions of Bash.

###-------------------------------------------###

### Pipe the output of this script to 'more' ###

###+ else it will scroll off the page. ###

### ###

### You may also redirect its output ###

###+ to a file for examination. ###

###-------------------------------------------###

Most of the following points are described at length in

+ the text of the foregoing "Advanced Bash Scripting Guide."

This demonstration script is mostly just a reorganized presentation.

-- msz

Variables are not typed unless otherwise specified.

Variables are named. Names must contain a non-digit.

File descriptor names (as in, for example: 2>&1)

+ contain ONLY digits.

Parameters and Bash array elements are numbered.

(Parameters are very similar to Bash arrays.)

A variable name may be undefined (null reference).

unset VarNull

A variable name may be defined but empty (null contents).

VarEmpty='' # Two, adjacent, single quotes.

A variable name may be defined and non-empty.

VarSomething='Literal'

A variable may contain:

* A whole number as a signed 32-bit (or larger) integer

* A string

A variable may also be an array.

A string may contain embedded blanks and may be treated

+ as if it where a function name with optional arguments.

The names of variables and the names of functions

+ are in different namespaces.

A variable may be defined as a Bash array either explicitly or

+ implicitly by the syntax of the assignment statement.

Explicit:

declare -a ArrayVar

The echo command is a builtin.

echo $VarSomething

The printf command is a builtin.

Translate %s as: String-Format

printf %s $VarSomething # No linebreak specified, none output.

echo # Default, only linebreak output.

The Bash parser word breaks on whitespace.

Whitespace, or the lack of it is significant.

(This holds true in general; there are, of course, exceptions.)

Translate the DOLLAR_SIGN character as: Content-Of.

Extended-Syntax way of writing Content-Of:

echo ${VarSomething}

The ${ ... } Extended-Syntax allows more than just the variable

+ name to be specified.

In general, $VarSomething can always be written as: ${VarSomething}.

Call this script with arguments to see the following in action.

Outside of double-quotes, the special characters @ and *

+ specify identical behavior.

May be pronounced as: All-Elements-Of.

Without specification of a name, they refer to the

+ pre-defined parameter Bash-Array.

Glob-Pattern references

echo $* # All parameters to script or function

echo ${*} # Same

Bash disables filename expansion for Glob-Patterns.

Only character matching is active.

All-Elements-Of references

echo $@ # Same as above

echo ${@} # Same as above

Within double-quotes, the behavior of Glob-Pattern references

+ depends on the setting of IFS (Input Field Separator).

Within double-quotes, All-Elements-Of references behave the same.

Specifying only the name of a variable holding a string refers

+ to all elements (characters) of a string.

To specify an element (character) of a string,

+ the Extended-Syntax reference notation (see below) MAY be used.

Specifying only the name of a Bash array references

+ the subscript zero element,

+ NOT the FIRST DEFINED nor the FIRST WITH CONTENTS element.

Additional qualification is needed to reference other elements,

+ which means that the reference MUST be written in Extended-Syntax.

The general form is: ${name[subscript]}.

The string forms may also be used: ${name:subscript}

+ for Bash-Arrays when referencing the subscript zero element.

Bash-Arrays are implemented internally as linked lists,

+ not as a fixed area of storage as in some programming languages.

Characteristics of Bash arrays (Bash-Arrays):

--------------------------------------------

If not otherwise specified, Bash-Array subscripts begin with

+ subscript number zero. Literally: [0]

This is called zero-based indexing.

If not otherwise specified, Bash-Arrays are subscript packed

+ (sequential subscripts without subscript gaps).

Negative subscripts are not allowed.

Elements of a Bash-Array need not all be of the same type.

Elements of a Bash-Array may be undefined (null reference).

That is, a Bash-Array may be "subscript sparse."

Elements of a Bash-Array may be defined and empty (null contents).

Elements of a Bash-Array may contain:

* A whole number as a signed 32-bit (or larger) integer

* A string

* A string formated so that it appears to be a function name

+ with optional arguments

Defined elements of a Bash-Array may be undefined (unset).

That is, a subscript packed Bash-Array may be changed

+ into a subscript sparse Bash-Array.

Elements may be added to a Bash-Array by defining an element

+ not previously defined.

For these reasons, I have been calling them "Bash-Arrays".

I'll return to the generic term "array" from now on.

-- msz

echo "========================================================="

Lines 202 - 334 supplied by Cliff Bamford. (Thanks!)

Demo --- Interaction with Arrays, quoting, IFS, echo, * and @ ---

+ all affect how things work

ArrayVar[0]='zero' # 0 normal

ArrayVar[1]=one # 1 unquoted literal

ArrayVar[2]='two' # 2 normal

ArrayVar[3]='three' # 3 normal

ArrayVar[4]='I am four' # 4 normal with spaces

ArrayVar[5]='five' # 5 normal

unset ArrayVar[6] # 6 undefined

ArrayValue[7]='seven' # 7 normal

ArrayValue[8]='' # 8 defined but empty

ArrayValue[9]='nine' # 9 normal

echo '--- Here is the array we are using for this test'

echo

echo "ArrayVar[0]='zero' # 0 normal"

echo "ArrayVar[1]=one # 1 unquoted literal"

echo "ArrayVar[2]='two' # 2 normal"

echo "ArrayVar[3]='three' # 3 normal"

echo "ArrayVar[4]='I am four' # 4 normal with spaces"

echo "ArrayVar[5]='five' # 5 normal"

echo "unset ArrayVar[6] # 6 undefined"

echo "ArrayValue[7]='seven' # 7 normal"

echo "ArrayValue[8]='' # 8 defined but empty"

echo "ArrayValue[9]='nine' # 9 normal"

echo

echo '---Case0: No double-quotes, Default IFS of space,tab,newline ---'

IFS= \x20' \x09' \x0A' # In exactly this order.

echo 'Here is: printf %q {${ArrayVar[*]}'

printf %q ${ArrayVar[*]}

echo

echo 'Here is: printf %q {${ArrayVar[@]}'

printf %q ${ArrayVar[@]}

echo

echo 'Here is: echo ${ArrayVar[*]}'

echo ${ArrayVar[@]}

echo 'Here is: echo {${ArrayVar[@]}'

echo ${ArrayVar[@]}

echo

echo '---Case1: Within double-quotes - Default IFS of space-tab-

newline ---'

IFS= \x20' \x09' \x0A' # These three bytes,

echo 'Here is: printf %q "{${ArrayVar[*]}"'

printf %q "${ArrayVar[*]}"

echo

echo 'Here is: printf %q "{${ArrayVar[@]}"'

printf %q "${ArrayVar[@]}"

echo

echo 'Here is: echo "${ArrayVar[*]}"'

echo "${ArrayVar[@]}"

echo 'Here is: echo "{${ArrayVar[@]}"'

echo "${ArrayVar[@]}"

echo

echo '---Case2: Within double-quotes - IFS is q'

IFS='q'

echo 'Here is: printf %q "{${ArrayVar[*]}"'

printf %q "${ArrayVar[*]}"

echo

echo 'Here is: printf %q "{${ArrayVar[@]}"'

printf %q "${ArrayVar[@]}"

echo

echo 'Here is: echo "${ArrayVar[*]}"'

echo "${ArrayVar[@]}"

echo 'Here is: echo "{${ArrayVar[@]}"'

echo "${ArrayVar[@]}"

echo

echo '---Case3: Within double-quotes - IFS is ^'

IFS='^'

echo 'Here is: printf %q "{${ArrayVar[*]}"'

printf %q "${ArrayVar[*]}"

echo

echo 'Here is: printf %q "{${ArrayVar[@]}"'

printf %q "${ArrayVar[@]}"

echo

echo 'Here is: echo "${ArrayVar[*]}"'

echo "${ArrayVar[@]}"

echo 'Here is: echo "{${ArrayVar[@]}"'

echo "${ArrayVar[@]}"

echo

echo '---Case4: Within double-quotes - IFS is ^ followed by

space,tab,newline'

IFS= ^' \x20' \x09' \x0A' # ^ + space tab newline

echo 'Here is: printf %q "{${ArrayVar[*]}"'

printf %q "${ArrayVar[*]}"

echo

echo 'Here is: printf %q "{${ArrayVar[@]}"'

printf %q "${ArrayVar[@]}"

echo

echo 'Here is: echo "${ArrayVar[*]}"'

echo "${ArrayVar[@]}"

echo 'Here is: echo "{${ArrayVar[@]}"'

echo "${ArrayVar[@]}"

echo

echo '---Case6: Within double-quotes - IFS set and empty '

IFS=''

echo 'Here is: printf %q "{${ArrayVar[*]}"'

printf %q "${ArrayVar[*]}"

echo

echo 'Here is: printf %q "{${ArrayVar[@]}"'

printf %q "${ArrayVar[@]}"

echo

echo 'Here is: echo "${ArrayVar[*]}"'

echo "${ArrayVar[@]}"

echo 'Here is: echo "{${ArrayVar[@]}"'

echo "${ArrayVar[@]}"

echo

echo '---Case7: Within double-quotes - IFS is unset'

unset IFS

echo 'Here is: printf %q "{${ArrayVar[*]}"'

printf %q "${ArrayVar[*]}"

echo

echo 'Here is: printf %q "{${ArrayVar[@]}"'

printf %q "${ArrayVar[@]}"

echo

echo 'Here is: echo "${ArrayVar[*]}"'

echo "${ArrayVar[@]}"

echo 'Here is: echo "{${ArrayVar[@]}"'

echo "${ArrayVar[@]}"

echo

echo '---End of Cases---'

echo "========================================================="; echo

Put IFS back to the default.

Default is exactly these three bytes.

IFS= \x20' \x09' \x0A' # In exactly this order.

Interpretation of the above outputs:

A Glob-Pattern is I/O; the setting of IFS matters.

An All-Elements-Of does not consider IFS settings.

Note the different output using the echo command and the

+ quoted format operator of the printf command.

Recall:

Parameters are similar to arrays and have the similar behaviors.

The above examples demonstrate the possible variations.

To retain the shape of a sparse array, additional script

+ programming is required.

The source code of Bash has a routine to output the

+ [subscript]=value array assignment format.

As of version 2.05b, that routine is not used,

+ but that might change in future releases.

The length of a string, measured in non-null elements (characters):

echo

echo '- - Non-quoted references - -'

echo 'Non-Null character count: '${#VarSomething}' characters.'

test='Lit' \x00''eral' # \x00' is a null character.

echo ${#test} # See that?

The length of an array, measured in defined elements,

+ including null content elements.

echo

echo 'Defined content count: '${#ArrayVar[@]}' elements.'

That is NOT the maximum subscript (4).

That is NOT the range of the subscripts (1 . . 4 inclusive).

It IS the length of the linked list.

Both the maximum subscript and the range of the subscripts may

+ be found with additional script programming.

The length of a string, measured in non-null elements (characters):

echo

echo '- - Quoted, Glob-Pattern references - -'

echo 'Non-Null character count: '"${#VarSomething}"' characters.'

The length of an array, measured in defined elements,

+ including null-content elements.

echo

echo 'Defined element count: '"${#ArrayVar[*]}"' elements.'

Interpretation: Substitution does not effect the ${# ... } operation.

Suggestion:

Always use the All-Elements-Of character

+ if that is what is intended (independence from IFS).

Define a simple function.

I include an underscore in the name

+ to make it distinctive in the examples below.

Bash separates variable names and function names

+ in different namespaces.

The Mark-One eyeball isn't that advanced.

_simple() {

echo -n 'SimpleFunc'$@ # Newlines are swallowed in

} #+ result returned in any case.

The ( ... ) notation invokes a command or function.

The $( ... ) notation is pronounced: Result-Of.

Invoke the function _simple

echo

echo '- - Output of function _simple - -'

_simple # Try passing arguments.

echo

or

(_simple) # Try passing arguments.

echo

echo '- Is there a variable of that name? -'

echo $_simple not defined # No variable by that name.

Invoke the result of function _simple (Error msg intended)

$(_simple) # Gives an error message:

line 436: SimpleFunc: command not found

---------------------------------------

echo

The first word of the result of function _simple

+ is neither a valid Bash command nor the name of a defined function.

This demonstrates that the output of _simple is subject to evaluation.

Interpretation:

A function can be used to generate in-line Bash commands.

A simple function where the first word of result IS a bash command:

_print() {

echo -n 'printf %q '$@

}

echo '- - Outputs of function _print - -'

_print parm1 parm2 # An Output NOT A Command.

echo

$(_print parm1 parm2) # Executes: printf %q parm1 parm2

# See above IFS examples for the

#+ various possibilities.

echo

$(_print $VarSomething) # The predictable result.

echo

Function variables

------------------

echo

echo '- - Function variables - -'

A variable may represent a signed integer, a string or an array.

A string may be used like a function name with optional arguments.

set -vx # Enable if desired

declare -f funcVar #+ in namespace of functions

funcVar=_print # Contains name of function.

$funcVar parm1 # Same as _print at this point.

echo

funcVar=$(_print ) # Contains result of function.

$funcVar # No input, No output.

$funcVar $VarSomething # The predictable result.

echo

funcVar=$(_print $VarSomething) # $VarSomething replaced HERE.

$funcVar # The expansion is part of the

echo #+ variable contents.

funcVar="$(_print $VarSomething)" # $VarSomething replaced HERE.

$funcVar # The expansion is part of the

echo #+ variable contents.

The difference between the unquoted and the double-quoted versions

+ above can be seen in the "protect_literal.sh" example.

The first case above is processed as two, unquoted, Bash-Words.

The second case above is processed as one, quoted, Bash-Word.

Delayed replacement

-------------------

echo

echo '- - Delayed replacement - -'

funcVar="$(_print '$VarSomething')" # No replacement, single Bash-Word.

eval $funcVar # $VarSomething replaced HERE.

echo

VarSomething='NewThing'

eval $funcVar # $VarSomething replaced HERE.

echo

Restore the original setting trashed above.

VarSomething=Literal

There are a pair of functions demonstrated in the

+ "protect_literal.sh" and "unprotect_literal.sh" examples.

These are general purpose functions for delayed replacement literals

+ containing variables.

REVIEW:

------

A string can be considered a Classic-Array of elements (characters).

A string operation applies to all elements (characters) of the string

+ (in concept, anyway).

The notation: ${array_name[@]} represents all elements of the

+ Bash-Array: array_name.

The Extended-Syntax string operations can be applied to all

+ elements of an array.

This may be thought of as a For-Each operation on a vector of strings.

Parameters are similar to an array.

The initialization of a parameter array for a script

+ and a parameter array for a function only differ

+ in the initialization of ${0}, which never changes its setting.

Subscript zero of the script's parameter array contains

+ the name of the script.

Subscript zero of a function's parameter array DOES NOT contain

+ the name of the function.

The name of the current function is accessed by the $FUNCNAME variable.

A quick, review list follows (quick, not short).

echo

echo '- - Test (but not change) - -'

echo '- null reference -'

echo -n ${VarNull-'NotSet'}' ' # NotSet

echo ${VarNull} # NewLine only

echo -n ${VarNull:-'NotSet'}' ' # NotSet

echo ${VarNull} # Newline only

echo '- null contents -'

echo -n ${VarEmpty-'Empty'}' ' # Only the space

echo ${VarEmpty} # Newline only

echo -n ${VarEmpty:-'Empty'}' ' # Empty

echo ${VarEmpty} # Newline only

echo '- contents -'

echo ${VarSomething-'Content'} # Literal

echo ${VarSomething:-'Content'} # Literal

echo '- Sparse Array -'

echo ${ArrayVar[@]-'not set'}

ASCII-Art time

State Y==yes, N==no

- :-

Unset Y Y ${# ... } == 0

Empty N Y ${# ... } == 0

Contents N N ${# ... } > 0

Either the first and/or the second part of the tests

+ may be a command or a function invocation string.

echo

echo '- - Test 1 for undefined - -'

declare -i t

_decT() {

t=$t-1

}

Null reference, set: t == -1

t=${#VarNull} # Results in zero.

${VarNull- _decT } # Function executes, t now -1.

echo $t

Null contents, set: t == 0

t=${#VarEmpty} # Results in zero.

${VarEmpty- _decT } # _decT function NOT executed.

echo $t

Contents, set: t == number of non-null characters

VarSomething='_simple' # Set to valid function name.

t=${#VarSomething} # non-zero length

${VarSomething- _decT } # Function _simple executed.

echo $t # Note the Append-To action.

Exercise: clean up that example.

unset t

unset _decT

VarSomething=Literal

echo

echo '- - Test and Change - -'

echo '- Assignment if null reference -'

echo -n ${VarNull='NotSet'}' ' # NotSet NotSet

echo ${VarNull}

unset VarNull

echo '- Assignment if null reference -'

echo -n ${VarNull:='NotSet'}' ' # NotSet NotSet

echo ${VarNull}

unset VarNull

echo '- No assignment if null contents -'

echo -n ${VarEmpty='Empty'}' ' # Space only

echo ${VarEmpty}

VarEmpty=''

echo '- Assignment if null contents -'

echo -n ${VarEmpty:='Empty'}' ' # Empty Empty

echo ${VarEmpty}

VarEmpty=''

echo '- No change if already has contents -'

echo ${VarSomething='Content'} # Literal

echo ${VarSomething:='Content'} # Literal

"Subscript sparse" Bash-Arrays

Bash-Arrays are subscript packed, beginning with

+ subscript zero unless otherwise specified.

The initialization of ArrayVar was one way

+ to "otherwise specify". Here is the other way:

echo

declare -a ArraySparse

ArraySparse=( [1]=one [2]='' [4]='four' )

[0]=null reference, [2]=null content, [3]=null reference

echo '- - Array-Sparse List - -'

Within double-quotes, default IFS, Glob-Pattern

IFS= \x20' \x09' \x0A'

printf %q "${ArraySparse[*]}"

echo

Note that the output does not distinguish between "null content"

+ and "null reference".

Both print as escaped whitespace.

Note also that the output does NOT contain escaped whitespace

+ for the "null reference(s)" prior to the first defined element.

This behavior of 2.04, 2.05a and 2.05b has been reported

+ and may change in a future version of Bash.

To output a sparse array and maintain the [subscript]=value

+ relationship without change requires a bit of programming.

One possible code fragment:

local l=${#ArraySparse[@]} # Count of defined elements

local f=0 # Count of found subscripts

local i=0 # Subscript to test

( # Anonymous in-line function

for (( l=${#ArraySparse[@]}, f = 0, i = 0 ; f < l ; i++ ))

# 'if defined then...'

${ArraySparse[$i]+ eval echo '\ ['$i']='${ArraySparse[$i]} ; (( f++ ))

}

done

)

The reader coming upon the above code fragment cold

+ might want to review "command lists" and "multiple commands on a line"

+ in the text of the foregoing "Advanced Bash Scripting Guide."

Note:

The "read -a array_name" version of the "read" command

+ begins filling array_name at subscript zero.

ArraySparse does not define a value at subscript zero.

The user needing to read/write a sparse array to either

+ external storage or a communications socket must invent

+ a read/write code pair suitable for their purpose.

Exercise: clean it up.

unset ArraySparse

echo

echo '- - Conditional alternate (But not change)- -'

echo '- No alternate if null reference -'

echo -n ${VarNull+'NotSet'}' '

echo ${VarNull}

unset VarNull

echo '- No alternate if null reference -'

echo -n ${VarNull:+'NotSet'}' '

echo ${VarNull}

unset VarNull

echo '- Alternate if null contents -'

echo -n ${VarEmpty+'Empty'}' ' # Empty

echo ${VarEmpty}

VarEmpty=''

echo '- No alternate if null contents -'

echo -n ${VarEmpty:+'Empty'}' ' # Space only

echo ${VarEmpty}

VarEmpty=''

echo '- Alternate if already has contents -'

Alternate literal

echo -n ${VarSomething+'Content'}' ' # Content Literal

echo ${VarSomething}

Invoke function

echo -n ${VarSomething:+ $(_simple) }' ' # SimpleFunc Literal

echo ${VarSomething}

echo

echo '- - Sparse Array - -'

echo ${ArrayVar[@]+'Empty'} # An array of 'Empty'(ies)

echo

echo '- - Test 2 for undefined - -'

declare -i t

_incT() {

t=$t+1

}

Note:

This is the same test used in the sparse array

+ listing code fragment.

Null reference, set: t == -1

t=${#VarNull}-1 # Results in minus-one.

${VarNull+ _incT } # Does not execute.

echo $t' Null reference'

Null contents, set: t == 0

t=${#VarEmpty}-1 # Results in minus-one.

${VarEmpty+ _incT } # Executes.

echo $t' Null content'

Contents, set: t == (number of non-null characters)

t=${#VarSomething}-1 # non-null length minus-one

${VarSomething+ _incT } # Executes.

echo $t' Contents'

Exercise: clean up that example.

unset t

unset _incT

${name?err_msg} ${name:?err_msg}

These follow the same rules but always exit afterwards

+ if an action is specified following the question mark.

The action following the question mark may be a literal

+ or a function result.

${name?} ${name:?} are test-only, the return can be tested.

Element operations

------------------

echo

echo '- - Trailing sub-element selection - -'

Strings, Arrays and Positional parameters

Call this script with multiple arguments

+ to see the parameter selections.

echo '- All -'

echo ${VarSomething:0} # all non-null characters

echo ${ArrayVar[@]:0} # all elements with content

echo ${@:0} # all parameters with content;

# ignoring parameter[0]

echo

echo '- All after -'

echo ${VarSomething:1} # all non-null after character[0]

echo ${ArrayVar[@]:1} # all after element[0] with content

echo ${@:2} # all after param[1] with content

echo

echo '- Range after -'

echo ${VarSomething:4:3} # ral

# Three characters after

# character[3]

echo '- Sparse array gotch -'

echo ${ArrayVar[@]:1:2} # four - The only element with content.

# Two elements after (if that many exist).

# the FIRST WITH CONTENTS

#+ (the FIRST WITH CONTENTS is being

#+ considered as if it

#+ were subscript zero).

Executed as if Bash considers ONLY array elements with CONTENT

printf %q "${ArrayVar[@]:0:3}" # Try this one

In versions 2.04, 2.05a and 2.05b,

+ Bash does not handle sparse arrays as expected using this notation.

The current Bash maintainer, Chet Ramey, has corrected this.

echo '- Non-sparse array -'

echo ${@:2:2} # Two parameters following parameter[1]

New victims for string vector examples:

stringZ=abcABC123ABCabc

arrayZ=( abcabc ABCABC 123123 ABCABC abcabc )

sparseZ=( [1]='abcabc' [3]='ABCABC' [4]='' [5]='123123' )

echo

echo ' - - Victim string - -'$stringZ'- - '

echo ' - - Victim array - -'${arrayZ[@]}'- - '

echo ' - - Sparse array - -'${sparseZ[@]}'- - '

echo ' - [0]==null ref, [2]==null ref, [4]==null content - '

echo ' - [1]=abcabc [3]=ABCABC [5]=123123 - '

echo ' - non-null-reference count: '${#sparseZ[@]}' elements'

echo

echo '- - Prefix sub-element removal - -'

echo '- - Glob-Pattern match must include the first character. - -'

echo '- - Glob-Pattern may be a literal or a function result. - -'

echo

Function returning a simple, Literal, Glob-Pattern

_abc() {

echo -n 'abc'

}

echo '- Shortest prefix -'

echo ${stringZ#123} # Unchanged (not a prefix).

echo ${stringZ#$(_abc)} # ABC123ABCabc

echo ${arrayZ[@]#abc} # Applied to each element.

echo ${sparseZ[@]#abc} # Version-2.05b core dumps.

Has since been fixed by Chet Ramey.

The -it would be nice- First-Subscript-Of

echo ${#sparseZ[@]#*} # This is NOT valid Bash.

echo

echo '- Longest prefix -'

echo ${stringZ##1*3} # Unchanged (not a prefix)

echo ${stringZ##a*C} # abc

echo ${arrayZ[@]##a*c} # ABCABC 123123 ABCABC

echo ${sparseZ[@]##a*c} # Version-2.05b core dumps.

Has since been fixed by Chet Ramey.

echo

echo '- - Suffix sub-element removal - -'

echo '- - Glob-Pattern match must include the last character. - -'

echo '- - Glob-Pattern may be a literal or a function result. - -'

echo

echo '- Shortest suffix -'

echo ${stringZ%1*3} # Unchanged (not a suffix).

echo ${stringZ%$(_abc)} # abcABC123ABC

echo ${arrayZ[@]%abc} # Applied to each element.

echo ${sparseZ[@]%abc} # Version-2.05b core dumps.

Has since been fixed by Chet Ramey.

The -it would be nice- Last-Subscript-Of

echo ${#sparseZ[@]%*} # This is NOT valid Bash.

echo

echo '- Longest suffix -'

echo ${stringZ%%1*3} # Unchanged (not a suffix)

echo ${stringZ%%b*c} # a

echo ${arrayZ[@]%%b*c} # a ABCABC 123123 ABCABC a

echo ${sparseZ[@]%%b*c} # Version-2.05b core dumps.

Has since been fixed by Chet Ramey.

echo

echo '- - Sub-element replacement - -'

echo '- - Sub-element at any location in string. - -'

echo '- - First specification is a Glob-Pattern - -'

echo '- - Glob-Pattern may be a literal or Glob-Pattern function result. - -'

echo '- - Second specification may be a literal or function result. - -'

echo '- - Second specification may be unspecified. Pronounce that'

echo ' as: Replace-With-Nothing (Delete) - -'

echo

Function returning a simple, Literal, Glob-Pattern

_123() {

echo -n '123'

}

echo '- Replace first occurrence -'

echo ${stringZ/$(_123)/999} # Changed (123 is a component).

echo ${stringZ/ABC/xyz} # xyzABC123ABCabc

echo ${arrayZ[@]/ABC/xyz} # Applied to each element.

echo ${sparseZ[@]/ABC/xyz} # Works as expected.

echo

echo '- Delete first occurrence -'

echo ${stringZ/$(_123)/}

echo ${stringZ/ABC/}

echo ${arrayZ[@]/ABC/}

echo ${sparseZ[@]/ABC/}

The replacement need not be a literal,

+ since the result of a function invocation is allowed.

This is general to all forms of replacement.

echo

echo '- Replace first occurrence with Result-Of -'

echo ${stringZ/$(_123)/$(_simple)} # Works as expected.

echo ${arrayZ[@]/ca/$(_simple)} # Applied to each element.

echo ${sparseZ[@]/ca/$(_simple)} # Works as expected.

echo

echo '- Replace all occurrences -'

echo ${stringZ//[b2]/X} # X-out b's and 2's

echo ${stringZ//abc/xyz} # xyzABC123ABCxyz

echo ${arrayZ[@]//abc/xyz} # Applied to each element.

echo ${sparseZ[@]//abc/xyz} # Works as expected.

echo

echo '- Delete all occurrences -'

echo ${stringZ//[b2]/}

echo ${stringZ//abc/}

echo ${arrayZ[@]//abc/}

echo ${sparseZ[@]//abc/}

echo

echo '- - Prefix sub-element replacement - -'

echo '- - Match must include the first character. - -'

echo

echo '- Replace prefix occurrences -'

echo ${stringZ/#[b2]/X} # Unchanged (neither is a prefix).

echo ${stringZ/#$(_abc)/XYZ} # XYZABC123ABCabc

echo ${arrayZ[@]/#abc/XYZ} # Applied to each element.

echo ${sparseZ[@]/#abc/XYZ} # Works as expected.

echo

echo '- Delete prefix occurrences -'

echo ${stringZ/#[b2]/}

echo ${stringZ/#$(_abc)/}

echo ${arrayZ[@]/#abc/}

echo ${sparseZ[@]/#abc/}

echo

echo '- - Suffix sub-element replacement - -'

echo '- - Match must include the last character. - -'

echo

echo '- Replace suffix occurrences -'

echo ${stringZ/%[b2]/X} # Unchanged (neither is a suffix).

echo ${stringZ/%$(_abc)/XYZ} # abcABC123ABCXYZ

echo ${arrayZ[@]/%abc/XYZ} # Applied to each element.

echo ${sparseZ[@]/%abc/XYZ} # Works as expected.

echo

echo '- Delete suffix occurrences -'

echo ${stringZ/%[b2]/}

echo ${stringZ/%$(_abc)/}

echo ${arrayZ[@]/%abc/}

echo ${sparseZ[@]/%abc/}

echo

echo '- - Special cases of null Glob-Pattern - -'

echo

echo '- Prefix all -'

null substring pattern means 'prefix'

echo ${stringZ/#/NEW} # NEWabcABC123ABCabc

echo ${arrayZ[@]/#/NEW} # Applied to each element.

echo ${sparseZ[@]/#/NEW} # Applied to null-content also.

# That seems reasonable.

echo

echo '- Suffix all -'

null substring pattern means 'suffix'

echo ${stringZ/%/NEW} # abcABC123ABCabcNEW

echo ${arrayZ[@]/%/NEW} # Applied to each element.

echo ${sparseZ[@]/%/NEW} # Applied to null-content also.

# That seems reasonable.

echo

echo '- - Special case For-Each Glob-Pattern - -'

echo '- - - - This is a nice-to-have dream - - - -'

echo

_GenFunc() {

echo -n ${0} # Illustration only.

# Actually, that would be an arbitrary computation.

}

All occurrences, matching the AnyThing pattern.

Currently //*/ does not match null-content nor null-reference.

/#/ and /%/ does match null-content but not null-reference.

echo ${sparseZ[@]//*/$(_GenFunc)}

A possible syntax would be to make

+ the parameter notation used within this construct mean:

${1} - The full element

${2} - The prefix, if any, to the matched sub-element

${3} - The matched sub-element

${4} - The suffix, if any, to the matched sub-element

echo ${sparseZ[@]//*/$(_GenFunc ${3})} # Same as ${1} here.

Perhaps it will be implemented in a future version of Bash.

exit 0

Example A-59. Testing execution times of various commands

!/bin/bash

test-execution-time.sh

Example by Erik Brandsberg, for testing execution time

+ of certain operations.

Referenced in the "Optimizations" section of "Miscellany" chapter.

count=50000

echo "Math tests"

echo "Math via \$(( ))"

time for (( i=0; i< $count; i++))

result=$(( $i%2 ))

done

echo "Math via *expr*:"

time for (( i=0; i< $count; i++))

result=`expr "$i%2"`

done

echo "Math via *let*:"

time for (( i=0; i< $count; i++))

let result=$i%2

done

echo

echo "Conditional testing tests"

echo "Test via case:"

time for (( i=0; i< $count; i++))

case $(( $i%2 )) in

0) : ;;

1) : ;;

esac

done

echo "Test with if [], no quotes:"

time for (( i=0; i< $count; i++))

if [ $(( $i%2 )) = 0 ]; then

else

done

echo "Test with if [], quotes:"

time for (( i=0; i< $count; i++))

if [ "$(( $i%2 ))" = "0" ]; then

else

done

echo "Test with if [], using -eq:"

time for (( i=0; i< $count; i++))

if [ $(( $i%2 )) -eq 0 ]; then

else

done

exit $?

Example A-60. Associative arrays vs. conventional arrays (execution

times)

!/bin/bash

assoc-arr-test.sh

Benchmark test script to compare execution times of

numeric-indexed array vs. associative array.

Thank you, Erik Brandsberg.

count=100000 # May take a while for some of the tests below.

declare simple # Can change to 20000, if desired.

declare -a array1

declare -A array2

declare -a array3

declare -A array4

echo "===Assignment tests==="

echo

echo "Assigning a simple variable:"

References $i twice to equalize lookup times.

time for (( i=0; i< $count; i++)); do

simple=$i$i

done

echo "---"

echo "Assigning a numeric index array entry:"

time for (( i=0; i< $count; i++)); do

array1[$i]=$i

done

echo "---"

echo "Overwriting a numeric index array entry:"

time for (( i=0; i< $count; i++)); do

array1[$i]=$i

done

echo "---"

echo "Linear reading of numeric index array:"

time for (( i=0; i< $count; i++)); do

simple=array1[$i]

done

echo "---"

echo "Assigning an associative array entry:"

time for (( i=0; i< $count; i++)); do

array2[$i]=$i

done

echo "---"

echo "Overwriting an associative array entry:"

time for (( i=0; i< $count; i++)); do

array2[$i]=$i

done

echo "---"

echo "Linear reading an associative array entry:"

time for (( i=0; i< $count; i++)); do

simple=array2[$i]

done

echo "---"

echo "Assigning a random number to a simple variable:"

time for (( i=0; i< $count; i++)); do

simple=$RANDOM

done

echo "---"

echo "Assign a sparse numeric index array entry randomly into 64k cells:"

time for (( i=0; i< $count; i++)); do

array3[$RANDOM]=$i

done

echo "---"

echo "Reading sparse numeric index array entry:"

time for value in "${array3[@]}"i; do

simple=$value

done

echo "---"

echo "Assigning a sparse associative array entry randomly into 64k cells:"

time for (( i=0; i< $count; i++)); do

array4[$RANDOM]=$i

done

echo "---"

echo "Reading sparse associative index array entry:"

time for value in "${array4[@]}"; do

simple=$value

done

exit $?

________________________________________________________________

Appendix B. Reference Cards

The following reference cards provide a useful summary of certain

scripting concepts. The foregoing text treats these matters in more

depth, as well as giving usage examples.

Table B-1. Special Shell Variables

Variable Meaning

$0 Filename of script

$1 Positional parameter #1

$2 - $9 Positional parameters #2 - #9

${10} Positional parameter #10

$# Number of positional parameters

"$*" All the positional parameters (as a single word) *

"$@" All the positional parameters (as separate strings)

${#*} Number of positional parameters

${#@} Number of positional parameters

$? Return value

$ Process ID (PID) of script

$- Flags passed to script (using set)

$_ Last argument of previous command

$! Process ID (PID) of last job run in background

* Must be quoted, otherwise it defaults to $@.

Table B-2. TEST Operators: Binary Comparison

Operator Meaning ----- Operator Meaning

Arithmetic Comparison String Comparison

-eq Equal to = Equal to

== Equal to

-ne Not equal to != Not equal to

-lt Less than \< Less than (ASCII) *

-le Less than or equal to

-gt Greater than \> Greater than (ASCII) *

-ge Greater than or equal to

-z String is empty

-n String is not empty

Arithmetic Comparison within double parentheses (( ... ))

> Greater than

>= Greater than or equal to

< Less than

<= Less than or equal to

* If within a double-bracket [[ ... ]] test construct, then no escape

\ is needed.

Table B-3. TEST Operators: Files

Operator Tests Whether ----- Operator Tests Whether

-e File exists -s File is not zero size

-f File is a regular file

-d File is a directory -r File has read permission

-h File is a symbolic link -w File has write permission

-L File is a symbolic link -x File has execute permission

-b File is a block device

-c File is a character device -g sgid flag set

-p File is a pipe -u suid flag set

-S File is a socket -k "sticky bit" set

-t File is associated with a terminal

-N File modified since it was last read F1 -nt F2 File F1 is newer

than F2 *

-O You own the file F1 -ot F2 File F1 is older than F2 *

-G Group id of file same as yours F1 -ef F2 Files F1 and F2 are

hard links to the same file *

! NOT (inverts sense of above tests)

* Binary operator (requires two operands).

Table B-4. Parameter Substitution and Expansion

Expression Meaning

${var} Value of var (same as $var)

${var-$DEFAULT} If var not set, evaluate expression as $DEFAULT *

${var:-$DEFAULT} If var not set or is empty, evaluate expression as

$DEFAULT *

${var=$DEFAULT} If var not set, evaluate expression as $DEFAULT *

${var:=$DEFAULT} If var not set or is empty, evaluate expression as

$DEFAULT *

${var+$OTHER} If var set, evaluate expression as $OTHER, otherwise as

null string

${var:+$OTHER} If var set, evaluate expression as $OTHER, otherwise

as null string

${var?$ERR_MSG} If var not set, print $ERR_MSG and abort script with

an exit status of 1.*

${var:?$ERR_MSG} If var not set, print $ERR_MSG and abort script with

an exit status of 1.*

${!varprefix*} Matches all previously declared variables beginning

with varprefix

${!varprefix@} Matches all previously declared variables beginning

with varprefix

* If var is set, evaluate the expression as $var with no

side-effects.

# Note that some of the above behavior of operators has changed from

earlier versions of Bash.

Table B-5. String Operations

Expression Meaning

${#string} Length of $string

${string:position} Extract substring from $string at $position

${string:position:length} Extract $length characters substring from

$string at $position [zero-indexed, first character is at position 0]

${string#substring} Strip shortest match of $substring from front of

$string

${string##substring} Strip longest match of $substring from front of

$string

${string%substring} Strip shortest match of $substring from back of

$string

${string%%substring} Strip longest match of $substring from back of

$string

${string/substring/replacement} Replace first match of $substring

with $replacement

${string//substring/replacement} Replace all matches of $substring

with $replacement

${string/#substring/replacement} If $substring matches front end of

$string, substitute $replacement for $substring

${string/%substring/replacement} If $substring matches back end of

$string, substitute $replacement for $substring

expr match "$string" '$substring' Length of matching $substring* at

beginning of $string

expr "$string" : '$substring' Length of matching $substring* at

beginning of $string

expr index "$string" $substring Numerical position in $string of

first character in $substring* that matches [0 if no match, first

character counts as position 1]

expr substr $string $position $length Extract $length characters from

$string starting at $position [0 if no match, first character counts

as position 1]

expr match "$string" '$$substring$' Extract $substring*, searching

from beginning of $string

expr "$string" : '$$substring$' Extract $substring* , searching

from beginning of $string

expr match "$string" '.*$$substring$' Extract $substring*,

searching from end of $string

expr "$string" : '.*$$substring$' Extract $substring*, searching

from end of $string

* Where $substring is a Regular Expression.

Table B-6. Miscellaneous Constructs

Expression Interpretation

Brackets

if [ CONDITION ] Test construct

if [[ CONDITION ]] Extended test construct

Array[1]=element1 Array initialization

[a-z] Range of characters within a Regular Expression

Curly Brackets

${variable} Parameter substitution

${!variable} Indirect variable reference

{ command1; command2; . . . commandN; } Block of code

{string1,string2,string3,...} Brace expansion

{a..z} Extended brace expansion

{} Text replacement, after find and xargs

Parentheses

( command1; command2 ) Command group executed within a subshell

Array=(element1 element2 element3) Array initialization

result=$(COMMAND) Command substitution, new style

>(COMMAND) Process substitution

<(COMMAND) Process substitution

Double Parentheses

(( var = 78 )) Integer arithmetic

var=$(( 20 + 5 )) Integer arithmetic, with variable assignment

(( var++ )) C-style variable increment

(( var-- )) C-style variable decrement

(( var0 = var1<98?9:21 )) C-style ternary operation

Quoting

"$variable" "Weak" quoting

'string' 'Strong' quoting

Back Quotes

result=`COMMAND` Command substitution, classic style

________________________________________________________________

Appendix C. A Sed and Awk Micro-Primer

This is a very brief introduction to the sed and awk text processing

utilities. We will deal with only a few basic commands here, but that

will suffice for understanding simple sed and awk constructs within

shell scripts.

sed: a non-interactive text file editor

awk: a field-oriented pattern processing language with a C-style

syntax

For all their differences, the two utilities share a similar

invocation syntax, use regular expressions , read input by default

from stdin, and output to stdout. These are well-behaved UNIX tools,

and they work together well. The output from one can be piped to the

other, and their combined capabilities give shell scripts some of the

power of Perl.

Note

One important difference between the utilities is that while shell

scripts can easily pass arguments to sed, it is more cumbersome for

awk (see Example 36-5 and Example 28-2).

________________________________________________________________

C.1. Sed

Sed is a non-interactive [141] stream editor. It receives text input,

whether from stdin or from a file, performs certain operations on

specified lines of the input, one line at a time, then outputs the

result to stdout or to a file. Within a shell script, sed is usually

one of several tool components in a pipe.

Sed determines which lines of its input that it will operate on from

the address range passed to it. [142] Specify this address range

either by line number or by a pattern to match. For example, 3d

signals sed to delete line 3 of the input, and /Windows/d tells sed

that you want every line of the input containing a match to "Windows"

deleted.

Of all the operations in the sed toolkit, we will focus primarily on

the three most commonly used ones. These are printing (to stdout),

deletion, and substitution.

Table C-1. Basic sed operators

Operator Name Effect

[address-range]/p print Print [specified address range]

[address-range]/d delete Delete [specified address range]

s/pattern1/pattern2/ substitute Substitute pattern2 for first

instance of pattern1 in a line

[address-range]/s/pattern1/pattern2/ substitute Substitute pattern2

for first instance of pattern1 in a line, over address-range

[address-range]/y/pattern1/pattern2/ transform replace any character

in pattern1 with the corresponding character in pattern2, over

address-range (equivalent of tr)

[address] i pattern Filename insert Insert pattern at address

indicated in file Filename. Usually used with -i in-place option.

g global Operate on every pattern match within each matched line of

input

Note

Unless the g (global) operator is appended to a substitute command,

the substitution operates only on the first instance of a pattern

match within each line.

From the command-line and in a shell script, a sed operation may

require quoting and certain options.

sed -e '/^$/d' $filename

The -e option causes the next string to be interpreted as an editing instruc

tion.

(If passing only a single instruction to sed, the "-e" is optional.)

The "strong" quotes ('') protect the RE characters in the instruction

+ from reinterpretation as special characters by the body of the script.

(This reserves RE expansion of the instruction for sed.)

Operates on the text contained in file $filename.

In certain cases, a sed editing command will not work with single

quotes.

filename=file1.txt

pattern=BEGIN

sed "/^$pattern/d" "$filename" # Works as specified.

sed '/^$pattern/d' "$filename" has unexpected results.

In this instance, with strong quoting (' ... '),

+ "$pattern" will not expand to "BEGIN".

Note

Sed uses the -e option to specify that the following string is an

instruction or set of instructions. If there is only a single

instruction contained in the string, then this may be omitted.

sed -n '/xzy/p' $filename

The -n option tells sed to print only those lines matching the pattern.

Otherwise all input lines would print.

The -e option not necessary here since there is only a single editing instru

ction.

Table C-2. Examples of sed operators

Notation Effect

8d Delete 8th line of input.

/^$/d Delete all blank lines.

1,/^$/d Delete from beginning of input up to, and including first

blank line.

/Jones/p Print only lines containing "Jones" (with -n option).

s/Windows/Linux/ Substitute "Linux" for first instance of "Windows"

found in each input line.

s/BSOD/stability/g Substitute "stability" for every instance of

"BSOD" found in each input line.

s/ *$// Delete all spaces at the end of every line.

s/00*/0/g Compress all consecutive sequences of zeroes into a single

zero.

echo "Working on it." | sed -e '1i How far are you along?' Prints

"How far are you along?" as first line, "Working on it" as second.

5i 'Linux is great.' file.txt Inserts 'Linux is great.' at line 5 of

the file file.txt.

/GUI/d Delete all lines containing "GUI".

s/GUI//g Delete all instances of "GUI", leaving the remainder of each

line intact.

Substituting a zero-length string for another is equivalent to

deleting that string within a line of input. This leaves the

remainder of the line intact. Applying s/GUI// to the line

The most important parts of any application are its GUI and sound effects

results in

The most important parts of any application are its and sound effects

A backslash forces the sed replacement command to continue on to the

next line. This has the effect of using the newline at the end of the

first line as the replacement string.

s/^ */\

This substitution replaces line-beginning spaces with a newline. The

net result is to replace paragraph indents with a blank line between

paragraphs.

An address range followed by one or more operations may require open

and closed curly brackets, with appropriate newlines.

/[0-9A-Za-z]/,/^$/{

/^$/d

}

This deletes only the first of each set of consecutive blank lines.

That might be useful for single-spacing a text file, but retaining

the blank line(s) between paragraphs.

Note

The usual delimiter that sed uses is /. However, sed allows other

delimiters, such as %. This is useful when / is part of a replacement

string, as in a file pathname. See Example 11-10 and Example 16-32.

Tip

A quick way to double-space a text file is sed G filename.

For illustrative examples of sed within shell scripts, see:

1. Example 36-1

2. Example 36-2

3. Example 16-3

4. Example A-2

5. Example 16-17

6. Example 16-27

7. Example A-12

8. Example A-16

9. Example A-17

10. Example 16-32

11. Example 11-10

12. Example 16-48

13. Example A-1

14. Example 16-14

15. Example 16-12

16. Example A-10

17. Example 19-12

18. Example 16-19

19. Example A-29

20. Example A-31

21. Example A-24

22. Example A-43

23. Example A-55

For a more extensive treatment of sed, refer to the pertinent

references in the Bibliography.

________________________________________________________________

C.2. Awk

Awk [143] is a full-featured text processing language with a syntax

reminiscent of C. While it possesses an extensive set of operators

and capabilities, we will cover only a few of these here - the ones

most useful in shell scripts.

Awk breaks each line of input passed to it into fields. By default, a

field is a string of consecutive characters delimited by whitespace,

though there are options for changing this. Awk parses and operates

on each separate field. This makes it ideal for handling structured

text files -- especially tables -- data organized into consistent

chunks, such as rows and columns.

Strong quoting and curly brackets enclose blocks of awk code within a

shell script.

$1 is field #1, $2 is field #2, etc.

echo one two | awk '{print $1}'

one

echo one two | awk '{print $2}'

two

But what is field #0 ($0)?

echo one two | awk '{print $0}'

one two

All the fields!

awk '{print $3}' $filename

Prints field #3 of file $filename to stdout.

awk '{print $1 $5 $6}' $filename

Prints fields #1, #5, and #6 of file $filename.

awk '{print $0}' $filename

Prints the entire file!

Same effect as: cat $filename . . . or . . . sed '' $filename

We have just seen the awk print command in action. The only other

feature of awk we need to deal with here is variables. Awk handles

variables similarly to shell scripts, though a bit more flexibly.

{ total += ${column_number} }

This adds the value of column_number to the running total of total>.

Finally, to print "total", there is an END command block, executed

after the script has processed all its input.

END { print total }

Corresponding to the END, there is a BEGIN, for a code block to be

performed before awk starts processing its input.

The following example illustrates how awk can add text-parsing tools

to a shell script.

Example C-1. Counting Letter Occurrences

! /bin/sh

letter-count2.sh: Counting letter occurrences in a text file.

Script by nyal [nyal@voila.fr].

Used in ABS Guide with permission.

Recommented and reformatted by ABS Guide author.

Version 1.1: Modified to work with gawk 3.1.3.

(Will still work with earlier versions.)

INIT_TAB_AWK=""

Parameter to initialize awk script.

count_case=0

FILE_PARSE=$1

E_PARAMERR=85

usage()

{

echo "Usage: letter-count.sh file letters" 2>&1

# For example: ./letter-count2.sh filename.txt a b c

exit $E_PARAMERR # Too few arguments passed to script.

}

if [ ! -f "$1" ] ; then

echo "$1: No such file." 2>&1

usage # Print usage message and exit.

if [ -z "$2" ] ; then

echo "$2: No letters specified." 2>&1

usage

shift # Letters specified.

for letter in `echo $@` # For each one . . .

INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \

\"$letter\"; final_tab[${count_case}] = 0; "

# Pass as parameter to awk script below.

count_case=`expr $count_case + 1`

done

DEBUG:

echo $INIT_TAB_AWK;

cat $FILE_PARSE |

Pipe the target file to the following awk script.

---------------------------------------------------------------------

Earlier version of script:

awk -v tab_search=0 -v final_tab=0 -v tab=0 -v \

nb_letter=0 -v chara=0 -v chara2=0 \

awk \

"BEGIN { $INIT_TAB_AWK } \

{ split(\$0, tab, \"\"); \

for (chara in tab) \

{ for (chara2 in tab_search) \

{ if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \

END { for (chara in final_tab) \

{ print tab_search[chara] \" => \" final_tab[chara] } }"

---------------------------------------------------------------------

Nothing all that complicated, just . . .

+ for-loops, if-tests, and a couple of specialized functions.

exit $?

Compare this script to letter-count.sh.

For simpler examples of awk within shell scripts, see:

1. Example 15-14

2. Example 20-8

3. Example 16-32

4. Example 36-5

5. Example 28-2

6. Example 15-20

7. Example 29-3

8. Example 29-4

9. Example 11-3

10. Example 16-61

11. Example 9-16

12. Example 16-4

13. Example 10-6

14. Example 36-19

15. Example 11-9

16. Example 36-4

17. Example 16-53

18. Example T-3

That's all the awk we'll cover here, folks, but there's lots more to

learn. See the appropriate references in the Bibliography.

________________________________________________________________

Appendix D. Parsing and Managing Pathnames

Emmanual Rouat contributed the following example of parsing and

transforming filenames and, in particular, pathnames. It draws

heavily on the functionality of sed.

!/usr/bin/env bash

-----------------------------------------------------------

Management of PATH, LD_LIBRARY_PATH, MANPATH variables...

By Emmanuel Rouat <no-email>

(Inspired by the bash documentation 'pathfuncs' and on

discussions found on stackoverflow:

http://stackoverflow.com/questions/370047/

http://stackoverflow.com/questions/273909/#346860 )

Last modified: Sat Sep 22 12:01:55 CEST 2012

The following functions handle spaces correctly.

These functions belong in .bash_profile rather than in

.bashrc, I guess.

The modular aspect of these functions should make it easy

to expand them to handle path substitutions instead

of path removal etc....

See http://www.catonmat.net/blog/awk-one-liners-explained-part-two/

(item 43) for an explanation of the 'duplicate-entries' removal

(it's a nice trick!)

-----------------------------------------------------------

Show $@ (usually PATH) as list.

function p_show() { local p="$@" && for p; do [[ ${!p} ]] &&

echo -e ${!p//:/\\n}; done }

Filter out empty lines, multiple/trailing slashes, and duplicate entries.

function p_filter()

{ awk '/^[ \t]*$/ {next} {sub(/\/+$/, "");gsub(/\/+/, "/")}!x[$0]++' ;}

Rebuild list of items into ':' separated word (PATH-like).

function p_build() { paste -sd: ;}

Clean $1 (typically PATH) and rebuild it

function p_clean()

{ local p=${1} && eval ${p}='$(p_show ${p} | p_filter | p_build)' ;}

Remove $1 from $2 (found on stackoverflow, with modifications).

function p_rm()

{ local d=$(echo $1 | p_filter) p=${2} &&

eval ${p}='$(p_show ${p} | p_filter | grep -xv "${d}" | p_build)' ;}

Same as previous, but filters on a pattern (dangerous...

+ don't use 'bin' or '/' as pattern!).

function p_rmpat()

{ local d=$(echo $1 | p_filter) p=${2} && eval ${p}='$(p_show ${p} |

p_filter | grep -v "${d}" | p_build)' ;}

Delete $1 from $2 and append it cleanly.

function p_append()

{ local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&

eval ${p}='$(p_show ${p} d | p_build)' ;}

Delete $1 from $2 and prepend it cleanly.

function p_prepend()

{ local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&

eval ${p}='$(p_show d ${p} | p_build)' ;}

Some tests:

echo

MYPATH="/bin:/usr/bin/:/bin://bin/"

p_append "/project//my project/bin" MYPATH

echo "Append '/project//my project/bin' to '/bin:/usr/bin/:/bin://bin/'"

echo "(result should be: /bin:/usr/bin:/project/my project/bin)"

echo $MYPATH

echo

MYOTHERPATH="/bin:/usr/bin/:/bin:/project//my project/bin"

p_prepend "/project//my project/bin" MYOTHERPATH

echo "Prepend '/project//my project/bin' \

to '/bin:/usr/bin/:/bin:/project//my project/bin/'"

echo "(result should be: /project/my project/bin:/bin:/usr/bin)"

echo $MYOTHERPATH

echo

p_prepend "/project//my project/bin" FOOPATH # FOOPATH doesn't exist.

echo "Prepend '/project//my project/bin' to an unset variable"

echo "(result should be: /project/my project/bin)"

echo $FOOPATH

echo

BARPATH="/a:/b/://b c://a:/my local pub"

p_clean BARPATH

echo "Clean BARPATH='/a:/b/://b c://a:/my local pub'"

echo "(result should be: /a:/b:/b c:/my local pub)"

echo $BARPATH

***

David Wheeler kindly permitted me to use his instructive examples.

Doing it correctly: A quick summary

by David Wheeler

http://www.dwheeler.com/essays/filenames-in-shell.html

So, how can you process filenames correctly in shell? Here's a quick

summary about how to do it correctly, for the impatient who "just want the

answer". In short: Double-quote to use "$variable" instead of $variable,

set IFS to just newline and tab, prefix all globs/filenames so they cannot

begin with "-" when expanded, and use one of a few templates that work

correctly. Here are some of those templates that work correctly:

IFS="$(printf '\n\t')"

# Remove SPACE, so filenames with spaces work well.

# Correct glob use:

#+ always use "for" loop, prefix glob, check for existence:

for file in ./* ; do # Use "./*" ... NEVER bare "*" ...

if [ -e "$file" ] ; then # Make sure it isn't an empty match.

COMMAND ... "$file" ...

done

# Correct glob use, but requires nonstandard bash extension.

shopt -s nullglob # Bash extension,

#+ so that empty glob matches will work.

for file in ./* ; do # Use "./*", NEVER bare "*"

COMMAND ... "$file" ...

done

# These handle all filenames correctly;

#+ can be unwieldy if COMMAND is large:

find ... -exec COMMAND... {} \;

find ... -exec COMMAND... {} \+ # If multiple files are okay for COMMAND.

# This skips filenames with control characters

#+ (including tab and newline).

IFS="$(printf '\n\t')"

controlchars="$(printf '*[\001-\037\177]*')"

for file in $(find . ! -name "$controlchars"') ; do

COMMAND "$file" ...

done

# Okay if filenames can't contain tabs or newlines --

#+ beware the assumption.

IFS="$(printf '\n\t')"

for file in $(find .) ; do

COMMAND "$file" ...

done

# Requires nonstandard but common extensions in find and xargs:

find . -print0 | xargs -0 COMMAND

# Requires nonstandard extensions to find and to shell (bash works).

# variables might not stay set once the loop ends:

find . -print0 | while IFS="" read -r -d "" file ; do ...

COMMAND "$file" # Use quoted "$file", not $file, everywhere.

done

# Requires nonstandard extensions to find and to shell (bash works).

# Underlying system must include named pipes (FIFOs)

#+ or the /dev/fd mechanism.

# In this version, variables *do* stay set after the loop ends,

# and you can read from stdin.

#+ (Change the 4 to another number if fd 4 is needed.)

while IFS="" read -r -d "" file <&4 ; do

COMMAND "$file" # Use quoted "$file" -- not $file, everywhere.

done 4< <(find . -print0)

# Named pipe version.

# Requires nonstandard extensions to find and to shell's read (bash ok).

# Underlying system must include named pipes (FIFOs).

# Again, in this version, variables *do* stay set after the loop ends,

# and you can read from stdin.

# (Change the 4 to something else if fd 4 needed).

mkfifo mypipe

find . -print0 > mypipe &

while IFS="" read -r -d "" file <&4 ; do

COMMAND "$file" # Use quoted "$file", not $file, everywhere.

done 4< mypipe

________________________________________________________________

Appendix E. Exit Codes With Special Meanings

Table E-1. Reserved Exit Codes

Exit Code Number Meaning Example Comments

1 Catchall for general errors let "var1 = 1/0" Miscellaneous errors,

such as "divide by zero" and other impermissible operations

2 Misuse of shell builtins (according to Bash documentation)

empty_function() {} Missing keyword or command, or permission problem

(and diff return code on a failed binary file comparison).

126 Command invoked cannot execute /dev/null Permission problem or

command is not an executable

127 "command not found" illegal_command Possible problem with $PATH

or a typo

128 Invalid argument to exit exit 3.14159 exit takes only integer

args in the range 0 - 255 (see first footnote)

128+n Fatal error signal "n" kill -9 $PPID of script $? returns 137

(128 + 9)

130 Script terminated by Control-C Ctl-C Control-C is fatal error

signal 2, (130 = 128 + 2, see above)

255* Exit status out of range exit -1 exit takes only integer args in

the range 0 - 255

According to the above table, exit codes 1 - 2, 126 - 165, and 255

[144] have special meanings, and should therefore be avoided for

user-specified exit parameters. Ending a script with exit 127 would

certainly cause confusion when troubleshooting (is the error code a

"command not found" or a user-defined one?). However, many scripts

use an exit 1 as a general bailout-upon-error. Since exit code 1

signifies so many possible errors, it is not particularly useful in

debugging.

There has been an attempt to systematize exit status numbers (see

/usr/include/sysexits.h), but this is intended for C and C++

programmers. A similar standard for scripting might be appropriate.

The author of this document proposes restricting user-defined exit

codes to the range 64 - 113 (in addition to 0, for success), to

conform with the C/C++ standard. This would allot 50 valid codes, and

make troubleshooting scripts more straightforward. [145] All

user-defined exit codes in the accompanying examples to this document

conform to this standard, except where overriding circumstances

exist, as in Example 9-2.

Note

Issuing a $? from the command-line after a shell script exits gives

results consistent with the table above only from the Bash or sh

prompt. Running the C-shell or tcsh may give different values in some

cases.

________________________________________________________________

Appendix F. A Detailed Introduction to I/O and I/O Redirection

written by Stéphane Chazelas, and revised by the document author

A command expects the first three file descriptors to be available.

The first, fd 0 (standard input, stdin), is for reading. The other

two (fd 1, stdout and fd 2, stderr) are for writing.

There is a stdin, stdout, and a stderr associated with each command.

ls 2>&1 means temporarily connecting the stderr of the ls command to

the same "resource" as the shell's stdout.

By convention, a command reads its input from fd 0 (stdin), prints

normal output to fd 1 (stdout), and error ouput to fd 2 (stderr). If

one of those three fd's is not open, you may encounter problems:

bash$ cat /etc/passwd >&-

cat: standard output: Bad file descriptor

For example, when xterm runs, it first initializes itself. Before

running the user's shell, xterm opens the terminal device

(/dev/pts/<n> or something similar) three times.

At this point, Bash inherits these three file descriptors, and each

command (child process) run by Bash inherits them in turn, except

when you redirect the command. Redirection means reassigning one of

the file descriptors to another file (or a pipe, or anything

permissible). File descriptors may be reassigned locally (for a

command, a command group, a subshell, a while or if or case or for

loop...), or globally, for the remainder of the shell (using exec).

ls > /dev/null means running ls with its fd 1 connected to /dev/null.

bash$ lsof -a -p $ -d0,1,2

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

bash 363 bozo 0u CHR 136,1 3 /dev/pts/1

bash 363 bozo 1u CHR 136,1 3 /dev/pts/1

bash 363 bozo 2u CHR 136,1 3 /dev/pts/1

bash$ exec 2> /dev/null

bash$ lsof -a -p $ -d0,1,2

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

bash 371 bozo 0u CHR 136,1 3 /dev/pts/1

bash 371 bozo 1u CHR 136,1 3 /dev/pts/1

bash 371 bozo 2w CHR 1,3 120 /dev/null

bash$ bash -c 'lsof -a -p $ -d0,1,2' | cat

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

lsof 379 root 0u CHR 136,1 3 /dev/pts/1

lsof 379 root 1w FIFO 0,0 7118 pipe

lsof 379 root 2u CHR 136,1 3 /dev/pts/1

bash$ echo "$(bash -c 'lsof -a -p $ -d0,1,2' 2>&1)"

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

lsof 426 root 0u CHR 136,1 3 /dev/pts/1

lsof 426 root 1w FIFO 0,0 7520 pipe

lsof 426 root 2w FIFO 0,0 7520 pipe

This works for different types of redirection.

Exercise: Analyze the following script.

! /usr/bin/env bash

mkfifo /tmp/fifo1 /tmp/fifo2

while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1

exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)

exec 3>&1

(

while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \

| tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2

echo 1st, to stdout

sleep 1

echo 2nd, to stderr >&2

sleep 1

echo 3rd, to fd 3 >&3

sleep 1

echo 4th, to fd 4 >&4

sleep 1

echo 5th, to fd 5 >&5

sleep 1

echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5

sleep 1

echo 7th, to fd 6 >&6

sleep 1

echo 8th, to fd 7 >&7

sleep 1

echo 9th, to fd 8 >&8

) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&-

) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&-

) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&-

rm -f /tmp/fifo1 /tmp/fifo2

For each command and subshell, figure out which fd points to what.

Good luck!

exit 0

________________________________________________________________

Appendix G. Command-Line Options

Many executables, whether binaries or script files, accept options to

modify their run-time behavior. For example: from the command-line,

typing command -o would invoke command, with option o.

________________________________________________________________

G.1. Standard Command-Line Options

Over time, there has evolved a loose standard for the meanings of

command-line option flags. The GNU utilities conform more closely to

this "standard" than older UNIX utilities.

Traditionally, UNIX command-line options consist of a dash, followed

by one or more lowercase letters. The GNU utilities added a

double-dash, followed by a complete word or compound word.

The two most widely-accepted options are:

* -h

--help

Help: Give usage message and exit.

* -v

--version

Version: Show program version and exit.

Other common options are:

* -a

--all

All: show all information or operate on all arguments.

* -l

--list

List: list files or arguments without taking other action.

* -o

Output filename

* -q

--quiet

Quiet: suppress stdout.

* -r

-R

--recursive

Recursive: Operate recursively (down directory tree).

* -v

--verbose

Verbose: output additional information to stdout or stderr.

* -z

--compress

Compress: apply compression (usually gzip).

However:

* In tar and gawk:

-f

--file

File: filename follows.

* In cp, mv, rm:

-f

--force

Force: force overwrite of target file(s).

Caution

Many UNIX and Linux utilities deviate from this "standard," so it is

dangerous to assume that a given option will behave in a standard

way. Always check the man page for the command in question when in

doubt.

A complete table of recommended options for the GNU utilities is

available at [http://www.gnu.org/prep/standards/] the GNU standards

page.

________________________________________________________________

G.2. Bash Command-Line Options

Bash itself has a number of command-line options. Here are some of

the more useful ones.

* -c

Read commands from the following string and assign any arguments

to the positional parameters.

bash$ bash -c 'set a b c d; IFS="+-;"; echo "$*"'

a+b+c+d

* -r

--restricted

Runs the shell, or a script, in restricted mode.

* --posix

Forces Bash to conform to POSIX mode.

* --version

Display Bash version information and exit.

* --

End of options. Anything further on the command line is an

argument, not an option.

________________________________________________________________

Appendix H. Important Files

startup files

These files contain the aliases and environmental variables

made available to Bash running as a user shell and to all Bash

scripts invoked after system initialization.

/etc/profile

Systemwide defaults, mostly setting the environment (all

Bourne-type shells, not just Bash [146])

/etc/bashrc

systemwide functions and aliases for Bash

$HOME/.bash_profile

user-specific Bash environmental default settings, found in

each user's home directory (the local counterpart to

/etc/profile)

$HOME/.bashrc

user-specific Bash init file, found in each user's home

directory (the local counterpart to /etc/bashrc). Only

interactive shells and user scripts read this file. See

Appendix M for a sample .bashrc file.

logout file

$HOME/.bash_logout

user-specific instruction file, found in each user's home

directory. Upon exit from a login (Bash) shell, the commands

in this file execute.

data files

/etc/passwd

A listing of all the user accounts on the system, their

identities, their home directories, the groups they belong to,

and their default shell. Note that the user passwords are not

stored in this file, [147] but in /etc/shadow in encrypted

form.

system configuration files

/etc/sysconfig/hwconf

Listing and description of attached hardware devices. This

information is in text form and can be extracted and parsed.

bash$ grep -A 5 AUDIO /etc/sysconfig/hwconf

class: AUDIO

bus: PCI

detached: 0

driver: snd-intel8x0

desc: "Intel Corporation 82801CA/CAM AC'97 Audio Controller"

vendorId: 8086

Note

This file is present on Red Hat and Fedora Core installations, but

may be missing from other distros.

________________________________________________________________

Appendix I. Important System Directories

Sysadmins and anyone else writing administrative scripts should be

intimately familiar with the following system directories.

* /bin

Binaries (executables). Basic system programs and utilities (such

as bash).

* /usr/bin [148]

More system binaries.

* /usr/local/bin

Miscellaneous binaries local to the particular machine.

* /sbin

System binaries. Basic system administrative programs and

utilities (such as fsck).

* /usr/sbin

More system administrative programs and utilities.

* /etc

Et cetera. Systemwide configuration scripts.

Of particular interest are the /etc/fstab (filesystem table),

/etc/mtab (mounted filesystem table), and the /etc/inittab files.

* /etc/rc.d

Boot scripts, on Red Hat and derivative distributions of Linux.

* /usr/share/doc

Documentation for installed packages.

* /usr/man

The systemwide manpages.

* /dev

Device directory. Entries (but not mount points) for physical and

virtual devices. See Chapter 29.

* /proc

Process directory. Contains information and statistics about

running processes and kernel parameters. See Chapter 29.

* /sys

Systemwide device directory. Contains information and statistics

about device and device names. This is newly added to Linux with

the 2.6.X kernels.

* /mnt

Mount. Directory for mounting hard drive partitions, such as

/mnt/dos, and physical devices. In newer Linux distros, the

/media directory has taken over as the preferred mount point for

I/O devices.

* /media

In newer Linux distros, the preferred mount point for I/O

devices, such as CD/DVD drives or USB flash drives.

* /var

Variable (changeable) system files. This is a catchall

"scratchpad" directory for data generated while a Linux/UNIX

machine is running.

* /var/log

Systemwide log files.

* /var/spool/mail

User mail spool.

* /lib

Systemwide library files.

* /usr/lib

More systemwide library files.

* /tmp

System temporary files.

* /boot

System boot directory. The kernel, module links, system map, and

boot manager reside here.

Warning

Altering files in this directory may result in an unbootable system.

________________________________________________________________

Appendix J. An Introduction to Programmable Completion

The programmable completion feature in Bash permits typing a partial

command, then pressing the [Tab] key to auto-complete the command

sequence. [149] If multiple completions are possible, then [Tab]

lists them all. Let's see how it works.

bash$ xtra[Tab]

xtraceroute xtrapin xtrapproto

xtraceroute.real xtrapinfo xtrapreset

xtrapchar xtrapout xtrapstats

bash$ xtrac[Tab]

xtraceroute xtraceroute.real

bash$ xtraceroute.r[Tab]

xtraceroute.real

Tab completion also works for variables and path names.

bash$ echo $BASH[Tab]

$BASH $BASH_COMPLETION $BASH_SUBSHELL

$BASH_ARGC $BASH_COMPLETION_DIR $BASH_VERSINFO

$BASH_ARGV $BASH_LINENO $BASH_VERSION

$BASH_COMMAND $BASH_SOURCE

bash$ echo /usr/local/[Tab]

bin/ etc/ include/ libexec/ sbin/ src/

doc/ games/ lib/ man/ share/

The Bash complete and compgen builtins make it possible for tab

completion to recognize partial parameters and options to commands.

In a very simple case, we can use complete from the command-line to

specify a short list of acceptable parameters.

bash$ touch sample_command

bash$ touch file1.txt file2.txt file2.doc file30.txt file4.zzz

bash$ chmod +x sample_command

bash$ complete -f -X '!*.txt' sample_command

bash$ ./sample[Tab][Tab]

sample_command

file1.txt file2.txt file30.txt

The -f option to complete specifies filenames, and -X the filter

pattern.

For anything more complex, we could write a script that specifies a

list of acceptable command-line parameters. The compgen builtin

expands a list of arguments to generate completion matches.

Let us take a modified version of the UseGetOpt.sh script as an

example command. This script accepts a number of command-line

parameters, preceded by either a single or double dash. And here is

the corresponding completion script, by convention given a filename

corresponding to its associated command.

Example J-1. Completion script for UseGetOpt.sh

file: UseGetOpt-2

UseGetOpt-2.sh parameter-completion

_UseGetOpt-2 () # By convention, the function name

{ #+ starts with an underscore.

local cur

# Pointer to current completion word.

# By convention, it's named "cur" but this isn't strictly necessary.

COMPREPLY=() # Array variable storing the possible completions.

cur=${COMP_WORDS[COMP_CWORD]}

case "$cur" in

-*)

COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \

--file --log --test --help --' -- $cur ) );;

Generate the completion matches and load them into $COMPREPLY array.

xx) May add more cases here.

yy)

zz)

esac

return 0

}

complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh

^^ ^^^^^^^^^^^^ Invokes the function _UseGetOpt-2.

Now, let's try it.

bash$ source UseGetOpt-2

bash$ ./UseGetOpt-2.sh -[Tab]

-- --aoption --debug --file --help --log --test

-a -d -f -h -l -t

bash$ ./UseGetOpt-2.sh --[Tab]

-- --aoption --debug --file --help --log --test

We begin by sourcing the "completion script." This sets the

command-line parameters. [150]

In the first instance, hitting [Tab] after a single dash, the output

is all the possible parameters preceded by one or more dashes.

Hitting [Tab] after two dashes gives the possible parameters preceded

by two or more dashes.

Now, just what is the point of having to jump through flaming hoops

to enable command-line tab completion? It saves keystrokes. [151]

Resources:

Bash [http://freshmeat.net/projects/bashcompletion] programmable

completion project

Mitch Frazier's Linux Journal article, More on Using the Bash

Complete Command

Steve's excellent two-part article, "An Introduction to Bash

Completion": Part 1 and

[http://www.debian-administration.org/article/An_introduction_to_bash

_completion_part_2] Part 2

________________________________________________________________

Appendix K. Localization

Localization is an undocumented Bash feature.

A localized shell script echoes its text output in the language

defined as the system's locale. A Linux user in Berlin, Germany,

would get script output in German, whereas his cousin in Berlin,

Maryland, would get output from the same script in English.

To create a localized script, use the following template to write all

messages to the user (error messages, prompts, etc.).

!/bin/bash

localized.sh

Script by Stéphane Chazelas,

+ modified by Bruno Haible, bugfixed by Alfredo Pironti.

. gettext.sh

E_CDERROR=65

error()

{

printf "$@" >&2

exit $E_CDERROR

}

cd $var || error "`eval_gettext \"Can\'t cd to \\\$var.\"`"

The triple backslashes (escapes) in front of $var needed

+ "because eval_gettext expects a string

+ where the variable values have not yet been substituted."

-- per Bruno Haible

read -p "`gettext \"Enter the value: \"`" var

...

------------------------------------------------------------------

Alfredo Pironti comments:

This script has been modified to not use the $"..." syntax in

+ favor of the "`gettext \"...\"`" syntax.

This is ok, but with the new localized.sh program, the commands

+ "bash -D filename" and "bash --dump-po-string filename"

+ will produce no output

+ (because those command are only searching for the $"..." strings)!

The ONLY way to extract strings from the new file is to use the

'xgettext' program. However, the xgettext program is buggy.

Note that 'xgettext' has another bug.

The shell fragment:

gettext -s "I like Bash"

will be correctly extracted, but . . .

xgettext -s "I like Bash"

. . . fails!

'xgettext' will extract "-s" because

+ the command only extracts the

+ very first argument after the 'gettext' word.

Escape characters:

To localize a sentence like

echo -e "Hello\tworld!"

+ you must use

echo -e "`gettext \"Hello\\tworld\"`"

The "double escape character" before the `t' is needed because

+ 'gettext' will search for a string like: 'Hello\tworld'

This is because gettext will read one literal `\')

+ and will output a string like "Bonjour\tmonde",

+ so the 'echo' command will display the message correctly.

You may not use

echo "`gettext -e \"Hello\tworld\"`"

+ due to the xgettext bug explained above.

Let's localize the following shell fragment:

echo "-h display help and exit"

First, one could do this:

echo "`gettext \"-h display help and exit\"`"

This way 'xgettext' will work ok,

+ but the 'gettext' program will read "-h" as an option!

One solution could be

echo "`gettext -- \"-h display help and exit\"`"

This way 'gettext' will work,

+ but 'xgettext' will extract "--", as referred to above.

The workaround you may use to get this string localized is

echo -e "`gettext \"\\0-h display help and exit\"`"

We have added a \0 (NULL) at the beginning of the sentence.

This way 'gettext' works correctly, as does 'xgettext.'

Moreover, the NULL character won't change the behavior

+ of the 'echo' command.

------------------------------------------------------------------

bash$ bash -D localized.sh

"Can't cd to %s."

"Enter the value: "

This lists all the localized text. (The -D option lists double-quoted

strings prefixed by a $, without executing the script.)

bash$ bash --dump-po-strings localized.sh

: a:6

msgid "Can't cd to %s."

msgstr ""

#: a:7

msgid "Enter the value: "

msgstr ""

The --dump-po-strings option to Bash resembles the -D option, but

uses gettext "po" format.

Note

Bruno Haible points out:

Starting with gettext-0.12.2, xgettext -o - localized.sh is

recommended instead of bash --dump-po-strings localized.sh, because

xgettext . . .

1. understands the gettext and eval_gettext commands (whereas bash

--dump-po-strings understands only its deprecated $"..." syntax)

2. can extract comments placed by the programmer, intended to be read

by the translator.

This shell code is then not specific to Bash any more; it works the

same way with Bash 1.x and other /bin/sh implementations.

Now, build a language.po file for each language that the script will

be translated into, specifying the msgstr. Alfredo Pironti gives the

following example:

fr.po:

: a:6

msgid "Can't cd to $var."

msgstr "Impossible de se positionner dans le repertoire $var."

: a:7

msgid "Enter the value: "

msgstr "Entrez la valeur : "

The string are dumped with the variable names, not with the %s syntax,

+ similar to C programs.

+ This is a very cool feature if the programmer uses

+ variable names that make sense!

Then, run msgfmt.

msgfmt -o localized.sh.mo fr.po

Place the resulting localized.sh.mo file in the

/usr/local/share/locale/fr/LC_MESSAGES directory, and at the

beginning of the script, insert the lines:

TEXTDOMAINDIR=/usr/local/share/locale

TEXTDOMAIN=localized.sh

If a user on a French system runs the script, she will get French

messages.

Note

With older versions of Bash or other shells, localization requires

gettext, using the -s option. In this case, the script becomes:

!/bin/bash

localized.sh

E_CDERROR=65

error() {

local format=$1

shift

printf "$(gettext -s "$format")" "$@" >&2

exit $E_CDERROR

}

cd $var || error "Can't cd to %s." "$var"

read -p "$(gettext -s "Enter the value: ")" var

...

The TEXTDOMAIN and TEXTDOMAINDIR variables need to be set and

exported to the environment. This should be done within the script

itself.

---

This appendix written by Stéphane Chazelas, with modifications

suggested by Alfredo Pironti, and by Bruno Haible, maintainer of GNU

gettext.

________________________________________________________________

Appendix L. History Commands

The Bash shell provides command-line tools for editing and

manipulating a user's command history. This is primarily a

convenience, a means of saving keystrokes.

Bash history commands:

1. history

2. fc

bash$ history

1 mount /mnt/cdrom

2 cd /mnt/cdrom

3 ls

...

Internal variables associated with Bash history commands:

1. $HISTCMD

2. $HISTCONTROL

3. $HISTIGNORE

4. $HISTFILE

5. $HISTFILESIZE

6. $HISTSIZE

7. $HISTTIMEFORMAT (Bash, ver. 3.0 or later)

8. !!

9. !$

10. !#

11. !N

12. !-N

13. !STRING

14. !?STRING?

15. ^STRING^string^

Unfortunately, the Bash history tools find no use in scripting.

!/bin/bash

history.sh

A (vain) attempt to use the 'history' command in a script.

history # No output.

var=$(history); echo "$var" # $var is empty.

History commands are, by default, disabled within a script.

However, as dhw points out,

+ set -o history

+ enables the history mechanism.

set -o history

var=$(history); echo "$var" # 1 var=$(history)

bash$ ./history.sh

(no output)

The Advancing in the Bash Shell site gives a good introduction to the

use of history commands in Bash.

________________________________________________________________

Appendix M. Sample .bashrc and .bash_profile Files

The ~/.bashrc file determines the behavior of interactive shells. A

good look at this file can lead to a better understanding of Bash.

Emmanuel Rouat contributed the following very elaborate .bashrc file,

written for a Linux system. He welcomes reader feedback on it.

Study the file carefully, and feel free to reuse code snippets and

functions from it in your own .bashrc file or even in your scripts.

Example M-1. Sample .bashrc file

=============================================================== #

PERSONAL $HOME/.bashrc FILE for bash-3.0 (or later)

By Emmanuel Rouat [no-email]

Last modified: Tue Nov 20 22:04:47 CET 2012

This file is normally read by interactive shells only.

+ Here is the place to define your aliases, functions and

+ other interactive features like your prompt.

The majority of the code here assumes you are on a GNU

+ system (most likely a Linux box) and is often based on code

+ found on Usenet or Internet.

See for instance:

http://tldp.org/LDP/abs/html/index.html

http://www.caliban.org/bash

http://www.shelldorado.com/scripts/categories.html

http://www.dotfiles.org

The choice of colors was done for a shell with a dark background

+ (white on black), and this is usually also suited for pure text-mode

+ consoles (no X server available). If you use a white background,

+ you'll have to do some other choices for readability.

This bashrc file is a bit overcrowded.

Remember, it is just just an example.

Tailor it to your needs.

=============================================================== #

--> Comments added by HOWTO author.

If not running interactively, don't do anything

[ -z "$PS1" ] && return

-------------------------------------------------------------

Source global definitions (if any)

-------------------------------------------------------------

if [ -f /etc/bashrc ]; then

. /etc/bashrc # --> Read /etc/bashrc, if present.

--------------------------------------------------------------

Automatic setting of $DISPLAY (if not set already).

This works for me - your mileage may vary. . . .

The problem is that different types of terminals give

+ different answers to 'who am i' (rxvt in particular can be

+ troublesome) - however this code seems to work in a majority

+ of cases.

--------------------------------------------------------------

function get_xserver ()

{

case $TERM in

xterm )

XSERVER=$(who am i | awk '{print $NF}' | tr -d ')''(' )

# Ane-Pieter Wieringa suggests the following alternative:

# I_AM=$(who am i)

# SERVER=${I_AM#*(}

# SERVER=${SERVER%*)}

XSERVER=${XSERVER%%:*}

;;

aterm | rxvt)

# Find some code that works here. ...

;;

esac

}

if [ -z ${DISPLAY:=""} ]; then

get_xserver

if [[ -z ${XSERVER} || ${XSERVER} == $(hostname) ||

${XSERVER} == "unix" ]]; then

DISPLAY=":0.0" # Display on local host.

else

DISPLAY=${XSERVER}:0.0 # Display on remote host.

export DISPLAY

-------------------------------------------------------------

Some settings

-------------------------------------------------------------

set -o nounset # These two options are useful for debugging.

set -o xtrace

alias debug="set -o nounset; set -o xtrace"

ulimit -S -c 0 # Don't want coredumps.

set -o notify

set -o noclobber

set -o ignoreeof

Enable options:

shopt -s cdspell

shopt -s cdable_vars

shopt -s checkhash

shopt -s checkwinsize

shopt -s sourcepath

shopt -s no_empty_cmd_completion

shopt -s cmdhist

shopt -s histappend histreedit histverify

shopt -s extglob # Necessary for programmable completion.

Disable options:

shopt -u mailwarn

unset MAILCHECK # Don't want my shell to warn me of incoming mail.

-------------------------------------------------------------

Greeting, motd etc. ...

-------------------------------------------------------------

Color definitions (taken from Color Bash Prompt HowTo).

Some colors might look different of some terminals.

For example, I see 'Bold Red' as 'orange' on my screen,

hence the 'Green' 'BRed' 'Red' sequence I often use in my prompt.

Normal Colors

Black='\e[0;30m' # Black

Red='\e[0;31m' # Red

Green='\e[0;32m' # Green

Yellow='\e[0;33m' # Yellow

Blue='\e[0;34m' # Blue

Purple='\e[0;35m' # Purple

Cyan='\e[0;36m' # Cyan

White='\e[0;37m' # White

Bold

BBlack='\e[1;30m' # Black

BRed='\e[1;31m' # Red

BGreen='\e[1;32m' # Green

BYellow='\e[1;33m' # Yellow

BBlue='\e[1;34m' # Blue

BPurple='\e[1;35m' # Purple

BCyan='\e[1;36m' # Cyan

BWhite='\e[1;37m' # White

Background

On_Black='\e[40m' # Black

On_Red='\e[41m' # Red

On_Green='\e[42m' # Green

On_Yellow='\e[43m' # Yellow

On_Blue='\e[44m' # Blue

On_Purple='\e[45m' # Purple

On_Cyan='\e[46m' # Cyan

On_White='\e[47m' # White

NC="\e[m" # Color Reset

ALERT=${BWhite}${On_Red} # Bold White on red background

echo -e "${BCyan}This is BASH ${BRed}${BASH_VERSION%.*}${BCyan}\

- DISPLAY on ${BRed}$DISPLAY${NC}\n"

date

if [ -x /usr/games/fortune ]; then

/usr/games/fortune -s # Makes our day a bit more fun.... :-)

function _exit() # Function to run upon exit of shell.

{

echo -e "${BRed}Hasta la vista, baby${NC}"

}

trap _exit EXIT

-------------------------------------------------------------

Shell Prompt - for many examples, see:

http://www.debian-administration.org/articles/205

http://www.askapache.com/linux/bash-power-prompt.html

http://tldp.org/HOWTO/Bash-Prompt-HOWTO

https://github.com/nojhan/liquidprompt

-------------------------------------------------------------

Current Format: [TIME USER@HOST PWD] >

TIME:

Green == machine load is low

Orange == machine load is medium

Red == machine load is high

ALERT == machine load is very high

USER:

Cyan == normal user

Orange == SU to user

Red == root

HOST:

Cyan == local session

Green == secured remote connection (via ssh)

Red == unsecured remote connection

PWD:

Green == more than 10% free disk space

Orange == less than 10% free disk space

ALERT == less than 5% free disk space

Red == current user does not have write privileges

Cyan == current filesystem is size zero (like /proc)

>:

White == no background or suspended jobs in this shell

Cyan == at least one background job in this shell

Orange == at least one suspended job in this shell

Command is added to the history file each time you hit enter,

so it's available to all shells (using 'history -a').

Test connection type:

if [ -n "${SSH_CONNECTION}" ]; then

CNX=${Green} # Connected on remote machine, via ssh (good).

elif [[ "${DISPLAY%%:0*}" != "" ]]; then

CNX=${ALERT} # Connected on remote machine, not via ssh (bad).

else

CNX=${BCyan} # Connected on local machine.

Test user type:

if [[ ${USER} == "root" ]]; then

SU=${Red} # User is root.

elif [[ ${USER} != $(logname) ]]; then

SU=${BRed} # User is not login user.

else

SU=${BCyan} # User is normal (well ... most of us are).

NCPU=$(grep -c 'processor' /proc/cpuinfo) # Number of CPUs

SLOAD=$(( 100*${NCPU} )) # Small load

MLOAD=$(( 200*${NCPU} )) # Medium load

XLOAD=$(( 400*${NCPU} )) # Xlarge load

Returns system load as percentage, i.e., '40' rather than '0.40)'.

function load()

{

local SYSLOAD=$(cut -d " " -f1 /proc/loadavg | tr -d '.')

# System load of the current host.

echo $((10#$SYSLOAD)) # Convert to decimal.

}

Returns a color indicating system load.

function load_color()

{

local SYSLOAD=$(load)

if [ ${SYSLOAD} -gt ${XLOAD} ]; then

echo -en ${ALERT}

elif [ ${SYSLOAD} -gt ${MLOAD} ]; then

echo -en ${Red}

elif [ ${SYSLOAD} -gt ${SLOAD} ]; then

echo -en ${BRed}

else

echo -en ${Green}

}

Returns a color according to free disk space in $PWD.

function disk_color()

{

if [ ! -w "${PWD}" ] ; then

echo -en ${Red}

# No 'write' privilege in the current directory.

elif [ -s "${PWD}" ] ; then

local used=$(command df -P "$PWD" |

awk 'END {print $5} {sub(/%/,"")}')

if [ ${used} -gt 95 ]; then

echo -en ${ALERT} # Disk almost full (>95%).

elif [ ${used} -gt 90 ]; then

echo -en ${BRed} # Free disk space almost gone.

else

echo -en ${Green} # Free disk space is ok.

else

echo -en ${Cyan}

# Current directory is size '0' (like /proc, /sys etc).

}

Returns a color according to running/suspended jobs.

function job_color()

{

if [ $(jobs -s | wc -l) -gt "0" ]; then

echo -en ${BRed}

elif [ $(jobs -r | wc -l) -gt "0" ] ; then

echo -en ${BCyan}

}

Adds some text in the terminal frame (if applicable).

Now we construct the prompt.

PROMPT_COMMAND="history -a"

case ${TERM} in

*term | rxvt | linux)

PS1="\[\$(load_color)\][\A\[${NC}\] "

# Time of day (with load info):

PS1="\[\$(load_color)\][\A\[${NC}\] "

# User@Host (with connection type info):

PS1=${PS1}"\[${SU}\]\u\[${NC}\]@\[${CNX}\]\h\[${NC}\] "

# PWD (with 'disk space' info):

PS1=${PS1}"\[\$(disk_color)\]\W]\[${NC}\] "

# Prompt (with 'job' info):

PS1=${PS1}"\[\$(job_color)\]>\[${NC}\] "

# Set title of current xterm:

PS1=${PS1}"\[\e]0;[\u@\h] \w\a\]"

;;

PS1="(\A \u@\h \W) > " # --> PS1="(\A \u@\h \w) > "

# --> Shows full pathname of current dir.

;;

esac

export TIMEFORMAT= \nreal %3R\tuser %3U\tsys %3S\tpcpu %P\n'

export HISTIGNORE="&:bg:fg:ll:h"

export HISTTIMEFORMAT="$(echo -e ${BCyan})[%d/%m %H:%M:%S]$(echo -e ${NC}) "

export HISTCONTROL=ignoredups

export HOSTFILE=$HOME/.hosts # Put a list of remote hosts in ~/.hosts

============================================================

ALIASES AND FUNCTIONS

Arguably, some functions defined here are quite big.

If you want to make this file smaller, these functions can

+ be converted into scripts and removed from here.

============================================================

-------------------

Personnal Aliases

-------------------

alias rm='rm -i'

alias cp='cp -i'

alias mv='mv -i'

-> Prevents accidentally clobbering files.

alias mkdir='mkdir -p'

alias h='history'

alias j='jobs -l'

alias which='type -a'

alias ..='cd ..'

Pretty-print of some PATH variables:

alias path='echo -e ${PATH//:/\\n}'

alias libpath='echo -e ${LD_LIBRARY_PATH//:/\\n}'

alias du='du -kh' # Makes a more readable output.

alias df='df -kTh'

-------------------------------------------------------------

The 'ls' family (this assumes you use a recent GNU ls).

-------------------------------------------------------------

Add colors for filetype and human-readable sizes by default on 'ls':

alias ls='ls -h --color'

alias lx='ls -lXB' # Sort by extension.

alias lk='ls -lSr' # Sort by size, biggest last.

alias lt='ls -ltr' # Sort by date, most recent last.

alias lc='ls -ltcr' # Sort by/show change time,most recent last.

alias lu='ls -ltur' # Sort by/show access time,most recent last.

The ubiquitous 'll': directories first, with alphanumeric sorting:

alias ll="ls -lv --group-directories-first"

alias lm='ll |more' # Pipe through 'more'

alias lr='ll -R' # Recursive ls.

alias la='ll -A' # Show hidden files.

alias tree='tree -Csuh' # Nice alternative to 'recursive ls' ...

-------------------------------------------------------------

Tailoring 'less'

-------------------------------------------------------------

alias more='less'

export PAGER=less

export LESSCHARSET='latin1'

export LESSOPEN='|/usr/bin/lesspipe.sh %s 2>&-'

# Use this if lesspipe.sh exists.

export LESS='-i -N -w -z-4 -g -e -M -X -F -R -P%t?f%f \

:stdin .?pb%pb\%:?lbLine %lb:?bbByte %bb:-...'

LESS man page colors (makes Man pages more readable).

export LESS_TERMCAP_mb= \E[01;31m'

export LESS_TERMCAP_md= \E[01;31m'

export LESS_TERMCAP_me= \E[0m'

export LESS_TERMCAP_se= \E[0m'

export LESS_TERMCAP_so= \E[01;44;33m'

export LESS_TERMCAP_ue= \E[0m'

export LESS_TERMCAP_us= \E[01;32m'

-------------------------------------------------------------

Spelling typos - highly personnal and keyboard-dependent :-)

-------------------------------------------------------------

alias xs='cd'

alias vf='cd'

alias moer='more'

alias moew='more'

alias kk='ll'

-------------------------------------------------------------

A few fun ones

-------------------------------------------------------------

Adds some text in the terminal frame (if applicable).

function xtitle()

{

case "$TERM" in

*term* | rxvt)

echo -en "\e]0;$*\a" ;;

*) ;;

esac

}

Aliases that use xtitle

alias top='xtitle Processes on $HOST && top'

alias make='xtitle Making $(basename $PWD) ; make'

.. and functions

function man()

{

for i ; do

xtitle The $(basename $1|tr -d .[:digit:]) manual

command man -a "$i"

done

}

-------------------------------------------------------------

Make the following commands run in background automatically:

-------------------------------------------------------------

function te() # wrapper around xemacs/gnuserv

{

if [ "$(gnuclient -batch -eval t 2>&-)" == "t" ]; then

gnuclient -q "$@";

else

( xemacs "$@" &);

}

function soffice() { command soffice "$@" & }

function firefox() { command firefox "$@" & }

function xpdf() { command xpdf "$@" & }

-------------------------------------------------------------

File & strings related functions:

-------------------------------------------------------------

Find a file with a pattern in name:

function ff() { find . -type f -iname '*'"$*"'*' -ls ; }

Find a file with pattern $1 in name and Execute $2 on it:

function fe() { find . -type f -iname '*'"${1:-}"'*' \

-exec ${2:-file} {} \; ; }

Find a pattern in a set of files and highlight them:

+ (needs a recent version of egrep).

function fstr()

{

OPTIND=1

local mycase=""

local usage="fstr: find string in files.

Usage: fstr [-i] \"pattern\" [\"filename pattern\"] "

while getopts :it opt

case "$opt" in

i) mycase="-i " ;;

*) echo "$usage"; return ;;

esac

done

shift $(( $OPTIND - 1 ))

if [ "$#" -lt 1 ]; then

echo "$usage"

return;

find . -type f -name "${2:-*}" -print0 | \

xargs -0 egrep --color=always -sn ${case} "$1" 2>&- | more

}

function swap()

{ # Swap 2 filenames around, if they exist (from Uzi's bashrc).

local TMPFILE=tmp.$

[ $# -ne 2 ] && echo "swap: 2 arguments needed" && return 1

[ ! -e $1 ] && echo "swap: $1 does not exist" && return 1

[ ! -e $2 ] && echo "swap: $2 does not exist" && return 1

mv "$1" $TMPFILE

mv "$2" "$1"

mv $TMPFILE "$2"

}

function extract() # Handy Extract Program

{

if [ -f $1 ] ; then

case $1 in

*.tar.bz2) tar xvjf $1 ;;

*.tar.gz) tar xvzf $1 ;;

*.bz2) bunzip2 $1 ;;

*.rar) unrar x $1 ;;

*.gz) gunzip $1 ;;

*.tar) tar xvf $1 ;;

*.tbz2) tar xvjf $1 ;;

*.tgz) tar xvzf $1 ;;

*.zip) unzip $1 ;;

*.Z) uncompress $1 ;;

*.7z) 7z x $1 ;;

*) echo "'$1' cannot be extracted via >extract<" ;;

esac

else

echo "'$1' is not a valid file!"

}

Creates an archive (*.tar.gz) from given directory.

function maketar() { tar cvzf "${1%%/}.tar.gz" "${1%%/}/"; }

Create a ZIP archive of a file or folder.

function makezip() { zip -r "${1%%/}.zip" "$1" ; }

Make your directories and files access rights sane.

function sanitize() { chmod -R u=rwX,g=rX,o= "$@" ;}

-------------------------------------------------------------

Process/system related functions:

-------------------------------------------------------------

function my_ps() { ps $@ -u $USER -o pid,%cpu,%mem,bsdtime,command ; }

function pp() { my_ps f | awk '!/awk/ && $0~var' var=${1:-".*"} ; }

function killps() # kill by process name

{

local pid pname sig="-TERM" # default signal

if [ "$#" -lt 1 ] || [ "$#" -gt 2 ]; then

echo "Usage: killps [-SIGNAL] pattern"

return;

if [ $# = 2 ]; then sig=$1 ; fi

for pid in $(my_ps| awk '!/awk/ && $0~pat { print $1 }' pat=${!#} )

pname=$(my_ps | awk '$1~var { print $5 }' var=$pid )

if ask "Kill process $pid <$pname> with signal $sig?"

then kill $sig $pid

done

}

function mydf() # Pretty-print of 'df' output.

{ # Inspired by 'dfc' utility.

for fs ; do

if [ ! -d $fs ]

then

echo -e $fs" :No such file or directory" ; continue

local info=( $(command df -P $fs | awk 'END{ print $2,$3,$5 }') )

local free=( $(command df -Pkh $fs | awk 'END{ print $4 }') )

local nbstars=$(( 20 * ${info[1]} / ${info[0]} ))

local out="["

for ((j=0;j<20;j++)); do

if [ ${j} -lt ${nbstars} ]; then

out=$out"*"

else

out=$out"-"

done

out=${info[2]}" "$out"] ("$free" free on "$fs")"

echo -e $out

done

}

function my_ip() # Get IP adress on ethernet.

{

MY_IP=$(/sbin/ifconfig eth0 | awk '/inet/ { print $2 } ' |

sed -e s/addr://)

echo ${MY_IP:-"Not connected"}

}

function ii() # Get current host related info.

{

echo -e "\nYou are logged on ${BRed}$HOST"

echo -e "\n${BRed}Additionnal information:$NC " ; uname -a

echo -e "\n${BRed}Users logged on:$NC " ; w -hs |

cut -d " " -f1 | sort | uniq

echo -e "\n${BRed}Current date :$NC " ; date

echo -e "\n${BRed}Machine stats :$NC " ; uptime

echo -e "\n${BRed}Memory stats :$NC " ; free

echo -e "\n${BRed}Diskspace :$NC " ; mydf / $HOME

echo -e "\n${BRed}Local IP Address :$NC" ; my_ip

echo -e "\n${BRed}Open connections :$NC "; netstat -pan --inet;

echo

}

-------------------------------------------------------------

Misc utilities:

-------------------------------------------------------------

function repeat() # Repeat n times command.

{

local i max

max=$1; shift;

for ((i=1; i <= max ; i++)); do # --> C-like syntax

eval "$@";

done

}

function ask() # See 'killps' for example of use.

{

echo -n "$@" '[y/n] ' ; read ans

case "$ans" in

y*|Y*) return 0 ;;

*) return 1 ;;

esac

}

function corename() # Get name of app that created a corefile.

{

for file ; do

echo -n $file : ; gdb --core=$file --batch | head -1

done

}

=========================================================================

PROGRAMMABLE COMPLETION SECTION

Most are taken from the bash 2.05 documentation and from Ian McDonald's

'Bash completion' package (http://www.caliban.org/bash/#completion)

You will in fact need bash more recent then 3.0 for some features.

Note that most linux distributions now provide many completions

'out of the box' - however, you might need to make your own one day,

so I kept those here as examples.

=========================================================================

if [ "${BASH_VERSION%.*}" \< "3.0" ]; then

echo "You will need to upgrade to version 3.0 for full \

programmable completion features"

return

shopt -s extglob # Necessary.

complete -A hostname rsh rcp telnet rlogin ftp ping disk

complete -A export printenv

complete -A variable export local readonly unset

complete -A enabled builtin

complete -A alias alias unalias

complete -A function function

complete -A user su mail finger

complete -A helptopic help # Currently same as builtins.

complete -A shopt shopt

complete -A stopped -P '%' bg

complete -A job -P '%' fg jobs disown

complete -A directory mkdir rmdir

complete -A directory -o default cd

Compression

complete -f -o default -X '*.+(zip|ZIP)' zip

complete -f -o default -X '!*.+(zip|ZIP)' unzip

complete -f -o default -X '*.+(z|Z)' compress

complete -f -o default -X '!*.+(z|Z)' uncompress

complete -f -o default -X '*.+(gz|GZ)' gzip

complete -f -o default -X '!*.+(gz|GZ)' gunzip

complete -f -o default -X '*.+(bz2|BZ2)' bzip2

complete -f -o default -X '!*.+(bz2|BZ2)' bunzip2

complete -f -o default -X '!*.+(zip|ZIP|z|Z|gz|GZ|bz2|BZ2)' extract

Documents - Postscript,pdf,dvi.....

complete -f -o default -X '!*.+(ps|PS)' gs ghostview ps2pdf ps2ascii

complete -f -o default -X \

'!*.+(dvi|DVI)' dvips dvipdf xdvi dviselect dvitype

complete -f -o default -X '!*.+(pdf|PDF)' acroread pdf2ps

complete -f -o default -X '!*.@(@(?(e)ps|?(E)PS|pdf|PDF)?\

(.gz|.GZ|.bz2|.BZ2|.Z))' gv ggv

complete -f -o default -X '!*.texi*' makeinfo texi2dvi texi2html texi2pdf

complete -f -o default -X '!*.tex' tex latex slitex

complete -f -o default -X '!*.lyx' lyx

complete -f -o default -X '!*.+(htm*|HTM*)' lynx html2ps

complete -f -o default -X \

'!*.+(doc|DOC|xls|XLS|ppt|PPT|sx?|SX?|csv|CSV|od?|OD?|ott|OTT)' soffice

Multimedia

complete -f -o default -X \

'!*.+(gif|GIF|jp*g|JP*G|bmp|BMP|xpm|XPM|png|PNG)' xv gimp ee gqview

complete -f -o default -X '!*.+(mp3|MP3)' mpg123 mpg321

complete -f -o default -X '!*.+(ogg|OGG)' ogg123

complete -f -o default -X \

'!*.@(mp[23]|MP[23]|ogg|OGG|wav|WAV|pls|\

m3u|xm|mod|s[3t]m|it|mtm|ult|flac)' xmms

complete -f -o default -X '!*.@(mp?(e)g|MP?(E)G|wma|avi|AVI|\

asf|vob|VOB|bin|dat|vcd|ps|pes|fli|viv|rm|ram|yuv|mov|MOV|qt|\

QT|wmv|mp3|MP3|ogg|OGG|ogm|OGM|mp4|MP4|wav|WAV|asx|ASX)' xine

complete -f -o default -X '!*.pl' perl perl5

This is a 'universal' completion function - it works when commands have

+ a so-called 'long options' mode , ie: 'ls --all' instead of 'ls -a'

Needs the '-o' option of grep

+ (try the commented-out version if not available).

First, remove '=' from completion word separators

+ (this will allow completions like 'ls --color=auto' to work correctly).

COMP_WORDBREAKS=${COMP_WORDBREAKS/=/}

_get_longopts()

{

#$1 --help | sed -e '/--/!d' -e 's/.*--$[^[:space:].,]*$.*/--\1/'| \

#grep ^"$2" |sort -u ;

$1 --help | grep -o -e "--[^[:space:].,]*" | grep -e "$2" |sort -u

}

_longopts()

{

local cur

cur=${COMP_WORDS[COMP_CWORD]}

case "${cur:-*}" in

-*) ;;

*) return ;;

esac

case "$1" in

\~*) eval cmd="$1" ;;

*) cmd="$1" ;;

esac

COMPREPLY=( $(_get_longopts ${1} ${cur} ) )

}

complete -o default -F _longopts configure bash

complete -o default -F _longopts wget id info a2ps ls recode

_tar()

{

local cur ext regex tar untar

COMPREPLY=()

cur=${COMP_WORDS[COMP_CWORD]}

# If we want an option, return the possible long options.

case "$cur" in

-*) COMPREPLY=( $(_get_longopts $1 $cur ) ); return 0;;

esac

if [ $COMP_CWORD -eq 1 ]; then

COMPREPLY=( $( compgen -W 'c t x u r d A' -- $cur ) )

return 0

case "${COMP_WORDS[1]}" in

?(-)c*f)

COMPREPLY=( $( compgen -f $cur ) )

return 0

;;

+([^Izjy])f)

ext='tar'

regex=$ext

;;

*z*f)

ext='tar.gz'

regex='t$ar\.$$gz\|Z$'

;;

*[Ijy]*f)

ext='t?(ar.)bz?(2)'

regex='t$ar\.$bz2\?'

;;

COMPREPLY=( $( compgen -f $cur ) )

return 0

;;

esac

if [[ "$COMP_LINE" == tar*.$ext' '* ]]; then

# Complete on files in tar file.

# Get name of tar file from command line.

tar=$( echo "$COMP_LINE" | \

sed -e 's|^.* $[^ ]*'$regex'$ .*$|\1|' )

# Devise how to untar and list it.

untar=t${COMP_WORDS[1]//[^Izjyf]/}

COMPREPLY=( $( compgen -W "$( echo $( tar $untar $tar \

2>/dev/null ) )" -- "$cur" ) )

return 0

else

# File completion on relevant files.

COMPREPLY=( $( compgen -G $cur\*.$ext ) )

return 0

}

complete -F _tar -o default tar

_make()

{

local mdef makef makef_dir="." makef_inc gcmd cur prev i;

COMPREPLY=();

cur=${COMP_WORDS[COMP_CWORD]};

prev=${COMP_WORDS[COMP_CWORD-1]};

case "$prev" in

-*f)

COMPREPLY=($(compgen -f $cur ));

return 0

;;

esac;

case "$cur" in

-*)

COMPREPLY=($(_get_longopts $1 $cur ));

return 0

;;

esac;

# ... make reads

# GNUmakefile,

# then makefile

# then Makefile ...

if [ -f ${makef_dir}/GNUmakefile ]; then

makef=${makef_dir}/GNUmakefile

elif [ -f ${makef_dir}/makefile ]; then

makef=${makef_dir}/makefile

elif [ -f ${makef_dir}/Makefile ]; then

makef=${makef_dir}/Makefile

else

makef=${makef_dir}/*.mk # Local convention.

# Before we scan for targets, see if a Makefile name was

#+ specified with -f.

for (( i=0; i < ${#COMP_WORDS[@]}; i++ )); do

if [[ ${COMP_WORDS[i]} == -f ]]; then

# eval for tilde expansion

eval makef=${COMP_WORDS[i+1]}

break

done

[ ! -f $makef ] && return 0

# Deal with included Makefiles.

makef_inc=$( grep -E '^-?include' $makef |

sed -e "s,^.* ,"$makef_dir"/," )

for file in $makef_inc; do

[ -f $file ] && makef="$makef $file"

done

# If we have a partial word to complete, restrict completions

#+ to matches of that word.

if [ -n "$cur" ]; then gcmd='grep "^$cur"' ; else gcmd=cat ; fi

COMPREPLY=( $( awk -F':' '/^[a-zA-Z0-9][^$#\/\t=]*:([^=]|$)/ \

{split($1,A,/ /);for(i in A)print A[i]}' \

$makef 2>/dev/null | eval $gcmd ))

}

complete -F _make -X '+($*|*.[cho])' make gmake pmake

_killall()

{

local cur prev

COMPREPLY=()

cur=${COMP_WORDS[COMP_CWORD]}

# Get a list of processes

#+ (the first sed evaluation

#+ takes care of swapped out processes, the second

#+ takes care of getting the basename of the process).

COMPREPLY=( $( ps -u $USER -o comm | \

sed -e '1,1d' -e 's#[]\[]##g' -e 's#^.*/##'| \

awk '{if ($0 ~ /^'$cur'/) print $0}' ))

return 0

}

complete -F _killall killall killps

Local Variables:

mode:shell-script

sh-shell:bash

End:

And, here is a snippet from Andrzej Szelachowski's instructive

.bash_profile file.

Example M-2. .bash_profile file

From Andrzej Szelachowski's ~/.bash_profile:

Note that a variable may require special treatment

+ if it will be exported.

DARKGRAY='\e[1;30m'

LIGHTRED='\e[1;31m'

GREEN='\e[32m'

YELLOW='\e[1;33m'

LIGHTBLUE='\e[1;34m'

NC='\e[m'

PCT="\`if [[ \$EUID -eq 0 ]]; then T='$LIGHTRED' ; else T='$LIGHTBLUE'; fi;

echo \$T \`"

For "literal" command substitution to be assigned to a variable,

+ use escapes and double quotes:

+ PCT="\` ... \`" . . .

Otherwise, the value of PCT variable is assigned only once,

+ when the variable is exported/read from .bash_profile,

+ and it will not change afterwards even if the user ID changes.

PS1="\n$GREEN[\w] \n$DARKGRAY($PCT\t$DARKGRAY)-($PCT\u$DARKGRAY)-($PCT\!

$DARKGRAY)$YELLOW-> $NC"

Escape a variables whose value changes:

if [[ \$EUID -eq 0 ]],

Otherwise the value of the EUID variable will be assigned only once,

+ as above.

When a variable is assigned, it should be called escaped:

+ echo \$T,

Otherwise the value of the T variable is taken from the moment the PCT

+ variable is exported/read from .bash_profile.

So, in this example it would be null.

When a variable's value contains a semicolon it should be strong quoted:

T='$LIGHTRED',

Otherwise, the semicolon will be interpreted as a command separator.

Variables PCT and PS1 can be merged into a new PS1 variable:

PS1="\`if [[ \$EUID -eq 0 ]]; then PCT='$LIGHTRED';

else PCT='$LIGHTBLUE'; fi;

echo '\n$GREEN[\w] \n$DARKGRAY('\$PCT'\t$DARKGRAY)-\

('\$PCT'\u$DARKGRAY)-('\$PCT'\!$DARKGRAY)$YELLOW-> $NC'\`"

The trick is to use strong quoting for parts of old PS1 variable.

________________________________________________________________

Appendix N. Converting DOS Batch Files to Shell Scripts

Quite a number of programmers learned scripting on a PC running DOS.

Even the crippled DOS batch file language allowed writing some fairly

powerful scripts and applications, though they often required

extensive kludges and workarounds. Occasionally, the need still

arises to convert an old DOS batch file to a UNIX shell script. This

is generally not difficult, as DOS batch file operators are only a

limited subset of the equivalent shell scripting ones.

Table N-1. Batch file keywords / variables / operators, and their

shell equivalents

Batch File Operator Shell Script Equivalent Meaning

% $ command-line parameter prefix

/ - command option flag

\ / directory path separator

== = (equal-to) string comparison test

!==! != (not equal-to) string comparison test

| | pipe

@ set +v do not echo current command

* * filename "wild card"

> > file redirection (overwrite)

>> >> file redirection (append)

< < redirect stdin

%VAR% $VAR environmental variable

REM # comment

NOT ! negate following test

NUL /dev/null "black hole" for burying command output

ECHO echo echo (many more option in Bash)

ECHO. echo echo blank line

ECHO OFF set +v do not echo command(s) following

FOR %%VAR IN (LIST) DO for var in [list]; do "for" loop

:LABEL none (unnecessary) label

GOTO none (use a function) jump to another location in the script

PAUSE sleep pause or wait an interval

CHOICE case or select menu choice

IF if if-test

IF EXIST FILENAME if [ -e filename ] test if file exists

IF !%N==! if [ -z "$N" ] if replaceable parameter "N" not present

CALL source or . (dot operator) "include" another script

COMMAND /C source or . (dot operator) "include" another script (same

as CALL)

SET export set an environmental variable

SHIFT shift left shift command-line argument list

SGN -lt or -gt sign (of integer)

ERRORLEVEL $? exit status

CON stdin "console" (stdin)

PRN /dev/lp0 (generic) printer device

LPT1 /dev/lp0 first printer device

COM1 /dev/ttyS0 first serial port

Batch files usually contain DOS commands. These must be translated

into their UNIX equivalents in order to convert a batch file into a

shell script.

Table N-2. DOS commands and their UNIX equivalents

DOS Command UNIX Equivalent Effect

ASSIGN ln link file or directory

ATTRIB chmod change file permissions

CD cd change directory

CHDIR cd change directory

CLS clear clear screen

COMP diff, comm, cmp file compare

COPY cp file copy

Ctl-C Ctl-C break (signal)

Ctl-Z Ctl-D EOF (end-of-file)

DEL rm delete file(s)

DELTREE rm -rf delete directory recursively

DIR ls -l directory listing

ERASE rm delete file(s)

EXIT exit exit current process

FC comm, cmp file compare

FIND grep find strings in files

MD mkdir make directory

MKDIR mkdir make directory

MORE more text file paging filter

MOVE mv move

PATH $PATH path to executables

REN mv rename (move)

RENAME mv rename (move)

RD rmdir remove directory

RMDIR rmdir remove directory

SORT sort sort file

TIME date display system time

TYPE cat output file to stdout

XCOPY cp (extended) file copy

Note

Virtually all UNIX and shell operators and commands have many more

options and enhancements than their DOS and batch file counterparts.

Many DOS batch files rely on auxiliary utilities, such as ask.com, a

crippled counterpart to read.

DOS supports only a very limited and incompatible subset of filename

wild-card expansion, recognizing just the * and ? characters.

Converting a DOS batch file into a shell script is generally

straightforward, and the result ofttimes reads better than the

original.

Example N-1. VIEWDATA.BAT: DOS Batch File

REM VIEWDATA

REM INSPIRED BY AN EXAMPLE IN "DOS POWERTOOLS"

REM BY PAUL SOMERSON

@ECHO OFF

IF !%1==! GOTO VIEWDATA

REM IF NO COMMAND-LINE ARG...

FIND "%1" C:\BOZO\BOOKLIST.TXT

GOTO EXIT0

REM PRINT LINE WITH STRING MATCH, THEN EXIT.

:VIEWDATA

TYPE C:\BOZO\BOOKLIST.TXT | MORE

REM SHOW ENTIRE FILE, 1 PAGE AT A TIME.

:EXIT0

The script conversion is somewhat of an improvement. [152]

Example N-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT

!/bin/bash

viewdata.sh

Conversion of VIEWDATA.BAT to shell script.

DATAFILE=/home/bozo/datafiles/book-collection.data

ARGNO=1

@ECHO OFF Command unnecessary here.

if [ $# -lt "$ARGNO" ] # IF !%1==! GOTO VIEWDATA

then

less $DATAFILE # TYPE C:\MYDIR\BOOKLIST.TXT | MORE

else

grep "$1" $DATAFILE # FIND "%1" C:\MYDIR\BOOKLIST.TXT

exit 0 # :EXIT0

GOTOs, labels, smoke-and-mirrors, and flimflam unnecessary.

The converted script is short, sweet, and clean,

+ which is more than can be said for the original.

Ted Davis' Shell Scripts on the PC site had a set of comprehensive

tutorials on the old-fashioned art of batch file programming.

Unfortunately the page has vanished without a trace.

________________________________________________________________

Appendix O. Exercises

The exercises that follow test and extend your knowledge of

scripting. Think of them as a challenge, as an entertaining way to

take you further along the stony path toward UNIX wizardry.

On a dingy side street in a run-down section of Hoboken, New Je

rsey,

there sits a nondescript squat two-story brick building with an

inscription

incised on a marble plate in its wall:

Bash Scripting Hall of Fame.

Inside, among various dusty uninteresting exhibits is a corrodi

ng,

cobweb-festooned brass plaque inscribed with a short, very shor

list of those few persons who have successfully mastered the ma

terial

in the Advanced Bash Scripting

Guide, as evidenced by their performance

on the following Exercise sections.

(Alas, the author of the ABS

Guide is not represented among the exhibits.

This is possibly due to malicious rumors about lack of

credentials and

deficient scripting skills.)

________________________________________________________________

O.1. Analyzing Scripts

Examine the following script. Run it, then explain what it does.

Annotate the script and rewrite it in a more compact and elegant

manner.

!/bin/bash

MAX=10000

for((nr=1; nr<$MAX; nr++))

let "t1 = nr % 5"

if [ "$t1" -ne 3 ]

then

continue

let "t2 = nr % 7"

if [ "$t2" -ne 4 ]

then

continue

let "t3 = nr % 9"

if [ "$t3" -ne 5 ]

then

continue

break # What happens when you comment out this line? Why?

done

echo "Number = $nr"

exit 0

---

Explain what the following script does. It is really just a

parameterized command-line pipe.

!/bin/bash

DIRNAME=/usr/bin

FILETYPE="shell script"

LOGFILE=logfile

file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l

exit 0

---

Examine and explain the following script. For hints, you might refer

to the listings for find and stat.

!/bin/bash

Author: Nathan Coulter

This code is released to the public domain.

The author gave permission to use this code snippet in the ABS Guide.

find -maxdepth 1 -type f -printf '%f\000' | {

while read -d \000'; do

mv "$REPLY" "$(date -d "$(stat -c '%y' "$REPLY") " '+%Y%m%d%H%M%S'

)-$REPLY"

done

}

Warning: Test-drive this script in a "scratch" directory.

It will somehow affect all the files there.

---

A reader sent in the following code snippet.

while read LINE

echo $LINE

done < `tail -f /var/log/messages`

He wished to write a script tracking changes to the system log file,

/var/log/messages. Unfortunately, the above code block hangs and does

nothing useful. Why? Fix this so it does work. (Hint: rather than

redirecting the stdin of the loop, try a pipe.)

---

Analyze the following "one-liner" (here split into two lines for

clarity) contributed by Rory Winston:

export SUM=0; for f in $(find src -name "*.java");

do export SUM=$(($SUM + $(wc -l $f | awk '{ print $1 }'))); done; echo $SUM

Hint: First, break the script up into bite-sized sections. Then,

carefully examine its use of double-parentheses arithmetic, the

export command, the find command, the wc command, and awk.

---

Analyze Example A-10, and reorganize it in a simplified and more

logical style. See how many of the variables can be eliminated, and

try to optimize the script to speed up its execution time.

Alter the script so that it accepts any ordinary ASCII text file as

input for its initial "generation". The script will read the first

$ROW*$COL characters, and set the occurrences of vowels as "living"

cells. Hint: be sure to translate the spaces in the input file to

underscore characters.

________________________________________________________________

O.2. Writing Scripts

Write a script to carry out each of the following tasks.

EASY

Self-reproducing Script

Write a script that backs itself up, that is, copies itself to

a file named backup.sh.

Hint: Use the cat command and the appropriate positional

parameter.

Home Directory Listing

Perform a recursive directory listing on the user's home

directory and save the information to a file. Compress the

file, have the script prompt the user to insert a USB flash

drive, then press ENTER. Finally, save the file to the flash

drive after making certain the flash drive has properly

mounted by parsing the output of df. Note that the flash drive

must be unmounted before it is removed.

Converting for loops to while and until loops

Convert the for loops in Example 11-1 to while loops. Hint:

store the data in an array and step through the array

elements.

Having already done the "heavy lifting," now convert the loops

in the example to until loops.

Changing the line spacing of a text file

Write a script that reads each line of a target file, then

writes the line back to stdout, but with an extra blank line

following. This has the effect of double-spacing the file.

Include all necessary code to check whether the script gets

the necessary command-line argument (a filename), and whether

the specified file exists.

When the script runs correctly, modify it to triple-space the

target file.

Finally, write a script to remove all blank lines from the

target file, single-spacing it.

Backwards Listing

Write a script that echoes itself to stdout, but backwards.

Automatically Decompressing Files

Given a list of filenames as input, this script queries each

target file (parsing the output of the file command) for the

type of compression used on it. Then the script automatically

invokes the appropriate decompression command (gunzip,

bunzip2, unzip, uncompress, or whatever). If a target file is

not compressed, the script emits a warning message, but takes

no other action on that particular file.

Unique System ID

Generate a "unique" 6-digit hexadecimal identifier for your

computer. Do not use the flawed hostid command. Hint: md5sum

/etc/passwd, then select the first 6 digits of output.

Backup

Archive as a "tarball" (*.tar.gz file) all the files in your

home directory tree (/home/your-name) that have been modified

in the last 24 hours. Hint: use find.

Optional: you may use this as the basis of a backup script.

Checking whether a process is still running

Given a process ID (PID) as an argument, this script will

check, at user-specified intervals, whether the given process

is still running. You may use the ps and sleep commands.

Primes

Print (to stdout) all prime numbers between 60000 and 63000.

The output should be nicely formatted in columns (hint: use

printf).

Lottery Numbers

One type of lottery involves picking five different numbers,

in the range of 1 - 50. Write a script that generates five

pseudorandom numbers in this range, with no duplicates. The

script will give the option of echoing the numbers to stdout

or saving them to a file, along with the date and time the

particular number set was generated. (If your script

consistently generates winning lottery numbers, then you can

retire on the proceeds and leave shell scripting to those of

us who have to work for a living.)

INTERMEDIATE

Integer or String

Write a script function that determines if an argument passed

to it is an integer or a string. The function will return TRUE

(0) if passed an integer, and FALSE (1) if passed a string.

Hint: What does the following expression return when $1 is not

an integer?

expr $1 + 0

ASCII to Integer

The atoi function in C converts a string character to an

integer. Write a shell script function that performs the same

operation. Likewise, write a shell script function that does

the inverse, mirroring the C itoa function which converts an

integer into an ASCII character.

Managing Disk Space

List, one at a time, all files larger than 100K in the

/home/username directory tree. Give the user the option to

delete or compress the file, then proceed to show the next

one. Write to a logfile the names of all deleted files and the

deletion times.

Banner

Simulate the functionality of the deprecated banner command in

a script.

Removing Inactive Accounts

Inactive accounts on a network server waste disk space and may

become a security risk. Write an administrative script (to be

invoked by root or the cron daemon) that checks for and

deletes user accounts that have not been accessed within the

last 90 days.

Enforcing Disk Quotas

Write a script for a multi-user system that checks users' disk

usage. If a user surpasses a preset limit (500 MB, for

example) in her /home/username directory, then the script

automatically sends her a "pigout" warning e-mail.

The script will use the du and mail commands. As an option, it

will allow setting and enforcing quotas using the quota and

setquota commands.

Logged in User Information

For all logged in users, show their real names and the time

and date of their last login.

Hint: use who, lastlog, and parse /etc/passwd.

Safe Delete

Implement, as a script, a "safe" delete command, sdel.sh.

Filenames passed as command-line arguments to this script are

not deleted, but instead gzipped if not already compressed

(use file to check), then moved to a ~/TRASH directory. Upon

invocation, the script checks the ~/TRASH directory for files

older than 48 hours and permanently deletes them. (An better

alternative might be to have a second script handle this,

periodically invoked by the cron daemon.)

Extra credit: Write the script so it can handle files and

directories recursively. This would give it the capability of

"safely deleting" entire directory structures.

Making Change

What is the most efficient way to make change for $1.68, using

only coins in common circulations (up to 25c)? It's 6

quarters, 1 dime, a nickel, and three cents.

Given any arbitrary command-line input in dollars and cents

($*.??), calculate the change, using the minimum number of

coins. If your home country is not the United States, you may

use your local currency units instead. The script will need to

parse the command-line input, then change it to multiples of

the smallest monetary unit (cents or whatever). Hint: look at

Example 24-8.

Quadratic Equations

Solve a quadratic equation of the form Ax^2 + Bx + C = 0. Have

a script take as arguments the coefficients, A, B, and C, and

return the solutions to five decimal places.

Hint: pipe the coefficients to bc, using the well-known

formula, x = ( -B +/- sqrt( B^2 - 4AC ) ) / 2A.

Table of Logarithms

Using the bc and printf commands, print out a nicely-formatted

table of eight-place natural logarithms in the interval

between 0.00 and 100.00, in steps of .01.

Hint: bc requires the -l option to load the math library.

Unicode Table

Using Example T-1 as a template, write a script that prints to

a file a complete Unicode table.

Hint: Use the -e option to echo: echo -e '\uXXXX', where XXXX

is the Unicode numerical character designation. This requires

version 4.2 or later of Bash.

Sum of Matching Numbers

Find the sum of all five-digit numbers (in the range 10000 -

99999) containing exactly two out of the following set of

digits: { 4, 5, 6 }. These may repeat within the same number,

and if so, they count once for each occurrence.

Some examples of matching numbers are 42057, 74638, and 89515.

Lucky Numbers

A lucky number is one whose individual digits add up to 7, in

successive additions. For example, 62431 is a lucky number (6

+ 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the lucky numbers

between 1000 and 10000.

Craps

Borrowing the ASCII graphics from Example A-40, write a script

that plays the well-known gambling game of craps. The script

will accept bets from one or more players, roll the dice, and

keep track of wins and losses, as well as of each player's

bankroll.

Tic-tac-toe

Write a script that plays the child's game of tic-tac-toe

against a human player. The script will let the human choose

whether to take the first move. The script will follow an

optimal strategy, and therefore never lose. To simplify

matters, you may use ASCII graphics:

o | x |

----------

| x |

----------

| o |

Your move, human (row, column)?

Alphabetizing a String

Alphabetize (in ASCII order) an arbitrary string read from the

command-line.

Parsing

Parse /etc/passwd, and output its contents in nice,

easy-to-read tabular form.

Logging Logins

Parse /var/log/messages to produce a nicely formatted file of

user logins and login times. The script may need to run as

root. (Hint: Search for the string "LOGIN.")

Pretty-Printing a Data File

Certain database and spreadsheet packages use save-files with

the fields separated by commas, commonly referred to as

comma-separated values or CSVs. Other applications often need

to parse these files.

Given a data file with comma-separated fields, of the form:

Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989

Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612

...

Reformat the data and print it out to stdout in labeled,

evenly-spaced columns.

Justification

Given ASCII text input either from stdin or a file, adjust the

word spacing to right-justify each line to a user-specified

line-width, then send the output to stdout.

Mailing List

Using the mail command, write a script that manages a simple

mailing list. The script automatically e-mails the monthly

company newsletter, read from a specified text file, and sends

it to all the addresses on the mailing list, which the script

reads from another specified file.

Generating Passwords

Generate pseudorandom 8-character passwords, using characters

in the ranges [0-9], [A-Z], [a-z]. Each password must contain

at least two digits.

Monitoring a User

You suspect that one particular user on the network has been

abusing her privileges and possibly attempting to hack the

system. Write a script to automatically monitor and log her

activities when she's signed on. The log file will save

entries for the previous week, and delete those entries more

than seven days old.

You may use last, lastlog, and lastcomm to aid your

surveillance of the suspected fiend.

Checking for Broken Links

Using lynx with the -traversal option, write a script that

checks a Web site for broken links.

DIFFICULT

Testing Passwords

Write a script to check and validate passwords. The object is

to flag "weak" or easily guessed password candidates.

A trial password will be input to the script as a command-line

parameter. To be considered acceptable, a password must meet

the following minimum qualifications:

+ Minimum length of 8 characters

+ Must contain at least one numeric character

+ Must contain at least one of the following non-alphabetic

characters: @, #, $, %, &, *, +, -, =

Optional:

+ Do a dictionary check on every sequence of at least four

consecutive alphabetic characters in the password under

test. This will eliminate passwords containing embedded

"words" found in a standard dictionary.

+ Enable the script to check all the passwords on your system.

These do not reside in /etc/passwd.

This exercise tests mastery of Regular Expressions.

Cross Reference

Write a script that generates a cross-reference (concordance)

on a target file. The output will be a listing of all word

occurrences in the target file, along with the line numbers in

which each word occurs. Traditionally, linked list constructs

would be used in such applications. Therefore, you should

investigate arrays in the course of this exercise. Example

16-12 is probably not a good place to start.

Square Root

Write a script to calculate square roots of numbers using

Newton's Method.

The algorithm for this, expressed as a snippet of Bash

pseudo-code is:

(Isaac) Newton's Method for speedy extraction

+ of square roots.

guess = $argument

$argument is the number to find the square root of.

$guess is each successive calculated "guess" -- or trial solution --

+ of the square root.

Our first "guess" at a square root is the argument itself.

oldguess = 0

$oldguess is the previous $guess.

tolerance = .000001

To how close a tolerance we wish to calculate.

loopcnt = 0

Let's keep track of how many times through the loop.

Some arguments will require more loop iterations than others.

while [ ABS( $guess $oldguess ) -gt $tolerance ]

^^^^^^^^^^^^^^^^^^^^^^^ Fix up syntax, of course.

"ABS" is a (floating point) function to find the absolute value

+ of the difference between the two terms.

So, as long as difference between current and previous

+ trial solution (guess) exceeds the tolerance, keep looping.

oldguess = $guess # Update $oldguess to previous $guess.

=======================================================

guess = ( $oldguess + ( $argument / $oldguess ) ) / 2.0

= 1/2 ( ($oldguess **2 + $argument) / $oldguess )

equivalent to:

= 1/2 ( $oldguess + $argument / $oldguess )

that is, "averaging out" the trial solution and

+ the proportion of argument deviation

+ (in effect, splitting the error in half).

This converges on an accurate solution

+ with surprisingly few loop iterations . . .

+ for arguments > $tolerance, of course.

=======================================================

(( loopcnt++ )) # Update loop counter.

done

It's a simple enough recipe, and seems at first glance easy

enough to convert into a working Bash script. The problem,

though, is that Bash has no native support for floating point

numbers. So, the script writer needs to use bc or possibly awk

to convert the numbers and do the calculations. It could get

rather messy . . .

Logging File Accesses

Log all accesses to the files in /etc during the course of a

single day. This information should include the filename, user

name, and access time. If any alterations to the files take

place, that will be flagged. Write this data as tabular

(tab-separated) formatted records in a logfile.

Monitoring Processes

Write a script to continually monitor all running processes

and to keep track of how many child processes each parent

spawns. If a process spawns more than five children, then the

script sends an e-mail to the system administrator (or root)

with all relevant information, including the time, PID of the

parent, PIDs of the children, etc. The script appends a report

to a log file every ten minutes.

Strip Comments

Strip all comments from a shell script whose name is specified

on the command-line. Note that the initial #! line must not be

stripped out.

Strip HTML Tags

Strip all the HTML tags from a specified HTML file, then

reformat it into lines between 60 and 75 characters in length.

Reset paragraph and block spacing, as appropriate, and convert

HTML tables to their approximate text equivalent.

XML Conversion

Convert an XML file to both HTML and text format.

Optional: A script that converts Docbook/SGML to XML.

Chasing Spammers

Write a script that analyzes a spam e-mail by doing DNS

lookups on the IP addresses in the headers to identify the

relay hosts as well as the originating ISP. The script will

forward the unaltered spam message to the responsible ISPs. Of

course, it will be necessary to filter out your own ISP's IP

address, so you don't end up complaining about yourself.

As necessary, use the appropriate network analysis commands.

For some ideas, see Example 16-41 and Example A-28.

Optional: Write a script that searches through a list of

e-mail messages and deletes the spam according to specified

filters.

Creating man pages

Write a script that automates the process of creating man

pages.

Given a text file which contains information to be formatted

into a man page, the script will read the file, then invoke

the appropriate groff commands to output the corresponding man

page to stdout. The text file contains blocks of information

under the standard man page headings, i.e., NAME, SYNOPSIS,

DESCRIPTION, etc.

Example A-39 is an instructive first step.

Hex Dump

Do a hex(adecimal) dump on a binary file specified as an

argument to the script. The output should be in neat tabular

fields, with the first field showing the address, each of the

next 8 fields a 4-byte hex number, and the final field the

ASCII equivalent of the previous 8 fields.

The obvious followup to this is to extend the hex dump script

into a disassembler. Using a lookup table, or some other

clever gimmick, convert the hex values into 80x86 op codes.

Emulating a Shift Register

Using Example 27-15 as an inspiration, write a script that

emulates a 64-bit shift register as an array. Implement

functions to load the register, shift left, shift right, and

rotate it. Finally, write a function that interprets the

Calculating Determinants

Write a script that calculates determinants [153] by

recursively expanding the minors. Use a 4 x 4 determinant as a

test case.

Hidden Words

Write a "word-find" puzzle generator, a script that hides 10

input words in a 10 x 10 array of random letters. The words

may be hidden across, down, or diagonally.

Optional: Write a script that solves word-find puzzles. To

keep this from becoming too difficult, the solution script

will find only horizontal and vertical words. (Hint: Treat

each row and column as a string, and search for substrings.)

Anagramming

Anagram 4-letter input. For example, the anagrams of word are:

do or rod row word. You may use /usr/share/dict/linux.words as

the reference list.

Word Ladders

A "word ladder" is a sequence of words, with each successive

word in the sequence differing from the previous one by a

single letter.

For example, to "ladder" from mark to vase:

mark --> park --> part --> past --> vast --> vase

^ ^ ^ ^ ^

Write a script that solves word ladder puzzles. Given a

starting and an ending word, the script will list all

intermediate steps in the "ladder." Note that all words in the

sequence must be legitimate dictionary words.

Fog Index

The "fog index" of a passage of text estimates its reading

difficulty, as a number corresponding roughly to a school

grade level. For example, a passage with a fog index of 12

should be comprehensible to anyone with 12 years of schooling.

The Gunning version of the fog index uses the following

algorithm.

1. Choose a section of the text at least 100 words in length.

2. Count the number of sentences (a portion of a sentence

truncated by the boundary of the text section counts as

one).

3. Find the average number of words per sentence.

AVE_WDS_SEN = TOTAL_WORDS / SENTENCES

4. Count the number of "difficult" words in the segment --

those containing at least 3 syllables. Divide this quantity

by total words to get the proportion of difficult words.

PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS

5. The Gunning fog index is the sum of the above two

quantities, multiplied by 0.4, then rounded to the nearest

integer.

G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )

Step 4 is by far the most difficult portion of the exercise.

There exist various algorithms for estimating the syllable

count of a word. A rule-of-thumb formula might consider the

number of letters in a word and the vowel-consonant mix.

A strict interpretation of the Gunning fog index does not

count compound words and proper nouns as "difficult" words,

but this would enormously complicate the script.

Calculating PI using Buffon's Needle

The Eighteenth Century French mathematician de Buffon came up

with a novel experiment. Repeatedly drop a needle of length n

onto a wooden floor composed of long and narrow parallel

boards. The cracks separating the equal-width floorboards are

a fixed distance d apart. Keep track of the total drops and

the number of times the needle intersects a crack on the

floor. The ratio of these two quantities turns out to be a

fractional multiple of PI.

In the spirit of Example 16-50, write a script that runs a

Monte Carlo simulation of Buffon's Needle. To simplify

matters, set the needle length equal to the distance between

the cracks, n = d.

Hint: there are actually two critical variables: the distance

from the center of the needle to the nearest crack, and the

inclination angle of the needle to that crack. You may use bc

to handle the calculations.

Playfair Cipher

Implement the Playfair (Wheatstone) Cipher in a script.

The Playfair Cipher encrypts text by substitution of digrams

(2-letter groupings). It is traditional to use a 5 x 5 letter

scrambled-alphabet key square for the encryption and

decryption.

C O D E S

A B F G H

I K L M N

P Q R T U

V W X Y Z

Each letter of the alphabet appears once, except "I" also represents

"J". The arbitrarily chosen key word, "CODES" comes first, then all

the rest of the alphabet, in order from left to right, skipping letters

already used.

To encrypt, separate the plaintext message into digrams (2-letter

groups). If a group has two identical letters, delete the second, and

form a new group. If there is a single letter left over at the end,

insert a "null" character, typically an "X."

THIS IS A TOP SECRET MESSAGE

TH IS IS AT OP SE CR ET ME SA GE

For each digram, there are three possibilities.

-----------------------------------------------

1) Both letters will be on the same row of the key square:

For each letter, substitute the one immediately to the right, in that

row. If necessary, wrap around left to the beginning of the row.

2) Both letters will be in the same column of the key square:

For each letter, substitute the one immediately below it, in that

row. If necessary, wrap around to the top of the column.

3) Both letters will form the corners of a rectangle within the key square:

For each letter, substitute the one on the other corner the rectangle

which lies on the same row.

The "TH" digram falls under case #3.

G H

M N

T U (Rectangle with "T" and "H" at corners)

T --> U

H --> G

The "SE" digram falls under case #1.

C O D E S (Row containing "S" and "E")

S --> C (wraps around left to beginning of row)

E --> S

=========================================================================

To decrypt encrypted text, reverse the above procedure under cases #1

and #2 (move in opposite direction for substitution). Under case #3,

just take the remaining two corners of the rectangle.

Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a

fairly detailed description of the Playfair Cipher and its solution methods.

This script will have three main sections

I. Generating the key square, based on a user-input keyword.

II. Encrypting a plaintext message.

III. Decrypting encrypted text.

The script will make extensive use of arrays and functions. You may

use Example A-56 as an inspiration.

Please do not send the author your solutions to these exercises.

There are more appropriate ways to impress him with your cleverness,

such as submitting bugfixes and suggestions for improving the book.

________________________________________________________________

Appendix P. Revision History

This document first appeared as a 60-page HOWTO in the late spring

of 2000. Since then, it has gone through quite a number of updates

and revisions. This book could not have been written without the

assistance of the Linux community, and especially of the volunteers

of the [http://www.tldp.org] Linux Documentation Project.

Here is the e-mail to the LDP requesting permission to submit version

0.1.

From thegrendel@theriver.com Sat Jun 10 09:05:33 2000 -0700

Date: Sat, 10 Jun 2000 09:05:28 -0700 (MST)

From: "M. Leo Cooper" <thegrendel@theriver.com>

X-Sender: thegrendel@localhost

To: ldp-discuss@lists.linuxdoc.org

Subject: Permission to submit HOWTO

Dear HOWTO Coordinator,

I am working on and would like to submit to the LDP a HOWTO on the subject

of "Bash Scripting" (shell scripting, using 'bash'). As it happens,

I have been writing this document, off and on, for about the last eight

months or so, and I could produce a first draft in ASCII text format in

a matter of just a few more days.

I began writing this out of frustration at being unable to find a

decent book on shell scripting. I managed to locate some pretty good

articles on various aspects of scripting, but nothing like a complete,

beginning-to-end tutorial. Well, in keeping with my philosophy, if all

else fails, do it yourself.

As it stands, this proposed "Bash-Scripting HOWTO" would serve as a

combination tutorial and reference, with the heavier emphasis on the

tutorial. It assumes Linux experience, but only a very basic level

of programming skills. Interspersed with the text are 79 illustrative

example scripts of varying complexity, all liberally commented. There

are even exercises for the reader.

At this stage, I'm up to 18,000+ words (124k), and that's over 50 pages of

text (whew!).

I haven't mentioned that I've previously authored an LDP HOWTO, the

"Software-Building HOWTO", which I wrote in Linuxdoc/SGML. I don't know

if I could handle Docbook/SGML, and I'm glad you have volunteers to do

the conversion. You people seem to have gotten on a more organized basis

these last few months. Working with Greg Hankins and Tim Bynum was nice,

but a professional team is even nicer.

Anyhow, please advise.

Mendel Cooper

thegrendel@theriver.com

Table P-1. Revision History

Release Date Comments

0.1 14 Jun 2000 Initial release.

0.2 30 Oct 2000 Bugs fixed, plus much additional material and more

example scripts.

0.3 12 Feb 2001 Major update.

0.4 08 Jul 2001 Complete revision and expansion of the book.

0.5 03 Sep 2001 Major update: Bugfixes, material added, sections

reorganized.

1.0 14 Oct 2001 Stable release: Bugfixes, reorganization, material

added.

1.1 06 Jan 2002 Bugfixes, material and scripts added.

1.2 31 Mar 2002 Bugfixes, material and scripts added.

1.3 02 Jun 2002 TANGERINE release: A few bugfixes, much more material

and scripts added.

1.4 16 Jun 2002 MANGO release: A number of typos fixed, more material

and scripts.

1.5 13 Jul 2002 PAPAYA release: A few bugfixes, much more material

and scripts added.

1.6 29 Sep 2002 POMEGRANATE release: Bugfixes, more material, one

more script.

1.7 05 Jan 2003 COCONUT release: A couple of bugfixes, more material,

one more script.

1.8 10 May 2003 BREADFRUIT release: A number of bugfixes, more

scripts and material.

1.9 21 Jun 2003 PERSIMMON release: Bugfixes, and more material.

2.0 24 Aug 2003 GOOSEBERRY release: Major update.

2.1 14 Sep 2003 HUCKLEBERRY release: Bugfixes, and more material.

2.2 31 Oct 2003 CRANBERRY release: Major update.

2.3 03 Jan 2004 STRAWBERRY release: Bugfixes and more material.

2.4 25 Jan 2004 MUSKMELON release: Bugfixes.

2.5 15 Feb 2004 STARFRUIT release: Bugfixes and more material.

2.6 15 Mar 2004 SALAL release: Minor update.

2.7 18 Apr 2004 MULBERRY release: Minor update.

2.8 11 Jul 2004 ELDERBERRY release: Minor update.

3.0 03 Oct 2004 LOGANBERRY release: Major update.

3.1 14 Nov 2004 BAYBERRY release: Bugfix update.

3.2 06 Feb 2005 BLUEBERRY release: Minor update.

3.3 20 Mar 2005 RASPBERRY release: Bugfixes, much material added.

3.4 08 May 2005 TEABERRY release: Bugfixes, stylistic revisions.

3.5 05 Jun 2005 BOXBERRY release: Bugfixes, some material added.

3.6 28 Aug 2005 POKEBERRY release: Bugfixes, some material added.

3.7 23 Oct 2005 WHORTLEBERRY release: Bugfixes, some material added.

3.8 26 Feb 2006 BLAEBERRY release: Bugfixes, some material added.

3.9 15 May 2006 SPICEBERRY release: Bugfixes, some material added.

4.0 18 Jun 2006 WINTERBERRY release: Major reorganization.

4.1 08 Oct 2006 WAXBERRY release: Minor update.

4.2 10 Dec 2006 SPARKLEBERRY release: Important update.

4.3 29 Apr 2007 INKBERRY release: Bugfixes, material added.

5.0 24 Jun 2007 SERVICEBERRY release: Major update.

5.1 10 Nov 2007 LINGONBERRY release: Minor update.

5.2 16 Mar 2008 SILVERBERRY release: Important update.

5.3 11 May 2008 GOLDENBERRY release: Minor update.

5.4 21 Jul 2008 ANGLEBERRY release: Major update.

5.5 23 Nov 2008 FARKLEBERRY release: Minor update.

5.6 26 Jan 2009 WORCESTERBERRY release: Minor update.

6.0 23 Mar 2009 THIMBLEBERRY release: Major update.

6.1 30 Sep 2009 BUFFALOBERRY release: Minor update.

6.2 17 Mar 2010 ROWANBERRY release: Minor update.

6.3 30 Apr 2011 SWOZZLEBERRY release: Major update.

6.4 30 Aug 2011 VORTEXBERRY release: Minor update.

6.5 05 Apr 2012 TUNGSTENBERRY release: Minor update.

6.6 27 Nov 2012 YTTERBIUMBERRY release: Minor update.

10 10 Mar 2014 YTTERBIUMBERRY release: License change.

________________________________________________________________

Appendix Q. Download and Mirror Sites

The latest update of this document, as an archived, bzip2-ed

"tarball" including both the SGML source and rendered HTML, may be

downloaded from the author's home site). A

[http://bash.deta.in/abs-guide.pdf] pdf version is also available

(mirror site). There is likewise an

[http://bash.deta.in/abs-guide.epub] epub version, courtesy of Craig

Barnes and Michael Satke. The change log gives a detailed revision

history. The ABS Guide even has its own freshmeat.net/freecode page

to keep track of major updates, user comments, and popularity ratings

for the project.

The legacy hosting site for this document is the

[http://www.tldp.org/LDP/abs/] Linux Documentation Project, which

maintains many other Guides and HOWTOs as well.

Many thanks to Ronny Bangsund for donating [http://bash.deta.in/]

server space to host this project.

________________________________________________________________

Appendix R. To Do List

* A comprehensive survey of incompatibilities between Bash and the

classic Bourne shell.

* Same as above, but for the Korn shell (ksh).

________________________________________________________________

Appendix S. Copyright

The Advanced Bash Scripting Guide is herewith granted to the PUBLIC

DOMAIN. This has the following implications and consequences.

A. All previous releases of the Advanced Bash Scripting Guide

are as well granted to the Public Domain.

A1. All printed editions, whether authorized by the author or not,

are as well granted to the Public Domain. This legally overrides

any stated intention or wishes of the publishers. Any statement

of copyright is void and invalid.

THERE ARE NO EXCEPTIONS TO THIS.

A2. Any release of the Advanced Bash Scripting Guide, whether in

electronic or print form is granted to the Public Domain by the

express directive of the author and previous copyright holder, Mendel

Cooper. No other person(s) or entities have ever held a valid copyright.

B. As a Public Domain document, unlimited copying and distribution rights

are granted. There can be NO restrictions. If anyone has published or will

in the future publish an original or modified version of this document,

then only additional original material may be copyrighted. The core

work will remain in the Public Domain.

By law, distributors and publishers (including on-line publishers)

are prohibited from imposing any conditions, strictures, or

provisions on this document, any previous versions, or any derivative

versions. The author asserts that he has not entered into any

contractual obligations that would alter the foregoing declarations.

Essentially, you may freely distribute this book or any derivative

thereof in electronic or printed form. If you have previously

purchased or are in possession of a printed copy of a current or

previous edition, you have the LEGAL RIGHT to copy and/or

redistribute it, regardless of any copyright notice. Any copyright

notice is void.

Additionally, the author wishes to state his intention that:

If you copy or distribute this book, kindly DO NOT

use the materials within, or any portion thereof, in a patent or copyright

lawsuit against the Open Source community, its developers, its

distributors, or against any of its associated software or documentation

including, but not limited to, the Linux kernel, Open Office, Samba,

and Wine. Kindly DO NOT use any of the materials within

this book in testimony or depositions as a plaintiff's "expert witness" in

any lawsuit against the Open Source community, any of its developers, its

distributors, or any of its associated software or documentation.

A Public Domain license essentially does not restrict ANY legitimate

distribution or use of this book. The author especially encourages

its (royalty-free!) use for classroom and instructional purposes.

To date, limited print rights (Lulu edition) have been granted to one

individual and to no one else. Neither that individual nor Lulu holds

or ever has held a valid copyright.

Warning

It has come to the attention of the author that unauthorized

electronic and print editions of this book are being sold

commercially on itunes®, amazon.com and elsewhere. These are illegal

and pirated editions produced without the author's permission, and

readers of this book are strongly urged not to purchase them. In

fact, these pirated editions are now legal, but necessarily fall into

the Public Domain, and any copyright notices contained within them

are invalid and void.

The author produced this book in a manner consistent with the spirit

of the LDP Manifesto.

Linux is a trademark registered to Linus Torvalds.

Fedora is a trademark registered to Red Hat.

Unix and UNIX are trademarks registered to the Open Group.

MS Windows is a trademark registered to the Microsoft Corp.

Solaris is a trademark registered to Oracle, Inc.

OSX is a trademark registered to Apple, Inc.

Yahoo is a trademark registered to Yahoo, Inc.

Pentium is a trademark registered to Intel, Inc.

Thinkpad is a trademark registered to Lenovo, Inc.

Scrabble is a trademark registered to Hasbro, Inc.

Librie, PRS-500, and PRS-505 are trademarks registered to Sony, Inc.

All other commercial trademarks mentioned in the body of this work

are registered to their respective owners.

Hyun Jin Cha has done a Korean translation of version 1.0.11 of this

book. Spanish, Portuguese, [http://abs.traduc.org/] French, German,

[http://it.tldp.org/guide/abs/index.html] Italian,

[http://gazette.linux.ru.net/rus/articles/index-abs-guide.html]

Russian, [http://premekvihan.net/bash] Czech,

[http://www.linuxsir.org/bbs/showthread.php?t=256887] Chinese,

Indonesian, Dutch, Romanian, Bulgarian, and Turkish translations are

also available or in progress. If you wish to translate this document

into another language, please feel free to do so, subject to the

terms stated above. The author wishes to be notified of such efforts.

Those generous readers desiring to make a donation to the author may

contribute a small amount via Paypal to my e-mail address,

<thegrendel.abs@gmail.com>. (An Honor Roll of Supporters is given at

the beginning of the [http://bash.deta.in/Change.log] Change Log.)

This is not a requirement. The ABS Guide is a free and freely

distributed document for the use and enjoyment of the Linux

community. However, in these difficult times, showing support for

voluntary projects and especially to authors of limited means is more

critically important than ever.

________________________________________________________________

Appendix T. ASCII Table

Traditionally, a book of this sort has an ASCII Table appendix. This

book does not. Instead, here are several short scripts, each of which

generates a complete ASCII table.

Example T-1. A script that generates an ASCII table

!/bin/bash

ascii.sh

ver. 0.2, reldate 26 Aug 2008

Patched by ABS Guide author.

Original script by Sebastian Arming.

Used with permission (thanks!).

exec >ASCII.txt # Save stdout to file,

#+ as in the example scripts

#+ reassign-stdout.sh and upperconv.sh.

MAXNUM=256

COLUMNS=5

OCT=8

OCTSQU=64

LITTLESPACE=-3

BIGSPACE=-5

i=1 # Decimal counter

o=1 # Octal counter

while [ "$i" -lt "$MAXNUM" ]; do # We don't have to count past 400 octal.

paddi=" $i"

echo -n "${paddi: $BIGSPACE} " # Column spacing.

paddo="00$o"

echo -ne "\\${paddo: $LITTLESPACE}" # Original.

echo -ne "\\0${paddo: $LITTLESPACE}" # Fixup.

^

echo -n " "

if (( i % $COLUMNS == 0)); then # New line.

echo

((i++, o++))

# The octal notation for 8 is 10, and 64 decimal is 100 octal.

(( i % $OCT == 0)) && ((o+=2))

(( i % $OCTSQU == 0)) && ((o+=20))

done

exit $?

Compare this script with the "pr-asc.sh" example.

This one handles "unprintable" characters.

Exercise:

Rewrite this script to use decimal numbers, rather than octal.

Example T-2. Another ASCII table script

!/bin/bash

Script author: Joseph Steinhauser

Lightly edited by ABS Guide author, but not commented.

Used in ABS Guide with permission.

-------------------------------------------------------------------------

-- File: ascii.sh Print ASCII chart, base 10/8/16 (JETS-2012)

-------------------------------------------------------------------------

-- Usage: ascii [oct|dec|hex|help|8|10|16]

--

-- This script prints out a summary of ASCII char codes from Zero to 127.

-- Numeric values may be printed in Base10, Octal, or Hex.

--

-- Format Based on: /usr/share/lib/pub/ascii with base-10 as default.

-- For more detail, man ascii . . .

-------------------------------------------------------------------------

[ -n "$BASH_VERSION" ] && shopt -s extglob

case "$1" in

oct|[Oo]?([Cc][Tt])|8) Obase=Octal; Numy=3o;;

hex|[Hh]?([Ee][Xx])|16|[Xx]) Obase=Hex; Numy=2X;;

help|?(-)[h?]) sed -n '2,/^[ ]*$/p' $0;exit;;

code|[Cc][Oo][Dd][Ee])sed -n '/case/,$p' $0;exit;;

*) Obase=Decimal

esac # CODE is actually shorter than the chart!

printf "\t\t## $Obase ASCII Chart ##\n\n"; FM1="|%0${Numy:-3d}"; LD=-1

AB="nul soh stx etx eot enq ack bel bs tab nl vt np cr so si dle"

AD="dc1 dc2 dc3 dc4 nak syn etb can em sub esc fs gs rs us sp"

for TOK in $AB $AD; do ABR[$((LD+=1))]=$TOK; done;

ABR[127]=del

IDX=0

while [ $IDX -le 127 ] && CHR="${ABR[$IDX]}"

do ((${#CHR}))&& FM2='%-3s'|| FM2=`printf '\\\\%o ' $IDX`

printf "$FM1 $FM2" "$IDX" $CHR; (( (IDX+=1)%8))||echo '|'

done

exit $?

Example T-3. A third ASCII table script, using awk

!/bin/bash

ASCII table script, using awk.

Author: Joseph Steinhauser

Used in ABS Guide with permission.

-------------------------------------------------------------------------

-- File: ascii Print ASCII chart, base 10/8/16 (JETS-2010)

-------------------------------------------------------------------------

-- Usage: ascii [oct|dec|hex|help|8|10|16]

--

-- This script prints a summary of ASCII char codes from Zero to 127.

-- Numeric values may be printed in Base10, Octal, or Hex (Base16).

--

-- Format Based on: /usr/share/lib/pub/ascii with base-10 as default.

-- For more detail, man ascii

-------------------------------------------------------------------------

[ -n "$BASH_VERSION" ] && shopt -s extglob

case "$1" in

oct|[Oo]?([Cc][Tt])|8) Obase=Octal; Numy=3o;;

hex|[Hh]?([Ee][Xx])|16|[Xx]) Obase=Hex; Numy=2X;;

help|?(-)[h?]) sed -n '2,/^[ ]*$/p' $0;exit;;

code|[Cc][Oo][Dd][Ee])sed -n '/case/,$p' $0;exit;;

*) Obase=Decimal

esac

export Obase # CODE is actually shorter than the chart!

awk 'BEGIN{print "\n\t\t## "ENVIRON["Obase"]" ASCII Chart ##\n"

ab="soh,stx,etx,eot,enq,ack,bel,bs,tab,nl,vt,np,cr,so,si,dle,"

ad="dc1,dc2,dc3,dc4,nak,syn,etb,can,em,sub,esc,fs,gs,rs,us,sp"

split(ab ad,abr,",");abr[0]="nul";abr[127]="del";

fm1="|%0'"${Numy:- 4d}"' %-3s"

for(idx=0;idx<128;idx++){fmt=fm1 (++colz%8?"":"|\n")

printf(fmt,idx,(idx in abr)?abr[idx]:sprintf("%c",idx))} }'

exit $?

________________________________________________________________

Index

This index / glossary / quick-reference lists many of the important

topics covered in the text. Terms are arranged in approximate ASCII

sorting order, modified as necessary for enhanced clarity.

Note that commands are indexed in Part 4.

* * *

^ (caret)

* Beginning-of-line, in a Regular Expression

* ^

Uppercase conversion in parameter substitution

~ Tilde

* ~ home directory, corresponds to $HOME

* ~/ Current user's home directory

* ~+ Current working directory

* ~- Previous working directory

= Equals sign

* = Variable assignment operator

* = String comparison operator

== String comparison operator

* =~ Regular Expression match operator

Example script

< Left angle bracket

* Is-less-than

String comparison

Integer comparison within double parentheses

* Redirection

< stdin

<< Here document

<<< Here string

<> Opening a file for both reading and writing

> Right angle bracket

* Is-greater-than

String comparison

Integer comparison, within double parentheses

* Redirection

> Redirect stdout to a file

>> Redirect stdout to a file, but append

i>&j Redirect file descriptor i to file descriptor j

>&j Redirect stdout to file descriptor j

>&2 Redirect stdout of a command to stderr

2>&1 Redirect stderr to stdout

&> Redirect both stdout and stderr of a command to a file

:> file Truncate file to zero length

| Pipe, a device for passing the output of a command to another

command or to the shell

|| Logical OR test operator

- (dash)

* Prefix to default parameter, in parameter substitution

* Prefix to option flag

* Indicating redirection from stdin or stdout

* -- (double-dash)

Prefix to long command options

C-style variable decrement within double parentheses

; (semicolon)

* As command separator

* \; Escaped semicolon, terminates a find command

* ;; Double-semicolon, terminator in a case option

Required when ...

do keyword is on the first line of loop

terminating curly-bracketed code block

* ;;& ;& Terminators in a case option (version 4+ of Bash).

: Colon

* :> filename Truncate file to zero length

* null command, equivalent to the true Bash builtin

* Used in an anonymous here document

* Used in an otherwise empty function

* Used as a function name

! Negation operator, inverts exit status of a test or command

* != not-equal-to String comparison operator

? (question mark)

* Match zero or one characters, in an Extended Regular Expression

* Single-character wild card, in globbing

* In a C-style Trinary operator

// Double forward slash, behavior of cd command toward

. (dot / period)

* . Load a file (into a script), equivalent to source command

* . Match single character, in a Regular Expression

* . Current working directory

./ Current working directory

* .. Parent directory

' ... ' (single quotes) strong quoting

" ... " (double quotes) weak quoting

* Double-quoting the backslash (\) character

* Comma operator

* ,

Lowercase conversion in parameter substitution

() Parentheses

* ( ... ) Command group; starts a subshell

* ( ... ) Enclose group of Extended Regular Expressions

* >( ... )

<( ... ) Process substitution

* ... ) Terminates test-condition in case construct

* (( ... )) Double parentheses, in arithmetic expansion

[ Left bracket, test construct

[ ]Brackets

* Array element

* Enclose character set to match in a Regular Expression

* Test construct

[[ ... ]] Double brackets, extended test construct

$ Anchor, in a Regular Expression

$ Prefix to a variable name

$( ... ) Command substitution, setting a variable with output of a

command, using parentheses notation

` ... ` Command substitution, using backquotes notation

$[ ... ] Integer expansion (deprecated)

${ ... } Variable manipulation / evaluation

* ${var} Value of a variable

* ${#var} Length of a variable

* ${#@}

${#*} Number of positional parameters

* ${parameter?err_msg} Parameter-unset message

* ${parameter-default}

${parameter:-default}

${parameter=default}

${parameter:=default} Set default parameter

* ${parameter+alt_value}

${parameter:+alt_value}

Alternate value of parameter, if set

* ${!var}

Indirect referencing of a variable, new notation

* ${!#}

Final positional parameter. (This is an indirect reference to

$#.)

* ${!varprefix*}

${!varprefix@}

Match names of all previously declared variables beginning with

varprefix

* ${string:position}

${string:position:length} Substring extraction

* ${var#Pattern}

${var##Pattern} Substring removal

* ${var%Pattern}

${var%%Pattern} Substring removal

* ${string/substring/replacement}

${string//substring/replacement}

${string/#substring/replacement}

${string/%substring/replacement} Substring replacement

... ' String expansion, using escaped characters.

\ Escape the character following

* \< ... \> Angle brackets, escaped, word boundary in a Regular

Expression

* \{ N \} "Curly" brackets, escaped, number of character sets to

match in an Extended RE

* \; Semicolon, escaped, terminates a find command

* \$ Indirect reverencing of a variable, old-style notation

* Escaping a newline, to write a multi-line command

* &> Redirect both stdout and stderr of a command to a file

* >&j Redirect stdout to file descriptor j

>&2 Redirect stdout of a command to stderr

* i>&j Redirect file descriptor i to file descriptor j

2>&1 Redirect stderr to stdout

* Closing file descriptors

n<&- Close input file descriptor n

0<&-, <&- Close stdin

n>&- Close output file descriptor n

1>&-, >&- Close stdout

* && Logical AND test operator

* Command & Run job in background

# Hashmark, special symbol beginning a script comment

#! Sha-bang, special string starting a shell script

* Asterisk

* Wild card, in globbing

* Any number of characters in a Regular Expression

* ** Exponentiation, arithmetic operator

* ** Extended globbing file-match operator

% Percent sign

* Modulo, division-remainder arithmetic operation

* Substring removal (pattern matching) operator

+ Plus sign

* Character match, in an extended Regular Expression

* Prefix to alternate parameter, in parameter substitution

* ++ C-style variable increment, within double parentheses

* * *

Shell Variables

$_ Last argument to previous command

$- Flags passed to script, using set

$! Process ID of last background job

$? Exit status of a command

$@ All the positional parameters, as separate words

$* All the positional parameters, as a single word

$ Process ID of the script

$# Number of arguments passed to a function, or to the script itself

$0 Filename of the script

$1 First argument passed to script

$9 Ninth argument passed to script

Table of shell variables

* * * * * *

-a Logical AND compound comparison test

Address database, script example

Advanced Bash Scripting Guide, where to download

Alias

* Removing an alias, using unalias

Anagramming

And list

* To supply default command-line argument

And logical operator &&

Angle brackets, escaped, \< . . . \> word boundary in a Regular

Expression

Anonymous here document, using :

Archiving

* rpm

* tar

Arithmetic expansion

* exit status of

* variations of

Arithmetic operators

* combination operators, C-style

+= -= *= /= %=

Note

In certain contexts, += can also function as a string concatenation

operator.

Arrays

* Associative arrays

more efficient than conventional arrays

* Bracket notation

* Concatenating, example script

* Copying

* Declaring

declare -a array_name

* Embedded arrays

* Empty arrays, empty elements, example script

* Indirect references

* Initialization

array=( element1 element2 ... elementN)

Example script

Using command substitution

* Loading a file into an array

* Multidimensional, simulating

* Nesting and embedding

* Notation and usage

* Number of elements in

${#array_name[@]}

${#array_name[*]}

* Operations

* Passing an array to a function

* As return value from a function

* Special properties, example script

* String operations, example script

* unset deletes array elements

Arrow keys, detecting

ASCII

* Definition

* Scripts for generating ASCII table

awk field-oriented text processing language

* rand(), random function

* String manipulation

* Using export to pass a variable to an embedded awk script

* * *

Backlight, setting the brightness

Backquotes, used in command substitution

Base conversion, example script

Bash

* Bad scripting practices

* Basics reviewed, script example

* Command-line options

Table

* Features that classic Bourne shell lacks

* Internal variables

* Version 2

* Version 3

* Version 4

Version 4.1

Version 4.2

.bashrc

$BASH_SUBSHELL

Basic commands, external

Batch files, DOS

Batch processing

bc, calculator utility

* In a here document

* Template for calculating a script variable

Bibliography

Bison utility

Bitwise operators

* Example script

Block devices

* testing for

Blocks of code

* Iterating / looping

* Redirection

Script example: Redirecting output of a a code block

Bootable flash drives, creating

Brace expansion

* Extended, {a..z}

* Parameterizing

* With increment and zero-padding (new feature in Bash, version 4)

Brackets, [ ]

* Array element

* Enclose character set to match in a Regular Expression

* Test construct

Brackets, curly, {}, used in

* Code block

* find

* Extended Regular Expressions

* Positional parameters

* xargs

break loop control command

* Parameter (optional)

Builtins in Bash

* Do not fork a subprocess

* * *

case construct

* Command-line parameters, handling

* Globbing, filtering strings with

cat, concatentate file(s)

* Abuse of

* cat scripts

* Less efficient than redirecting stdin

* Piping the output of, to a read

* Uses of

Character devices

* testing for

Checksum

Child processes

Colon, : , equivalent to the true Bash builtin

Colorizing scripts

* Cycling through the background colors, example script

* Table of color escape sequences

* Template, colored text on colored background

Comma operator, linking commands or operations

Command-line options

command_not_found_handle () builtin error-handling function (version

4+ of Bash)

Command substitution

* $( ... ), preferred notation

* Backquotes

* Extending the Bash toolset

* Invokes a subshell

* Nesting

* Removes trailing newlines

* Setting variable from loop output

* Word splitting

Comment headers, special purpose

Commenting out blocks of code

* Using an anonymous here document

* Using an if-then construct

Communications and hosts

Compound comparison operators

Compression utilities

* bzip2

* compress

* gzip

* zip

continue loop control command

Control characters

* Control-C, break

* Control-D, terminate / log out / erase

* Control-G, BEL (beep)

* Control-H, rubout

* Control-J, newline

* Control-M, carriage return

Coprocesses

cron, scheduling daemon

C-style syntax , for handling variables

Crossword puzzle solver

Cryptography

Curly brackets {}

* in find command

* in an Extended Regular Expression

* in xargs

* * *

Daemons, in UNIX-type OS

date

dc, calculator utility

dd, data duplicator command

* Conversions

* Copying raw data to/from devices

* File deletion, secure

* Keystrokes, capturing

* Options

* Random access on a data stream

* Raspberry Pi, script for preparing a bootable SD card

* Swapfiles, initializing

* Thread on www.linuxquestions.org

Debugging scripts

* Tools

* Trapping at exit

* Trapping signals

Decimal number, Bash interprets numbers as

declare builtin

* options

case-modification options (version 4+ of Bash)

Default parameters

/dev directory

* /dev/null pseudo-device file

* /dev/urandom pseudo-device file, generating pseudorandom numbers

with

* /dev/zero, pseudo-device file

Device file

dialog, utility for generating dialog boxes in a script

$DIRSTACK directory stack

Disabled commands, in restricted shells

do keyword, begins execution of commands within a loop

done keyword, terminates a loop

DOS batch files, converting to shell scripts

DOS commands, UNIX equivalents of (table)

dot files, "hidden" setup and configuration files

Double brackets [[ ... ]] test construct

* and evaluation of octal/hex constants

Double parentheses (( ... )) arithmetic expansion/evaluation

construct

Double quotes " ... " weak quoting

* Double-quoting the backslash (\) character

Double-spacing a text file, using sed

* * *

-e File exists test

echo

* Feeding commands down a pipe

* Setting a variable using command substitution

* /bin/echo, external echo command

elif, Contraction of else and if

else

Encrypting files, using openssl

esac, keyword terminating case construct

Environmental variables

-eq , is-equal-to integer comparison test

Eratosthenes, Sieve of, algorithm for generating prime numbers

Escaped characters, special meanings of

* Within ... ' string expansion

* Used with Unicode characters

/etc/fstab (filesystem mount) file

/etc/passwd (user account) file

$EUID, Effective user ID

eval, Combine and evaluate expression(s), with variable expansion

* Effects of, Example script

* Forces reevaluation of arguments

* And indirect references

* Risk of using

* Using eval to convert array elements into a command list

* Using eval to select among variables

Evaluation of octal/hex constants within [[ ... ]]

exec command, using in redirection

Exercises

Exit and Exit status

* exit command

* Exit status (exit code, return status of a command)

Table, Exit codes with special meanings

Anomalous

Out of range

Pipe exit status

Specified by a function return

Successful, 0

/usr/include/sysexits.h, system file listing C/C++ standard exit

codes

Export, to make available variables to child processes

* Passing a variable to an embedded awk script

expr, Expression evaluator

* Substring extraction

* Substring index (numerical position in string)

* Substring matching

Extended Regular Expressions

* ? (question mark) Match zero / one characters

* ( ... ) Group of expressions

* \{ N \} "Curly" brackets, escaped, number of character sets to

match

* + Character match

* * *

factor, decomposes an integer into its prime factors

* Application: Generating prime numbers

false, returns unsuccessful (1) exit status

Field, a group of characters that comprises an item of data

Files / Archiving

File descriptors

* Closing

n<&- Close input file descriptor n

0<&-, <&- Close stdin

n>&- Close output file descriptor n

1>&-, >&- Close stdout

* File handles in C, similarity to

File encryption

find

* {} Curly brackets

* \; Escaped semicolon

Filter

* Using - with file-processing utility as a filter

* Feeding output of a filter back to same filter

Floating point numbers, Bash does not recognize

fold, a filter to wrap lines of text

Forking a child process

for loops

Functions

* Arguments passed referred to by position

* Capturing the return value of a function using echo

* Colon as function name

* Definition must precede first call to function

* Exit status

* Local variables

and recursion

* Passing an array to a function

* Passing pointers to a function

* Positional parameters

* Recursion

* Redirecting stdin of a function

* return

Multiple return values from a function, example script

Returning an array from a function

Return range limits, workarounds

* Shift arguments passed to a function

* Unusual function names

* * *

Games and amusements

* Anagrams

* Anagrams, again

* Bingo Number Generator

* Crossword puzzle solver

* Crypto-Quotes

* Dealing a deck of cards

* Fifteen Puzzle

* Horse race

* Knight's Tour

* "Life" game

* Magic Squares

* Music-playing script

* Nim

* Pachinko

* Perquackey

* Petals Around the Rose

* Podcasting

* Poem

* Speech generation

* Towers of Hanoi

Graphic version

Alternate graphic version

getopt, external command for parsing script command-line arguments

* Emulated in a script

getopts, Bash builtin for parsing script command-line arguments

* $OPTIND / $OPTARG

Global variable

Globbing, filename expansion

* Handling filenames correctly

* Wild cards

* Will not match dot files

Golden Ratio (Phi)

-ge , greater-than or equal integer comparison test

-gt , greater-than integer comparison test

groff, text markup and formatting language

Gronsfeld cipher

$GROUPS, Groups user belongs to

gzip, compression utility

* * *

Hashing, creating lookup keys in a table

* Example script

head, echo to stdout lines at the beginning of a text file

help, gives usage summary of a Bash builtin

Here documents

* Anonymous here documents, using :

Commenting out blocks of code

Self-documenting scripts

* bc in a here document

* cat scripts

* Command substitution

* ex scripts

* Function, supplying input to

* Here strings

Calculating the Golden Ratio

Prepending text

As the stdin of a loop

Using read

* Limit string

! as a limit string

Closing limit string may not be indented

Dash option to limit string, <<-LimitString

* Literal text output, for generating program code

* Parameter substitution

Disabling parameter substitution

* Passing parameters

* Temporary files

* Using vi non-interactively

History commands

$HOME, user's home directory

Homework assignment solver

$HOSTNAME, system host name

* * *

$Id parameter, in rcs (Revision Control System)

if [ condition ]; then ... test construct

* if-grep, if and grep in combination

Fixup for if-grep test

$IFS, Internal field separator variable

* Defaults to whitespace

Integer comparison operators

in, keyword preceding [list] in a for loop

Initialization table, /etc/inittab

Inline group, i.e., code block

Interactive script, test for

I/O redirection

Indirect referencing of variables

* New notation, introduced in version 2 of Bash ( example script)

iptables, packet filtering and firewall utility

* Usage example

* Example script

Iteration

* * *

Job IDs, table

jot, Emit a sequence of integers. Equivalent to seq.

* Random sequence generation

Just another Bash hacker!

* * *

Keywords

* error, if missing

kill, terminate a process by process ID

* Options (-l, -9)

killall, terminate a process by name

killall script in /etc/rc.d/init.d

* * *

lastpipe shell option

-le , less-than or equal integer comparison test

let, setting and carrying out arithmetic operations on variables

* C-style increment and decrement operators

Limit string, in a here document

$LINENO, variable indicating the line number where it appears in a

script

Link, file (using ln command)

* Invoking script with multiple names, using ln

* symbolic links, ln -s

List constructs

* And list

* Or list

Local variables

* and recursion

Localization

Logical operators (&&, ||, etc.)

Logout file, the ~/.bash_logout file

Loopback device, mounting a file on a block device

Loops

* break loop control command

* continue loop control command

* C-style loop within double parentheses

for loop

while loop

* do (keyword), begins execution of commands within a loop

* done (keyword), terminates a loop

* for loops

for arg in [list]; do

Command substitution to generate [list]

Filename expansion in [list]

Multiple parameters in each [list] element

Omitting [list], defaults to positional parameters

Parameterizing [list]

Redirection

* in, (keyword) preceding [list] in a for loop

* Nested loops

* Running a loop in the background, script example

* Semicolon required, when do is on first line of loop

for loop

while loop

* until loop

until [ condition-is-true ]; do

* while loop

while [ condition ]; do

Function call inside test brackets

Multiple conditions

Omitting test brackets

Redirection

while read construct

* Which type of loop to use

Loopback devices

* In /dev directory

* Mounting an ISO image

-lt , less-than integer comparison test

* * *

m4, macro processing language

$MACHTYPE, Machine type

Magic number, marker at the head of a file indicating the file type

Makefile, file containing the list of dependencies used by make

command

man, manual page (lookup)

* Man page editor (script)

mapfile builtin, loads an array with a text file

Math commands

Meta-meaning

Morse code training script

Modulo, arithmetic remainder operator

* Application: Generating prime numbers

Mortgage calculations, example script

* * *

-n String not null test

Named pipe, a temporary FIFO buffer

* Example script

nc, netcat, a network toolkit for TCP and UDP ports

-ne, not-equal-to integer comparison test

Negation operator, !, reverses the sense of a test

netstat, Network statistics

Network programming

nl, a filter to number lines of text

Noclobber, -C option to Bash to prevent overwriting of files

NOT logical operator, !

null variable assignment, avoiding

* * *

-o Logical OR compound comparison test

Obfuscation

* Colon as function name

* Homework assignment

* Just another Bash hacker!

octal, base-8 numbers

od, octal dump

$OLDPWD Previous working directory

openssl encryption utility

Operator

* Definition of

* Precedence

Options, passed to shell or script on command line or by set command

Or list

Or logical operator, ||

* * *

Parameter substitution

* ${parameter+alt_value}

${parameter:+alt_value}

Alternate value of parameter, if set

* ${parameter-default}

${parameter:-default}

${parameter=default}

${parameter:=default}

Default parameters

* ${!varprefix*}

${!varprefix@}

Parameter name match

* ${parameter?err_msg}

Parameter-unset message

* ${parameter}

Value of parameter

* Case modification (version 4+ of Bash).

* Script example

* Table of parameter substitution

Parent / child process problem, a child process cannot export

variables to a parent process

Parentheses

* Command group

* Enclose group of Extended Regular Expressions

* Double parentheses, in arithmetic expansion

$PATH, the path (location of system binaries)

* Appending directories to $PATH using the += operator.

Pathname, a filename that incorporates the complete path of a given

file.

* Parsing pathnames

Perl, programming language

* Combined in the same file with a Bash script

* Embedded in a Bash script

Perquackey-type anagramming game (Quackey script)

Petals Around the Rose

PID, Process ID, an identification number assigned to a running

process.

Pipe, | , a device for passing the output of a command to another

command or to the shell

* Avoiding unnecessary commands in a pipe

* Comments embedded within

* Exit status of a pipe

* Pipefail, set -o pipefail option to indicate exit status within a

pipe

* $PIPESTATUS, exit status of last executed pipe

* Piping output of a command to a script

* Redirecting stdin, rather than using cat in a pipe

Pitfalls

* - (dash) is not redirection operator

* // (double forward slash), behavior of cd command toward

* #!/bin/sh script header disables extended Bash features

* Abuse of cat

* CGI programming, using scripts for

* Closing limit string in a here document, indenting

* DOS-type newlines (\r\n) crash a script

* Double-quoting the backslash (\) character

* eval, risk of using

* Execute permission lacking for commands within a script

* Exit status, anomalous

* Exit status of arithmetic expression not equivalent to an error

code

* Export problem, child process to parent process

* Extended Bash features not available

* Failing to quote variables within test brackets

* GNU command set, in cross-platform scripts

* let misuse: attempting to set string variables

* Multiple echo statements in a function whose output is captured

* null variable assignment

* Numerical and string comparison operators not equivalent

= and -eq not interchangeable

* Omitting terminal semicolon, in a curly-bracketed code block

* Piping

echo to a loop

echo to read (however, this problem can be circumvented)

tail -f to grep

* Preserving whitespace within a variable, unintended consequences

* suid commands inside a script

* Undocumented Bash features, danger of

* Updates to Bash breaking older scripts

* Uninitialized variables

* Variable names, inappropriate

* Variables in a subshell, scope limited

* Subshell in while-read loop

* Whitespace, misuse of

Pointers

* and file descriptors

* and functions

* and indirect references

* and variables

Portability issues in shell scripting

* Setting path and umask

* A test suite script (Bash versus classic Bourne shell)

* Using whatis

Positional parameters

* $@, as separate words

* $*, as a single word

* in functions

POSIX, Portable Operating System Interface / UNIX

* --posix option

* 1003.2 standard

* Character classes

$PPID, process ID of parent process

Precedence, operator

Prepending lines at head of a file, script example

Prime numbers

* Generating primes using the factor command

* Generating primes using the modulo operator

* Sieve of Eratosthenes, example script

printf, formatted print command

/proc directory

* Running processes, files describing

* Writing to files in /proc, warning

Process

* Child process

* Parent process

* Process ID (PID)

Process substitution

* To compare contents of directories

* To supply stdin of a command

* Template

* while-read loop without a subshell

Programmable completion (tab expansion)

Prompt

* $PS1, Main prompt, seen at command line

* $PS2, Secondary prompt

Pseudo-code, as problem-solving method

$PWD, Current working directory

* * *

Quackey, a Perquackey-type anagramming game (script)

Question mark, ?

* Character match in an Extended Regular Expression

* Single-character wild card, in globbing

* In a C-style Trinary (ternary) operator

Quoting

* Character string

* Variables

within test brackets

* Whitespace, using quoting to preserve

* * *

Random numbers

* /dev/urandom

* rand(), random function in awk

* $RANDOM, Bash function that returns a pseudorandom integer

* Random sequence generation, using date command

* Random sequence generation, using jot

* Random string, generating

Raspberry Pi (single-board computer)

* Script for preparing a bootable SD card

rcs

read, set value of a variable from stdin

* Detecting arrow keys

* Options

* Piping output of cat to read

* "Prepending" text

* Problems piping echo to read

* Redirection from a file to read

* $REPLY, default read variable

* Timed input

* while read construct

readline library

Recursion

* Demonstration of

* Factorial

* Fibonacci sequence

* Local variables

* Script calling itself recursively

* Towers of Hanoi

Redirection

* Code blocks

* exec <filename,

to reassign file descriptors

* Introductory-level explanation of I/O redirection

* Open a file for both reading and writing

<>filename

* read input redirected from a file

* stderr to stdout

2>&1

* stdin / stdout, using -

* stdinof a function

* stdout to a file

> ... >>

* stdout to file descriptor j

>&j

* file descriptori to file descriptor j

i>&j

* stdout of a command to stderr

>&2

* stdout and stderr of a command to a file

* tee, redirect to a file output of command(s) partway through a

pipe

Reference Cards

* Miscellaneous constructs

* Parameter substitution/expansion

* Special shell variables

* String operations

* Test operators

Binary comparison

Files

Regular Expressions

* ^ (caret) Beginning-of-line

* $ (dollar sign) Anchor

* . (dot) Match single character

* * (asterisk) Any number of characters

* [ ] (brackets) Enclose character set to match

* \ (backslash) Escape, interpret following character literally

* \< ... \> (angle brackets, escaped) Word boundary

* Extended REs

+ Character match

\{ \} Escaped "curly" brackets

[: :] POSIX character classes

$REPLY, Default value associated with read command

Restricted shell, shell (or script) with certain commands disabled

return, command that terminates a function

run-parts

* Running scripts in sequence, without user intervention

* * *

Scope of a variable, definition

Script options, set at command line

Scripting routines, library of useful definitions and functions

Secondary prompt, $PS2

Security issues

* nmap, network mapper / port scanner

* sudo

* suid commands inside a script

* Viruses, trojans, and worms in scripts

* Writing secure scripts

sed, pattern-based programming language

* Table, basic operators

* Table, examples of operators

select, construct for menu building

* in list omitted

Semaphore

Semicolon required, when do keyword is on first line of loop

* When terminating curly-bracketed code block

seq, Emit a sequence of integers. Equivalent to jot.

set, Change value of internal script variables

* set -u, Abort script with error message if attempting to use an

undeclared variable.

Shell script, definition of

Shell wrapper, script embedding a command or utility

shift, reassigning positional parameters

$SHLVL, shell level, depth to which the shell (or script) is nested

shopt, change shell options

Signal, a message sent to a process

Simulations

* Brownian motion

* Galton board

* Horserace

* Life, game of

* PI, approximating by firing cannonballs

* Pushdown stack

Single quotes (' ... ') strong quoting

Socket, a communication node associated with an I/O port

Sorting

* Bubble sort

* Insertion sort

source, execute a script or, within a script, import a file

* Passing positional parameters

Spam, dealing with

* Example script

Special characters

Stack

* Definition

* Emulating a push-down stack, example script

Standard Deviation, example script

Startup files, Bash

stdin and stdout

Stopwatch, example script

Strings

* =~ String match operator

* Comparison

* Length

${#string}

* Manipulation

* Manipulation, using awk

* Null string, testing for

* Protecting strings from expansion and/or reinterpretation, script

example

Unprotecting strings, script example

* strchr(), equivalent of

* strlen(), equivalent of

* strings command, find printable strings in a binary or data file

* Substring extraction

${string:position}

${string:position:length}

Using expr

* Substring index (numerical position in string)

* Substring matching, using expr

* Substring removal

${var#Pattern}

${var##Pattern}

${var%Pattern}

${var%%Pattern}

* Substring replacement

${string/substring/replacement}

${string//substring/replacement}

${string/#substring/replacement}

${string/%substring/replacement}

Script example

* Table of string/substring manipulation and extraction operators

Strong quoting ' ... '

Stylesheet for writing scripts

Subshell

* Command list within parentheses

* Variables, $BASH_SUBSHELL and $SHLVL

* Variables in a subshell

scope limited, but ...

... can be accessed outside the subshell?

su Substitute user, log on as a different user or as root

suid (set user id) file flag

* suid commands inside a script, not advisable

Symbolic links

Swapfiles

* * *

Tab completion

Table lookup, script example

tail, echo to stdout lines at the (tail) end of a text file

tar, archiving utility

tee, redirect to a file output of command(s) partway through a pipe

Terminals

* setserial

* setterm

* stty

* tput

* wall

test command

* Bash builtin

* external command, /usr/bin/test (equivalent to /usr/bin/[)

Test constructs

Test operators

* -a Logical AND compound comparison

* -e File exists

* -eq is-equal-to (integer comparison)

* -f File is a regular file

* -ge greater-than or equal (integer comparison)

* -gt greater-than (integer comparison)

* -le less-than or equal (integer comparison)

* -lt less-than (integer comparison)

* -n not-zero-length (string comparison)

* -ne not-equal-to (integer comparison)

* -o Logical OR compound comparison

* -u suid flag set, file test

* -z is-zero-length (string comparison)

* = is-equal-to (string comparison)

== is-equal-to (string comparison)

* < less-than (string comparison)

* < less-than, (integer comparison, within double parentheses)

* <= less-than-or-equal, (integer comparison, within double

parentheses)

* > greater-than (string comparison)

* > greater-than, (integer comparison, within double parentheses)

* >= greater-than-or-equal, (integer comparison, within double

parentheses)

* || Logical OR

* && Logical AND

* ! Negation operator, inverts exit status of a test

!= not-equal-to (string comparison)

* Tables of test operators

Binary comparison

File

Text and text file processing

Time / Date

Timed input

* Using read -t

* Using stty

* Using timing loop

* Using $TMOUT

Tips and hints for Bash scripts

* Array, as return value from a function

Associative array more efficient than a numerically-indexed array

* Capturing the return value of a function, using echo

* CGI programming, using scripts for

* Comment blocks

Using anonymous here documents

Using if-then constructs

* Comment headers, special purpose

* C-style syntax , for manipulating variables

* Double-spacing a text file

* Filenames prefixed with a dash, removing

* Filter, feeding output back to same filter

* Function return value workarounds

* if-grep test fixup

* Library of useful definitions and functions

* null variable assignment, avoiding

* Passing an array to a function

* $PATH, appending to, using the += operator.

* Prepending lines at head of a file

* Progress bar template

* Pseudo-code

* rcs

* Redirecting a test to /dev/null to suppress output

* Running scripts in sequence without user intervention, using

run-parts

* Script as embedded command

* Script portability

Setting path and umask

Using whatis

* Setting script variable to a block of embedded sed or awk code

* Speeding up script execution by disabling unicode

* Subshell variable, accessing outside the subshell

* Testing a variable to see if it contains only digits

* Testing whether a command exists, using type

* Tracking script usage

* while-read loop without a subshell

* Widgets, invoking from a script

$TMOUT, Timeout interval

Token, a symbol that may expand to a keyword or command

tput, terminal-control command

tr, character translation filter

* DOS to Unix text file conversion

* Options

* Soundex, example script

* Variants

Trap, specifying an action upon receipt of a signal

Trinary (ternary) operator, C-style, var>10?88:99

* in double-parentheses construct

* in let construct

true, returns successful (0) exit status

typeset builtin

* options

* * *

$UID, User ID number

unalias, to remove an alias

uname, output system information

Unicode, encoding standard for representing letters and symbols

* Disabling unicode to optimize script

Uninitialized variables

uniq, filter to remove duplicate lines from a sorted file

unset, delete a shell variable

until loop

until [ condition-is-true ]; do

* * *

Variables

* Array operations on

* Assignment

Script example

* Bash internal variables

* Block of sed or awk code, setting a variable to

* C-style increment/decrement/trinary operations

* Change value of internal script variables using set

* declare, to modify the properties of variables

* Deleting a shell variable using unset

* Environmental

* Expansion / Substring replacement operators

* Indirect referencing

eval variable1=\$variable2

Newer notation

${!variable}

* Integer

* Integer / string (variables are untyped)

* Length

${#var}

* Lvalue

* Manipulating and expanding

* Name and value of a variable, distinguishing between

* Null string, testing for

* Null variable assignment, avoiding

* Quoting

within test brackets

to preserve whitespace

* rvalue

* Setting to null value

* In subshell not visible to parent shell

* Testing a variable if it contains only digits

* Typing, restricting the properties of a variable

* Undeclared, error message

* Uninitialized

* Unquoted variable, splitting

* Unsetting

* Untyped

* * *

wait, suspend script execution

* To remedy script hang

Weak quoting " ... "

while loop

while [ condition ]; do

* C-style syntax

* Calling a function within test brackets

* Multiple conditions

* Omitting test brackets

* while read construct

Avoiding a subshell

Whitespace, spaces, tabs, and newline characters

* $IFS defaults to

* Inappropriate use of

* Preceding closing limit string in a here document, error

* Preceding script comments

* Quoting, to preserve whitespace within strings or variables

* [:space:], POSIX character class

who, information about logged on users

* w

* whoami

* logname

Widgets

Wild card characters

* Asterisk *

* In [list] constructs

* Question mark ?

* Will not match dot files

Word splitting

* Definition

* Resulting from command substitution

Wrapper, shell

* * *

xargs, Filter for grouping arguments

* Curly brackets

* Limiting arguments passed

* Options

* Processes arguments one at a time

* Whitespace, handling

* * *

yes

* Emulation

* * *

-z String is null

Zombie, a process that has terminated, but not yet been killed by its

parent

Notes

[1]

These are referred to as builtins, features internal to the shell.

[2]

Although recursion is possible in a shell script, it tends to be slow

and its implementation is often an ugly kludge.

[3]

An acronym is an ersatz word formed by pasting together the initial

letters of the words into a tongue-tripping phrase. This morally

corrupt and pernicious practice deserves appropriately severe

punishment. Public flogging suggests itself.

[4]

Many of the features of ksh88, and even a few from the updated ksh93

have been merged into Bash.

[5]

By convention, user-written shell scripts that are Bourne shell

compliant generally take a name with a .sh extension. System scripts,

such as those found in /etc/rc.d, do not necessarily conform to this

nomenclature.

[6]

More commonly seen in the literature as she-bang or sh-bang. This

derives from the concatenation of the tokens sharp (#) and bang (!).

[7]

Some flavors of UNIX (those based on 4.2 BSD) allegedly take a

four-byte magic number, requiring a blank after the ! -- #! /bin/sh.

[http://www.in-ulm.de/~mascheck/various/shebang/#details] According

to Sven Mascheck this is probably a myth.

[8]

The #! line in a shell script will be the first thing the command

interpreter (sh or bash) sees. Since this line begins with a #, it

will be correctly interpreted as a comment when the command

interpreter finally executes the script. The line has already served

its purpose - calling the command interpreter.

If, in fact, the script includes an extra #! line, then bash will

interpret it as a comment.

!/bin/bash

echo "Part 1 of script."

a=1

!/bin/bash

This does not launch a new script.

echo "Part 2 of script."

echo $a # Value of $a stays at 1.

[9]

This allows some cute tricks.

!/bin/rm

Self-deleting script.

Nothing much seems to happen when you run this... except that the file disap

pears.

WHATEVER=85

echo "This line will never print (betcha!)."

exit $WHATEVER # Doesn't matter. The script will not exit here.

# Try an echo $? after script termination.

# You'll get a 0, not a 85.

Also, try starting a README file with a #!/bin/more, and making it

executable. The result is a self-listing documentation file. (A here

document using cat is possibly a better alternative -- see Example

19-3).

[10]

Portable Operating System Interface, an attempt to standardize

UNIX-like OSes. The POSIX specifications are listed on the Open Group

site.

[11]

To avoid this possibility, a script may begin with a #!/bin/env bash

sha-bang line. This may be useful on UNIX machines where bash is not

located in /bin

[12]

If Bash is your default shell, then the #! isn't necessary at the

beginning of a script. However, if launching a script from a

different shell, such as tcsh, then you will need the #!.

[13]

Caution: invoking a Bash script by sh scriptname turns off

Bash-specific extensions, and the script may therefore fail to

execute.

[14]

A script needs read, as well as execute permission for it to run,

since the shell needs to be able to read it.

[15]

Why not simply invoke the script with scriptname? If the directory

you are in ($PWD) is where scriptname is located, why doesn't this

work? This fails because, for security reasons, the current directory

(./) is not by default included in a user's $PATH. It is therefore

necessary to explicitly invoke the script in the current directory

with a ./scriptname.

[16]

An operator is an agent that carries out an operation. Some examples

are the common arithmetic operators, + - * /. In Bash, there is some

overlap between the concepts of operator and keyword.

[17]

This is more commonly known as the ternary operator. Unfortunately,

ternary is an ugly word. It doesn't roll off the tongue, and it

doesn't elucidate. It obfuscates. Trinary is by far the more elegant

usage.

[18]

American Standard Code for Information Interchange. This is a system

for encoding text characters (alphabetic, numeric, and a limited set

of symbols) as 7-bit numbers that can be stored and manipulated by

computers. Many of the ASCII characters are represented on a standard

keyboard.

[19]

A PID, or process ID, is a number assigned to a running process. The

PIDs of running processes may be viewed with a ps command.

Definition: A process is a currently executing command (or program),

sometimes referred to as a job.

[20]

The shell does the brace expansion. The command itself acts upon the

result of the expansion.

[21]

Exception: a code block in braces as part of a pipe may run as a

subshell.

ls | { read firstline; read secondline; }

Error. The code block in braces runs as a subshell,

+ so the output of "ls" cannot be passed to variables within the block.

echo "First line is $firstline; second line is $secondline" # Won't work.

Thanks, S.C.

[22]

Even as in olden times a philtre denoted a potion alleged to have

magical transformative powers, so does a UNIX filter transform its

target in (roughly) analogous fashion. (The coder who comes up with a

"love philtre" that runs on a Linux machine will likely win accolades

and honors.)

[23]

Bash stores a list of commands previously issued from the

command-line in a buffer, or memory space, for recall with the

builtin history commands.

[24]

A linefeed (newline) is also a whitespace character. This explains

why a blank line, consisting only of a linefeed, is considered

whitespace.

[25]

Technically, the name of a variable is called an lvalue, meaning that

it appears on the left side of an assignment statment, as in

VARIABLE=23. A variable's value is an rvalue, meaning that it appears

on the right side of an assignment statement, as in VAR2=$VARIABLE.

A variable's name is, in fact, a reference, a pointer to the memory

location(s) where the actual data associated with that variable is

kept.

[26]

Note that functions also take positional parameters.

[27]

The process calling the script sets the $0 parameter. By convention,

this parameter is the name of the script. See the manpage (manual

page) for execv.

From the command-line, however, $0 is the name of the shell.

bash$ echo $0

bash

tcsh% echo $0

tcsh

[28]

If the the script is sourced or symlinked, then this will not work.

It is safer to check $BASH_Source.

[29]

Unless there is a file named first in the current working directory.

Yet another reason to quote. (Thank you, Harald Koenig, for pointing

this out.

[30]

Encapsulating "!" within double quotes gives an error when used from

the command line. This is interpreted as a history command. Within a

script, though, this problem does not occur, since the Bash history

mechanism is disabled then.

Of more concern is the apparently inconsistent behavior of \ within

double quotes, and especially following an echo -e command.

bash$ echo hello\!

hello!

bash$ echo "hello\!"

hello\!

bash$ echo \

bash$ echo "\"

bash$ echo \a

bash$ echo "\a"

bash$ echo x\ty

xty

bash$ echo "x\ty"

x\ty

bash$ echo -e x\ty

xty

bash$ echo -e "x\ty"

x y

Double quotes following an echo sometimes escape \. Moreover, the -e

option to echo causes the "\t" to be interpreted as a tab.

(Thank you, Wayne Pollock, for pointing this out, and Geoff Lee and

Daniel Barclay for explaining it.)

[31]

"Word splitting," in this context, means dividing a character string

into separate and discrete arguments.

[32]

In those instances when there is no return terminating the function.

[33]

A token is a symbol or short string with a special meaning attached

to it (a meta-meaning). In Bash, certain tokens, such as [ and .

(dot-command), may expand to keywords and commands.

[34]

Per the 1913 edition of Webster's Dictionary:

Deprecate

...

To pray against, as an evil;

to seek to avert by prayer;

to desire the removal of;

to seek deliverance from;

to express deep regret for;

to disapprove of strongly.

[35]

Be aware that suid binaries may open security holes. The suid flag

has no effect on shell scripts.

[36]

On Linux systems, the sticky bit is no longer used for files, only on

directories.

[37]

As S.C. points out, in a compound test, even quoting the string

variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause

an error with some versions of Bash if $string is empty. The safe way

is to append an extra character to possibly empty variables, [

"x$string" != x -o "x$a" = "x$b" ] (the "x's" cancel out).

[38]

In a different context, += can serve as a string concatenation

operator. This can be useful for modifying environmental variables.

[39]

Side effects are, of course, unintended -- and usually undesirable --

consequences.

[40]

Precedence, in this context, has approximately the same meaning as

priority

[41]

A stack register is a set of consecutive memory locations, such that

the values stored (pushed) are retrieved (popped) in reverse order.

The last value stored is the first retrieved. This is sometimes

called a LIFO (last-in-first-out) or pushdown stack.

[42]

The PID of the currently running script is $, of course.

[43]

Somewhat analogous to recursion, in this context nesting refers to a

pattern embedded within a larger pattern. One of the definitions of

nest, according to the 1913 edition of Webster's Dictionary,

illustrates this beautifully: "A collection of boxes, cases, or the

like, of graduated size, each put within the one next larger."

[44]

The words "argument" and "parameter" are often used interchangeably.

In the context of this document, they have the same precise meaning:

a variable passed to a script or function.

[45]

Within a script, inside a subshell, $ returns the PID of the script,

not the subshell.

[46]

In this context, typing a variable means to classify it and restrict

its properties. For example, a variable declared or typed as an

integer is no longer available for string operations.

declare -i intvar

intvar=23

echo "$intvar" # 23

intvar=stringval

echo "$intvar" # 0

[47]

True "randomness," insofar as it exists at all, can only be found in

certain incompletely understood natural phenomena, such as

radioactive decay. Computers only simulate randomness, and

computer-generated sequences of "random" numbers are therefore

referred to as pseudorandom.

[48]

The seed of a computer-generated pseudorandom number series can be

considered an identification label. For example, think of the

pseudorandom series with a seed of 23 as Series #23.

A property of a pseurandom number series is the length of the cycle

before it starts repeating itself. A good pseurandom generator will

produce series with very long cycles.

[49]

This applies to either command-line arguments or parameters passed to

a function.

[50]

Note that $substring and $replacement may refer to either literal

strings or variables, depending on context. See the first usage

example.

[51]

If $parameter is null in a non-interactive script, it will terminate

with a 127 exit status (the Bash error code for "command not found").

[52]

Iteration: Repeated execution of a command or group of commands,

usually -- but not always, while a given condition holds, or until a

given condition is met.

[53]

These are shell builtins, whereas other loop commands, such as while

and case, are keywords.

[54]

Pattern-match lines may also start with a ( left paren to give the

layout a more structured appearance.

case $( arch ) in # $( arch ) returns machine architecture.

( i386 ) echo "80386-based machine";;

^ ^

( i486 ) echo "80486-based machine";;

( i586 ) echo "Pentium-based machine";;

( i686 ) echo "Pentium2+-based machine";;

( * ) echo "Other type of machine";;

esac

[55]

For purposes of command substitution, a command may be an external

system command, an internal scripting builtin, or even a script

function.

[56]

In a more technically correct sense, command substitution extracts

the stdout of a command, then assigns it to a variable using the =

operator.

[57]

In fact, nesting with backticks is also possible, but only by

escaping the inner backticks, as John Default points out.

word_count=` wc -w \`echo * | awk '{print $8}'\` `

[58]

As Nathan Coulter points out, "while forking a process is a low-cost

operation, executing a new program in the newly-forked child process

adds more overhead."

[59]

An exception to this is the time command, listed in the official Bash

documentation as a keyword ("reserved word").

[60]

Note that let cannot be used for setting string variables.

[61]

To Export information is to make it available in a more general

context. See also scope.

[62]

An option is an argument that acts as a flag, switching script

behaviors on or off. The argument associated with a particular option

indicates the behavior that the option (flag) switches on or off.

[63]

Technically, an exit only terminates the process (or shell) in which

it is running, not the parent process.

[64]

Unless the exec is used to reassign file descriptors.

[65]

Hashing is a method of creating lookup keys for data stored in a

table. The data items themselves are "scrambled" to create keys,

using one of a number of simple mathematical algorithms (methods, or

recipes).

An advantage of hashing is that it is fast. A disadvantage is that

collisions -- where a single key maps to more than one data item --

are possible.

For examples of hashing see Example A-20 and Example A-21.

[66]

The readline library is what Bash uses for reading input in an

interactive shell.

[67]

This only applies to child processes, of course.

[68]

The C source for a number of loadable builtins is typically found in

the /usr/share/doc/bash-?.??/functions directory.

Note that the -f option to enable is not portable to all systems.

[69]

The same effect as autoload can be achieved with typeset -fu.

[70]

The -v option also orders the sort by upper- and lowercase prefixed

filenames.

[71]

Dotfiles are files whose names begin with a dot, such as

~/.Xdefaults. Such filenames do not appear in a normal ls listing

(although an ls -a will show them), and they cannot be deleted by an

accidental rm -rf *. Dotfiles are generally used as setup and

configuration files in a user's home directory.

[72]

This particular feature may not yet be implemented in the version of

the ext2/ext3 filesystem installed on your system. Check the

documentation for your Linux distro.

[73]

And even when xargs is not strictly necessary, it can speed up

execution of a command involving batch-processing of multiple files.

[74]

This is only true of the GNU version of tr, not the generic version

often found on commercial UNIX systems.

[75]

An archive, in the sense discussed here, is simply a set of related

files stored in a single location.

[76]

A tar czvf ArchiveName.tar.gz * will include dotfiles in

subdirectories below the current working directory. This is an

undocumented GNU tar "feature."

[77]

The checksum may be expressed as a hexadecimal number, or to some

other base.

[78]

For even better security, use the sha256sum, sha512, and sha1pass

commands.

[79]

This is a symmetric block cipher, used to encrypt files on a single

system or local network, as opposed to the public key cipher class,

of which pgp is a well-known example.

[80]

Creates a temporary directory when invoked with the -d option.

[81]

A daemon is a background process not attached to a terminal session.

Daemons perform designated services either at specified times or

explicitly triggered by certain events.

The word "daemon" means ghost in Greek, and there is certainly

something mysterious, almost supernatural, about the way UNIX daemons

wander about behind the scenes, silently carrying out their appointed

tasks.

[82]

This is actually a script adapted from the Debian Linux distribution.

[83]

The print queue is the group of jobs "waiting in line" to be printed.

[84]

Large mechanical line printers printed a single line of type at a

time onto joined sheets of greenbar paper, to the accompaniment of a

great deal of noise. The hardcopy thusly printed was referred to as a

printout.

[85]

For an excellent overview of this topic, see Andy Vaught's article,

Introduction to Named Pipes, in the September, 1997 issue of Linux

Journal.

[86]

EBCDIC (pronounced "ebb-sid-ick") is an acronym for Extended Binary

Coded Decimal Interchange Code, an obsolete IBM data format. A

bizarre application of the conv=ebcdic option of dd is as a quick 'n

easy, but not very secure text file encoder.

cat $file | dd conv=swab,ebcdic > $file_encrypted

Encode (looks like gibberish).

Might as well switch bytes (swab), too, for a little extra obscurity.

cat $file_encrypted | dd conv=swab,ascii > $file_plaintext

Decode.

[87]

A macro is a symbolic constant that expands into a command string or

a set of operations on parameters. Simply put, it's a shortcut or

abbreviation.

[88]

This is the case on a Linux machine or a UNIX system with disk

quotas.

[89]

The userdel command will fail if the particular user being deleted is

still logged on.

[90]

For more detail on burning CDRs, see Alex Withers' article, Creating

CDs, in the October, 1999 issue of Linux Journal.

[91]

The -c option to mke2fs also invokes a check for bad blocks.

[92]

Since only root has write permission in the /var/lock directory, a

user script cannot set a lock file there.

[93]

Operators of single-user Linux systems generally prefer something

simpler for backups, such as tar.

[94]

As of the version 4 update of Bash, the -f and -c options take a

block size of 512 when in POSIX mode. Additionally, there are two new

options: -b for socket buffer size, and -T for the limit on the

number of threads.

[95]

NAND is the logical not-and operator. Its effect is somewhat similar

to subtraction.

[96]

In Bash and other Bourne shell derivatives, it is possible to set

variables in a single command's environment.

var1=value1 var2=value2 commandXXX

$var1 and $var2 set in the environment of 'commandXXX' only.

[97]

The killall system script should not be confused with the killall

command in /usr/bin.

[98]

A meta-meaning is the meaning of a term or expression on a higher

level of abstraction. For example, the literal meaning of regular

expression is an ordinary expression that conforms to accepted usage.

The meta-meaning is drastically different, as discussed at length in

this chapter.

[99]

Since sed, awk, and grep process single lines, there will usually not

be a newline to match. In those cases where there is a newline in a

multiple line expression, the dot will match the newline.

!/bin/bash

sed -e 'N;s/.*/[&]/' << EOF # Here Document

line1

line2

EOF

OUTPUT:

[line1

line2]

echo

awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF

line 1

line 2

EOF

OUTPUT:

line

1 Thanks, S.C.

exit 0

[100]

Filename expansion means expanding filename patterns or templates

containing special characters. For example, example.??? might expand

to example.001 and/or example.txt.

[101]

A wild card character, analogous to a wild card in poker, can

represent (almost) any other character.

[102]

Filename expansion can match dotfiles, but only if the pattern

explicitly includes the dot as a literal character.

~/[.]bashrc # Will not expand to ~/.bashrc

~/?bashrc # Neither will this.

# Wild cards and metacharacters will NOT

#+ expand to a dot in globbing.

~/.[b]ashrc # Will expand to ~/.bashrc

~/.ba?hrc # Likewise.

~/.bashr* # Likewise.

Setting the "dotglob" option turns this off.

Thanks, S.C.

[103]

Except, as Dennis Benzinger points out, if using <<- to suppress

tabs.

[104]

By convention in UNIX and Linux, data streams and peripherals (device

files) are treated as files, in a fashion analogous to ordinary

files.

[105]

A file descriptor is simply a number that the operating system

assigns to an open file to keep track of it. Consider it a simplified

type of file pointer. It is analogous to a file handle in C.

[106]

Using file descriptor 5 might cause problems. When Bash creates a

child process, as with exec, the child inherits fd 5 (see Chet

Ramey's archived e-mail,

[http://groups.google.com/group/gnu.bash.bug/browse_thread/thread/139

55daafded3b5c/18c17050087f9f37] SUBJECT: RE: File descriptor 5 is

held open). Best leave this particular fd alone.

[107]

An external command invoked with an exec does not (usually) fork off

a subprocess / subshell.

[108]

This has the same effect as a named pipe (temp file), and, in fact,

named pipes were at one time used in process substitution.

[109]

The return command is a Bash builtin.

[110]

However, as Thomas Braunberger points out, a local variable declared

in a function is also visible to functions called by the parent

function.

!/bin/bash

function1 ()

{

local func1var=20

echo "Within function1, \$func1var = $func1var."

function2

}

function2 ()

{

echo "Within function2, \$func1var = $func1var."

}

function1

exit 0

Output of the script:

Within function1, $func1var = 20.

Within function2, $func1var = 20.

This is documented in the Bash manual:

"Local can only be used within a function; it makes the variable name

have a visible scope restricted to that function and its children."

[emphasis added] The ABS Guide author considers this behavior to be a

bug.

[111]

Otherwise known as redundancy.

[112]

Otherwise known as tautology.

[113]

Otherwise known as a metaphor.

[114]

Otherwise known as a recursive function.

[115]

Too many levels of recursion may crash a script with a segfault.

!/bin/bash

Warning: Running this script could possibly lock up your system!

If you're lucky, it will segfault before using up all available memory.

recursive_function ()

{

echo "$1" # Makes the function do something, and hastens the segfault.

(( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;

As long as 1st parameter is less than 2nd,

+ increment 1st and recurse.

}

recursive_function 1 50000 # Recurse 50,000 levels!

Most likely segfaults (depending on stack size, set by ulimit -m).

Recursion this deep might cause even a C program to segfault,

+ by using up all the memory allotted to the stack.

echo "This will probably not print."

exit 0 # This script will not exit normally.

Thanks, Stéphane Chazelas.

[116]

... as the first word of a command string. Obviously, an alias is

only meaningful at the beginning of a command.

[117]

However, aliases do seem to expand positional parameters.

[118]

The entries in /dev provide mount points for physical and virtual

devices. These entries use very little drive space.

Some devices, such as /dev/null, /dev/zero, and /dev/urandom are

virtual. They are not actual physical devices and exist only in

software.

[119]

A block device reads and/or writes data in chunks, or blocks, in

contrast to a character device, which acesses data in character

units. Examples of block devices are hard drives, CDROM drives, and

flash drives. Examples of character devices are keyboards, modems,

sound cards.

[120]

Of course, the mount point /mnt/flashdrive must exist. If not, then,

as root, mkdir /mnt/flashdrive.

To actually mount the drive, use the following command: mount

/mnt/flashdrive

Newer Linux distros automount flash drives in the /media directory

without user intervention.

[121]

Certain system commands, such as procinfo, free, vmstat, lsdev, and

uptime do this as well.

[122]

By convention, signal 0 is assigned to exit.

[123]

Setting the suid permission on the script itself has no effect in

Linux and most other UNIX flavors.

[124]

In this context, "magic numbers" have an entirely different meaning

than the magic numbers used to designate file types.

[125]

Quite a number of Linux utilities are, in fact, shell wrappers. Some

examples are /usr/bin/pdf2ps, /usr/bin/batch, and /usr/bin/xmkmf.

[126]

ANSI is, of course, the acronym for the American National Standards

Institute. This august body establishes and maintains various

technical and industrial standards.

[127]

This usually means liberal use of functions.

[128]

See Marius van Oers' article, Unix Shell Scripting Malware, and also

the Denning reference in the bibliography.

[129]

Or, better yet, #!/bin/env sh.

[130]

To be more specific, Bash 4+ has limited support for associative

arrays. It's a bare-bones implementation, and it lacks the much of

the functionality of such arrays in other programming languages.

Note, however, that associative arrays in Bash seem to execute faster

and more efficiently than numerically-indexed arrays.

[131]

[132]

This only works with pipes and certain other special files.

[133]

But only in conjunction with readline, i.e., from the command-line.

[134]

And while you're at it, consider fixing the notorious piped read

problem.

[135]

This is the notorious flog it to death technique that works so well

with slow learners, eccentrics, odd ducks, fools and geniuses.

[136]

In fact, he has no credentials or special qualifications. He's a

school dropout with no formal credentials or professional experience

whatsoever. None. Zero. Nada. Aside from the ABS Guide, his major

claim to fame is a First Place in the sack race at the Colfax

Elementary School Field Day in June, 1958.

[137]

Those who can, do. Those who can't . . . get an MCSE.

[138]

Sometimes it seems as if he has spent his entire life flouting

conventional wisdom and defying the sonorous Voice of Authority:

"Hey, you can't do that!"

[139]

Well, if you absolutely insist, you can try modifying Example A-44 to

suit your purposes.

[140]

It was hard to resist the obvious pun. No slight intended, since the

book is a pretty decent introduction to the basic concepts of shell

scripting.

[141]

Sed executes without user intervention.

[142]

If no address range is specified, the default is all lines.

[143]

Its name derives from the initials of its authors, Aho, Weinberg, and

Kernighan.

[144]

Out of range exit values can result in unexpected exit codes. An exit

value greater than 255 returns an exit code modulo 256. For example,

exit 3809 gives an exit code of 225 (3809 % 256 = 225).

[145]

An update of /usr/include/sysexits.h allocates previously unused exit

codes from 64 - 78. It may be anticipated that the range of

unallotted exit codes will be further restricted in the future. The

author of this document will not do fixups on the scripting examples

to conform to the changing standard. This should not cause any

problems, since there is no overlap or conflict in usage of exit

codes between compiled C/C++ binaries and shell scripts.

[146]

This does not apply to csh, tcsh, and other shells not related to or

descended from the classic Bourne shell (sh).

[147]

In older versions of UNIX, passwords were stored in /etc/passwd, and

that explains the name of the file.

[148]

Some early UNIX systems had a fast, small-capacity fixed disk

(containing /, the root partition), and a second drive which was

larger, but slower (containing /usr and other partitions). The most

frequently used programs and utilities therefore resided on the

small-but-fast drive, in /bin, and the others on the slower drive, in

/usr/bin.

This likewise accounts for the split between /sbin and /usr/sbin,

/lib and /usr/lib, etc.

[149]

This works only from the command line, of course, and not within a

script.

[150]

Normally the default parameter completion files reside in either the

/etc/profile.d directory or in /etc/bash_completion. These autoload

on system startup. So, after writing a useful completion script, you

might wish to move it (as root, of course) to one of these

directories.

[151]

It has been extensively documented that programmers are willing to

put in long hours of effort in order to save ten minutes of

"unnecessary" labor. This is known as optimization.

[152]

Various readers have suggested modifications of the above batch file

to prettify it and make it more compact and efficient. In the opinion

of the ABS Guide author, this is wasted effort. A Bash script can

access a DOS filesystem, or even an NTFS partition (with the help of

[http://www.ntfs-3g.org] ntfs-3g) to do batch or scripted operations.

[153]

For all you clever types who failed intermediate algebra, a

determinant is a numerical value associated with a multidimensional

matrix (array of numbers).

For the simple case of a 2 x 2 determinant:

|a b|

|b a|

The solution is a*a - b*b, where "a" and "b" represent numbers.

Cleanup

Run as root, of course.

!/bin/bash

Proper header for a Bash script.

Cleanup, version 2

Run as root, of course.

Insert code here to print error message and exit if not root.

Variables are better than hard-coded values.

!/bin/bash

Cleanup, version 3

Warning:

-------

This script uses quite a number of features that will be explained

+ later on.

By the time you've finished the first half of the book,

+ there should be nothing mysterious about it.

Run as root, of course.

Test whether command-line argument is present (non-empty).

Stephane Chazelas suggests the following,

+ as a better way of checking command-line arguments,

+ but this is still a bit advanced for this stage of the tutorial.

E_WRONGARGS=85 # Non-numerical argument (bad argument format).

case "$1" in

"" ) lines=50;;

*[!0-9]*) echo "Usage: `basename $0` lines-to-cleanup";

exit $E_WRONGARGS;;

* ) lines=$1;;

esac

* Skip ahead to "Loops" chapter to decipher all this.

Far more efficient is:

cd /var/log || {

echo "Cannot change to necessary directory." >&2

exit $E_XCD;

}

cat /dev/null > messages

* No longer needed, as the above method is safer.

Note that there are other log files in /var/log not affected

+ by this script.

A zero return value from the script upon exit indicates success

+ to the shell.

!/bin/sh

!/bin/bash

!/usr/bin/perl

!/usr/bin/tcl

!/bin/sed -f

!/bin/awk -f

-a = all, -h = help, etc.

This line is a comment.

^ Note whitespace before #

Delete lines containing '#' comment character.

Excerpted from life.sh script

Thanks, S.C.

+ ^^

Set "a = 9" and "t2 = 15 / 3"

^ Find all executable files ending in "calc"

+ in /bin and /usr/bin directories.

/bin/ipcalc

/usr/bin/kcalc

/usr/bin/oidcalc

/usr/bin/oocalc

Thank you, Rory Winston, for pointing this out.

Same as:

while true

do

...

done

${username=`whoami`} Gives an error without the leading :

unless "username" is a command or builtin...

Prints error message

+ if one or more of essential environmental variables not set.

Same effect as cat /dev/null >data.xxx

However, this does not fork a new process, since ":" is a builtin.

The name of this function is :

^ ^

if [ "$var1" -lt 98 ]

then

var0=9

else

var0=21

fi

[!0-9]) echo "Usage: `basename $0` lines-to-cleanup";