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An introduction to the C shell

William Joy

Computer Science Division
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, California 94720

_A_B_S_T_R_A_C_T

_C_s_h is a new command language interpreter for
UNIX* systems. It incorporates good features of
other shells and a _h_i_s_t_o_r_y mechanism similar to
the _r_e_d_o of INTERLISP. While incorporating many
features of other shells which make writing shell
programs (shell scripts) easier, most of the
features unique to _c_s_h are designed more for the
interactive UNIX user.

UNIX users who have read a general introduc-
tion to the system will find a valuable basic
explanation of the shell here. Simple terminal
interaction with _c_s_h is possible after reading
just the first section of this document. The
second section describes the shells capabilities
which you can explore after you have begun to
become acquainted with the shell. Later sections
introduce features which are useful, but not
necessary for all users of the shell.

Back matter includes an appendix listing spe-
cial characters of the shell and a glossary of
terms and commands introduced in this manual.

March 10, 1984

__________________________

UNIX is a Trademark of Bell Laboratories.

An introduction to the C shell

William Joy

Computer Science Division
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, California 94720

_I_n_t_r_o_d_u_c_t_i_o_n

A _s_h_e_l_l is a command language interpreter. _C_s_h is the
name of one particular command interpreter on UNIX. The
primary purpose of _c_s_h is to translate command lines typed
at a terminal into system actions, such as invocation of
other programs. _C_s_h is a user program just like any you
might write. Hopefully, _c_s_h will be a very useful program
for you in interacting with the UNIX system.

In addition to this document, you will want to refer to
a copy of the UNIX programmer's manual. The _c_s_h documenta-
tion in the manual provides a full description of all
features of the shell and is a final reference for questions
about the shell.

Many words in this document are shown in _i_t_a_l_i_c_s.
These are important words; names of commands, and words
which have special meaning in discussing the shell and UNIX.
Many of the words are defined in a glossary at the end of
this document. If you don't know what is meant by a word,
you should look for it in the glossary.

_A_c_k_n_o_w_l_e_d_g_e_m_e_n_t_s

Numerous people have provided good input about previous
versions of _c_s_h and aided in its debugging and in the debug-
ging of its documentation. I would especially like to thank
Michael Ubell who made the crucial observation that history
commands could be done well over the word structure of input
text, and implemented a prototype history mechanism in an
older version of the shell. Eric Allman has also provided a
large number of useful comments on the shell, helping to
unify those concepts which are present and to identify and
eliminate useless and marginally useful features. Mike
O'Brien suggested the pathname hashing mechanism which
speeds command execution. Jim Kulp added the job control
and directory stack primitives and added their documentation
to this introduction.

- 2 -

_1. _T_e_r_m_i_n_a_l _u_s_a_g_e _o_f _t_h_e _s_h_e_l_l

_1._1. _T_h_e _b_a_s_i_c _n_o_t_i_o_n _o_f _c_o_m_m_a_n_d_s

A _s_h_e_l_l in UNIX acts mostly as a medium through which
other _p_r_o_g_r_a_m_s are invoked. While it has a set of _b_u_i_l_t_i_n
functions which it performs directly, most commands cause
execution of programs that are, in fact, external to the
shell. The shell is thus distinguished from the command
interpreters of other systems both by the fact that it is
just a user program, and by the fact that it is used almost
exclusively as a mechanism for invoking other programs.

_C_o_m_m_a_n_d_s in the UNIX system consist of a list of
strings or _w_o_r_d_s interpreted as a _c_o_m_m_a_n_d _n_a_m_e followed by
_a_r_g_u_m_e_n_t_s. Thus the command

mail bill

consists of two words. The first word _m_a_i_l names the com-
mand to be executed, in this case the mail program which
sends messages to other users. The shell uses the name of
the command in attempting to execute it for you. It will
look in a number of _d_i_r_e_c_t_o_r_i_e_s for a file with the name
_m_a_i_l which is expected to contain the mail program.

The rest of the words of the command are given as _a_r_g_u_-
_m_e_n_t_s to the command itself when it is executed. In this
case we specified also the argument _b_i_l_l which is inter-
preted by the _m_a_i_l program to be the name of a user to whom
mail is to be sent. In normal terminal usage we might use
the _m_a_i_l command as follows.

% mail bill
I have a question about the csh documentation.
My document seems to be missing page 5.
Does a page five exist?
Bill
EOT
%

Here we typed a message to send to _b_i_l_l and ended this
message with a |^D which sent an end-of-file to the mail pro-
gram. (Here and throughout this document, the notation
``|^_x'' is to be read ``control-_x'' and represents the strik-
ing of the _x key while the control key is held down.) The
mail program then echoed the characters `EOT' and transmit-
ted our message. The characters `% ' were printed before
and after the mail command by the shell to indicate that
input was needed.

After typing the `% ' prompt the shell was reading com-
mand input from our terminal. We typed a complete command

- 3 -

`mail bill'. The shell then executed the _m_a_i_l program with
argument _b_i_l_l and went dormant waiting for it to complete.
The mail program then read input from our terminal until we
signalled an end-of-file via typing a |^D after which the
shell noticed that mail had completed and signaled us that
it was ready to read from the terminal again by printing
another `% ' prompt.

This is the essential pattern of all interaction with
UNIX through the shell. A complete command is typed at the
terminal, the shell executes the command and when this exe-
cution completes, it prompts for a new command. If you run
the editor for an hour, the shell will patiently wait for
you to finish editing and obediently prompt you again when-
ever you finish editing.

An example of a useful command you can execute now is
the _t_s_e_t command, which sets the default _e_r_a_s_e and _k_i_l_l
characters on your terminal - the erase character erases the
last character you typed and the kill character erases the
entire line you have entered so far. By default, the erase
character is `#' and the kill character is `@'. Most people
who use CRT displays prefer to use the backspace (|^H) char-
acter as their erase character since it is then easier to
see what you have typed so far. You can make this be true
by typing

tset -e

which tells the program _t_s_e_t to set the erase character, and
its default setting for this character is a backspace.

_1._2. _F_l_a_g _a_r_g_u_m_e_n_t_s

A useful notion in UNIX is that of a _f_l_a_g argument.
While many arguments to commands specify file names or user
names some arguments rather specify an optional capability
of the command which you wish to invoke. By convention,
such arguments begin with the character `-' (hyphen). Thus
the command

will produce a list of the files in the current _w_o_r_k_i_n_g
_d_i_r_e_c_t_o_r_y. The option -_s is the size option, and

ls -s

causes _l_s to also give, for each file the size of the file
in blocks of 512 characters. The manual section for each
command in the UNIX reference manual gives the available
options for each command. The _l_s command has a large number
of useful and interesting options. Most other commands have
either no options or only one or two options. It is hard to

- 4 -

remember options of commands which are not used very fre-
quently, so most UNIX utilities perform only one or two
functions rather than having a large number of hard to
remember options.

_1._3. _O_u_t_p_u_t _t_o _f_i_l_e_s

Commands that normally read input or write output on
the terminal can also be executed with this input and/or
output done to a file.

Thus suppose we wish to save the current date in a file
called `now'. The command

date

will print the current date on our terminal. This is
because our terminal is the default _s_t_a_n_d_a_r_d _o_u_t_p_u_t for the
date command and the date command prints the date on its
standard output. The shell lets us _r_e_d_i_r_e_c_t the _s_t_a_n_d_a_r_d
_o_u_t_p_u_t of a command through a notation using the _m_e_t_a_c_h_a_r_a_c_-
_t_e_r `>' and the name of the file where output is to be
placed. Thus the command

date > now

runs the _d_a_t_e command such that its standard output is the
file `now' rather than the terminal. Thus this command
places the current date and time into the file `now'. It is
important to know that the _d_a_t_e command was unaware that its
output was going to a file rather than to the terminal. The
shell performed this _r_e_d_i_r_e_c_t_i_o_n before the command began
executing.

One other thing to note here is that the file `now'
need not have existed before the _d_a_t_e command was executed;
the shell would have created the file if it did not exist.
And if the file did exist? If it had existed previously
these previous contents would have been discarded! A shell
option _n_o_c_l_o_b_b_e_r exists to prevent this from happening
accidentally; it is discussed in section 2.2.

The system normally keeps files which you create with
`>' and all other files. Thus the default is for files to
be permanent. If you wish to create a file which will be
removed automatically, you can begin its name with a `#'
character, this `scratch' character denotes the fact that
the file will be a scratch file.* The system will remove
__________________________

Note that if your erase character is a `#', you will

have to precede the `#' with a `\'. The fact that the
`#' character is the old (pre-CRT) standard erase char-
acter means that it seldom appears in a file name, and
allows this convention to be used for scratch files.

- 5 -

such files after a couple of days, or sooner if file space
becomes very tight. Thus, in running the _d_a_t_e command
above, we don't really want to save the output forever, so
we would more likely do

date > #now

_1._4. _M_e_t_a_c_h_a_r_a_c_t_e_r_s _i_n _t_h_e _s_h_e_l_l

The shell has a large number of special characters
(like `>') which indicate special functions. We say that
these notations have _s_y_n_t_a_c_t_i_c and _s_e_m_a_n_t_i_c meaning to the
shell. In general, most characters which are neither
letters nor digits have special meaning to the shell. We
shall shortly learn a means of _q_u_o_t_a_t_i_o_n which allows us to
use _m_e_t_a_c_h_a_r_a_c_t_e_r_s without the shell treating them in any
special way.

Metacharacters normally have effect only when the shell
is reading our input. We need not worry about placing shell
metacharacters in a letter we are sending via _m_a_i_l, or when
we are typing in text or data to some other program. Note
that the shell is only reading input when it has prompted
with `% '.

_1._5. _I_n_p_u_t _f_r_o_m _f_i_l_e_s; _p_i_p_e_l_i_n_e_s

We learned above how to _r_e_d_i_r_e_c_t the _s_t_a_n_d_a_r_d _o_u_t_p_u_t of
a command to a file. It is also possible to redirect the
_s_t_a_n_d_a_r_d _i_n_p_u_t of a command from a file. This is not often
necessary since most commands will read from a file whose
name is given as an argument. We can give the command

sort < data

to run the _s_o_r_t command with standard input, where the com-
mand normally reads its input, from the file `data'. We
would more likely say

sort data

letting the _s_o_r_t command open the file `data' for input
itself since this is less to type.

We should note that if we just typed

sort

__________________________
If you are using a CRT, your erase character should be
a |^H, as we demonstrated in section 1.1 how this could
be set up.

- 6 -

then the sort program would sort lines from its _s_t_a_n_d_a_r_d
_i_n_p_u_t. Since we did not _r_e_d_i_r_e_c_t the standard input, it
would sort lines as we typed them on the terminal until we
typed a |^D to indicate an end-of-file.

A most useful capability is the ability to combine the
standard output of one command with the standard input of
another, i.e. to run the commands in a sequence known as a
_p_i_p_e_l_i_n_e. For instance the command

ls -s

normally produces a list of the files in our directory with
the size of each in blocks of 512 characters. If we are
interested in learning which of our files is largest we may
wish to have this sorted by size rather than by name, which
is the default way in which _l_s sorts. We could look at the
many options of _l_s to see if there was an option to do this
but would eventually discover that there is not. Instead we
can use a couple of simple options of the _s_o_r_t command, com-
bining it with _l_s to get what we want.

The -_n option of sort specifies a numeric sort rather
than an alphabetic sort. Thus

ls -s | sort -n

specifies that the output of the _l_s command run with the
option -_s is to be _p_i_p_e_d to the command _s_o_r_t run with the
numeric sort option. This would give us a sorted list of
our files by size, but with the smallest first. We could
then use the -_r reverse sort option and the _h_e_a_d command in
combination with the previous command doing

ls -s | sort -n -r | head -5

Here we have taken a list of our files sorted alphabeti-
cally, each with the size in blocks. We have run this to
the standard input of the _s_o_r_t command asking it to sort
numerically in reverse order (largest first). This output
has then been run into the command _h_e_a_d which gives us the
first few lines. In this case we have asked _h_e_a_d for the
first 5 lines. Thus this command gives us the names and
sizes of our 5 largest files.

The notation introduced above is called the _p_i_p_e
mechanism. Commands separated by `|' characters are con-
nected together by the shell and the standard output of each
is run into the standard input of the next. The leftmost
command in a pipeline will normally take its standard input
from the terminal and the rightmost will place its standard
output on the terminal. Other examples of pipelines will be
given later when we discuss the history mechanism; one
important use of pipes which is illustrated there is in the

- 7 -

routing of information to the line printer.

_1._6. _F_i_l_e_n_a_m_e_s

Many commands to be executed will need the names of
files as arguments. UNIX _p_a_t_h_n_a_m_e_s consist of a number of
_c_o_m_p_o_n_e_n_t_s separated by `/'. Each component except the last
names a directory in which the next component resides, in
effect specifying the _p_a_t_h of directories to follow to reach
the file. Thus the pathname

/etc/motd

specifies a file in the directory `etc' which is a subdirec-
tory of the _r_o_o_t directory `/'. Within this directory the
file named is `motd' which stands for `message of the day'.
A _p_a_t_h_n_a_m_e that begins with a slash is said to be an _a_b_s_o_-
_l_u_t_e pathname since it is specified from the absolute top of
the entire directory hierarchy of the system (the _r_o_o_t).
_P_a_t_h_n_a_m_e_s which do not begin with `/' are interpreted as
starting in the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y, which is, by
default, your _h_o_m_e directory and can be changed dynamically
by the _c_d change directory command. Such pathnames are said
to be _r_e_l_a_t_i_v_e to the working directory since they are found
by starting in the working directory and descending to lower
levels of directories for each _c_o_m_p_o_n_e_n_t of the pathname.
If the pathname contains no slashes at all then the file is
contained in the working directory itself and the pathname
is merely the name of the file in this directory. Absolute
pathnames have no relation to the working directory.

Most filenames consist of a number of alphanumeric
characters and `.'s (periods). In fact, all printing char-
acters except `/' (slash) may appear in filenames. It is
inconvenient to have most non-alphabetic characters in
filenames because many of these have special meaning to the
shell. The character `.' (period) is not a shell-
metacharacter and is often used to separate the _e_x_t_e_n_s_i_o_n of
a file name from the base of the name. Thus

prog.c prog.o prog.errs prog.output

are four related files. They share a _b_a_s_e portion of a name
(a base portion being that part of the name that is left
when a trailing `.' and following characters which are not
`.' are stripped off). The file `prog.c' might be the
source for a C program, the file `prog.o' the corresponding
object file, the file `prog.errs' the errors resulting from
a compilation of the program and the file `prog.output' the
output of a run of the program.

If we wished to refer to all four of these files in a
command, we could use the notation

- 8 -

prog.*

This word is expanded by the shell, before the command to
which it is an argument is executed, into a list of names
which begin with `prog.'. The character `*' here matches
any sequence (including the empty sequence) of characters in
a file name. The names which match are alphabetically
sorted and placed in the _a_r_g_u_m_e_n_t _l_i_s_t of the command. Thus
the command

echo prog.*

will echo the names

prog.c prog.errs prog.o prog.output

Note that the names are in sorted order here, and a dif-
ferent order than we listed them above. The _e_c_h_o command
receives four words as arguments, even though we only typed
one word as as argument directly. The four words were gen-
erated by _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n of the one input word.

Other notations for _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n are also avail-
able. The character `?' matches any single character in acan't open file tabs

filename. Thus

echo ? ?? ???

will echo a line of filenames; first those with one charac-
ter names, then those with two character names, and finally
those with three character names. The names of each length
will be independently sorted.

Another mechanism consists of a sequence of characters
between `[' and `]'. This metasequence matches any single
character from the enclosed set. Thus

prog.[co]

will match

prog.c prog.o

in the example above. We can also place two characters
around a `-' in this notation to denote a range. Thus

chap.[1-5]

might match files

chap.1 chap.2 chap.3 chap.4 chap.5

if they existed. This is shorthand for

- 9 -

chap.[12345]

and otherwise equivalent.

An important point to note is that if a list of argu-
ment words to a command (an _a_r_g_u_m_e_n_t _l_i_s_t) contains filename
expansion syntax, and if this filename expansion syntax
fails to match any existing file names, then the shell con-
siders this to be an error and prints a diagnostic

No match.

and does not execute the command.

Another very important point is that files with the
character `.' at the beginning are treated specially. Nei-
ther `*' or `?' or the `[' `]' mechanism will match it.
This prevents accidental matching of the filenames `.' and
`..' in the working directory which have special meaning to
the system, as well as other files such as ._c_s_h_r_c which are
not normally visible. We will discuss the special role of
the file ._c_s_h_r_c later.

Another filename expansion mechanism gives access to
the pathname of the _h_o_m_e directory of other users. This
notation consists of the character `~' (tilde) followed by
another users' login name. For instance the word `~bill'
would map to the pathname `/usr/bill' if the home directory
for `bill' was `/usr/bill'. Since, on large systems, users
may have login directories scattered over many different
disk volumes with different prefix directory names, this
notation provides a reliable way of accessing the files of
other users.

A special case of this notation consists of a `~'
alone, e.g. `~/mbox'. This notation is expanded by the
shell into the file `mbox' in your _h_o_m_e directory, i.e. into
`/usr/bill/mbox' for me on Ernie Co-vax, the UCB Computer
Science Department VAX machine, where this document was
prepared. This can be very useful if you have used _c_d to
change to another directory and have found a file you wish
to copy using _c_p. If I give the command

cp thatfile ~

the shell will expand this command to

cp thatfile /usr/bill

since my home directory is /usr/bill.

There also exists a mechanism using the characters `{'
and `}' for abbreviating a set of words which have common
parts but cannot be abbreviated by the above mechanisms

- 10 -

because they are not files, are the names of files which do
not yet exist, are not thus conveniently described. This
mechanism will be described much later, in section 4.2, as
it is used less frequently.

_1._7. _Q_u_o_t_a_t_i_o_n

We have already seen a number of metacharacters used by
the shell. These metacharacters pose a problem in that we
cannot use them directly as parts of words. Thus the com-
mand

echo *

will not echo the character `*'. It will either echo an
sorted list of filenames in the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y,
or print the message `No match' if there are no files in the
working directory.

The recommended mechanism for placing characters which
are neither numbers, digits, `/', `.' or `-' in an argument
word to a command is to enclose it with single quotation
characters `'', i.e.

echo '*'

There is one special character `!' which is used by the _h_i_s_-
_t_o_r_y mechanism of the shell and which cannot be _e_s_c_a_p_e_d by
placing it within `'' characters. It and the character `''
itself can be preceded by a single `\' to prevent their spe-
cial meaning. Thus

echo \'\!

prints

These two mechanisms suffice to place any printing character
into a word which is an argument to a shell command. They
can be combined, as in

echo \''*'

which prints

since the first `\' escaped the first `'' and the `*' was
enclosed between `'' characters.

- 11 -

_1._8. _T_e_r_m_i_n_a_t_i_n_g _c_o_m_m_a_n_d_s

When you are executing a command and the shell is wait-
ing for it to complete there are several ways to force it to
stop. For instance if you type the command

cat /etc/passwd

the system will print a copy of a list of all users of the
system on your terminal. This is likely to continue for
several minutes unless you stop it. You can send an INTER-
RUPT _s_i_g_n_a_l to the _c_a_t command by typing the DEL or RUBOUT
key on your terminal.* Since _c_a_t does not take any precau-
tions to avoid or otherwise handle this signal the INTERRUPT
will cause it to terminate. The shell notices that _c_a_t has
terminated and prompts you again with `% '. If you hit
INTERRUPT again, the shell will just repeat its prompt since
it handles INTERRUPT signals and chooses to continue to exe-
cute commands rather than terminating like _c_a_t did, which
would have the effect of logging you out.

Another way in which many programs terminate is when
they get an end-of-file from their standard input. Thus the
_m_a_i_l program in the first example above was terminated when
we typed a |^D which generates an end-of-file from the stan-
dard input. The shell also terminates when it gets an end-
of-file printing `logout'; UNIX then logs you off the sys-
tem. Since this means that typing too many |^D's can
accidentally log us off, the shell has a mechanism for
preventing this. This _i_g_n_o_r_e_e_o_f option will be discussed in
section 2.2.

If a command has its standard input redirected from a
file, then it will normally terminate when it reaches the
end of this file. Thus if we execute

mail bill < prepared.text

the mail command will terminate without our typing a |^D.
This is because it read to the end-of-file of our file
`prepared.text' in which we placed a message for `bill' with
an editor program. We could also have done

cat prepared.text | mail bill

since the _c_a_t command would then have written the text
through the pipe to the standard input of the mail command.
When the _c_a_t command completed it would have terminated,
closing down the pipeline and the _m_a_i_l command would have
received an end-of-file from it and terminated. Using a
__________________________

Many users use _s_t_t_y(1) to change the interrupt charac-

ter to |^C.

- 12 -

pipe here is more complicated than redirecting input so we
would more likely use the first form. These commands could
also have been stopped by sending an INTERRUPT.

Another possibility for stopping a command is to
suspend its execution temporarily, with the possibility of
continuing execution later. This is done by sending a STOP
signal via typing a |^Z. This signal causes all commands
running on the terminal (usually one but more if a pipeline
is executing) to become suspended. The shell notices that
the command(s) have been suspended, types `Stopped' and then
prompts for a new command. The previously executing command
has been suspended, but otherwise unaffected by the STOP
signal. Any other commands can be executed while the origi-
nal command remains suspended. The suspended command can be
continued using the _f_g command with no arguments. The shell
will then retype the command to remind you which command is
being continued, and cause the command to resume execution.
Unless any input files in use by the suspended command have
been changed in the meantime, the suspension has no effect
whatsoever on the execution of the command. This feature
can be very useful during editing, when you need to look at
another file before continuing. An example of command
suspension follows.

_2. _D_e_t_a_i_l_s _o_n _t_h_e _s_h_e_l_l _f_o_r _t_e_r_m_i_n_a_l _u_s_e_r_s

_2._1. _S_h_e_l_l _s_t_a_r_t_u_p _a_n_d _t_e_r_m_i_n_a_t_i_o_n

When you login, the shell is started by the system in
your _h_o_m_e directory and begins by reading commands from a
file ._c_s_h_r_c in this directory. All shells which you may
start during your terminal session will read from this file.
We will later see what kinds of commands are usefully placed
there. For now we need not have this file and the shell
does not complain about its absence.

A _l_o_g_i_n _s_h_e_l_l, executed after you login to the system,
will, after it reads commands from ._c_s_h_r_c, read commands
from a file ._l_o_g_i_n also in your home directory. This file
contains commands which you wish to do each time you login
to the UNIX system. My ._l_o_g_i_n file looks something like:

set ignoreeof
set mail=(/usr/spool/mail/bill)
echo "${prompt}users" ; users
alias ts \
'set noglob ; eval `tset -s -m dialup:c100rv4pna -m plugboard:?hp2621nl *`';
ts; stty intr |^C kill |^U crt
set time=15 history=10
msgs -f
if (-e $mail) then
echo "${prompt}mail"
mail
endif

This file contains several commands to be executed by
UNIX each time I login. The first is a _s_e_t command which is
interpreted directly by the shell. It sets the shell vari-
able _i_g_n_o_r_e_e_o_f which causes the shell to not log me off if I
hit |^D. Rather, I use the _l_o_g_o_u_t command to log off of the
system. By setting the _m_a_i_l variable, I ask the shell to
watch for incoming mail to me. Every 5 minutes the shell
looks for this file and tells me if more mail has arrived
there. An alternative to this is to put the command

biff y

in place of this _s_e_t; this will cause me to be notified
immediately when mail arrives, and to be shown the first few
lines of the new message.

Next I set the shell variable `time' to `15' causing
the shell to automatically print out statistics lines for
commands which execute for at least 15 seconds of CPU time.
The variable `history' is set to 10 indicating that I want
the shell to remember the last 10 commands I type in its
_h_i_s_t_o_r_y _l_i_s_t, (described later).

March 11, 1984

- 2 -

I create an _a_l_i_a_s ``ts'' which executes a _t_s_e_t(1) com-
mand setting up the modes of the terminal. The parameters
to _t_s_e_t indicate the kinds of terminal which I usually use
when not on a hardwired port. I then execute ``ts'' and
also use the _s_t_t_y command to change the interrupt character
to |^C and the line kill character to |^U.

I then run the `msgs' program, which provides me with
any system messages which I have not seen before; the `-f'
option here prevents it from telling me anything if there
are no new messages. Finally, if my mailbox file exists,
then I run the `mail' program to process my mail.

When the `mail' and `msgs' programs finish, the shell
will finish processing my ._l_o_g_i_n file and begin reading com-
mands from the terminal, prompting for each with `% '. When
I log off (by giving the _l_o_g_o_u_t command) the shell will
print `logout' and execute commands from the file `.logout'
if it exists in my home directory. After that the shell
will terminate and UNIX will log me off the system. If the
system is not going down, I will receive a new login mes-
sage. In any case, after the `logout' message the shell is
committed to terminating and will take no further input from
my terminal.

_2._2. _S_h_e_l_l _v_a_r_i_a_b_l_e_s

The shell maintains a set of _v_a_r_i_a_b_l_e_s. We saw above
the variables _h_i_s_t_o_r_y and _t_i_m_e which had values `10' and
`15'. In fact, each shell variable has as value an array of
zero or more _s_t_r_i_n_g_s. Shell variables may be assigned
values by the set command. It has several forms, the most
useful of which was given above and is

set name=value

Shell variables may be used to store values which are
to be used in commands later through a substitution mechan-
ism. The shell variables most commonly referenced are, how-
ever, those which the shell itself refers to. By changing
the values of these variables one can directly affect the
behavior of the shell.

One of the most important variables is the variable
_p_a_t_h. This variable contains a sequence of directory names
where the shell searches for commands. The _s_e_t command with
no arguments shows the value of all variables currently
defined (we usually say _s_e_t) in the shell. The default
value for path will be shown by _s_e_t to be

March 11, 1984

_3. _S_h_e_l_l _c_o_n_t_r_o_l _s_t_r_u_c_t_u_r_e_s _a_n_d _c_o_m_m_a_n_d _s_c_r_i_p_t_s

_3._1. _I_n_t_r_o_d_u_c_t_i_o_n

It is possible to place commands in files and to cause
shells to be invoked to read and execute commands from these
files, which are called _s_h_e_l_l _s_c_r_i_p_t_s. We here detail those
features of the shell useful to the writers of such scripts.

_3._2. _M_a_k_e

It is important to first note what shell scripts are
_n_o_t useful for. There is a program called _m_a_k_e which is
very useful for maintaining a group of related files or per-
forming sets of operations on related files. For instance a
large program consisting of one or more files can have its
dependencies described in a _m_a_k_e_f_i_l_e which contains defini-
tions of the commands used to create these different files
when changes occur. Definitions of the means for printing
listings, cleaning up the directory in which the files
reside, and installing the resultant programs are easily,
and most appropriately placed in this _m_a_k_e_f_i_l_e. This format
is superior and preferable to maintaining a group of shell
procedures to maintain these files.

Similarly when working on a document a _m_a_k_e_f_i_l_e may be
created which defines how different versions of the document
are to be created and which options of _n_r_o_f_f or _t_r_o_f_f are
appropriate.

_3._3. _I_n_v_o_c_a_t_i_o_n _a_n_d _t_h_e _a_r_g_v _v_a_r_i_a_b_l_e

A _c_s_h command script may be interpreted by saying

% csh script ...

where _s_c_r_i_p_t is the name of the file containing a group of
_c_s_h commands and `...' is replaced by a sequence of argu-
ments. The shell places these arguments in the variable
_a_r_g_v and then begins to read commands from the script.
These parameters are then available through the same mechan-
isms which are used to reference any other shell variables.

If you make the file `script' executable by doing

chmod 755 script

and place a shell comment at the beginning of the shell
script (i.e. begin the file with a `#' character) then a
`/bin/csh' will automatically be invoked to execute `script'
when you type

script

March 11, 1984

- 2 -

If the file does not begin with a `#' then the standard
shell `/bin/sh' will be used to execute it. This allows you
to convert your older shell scripts to use _c_s_h at your con-
venience.

_3._4. _V_a_r_i_a_b_l_e _s_u_b_s_t_i_t_u_t_i_o_n

After each input line is broken into words and history
substitutions are done on it, the input line is parsed into
distinct commands. Before each command is executed a
mechanism know as _v_a_r_i_a_b_l_e _s_u_b_s_t_i_t_u_t_i_o_n is done on these
words. Keyed by the character `

this substitution
replaces the names of variables by their values. Thus

echo $argv

when placed in a command script would cause the current
value of the variable _a_r_g_v to be echoed to the output of the
shell script. It is an error for _a_r_g_v to be unset at this
point.

A number of notations are provided for accessing com-
ponents and attributes of variables. The notation

$?name

expands to `1' if name is _s_e_t or to `0' if name is not _s_e_t.
It is the fundamental mechanism used for checking whether
particular variables have been assigned values. All other
forms of reference to undefined variables cause errors.

The notation

$#name

expands to the number of elements in the variable _n_a_m_e.
Thus

% set argv=(a b c)
% echo $?argv
1
% echo $#argv
3
% unset argv
% echo $?argv
0
% echo $argv
Undefined variable: argv.
%

It is also possible to access the components of a vari-
able which has several values. Thus

March 11, 1984

- 3 -

$argv[1]

gives the first component of _a_r_g_v or in the example above
`a'. Similarly

$argv[$#argv]

would give `c', and

$argv[1-2]

would give `a b'. Other notations useful in shell scripts
are

$_n

where _n is an integer as a shorthand for

$argv[_n]

the _n_t_h parameter and

which is a shorthand for

$argv

The form

expands to the process number of the current shell. Since
this process number is unique in the system it can be used
in generation of unique temporary file names. The form

{body}lt;

is quite special and is replaced by the next line of input
read from the shell's standard input (not the script it is
reading). This is useful for writing shell scripts that are
interactive, reading commands from the terminal, or even
writing a shell script that acts as a filter, reading lines
from its input file. Thus the sequence

echo 'yes or no?\c'
set a=({body}lt;)

would write out the prompt `yes or no?' without a newline
and then read the answer into the variable `a'. In this
case `$#a' would be `0' if either a blank line or end-of-
file (|^D) was typed.

One minor difference between `$_n' and `$argv[_n]' should

March 11, 1984

- 4 -

be noted here. The form `$argv[_n]' will yield an error if _n
is not in the range `1-$#argv' while `$n' will never yield
an out of range subscript error. This is for compatibility
with the way older shells handled parameters.

Another important point is that it is never an error to
give a subrange of the form `n-'; if there are less than _n
components of the given variable then no words are substi-
tuted. A range of the form `m-n' likewise returns an empty
vector without giving an error when _m exceeds the number of
elements of the given variable, provided the subscript _n is
in range.

_3._5. _E_x_p_r_e_s_s_i_o_n_s

In order for interesting shell scripts to be con-
structed it must be possible to evaluate expressions in the
shell based on the values of variables. In fact, all the
arithmetic operations of the language C are available in the
shell with the same precedence that they have in C. In par-
ticular, the operations `==' and `!=' compare strings and
the operators `&&' and `||' implement the boolean and/or
operations. The special operators `=~' and `!~' are similar
to `==' and `!=' except that the string on the right side
can have pattern matching characters (like *, ? or []) and
the test is whether the string on the left matches the pat-
tern on the right.

The shell also allows file enquiries of the form

-? filename

where `?' is replace by a number of single characters. For
instance the expression primitive

-e filename

tell whether the file `filename' exists. Other primitives
test for read, write and execute access to the file, whether
it is a directory, or has non-zero length.

It is possible to test whether a command terminates
normally, by a primitive of the form `{ command }' which
returns true, i.e. `1' if the command succeeds exiting nor-
mally with exit status 0, or `0' if the command terminates
abnormally or with exit status non-zero. If more detailed
information about the execution status of a command is
required, it can be executed and the variable `$status'
examined in the next command. Since `$status' is set by
every command, it is very transient. It can be saved if it
is inconvenient to use it only in the single immediately
following command.

For a full list of expression components available see

March 11, 1984

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the manual section for the shell.

_3._6. _S_a_m_p_l_e _s_h_e_l_l _s_c_r_i_p_t

A sample shell script which makes use of the expression
mechanism of the shell and some of its control structure
follows:

% cat copyc
#
# Copyc copies those C programs in the specified list
# to the directory ~/backup if they differ from the files
# already in ~/backup
#
set noglob
foreach i ($argv)

if ($i !~ *.c) continue # not a .c file so do nothing

if (! -r ~/backup/$i:t) then
echo $i:t not in backup... not cp\'ed
continue
endif

cmp -s $i ~/backup/$i:t # to set $status

if ($status != 0) then
echo new backup of $i
cp $i ~/backup/$i:t
endif
end

This script makes use of the _f_o_r_e_a_c_h command, which
causes the shell to execute the commands between the _f_o_r_e_a_c_h
and the matching _e_n_d for each of the values given between
`(' and `)' with the named variable, in this case `i' set to
successive values in the list. Within this loop we may use
the command _b_r_e_a_k to stop executing the loop and _c_o_n_t_i_n_u_e to
prematurely terminate one iteration and begin the next.
After the _f_o_r_e_a_c_h loop the iteration variable (_i in this
case) has the value at the last iteration.

We set the variable _n_o_g_l_o_b here to prevent filename
expansion of the members of _a_r_g_v. This is a good idea, in
general, if the arguments to a shell script are filenames
which have already been expanded or if the arguments may
contain filename expansion metacharacters. It is also pos-
sible to quote each use of a `

variable expansion, but
this is harder and less reliable.

The other control construct used here is a statement of
the form

March 11, 1984

- 6 -

if ( expression ) then
command
...
endif

The placement of the keywords here is _n_o_t flexible due to
the current implementation of the shell.

The shell does have another form of the if statement of
the form

if ( expression ) command

which can be written

if ( expression ) \
command

Here we have escaped the newline for the sake of appearance.
The command must not involve `|', `&' or `;' and must not be
another control command. The second form requires the final
`\' to _i_m_m_e_d_i_a_t_e_l_y precede the end-of-line.

The more general _i_f statements above also admit a
sequence of _e_l_s_e-_i_f pairs followed by a single _e_l_s_e and an
_e_n_d_i_f, e.g.:

if ( expression ) then
commands
else if (expression ) then
commands
...

else
commands
endif

Another important mechanism used in shell scripts is
the `:' modifier. We can use the modifier `:r' here to
__________________________
The following two formats are not currently acceptable
to the shell:

if ( expression ) # Won't work!
then
command
...
endif

and

if ( expression ) then command endif # Won't work

March 11, 1984

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extract a root of a filename or `:e' to extract the _e_x_t_e_n_-
_s_i_o_n. Thus if the variable _i has the value `/mnt/foo.bar'
then

% echo $i $i:r $i:e
/mnt/foo.bar /mnt/foo bar
%

shows how the `:r' modifier strips off the trailing `.bar'
and the the `:e' modifier leaves only the `bar'. Other
modifiers will take off the last component of a pathname
leaving the head `:h' or all but the last component of a
pathname leaving the tail `:t'. These modifiers are fully
described in the _c_s_h manual pages in the programmers manual.
It is also possible to use the _c_o_m_m_a_n_d _s_u_b_s_t_i_t_u_t_i_o_n mechan-
ism described in the next major section to perform modifica-
tions on strings to then reenter the shells environment.
Since each usage of this mechanism involves the creation of
a new process, it is much more expensive to use than the `:'
modification mechanism.# Finally, we note that the character
`#' lexically introduces a shell comment in shell scripts
(but not from the terminal). All subsequent characters on
the input line after a `#' are discarded by the shell. This
character can be quoted using `'' or `\' to place it in an
argument word.

_3._7. _O_t_h_e_r _c_o_n_t_r_o_l _s_t_r_u_c_t_u_r_e_s

The shell also has control structures _w_h_i_l_e and _s_w_i_t_c_h
similar to those of C. These take the forms

while ( expression )
commands
end

and

__________________________
#It is also important to note that the current imple-
mentation of the shell limits the number of `:' modif-
iers on a `

substitution to 1. Thus

% echo $i $i:h:t
/a/b/c /a/b:t
%

does not do what one would expect.

March 11, 1984

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switch ( word )

case str1:
commands
breaksw

...

case strn:
commands
breaksw

default:
commands
breaksw

endsw

For details see the manual section for _c_s_h. C programmers
should note that we use _b_r_e_a_k_s_w to exit from a _s_w_i_t_c_h while
_b_r_e_a_k exits a _w_h_i_l_e or _f_o_r_e_a_c_h loop. A common mistake to
make in _c_s_h scripts is to use _b_r_e_a_k rather than _b_r_e_a_k_s_w in
switches.

Finally, _c_s_h allows a _g_o_t_o statement, with labels look-
ing like they do in C, i.e.:

loop:
commands
goto loop

_3._8. _S_u_p_p_l_y_i_n_g _i_n_p_u_t _t_o _c_o_m_m_a_n_d_s

Commands run from shell scripts receive by default the
standard input of the shell which is running the script.
This is different from previous shells running under UNIX.
It allows shell scripts to fully participate in pipelines,
but mandates extra notation for commands which are to take
inline data.

Thus we need a metanotation for supplying inline data
to commands in shell scripts. As an example, consider this
script which runs the editor to delete leading blanks from
the lines in each argument file

March 11, 1984

- 9 -

% cat deblank
# deblank -- remove leading blanks
foreach i ($argv)
ed - $i << 'EOF'
1,$s/|^[ ]*//
w
q
'EOF'
end
%

The notation `<< 'EOF'' means that the standard input for
the _e_d command is to come from the text in the shell script
file up to the next line consisting of exactly `'EOF''. The
fact that the `EOF' is enclosed in `'' characters, i.e.
quoted, causes the shell to not perform variable substitu-
tion on the intervening lines. In general, if any part of
the word following the `<<' which the shell uses to ter-
minate the text to be given to the command is quoted then
these substitutions will not be performed. In this case
since we used the form `1,

in our editor script we needed
to insure that this `

was not variable substituted. We
could also have insured this by preceding the `

here with
a `\', i.e.:

1,\$s/|^[ ]*//

but quoting the `EOF' terminator is a more reliable way of
achieving the same thing.

_3._9. _C_a_t_c_h_i_n_g _i_n_t_e_r_r_u_p_t_s

If our shell script creates temporary files, we may
wish to catch interruptions of the shell script so that we
can clean up these files. We can then do

onintr label

where _l_a_b_e_l is a label in our program. If an interrupt is
received the shell will do a `goto label' and we can remove
the temporary files and then do an _e_x_i_t command (which is
built in to the shell) to exit from the shell script. If we
wish to exit with a non-zero status we can do

exit(1)

e.g. to exit with status `1'.

_3._1_0. _W_h_a_t _e_l_s_e?

There are other features of the shell useful to writers
of shell procedures. The _v_e_r_b_o_s_e and _e_c_h_o options and the
related -_v and -_x command line options can be used to help
trace the actions of the shell. The -_n option causes the

March 11, 1984

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shell only to read commands and not to execute them and may
sometimes be of use.

One other thing to note is that _c_s_h will not execute
shell scripts which do not begin with the character `#',
that is shell scripts that do not begin with a comment.
Similarly, the `/bin/sh' on your system may well defer to
`csh' to interpret shell scripts which begin with `#'. This
allows shell scripts for both shells to live in harmony.

There is also another quotation mechanism using `"'
which allows only some of the expansion mechanisms we have
so far discussed to occur on the quoted string and serves to
make this string into a single word as `'' does.

March 11, 1984

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_4. _O_t_h_e_r, _l_e_s_s _c_o_m_m_o_n_l_y _u_s_e_d, _s_h_e_l_l _f_e_a_t_u_r_e_s

_4._1. _L_o_o_p_s _a_t _t_h_e _t_e_r_m_i_n_a_l; _v_a_r_i_a_b_l_e_s _a_s _v_e_c_t_o_r_s

It is occasionally useful to use the _f_o_r_e_a_c_h control
structure at the terminal to aid in performing a number of
similar commands. For instance, there were at one point
three shells in use on the Cory UNIX system at Cory Hall,
`/bin/sh', `/bin/nsh', and `/bin/csh'. To count the number
of persons using each shell one could have issued the com-
mands

% grep -c csh$ /etc/passwd
27
% grep -c nsh$ /etc/passwd
128
% grep -c -v sh$ /etc/passwd
430
%

Since these commands are very similar we can use _f_o_r_e_a_c_h to
do this more easily.

% foreach i ('sh

'csh

'-v sh

)
? grep -c $i /etc/passwd
? end
27
128
430
%

Note here that the shell prompts for input with `? ' when
reading the body of the loop.

Very useful with loops are variables which contain
lists of filenames or other words. You can, for example, do

% set a=(`ls`)
% echo $a
csh.n csh.rm
% ls
csh.n
csh.rm
% echo $#a
2
%

The _s_e_t command here gave the variable _a a list of all the
filenames in the current directory as value. We can then
iterate over these names to perform any chosen function.

The output of a command within ``' characters is con-
verted by the shell to a list of words. You can also place
the ``' quoted string within `"' characters to take each

March 11, 1984

- 2 -

(non-empty) line as a component of the variable; preventing
the lines from being split into words at blanks and tabs. A
modifier `:x' exists which can be used later to expand each
component of the variable into another variable splitting it
into separate words at embedded blanks and tabs.

_4._2. _B_r_a_c_e_s { ... } _i_n _a_r_g_u_m_e_n_t _e_x_p_a_n_s_i_o_n

Another form of filename expansion, alluded to before
involves the characters `{' and `}'. These characters
specify that the contained strings, separated by `,' are to
be consecutively substituted into the containing characters
and the results expanded left to right. Thus

A{str1,str2,...strn}B

expands to

Astr1B Astr2B ... AstrnB

This expansion occurs before the other filename expansions,
and may be applied recursively (i.e. nested). The results
of each expanded string are sorted separately, left to right
order being preserved. The resulting filenames are not
required to exist if no other expansion mechanisms are used.
This means that this mechanism can be used to generate argu-
ments which are not filenames, but which have common parts.

A typical use of this would be

mkdir ~/{hdrs,retrofit,csh}

to make subdirectories `hdrs', `retrofit' and `csh' in your
home directory. This mechanism is most useful when the com-
mon prefix is longer than in this example, i.e.

chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

_4._3. _C_o_m_m_a_n_d _s_u_b_s_t_i_t_u_t_i_o_n

A command enclosed in ``' characters is replaced, just
before filenames are expanded, by the output from that com-
mand. Thus it is possible to do

set pwd=`pwd`

to save the current directory in the variable _p_w_d or to do

ex `grep -l TRACE *.c`

to run the editor _e_x supplying as arguments those files
whose names end in `.c' which have the string `TRACE' in
them.*
__________________________

Command expansion also occurs in input redirected with

March 11, 1984

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_4._4. _O_t_h_e_r _d_e_t_a_i_l_s _n_o_t _c_o_v_e_r_e_d _h_e_r_e

In particular circumstances it may be necessary to know
the exact nature and order of different substitutions per-
formed by the shell. The exact meaning of certain combina-
tions of quotations is also occasionally important. These
are detailed fully in its manual section.

The shell has a number of command line option flags
mostly of use in writing UNIX programs, and debugging shell
scripts. See the shells manual section for a list of these
options.

__________________________
`<<' and within `"' quotations. Refer to the shell
manual section for full details.

March 11, 1984

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_A_p_p_e_n_d_i_x - _S_p_e_c_i_a_l _c_h_a_r_a_c_t_e_r_s

The following table lists the special characters of _c_s_h and
the UNIX system, giving for each the section(s) in which it
is discussed. A number of these characters also have spe-
cial meaning in expressions. See the _c_s_h manual section for
a complete list.

Syntactic metacharacters

; 2.4 separates commands to be executed sequentially
| 1.5 separates commands in a pipeline
( ) 2.2,3.6 brackets expressions and variable values
& 2.5 follows commands to be executed without waiting for completion

Filename metacharacters

/ 1.6 separates components of a file's pathname
? 1.6 expansion character matching any single character
* 1.6 expansion character matching any sequence of characters
[ ] 1.6 expansion sequence matching any single character from a set
~ 1.6 used at the beginning of a filename to indicate home directories
{ } 4.2 used to specify groups of arguments with common parts

Quotation metacharacters

\ 1.7 prevents meta-meaning of following single character
' 1.7 prevents meta-meaning of a group of characters
" 4.3 like ', but allows variable and command expansion

Input/output metacharacters

< 1.5 indicates redirected input
> 1.3 indicates redirected output

Expansion/substitution metacharacters

$ 3.4 indicates variable substitution
! 2.3 indicates history substitution
: 3.6 precedes substitution modifiers
|^ 2.3 used in special forms of history substitution
` 4.3 indicates command substitution

Other metacharacters

# 1.3,3.6 begins scratch file names; indicates shell comments
- 1.2 prefixes option (flag) arguments to commands
% 2.6 prefixes job