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AWK(1P) POSIX Programmer's Manual AWK(1P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
awk -- pattern scanning and processing language
SYNOPSIS
awk [-F sepstring] [-v assignment]... program [argument...]
awk [-F sepstring] -f progfile [-f progfile]... [-v assignment]...
[argument...]
DESCRIPTION
The awk utility shall execute programs written in the awk
programming language, which is specialized for textual data
manipulation. An awk program is a sequence of patterns and
corresponding actions. When input is read that matches a pattern,
the action associated with that pattern is carried out.
Input shall be interpreted as a sequence of records. By default,
a record is a line, less its terminating <newline>, but this can
be changed by using the RS built-in variable. Each record of
input shall be matched in turn against each pattern in the
program. For each pattern matched, the associated action shall be
executed.
The awk utility shall interpret each input record as a sequence
of fields where, by default, a field is a string of non-<blank>
non-<newline> characters. This default <blank> and <newline>
field delimiter can be changed by using the FS built-in variable
or the -F sepstring option. The awk utility shall denote the
first field in a record $1, the second $2, and so on. The symbol
$0 shall refer to the entire record; setting any other field
causes the re-evaluation of $0. Assigning to $0 shall reset the
values of all other fields and the NF built-in variable.
OPTIONS
The awk utility shall conform to the Base Definitions volume of
POSIX.1-2017, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-F sepstring
Define the input field separator. This option shall be
equivalent to:
-v FS=sepstring
except that if -F sepstring and -v FS=sepstring are
both used, it is unspecified whether the FS assignment
resulting from -F sepstring is processed in command
line order or is processed after the last -v
FS=sepstring. See the description of the FS built-in
variable, and how it is used, in the EXTENDED
DESCRIPTION section.
-f progfile
Specify the pathname of the file progfile containing an
awk program. A pathname of '-' shall denote the
standard input. If multiple instances of this option
are specified, the concatenation of the files specified
as progfile in the order specified shall be the awk
program. The awk program can alternatively be specified
in the command line as a single argument.
-v assignment
The application shall ensure that the assignment
argument is in the same form as an assignment operand.
The specified variable assignment shall occur prior to
executing the awk program, including the actions
associated with BEGIN patterns (if any). Multiple
occurrences of this option can be specified.
OPERANDS
The following operands shall be supported:
program If no -f option is specified, the first operand to awk
shall be the text of the awk program. The application
shall supply the program operand as a single argument
to awk. If the text does not end in a <newline>, awk
shall interpret the text as if it did.
argument Either of the following two types of argument can be
intermixed:
file A pathname of a file that contains the input
to be read, which is matched against the set
of patterns in the program. If no file
operands are specified, or if a file operand
is '-', the standard input shall be used.
assignment
An operand that begins with an <underscore>
or alphabetic character from the portable
character set (see the table in the Base
Definitions volume of POSIX.1-2017, Section
6.1, Portable Character Set), followed by a
sequence of underscores, digits, and
alphabetics from the portable character set,
followed by the '=' character, shall specify
a variable assignment rather than a pathname.
The characters before the '=' represent the
name of an awk variable; if that name is an
awk reserved word (see Grammar) the behavior
is undefined. The characters following the
<equals-sign> shall be interpreted as if they
appeared in the awk program preceded and
followed by a double-quote ('"') character,
as a STRING token (see Grammar), except that
if the last character is an unescaped
<backslash>, it shall be interpreted as a
literal <backslash> rather than as the first
character of the sequence "\"". The variable
shall be assigned the value of that STRING
token and, if appropriate, shall be
considered a numeric string (see Expressions
in awk), the variable shall also be assigned
its numeric value. Each such variable
assignment shall occur just prior to the
processing of the following file, if any.
Thus, an assignment before the first file
argument shall be executed after the BEGIN
actions (if any), while an assignment after
the last file argument shall occur before the
END actions (if any). If there are no file
arguments, assignments shall be executed
before processing the standard input.
STDIN
The standard input shall be used only if no file operands are
specified, or if a file operand is '-', or if a progfile option-
argument is '-'; see the INPUT FILES section. If the awk program
contains no actions and no patterns, but is otherwise a valid awk
program, standard input and any file operands shall not be read
and awk shall exit with a return status of zero.
INPUT FILES
Input files to the awk program from any of the following sources
shall be text files:
* Any file operands or their equivalents, achieved by modifying
the awk variables ARGV and ARGC
* Standard input in the absence of any file operands
* Arguments to the getline function
Whether the variable RS is set to a value other than a <newline>
or not, for these files, implementations shall support records
terminated with the specified separator up to {LINE_MAX} bytes
and may support longer records.
If -f progfile is specified, the application shall ensure that
the files named by each of the progfile option-arguments are text
files and their concatenation, in the same order as they appear
in the arguments, is an awk program.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of
awk:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base
Definitions volume of POSIX.1-2017, Section 8.2,
Internationalization Variables for the precedence of
internationalization variables used to determine the
values of locale categories.)
LC_ALL If set to a non-empty string value, override the values
of all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges,
equivalence classes, and multi-character collating
elements within regular expressions and in comparisons
of string values.
LC_CTYPE Determine the locale for the interpretation of
sequences of bytes of text data as characters (for
example, single-byte as opposed to multi-byte
characters in arguments and input files), the behavior
of character classes within regular expressions, the
identification of characters as letters, and the
mapping of uppercase and lowercase characters for the
toupper and tolower functions.
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
LC_NUMERIC
Determine the radix character used when interpreting
numeric input, performing conversions between numeric
and string values, and formatting numeric output.
Regardless of locale, the <period> character (the
decimal-point character of the POSIX locale) is the
decimal-point character recognized in processing awk
programs (including assignments in command line
arguments).
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
PATH Determine the search path when looking for commands
executed by system(expr), or input and output pipes;
see the Base Definitions volume of POSIX.1-2017,
Chapter 8, Environment Variables.
In addition, all environment variables shall be visible via the
awk variable ENVIRON.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The nature of the output files depends on the awk program.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
The nature of the output files depends on the awk program.
EXTENDED DESCRIPTION
Overall Program Structure
An awk program is composed of pairs of the form:
pattern { action }
Either the pattern or the action (including the enclosing brace
characters) can be omitted.
A missing pattern shall match any record of input, and a missing
action shall be equivalent to:
{ print }
Execution of the awk program shall start by first executing the
actions associated with all BEGIN patterns in the order they
occur in the program. Then each file operand (or standard input
if no files were specified) shall be processed in turn by reading
data from the file until a record separator is seen (<newline> by
default). Before the first reference to a field in the record is
evaluated, the record shall be split into fields, according to
the rules in Regular Expressions, using the value of FS that was
current at the time the record was read. Each pattern in the
program then shall be evaluated in the order of occurrence, and
the action associated with each pattern that matches the current
record executed. The action for a matching pattern shall be
executed before evaluating subsequent patterns. Finally, the
actions associated with all END patterns shall be executed in the
order they occur in the program.
Expressions in awk
Expressions describe computations used in patterns and actions.
In the following table, valid expression operations are given in
groups from highest precedence first to lowest precedence last,
with equal-precedence operators grouped between horizontal lines.
In expression evaluation, where the grammar is formally
ambiguous, higher precedence operators shall be evaluated before
lower precedence operators. In this table expr, expr1, expr2, and
expr3 represent any expression, while lvalue represents any
entity that can be assigned to (that is, on the left side of an
assignment operator). The precise syntax of expressions is given
in Grammar.
Table 4-1: Expressions in Decreasing Precedence in awk
+---------------------+-------------------------+----------------+--------------+
| Syntax | Name | Type of Result |Associativity |
+---------------------+-------------------------+----------------+--------------+
|( expr ) |Grouping |Type of expr |N/A |
+---------------------+-------------------------+----------------+--------------+
|$expr |Field reference |String |N/A |
+---------------------+-------------------------+----------------+--------------+
|lvalue ++ |Post-increment |Numeric |N/A |
|lvalue -- |Post-decrement |Numeric |N/A |
+---------------------+-------------------------+----------------+--------------+
|++ lvalue |Pre-increment |Numeric |N/A |
|-- lvalue |Pre-decrement |Numeric |N/A |
+---------------------+-------------------------+----------------+--------------+
|expr ^ expr |Exponentiation |Numeric |Right |
+---------------------+-------------------------+----------------+--------------+
|! expr |Logical not |Numeric |N/A |
|+ expr |Unary plus |Numeric |N/A |
|- expr |Unary minus |Numeric |N/A |
+---------------------+-------------------------+----------------+--------------+
|expr * expr |Multiplication |Numeric |Left |
|expr / expr |Division |Numeric |Left |
|expr % expr |Modulus |Numeric |Left |
+---------------------+-------------------------+----------------+--------------+
|expr + expr |Addition |Numeric |Left |
|expr - expr |Subtraction |Numeric |Left |
+---------------------+-------------------------+----------------+--------------+
|expr expr |String concatenation |String |Left |
+---------------------+-------------------------+----------------+--------------+
|expr < expr |Less than |Numeric |None |
|expr <= expr |Less than or equal to |Numeric |None |
|expr != expr |Not equal to |Numeric |None |
|expr == expr |Equal to |Numeric |None |
|expr > expr |Greater than |Numeric |None |
|expr >= expr |Greater than or equal to |Numeric |None |
+---------------------+-------------------------+----------------+--------------+
|expr ~ expr |ERE match |Numeric |None |
|expr !~ expr |ERE non-match |Numeric |None |
+---------------------+-------------------------+----------------+--------------+
|expr in array |Array membership |Numeric |Left |
|( index ) in array |Multi-dimension array |Numeric |Left |
| |membership | | |
+---------------------+-------------------------+----------------+--------------+
|expr && expr |Logical AND |Numeric |Left |
+---------------------+-------------------------+----------------+--------------+
|expr || expr |Logical OR |Numeric |Left |
+---------------------+-------------------------+----------------+--------------+
|expr1 ? expr2 : expr3|Conditional expression |Type of selected|Right |
| | |expr2 or expr3 | |
+---------------------+-------------------------+----------------+--------------+
|lvalue ^= expr |Exponentiation assignment|Numeric |Right |
|lvalue %= expr |Modulus assignment |Numeric |Right |
|lvalue *= expr |Multiplication assignment|Numeric |Right |
|lvalue /= expr |Division assignment |Numeric |Right |
|lvalue += expr |Addition assignment |Numeric |Right |
|lvalue -= expr |Subtraction assignment |Numeric |Right |
|lvalue = expr |Assignment |Type of expr |Right |
+---------------------+-------------------------+----------------+--------------+
Each expression shall have either a string value, a numeric
value, or both. Except as stated for specific contexts, the value
of an expression shall be implicitly converted to the type needed
for the context in which it is used. A string value shall be
converted to a numeric value either by the equivalent of the
following calls to functions defined by the ISO C standard:
setlocale(LC_NUMERIC, "");
numeric_value = atof(string_value);
or by converting the initial portion of the string to type double
representation as follows:
The input string is decomposed into two parts: an initial,
possibly empty, sequence of white-space characters (as
specified by isspace()) and a subject sequence interpreted
as a floating-point constant.
The expected form of the subject sequence is an optional
'+' or '-' sign, then a non-empty sequence of digits
optionally containing a <period>, then an optional
exponent part. An exponent part consists of 'e' or 'E',
followed by an optional sign, followed by one or more
decimal digits.
The sequence starting with the first digit or the <period>
(whichever occurs first) is interpreted as a floating
constant of the C language, and if neither an exponent
part nor a <period> appears, a <period> is assumed to
follow the last digit in the string. If the subject
sequence begins with a <hyphen-minus>, the value resulting
from the conversion is negated.
A numeric value that is exactly equal to the value of an integer
(see Section 1.1.2, Concepts Derived from the ISO C Standard)
shall be converted to a string by the equivalent of a call to the
sprintf function (see String Functions) with the string "%d" as
the fmt argument and the numeric value being converted as the
first and only expr argument. Any other numeric value shall be
converted to a string by the equivalent of a call to the sprintf
function with the value of the variable CONVFMT as the fmt
argument and the numeric value being converted as the first and
only expr argument. The result of the conversion is unspecified
if the value of CONVFMT is not a floating-point format
specification. This volume of POSIX.1-2017 specifies no explicit
conversions between numbers and strings. An application can force
an expression to be treated as a number by adding zero to it, or
can force it to be treated as a string by concatenating the null
string ("") to it.
A string value shall be considered a numeric string if it comes
from one of the following:
1. Field variables
2. Input from the getline() function
3. FILENAME
4. ARGV array elements
5. ENVIRON array elements
6. Array elements created by the split() function
7. A command line variable assignment
8. Variable assignment from another numeric string variable
and an implementation-dependent condition corresponding to either
case (a) or (b) below is met.
a. After the equivalent of the following calls to functions
defined by the ISO C standard, string_value_end would differ
from string_value, and any characters before the terminating
null character in string_value_end would be <blank>
characters:
char *string_value_end;
setlocale(LC_NUMERIC, "");
numeric_value = strtod (string_value, &string_value_end);
b. After all the following conversions have been applied, the
resulting string would lexically be recognized as a NUMBER
token as described by the lexical conventions in Grammar:
-- All leading and trailing <blank> characters are
discarded.
-- If the first non-<blank> is '+' or '-', it is discarded.
-- Each occurrence of the decimal point character from the
current locale is changed to a <period>.
In case (a) the numeric value of the numeric string shall be the
value that would be returned by the strtod() call. In case (b) if
the first non-<blank> is '-', the numeric value of the numeric
string shall be the negation of the numeric value of the
recognized NUMBER token; otherwise, the numeric value of the
numeric string shall be the numeric value of the recognized
NUMBER token. Whether or not a string is a numeric string shall
be relevant only in contexts where that term is used in this
section.
When an expression is used in a Boolean context, if it has a
numeric value, a value of zero shall be treated as false and any
other value shall be treated as true. Otherwise, a string value
of the null string shall be treated as false and any other value
shall be treated as true. A Boolean context shall be one of the
following:
* The first subexpression of a conditional expression
* An expression operated on by logical NOT, logical AND, or
logical OR
* The second expression of a for statement
* The expression of an if statement
* The expression of the while clause in either a while or
do...while statement
* An expression used as a pattern (as in Overall Program
Structure)
All arithmetic shall follow the semantics of floating-point
arithmetic as specified by the ISO C standard (see Section 1.1.2,
Concepts Derived from the ISO C Standard).
The value of the expression:
expr1 ^ expr2
shall be equivalent to the value returned by the ISO C standard
function call:
pow(expr1, expr2)
The expression:
lvalue ^= expr
shall be equivalent to the ISO C standard expression:
lvalue = pow(lvalue, expr)
except that lvalue shall be evaluated only once. The value of the
expression:
expr1 % expr2
shall be equivalent to the value returned by the ISO C standard
function call:
fmod(expr1, expr2)
The expression:
lvalue %= expr
shall be equivalent to the ISO C standard expression:
lvalue = fmod(lvalue, expr)
except that lvalue shall be evaluated only once.
Variables and fields shall be set by the assignment statement:
lvalue = expression
and the type of expression shall determine the resulting variable
type. The assignment includes the arithmetic assignments ("+=",
"-=", "*=", "/=", "%=", "^=", "++", "--") all of which shall
produce a numeric result. The left-hand side of an assignment and
the target of increment and decrement operators can be one of a
variable, an array with index, or a field selector.
The awk language supplies arrays that are used for storing
numbers or strings. Arrays need not be declared. They shall
initially be empty, and their sizes shall change dynamically. The
subscripts, or element identifiers, are strings, providing a type
of associative array capability. An array name followed by a
subscript within square brackets can be used as an lvalue and
thus as an expression, as described in the grammar; see Grammar.
Unsubscripted array names can be used in only the following
contexts:
* A parameter in a function definition or function call
* The NAME token following any use of the keyword in as
specified in the grammar (see Grammar); if the name used in
this context is not an array name, the behavior is undefined
A valid array index shall consist of one or more
<comma>-separated expressions, similar to the way in which multi-
dimensional arrays are indexed in some programming languages.
Because awk arrays are really one-dimensional, such a
<comma>-separated list shall be converted to a single string by
concatenating the string values of the separate expressions, each
separated from the other by the value of the SUBSEP variable.
Thus, the following two index operations shall be equivalent:
var[expr1, expr2, ... exprn]
var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]
The application shall ensure that a multi-dimensioned index used
with the in operator is parenthesized. The in operator, which
tests for the existence of a particular array element, shall not
cause that element to exist. Any other reference to a nonexistent
array element shall automatically create it.
Comparisons (with the '<', "<=", "!=", "==", '>', and ">="
operators) shall be made numerically if both operands are
numeric, if one is numeric and the other has a string value that
is a numeric string, or if one is numeric and the other has the
uninitialized value. Otherwise, operands shall be converted to
strings as required and a string comparison shall be made as
follows:
* For the "!=" and "==" operators, the strings should be
compared to check if they are identical but may be compared
using the locale-specific collation sequence to check if they
collate equally.
* For the other operators, the strings shall be compared using
the locale-specific collation sequence.
The value of the comparison expression shall be 1 if the relation
is true, or 0 if the relation is false.
Variables and Special Variables
Variables can be used in an awk program by referencing them. With
the exception of function parameters (see User-Defined
Functions), they are not explicitly declared. Function parameter
names shall be local to the function; all other variable names
shall be global. The same name shall not be used as both a
function parameter name and as the name of a function or a
special awk variable. The same name shall not be used both as a
variable name with global scope and as the name of a function.
The same name shall not be used within the same scope both as a
scalar variable and as an array. Uninitialized variables,
including scalar variables, array elements, and field variables,
shall have an uninitialized value. An uninitialized value shall
have both a numeric value of zero and a string value of the empty
string. Evaluation of variables with an uninitialized value, to
either string or numeric, shall be determined by the context in
which they are used.
Field variables shall be designated by a '