usr.bin/awk/awk.1

.\"	$OpenBSD: awk.1,v 1.44 2015/09/14 20:06:58 schwarze Exp $
.\"
.\" Copyright (C) Lucent Technologies 1997
.\" All Rights Reserved
.\"
.\" Permission to use, copy, modify, and distribute this software and
.\" its documentation for any purpose and without fee is hereby
.\" granted, provided that the above copyright notice appear in all
.\" copies and that both that the copyright notice and this
.\" permission notice and warranty disclaimer appear in supporting
.\" documentation, and that the name Lucent Technologies or any of
.\" its entities not be used in advertising or publicity pertaining
.\" to distribution of the software without specific, written prior
.\" permission.
.\"
.\" LUCENT DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
.\" INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS.
.\" IN NO EVENT SHALL LUCENT OR ANY OF ITS ENTITIES BE LIABLE FOR ANY
.\" SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER
.\" IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
.\" ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
.\" THIS SOFTWARE.
.Dd September 3, 2025
.Dt AWK 1
.Os
.Sh NAME
.Nm awk
.Nd pattern-directed scanning and processing language
.Sh SYNOPSIS
.Nm awk
.Op Fl safe
.Op Fl version
.Op Fl d Ns Op Ar n
.Op Fl F Ar fs | Fl -csv
.Op Fl v Ar var Ns = Ns Ar value
.Op Ar prog | Fl f Ar progfile
.Ar
.Sh DESCRIPTION
.Nm
scans each input
.Ar file
for lines that match any of a set of patterns specified literally in
.Ar prog
or in one or more files
specified as
.Fl f Ar progfile .
With each pattern
there can be an associated action that will be performed
when a line of a
.Ar file
matches the pattern.
Each line is matched against the
pattern portion of every pattern-action statement;
the associated action is performed for each matched pattern.
The file name
.Sq -
means the standard input.
Any
.Ar file
of the form
.Ar var Ns = Ns Ar value
is treated as an assignment, not a filename,
and is executed at the time it would have been opened if it were a filename.
.Pp
The options are as follows:
.Bl -tag -width "-safe "
.It Fl d Ns Op Ar n
Debug mode.
Set debug level to
.Ar n ,
or 1 if
.Ar n
is not specified.
A value greater than 1 causes
.Nm
to dump core on fatal errors.
.It Fl F Ar fs
Define the input field separator to be the regular expression
.Ar fs .
.It Fl -csv
causes
.Nm
to process records using (more or less) standard comma-separated values
(CSV) format.
.It Fl f Ar progfile
Read program code from the specified file
.Ar progfile
instead of from the command line.
.It Fl safe
Disable file output
.Pf ( Ic print No > ,
.Ic print No >> ) ,
process creation
.Po
.Ar cmd | Ic getline ,
.Ic print | ,
.Ic system
.Pc
and access to the environment
.Pf ( Va ENVIRON ;
see the section on variables below).
This is a first
.Pq and not very reliable
approximation to a
.Dq safe
version of
.Nm .
.It Fl version
Print the version number of
.Nm
to standard output and exit.
.It Fl v Ar var Ns = Ns Ar value
Assign
.Ar value
to variable
.Ar var
before
.Ar prog
is executed;
any number of
.Fl v
options may be present.
.El
.Pp
The input is normally made up of input lines
.Pq records
separated by newlines, or by the value of
.Va RS .
If
.Va RS
is null, then any number of blank lines are used as the record separator,
and newlines are used as field separators
(in addition to the value of
.Va FS ) .
This is convenient when working with multi-line records.
.Pp
An input line is normally made up of fields separated by whitespace,
or by the extended regular expression
.Va FS
as described below.
The fields are denoted
.Va $1 , $2 , ... ,
while
.Va $0
refers to the entire line.
If
.Va FS
is null, the input line is split into one field per character.
While both gawk and mawk have the same behavior, it is unspecified in the
.St -p1003.1-2008
standard.
If
.Va FS
is a single space, then leading and trailing blank and newline characters are
skipped.
Fields are delimited by one or more blank or newline characters.
A blank character is a space or a tab.
If
.Va FS
is a single character, other than space, fields are delimited by each single
occurrence of that character.
The
.Va FS
variable defaults to a single space.
.Pp
Normally, any number of blanks separate fields.
In order to set the field separator to a single blank, use the
.Fl F
option with a value of
.Sq [\ \&] .
If a field separator of
.Sq t
is specified,
.Nm
treats it as if
.Sq \et
had been specified and uses
.Aq TAB
as the field separator.
In order to use a literal
.Sq t
as the field separator, use the
.Fl F
option with a value of
.Sq [t] .
.Pp
A pattern-action statement has the form:
.Pp
.D1 Ar pattern Ic \&{ Ar action Ic \&}
.Pp
A missing
.Ic \&{ Ar action Ic \&}
means print the line;
a missing pattern always matches.
Pattern-action statements are separated by newlines or semicolons.
.Pp
Newlines are permitted after a terminating statement or following a comma
.Pq Sq ,\& ,
an open brace
.Pq Sq { ,
a logical AND
.Pq Sq && ,
a logical OR
.Pq Sq || ,
after the
.Sq do
or
.Sq else
keywords,
or after the closing parenthesis of an
.Sq if ,
.Sq for ,
or
.Sq while
statement.
Additionally, a backslash
.Pq Sq \e
can be used to escape a newline between tokens.
.Pp
An action is a sequence of statements.
A statement can be one of the following:
.Pp
.Bl -tag -width Ds -offset indent -compact
.It Ic if Ar ( expression ) Ar statement Op Ic else Ar statement
.It Ic while Ar ( expression ) Ar statement
.It Ic for Ar ( expression ; expression ; expression ) statement
.It Ic for Ar ( var Ic in Ar array ) statement
.It Ic do Ar statement Ic while Ar ( expression )
.It Ic break
.It Ic continue
.It Xo Ic {
.Op Ar statement ...
.Ic }
.Xc
.It Xo Ar expression
.No # commonly
.Ar var No = Ar expression
.Xc
.It Xo Ic print
.Op Ar expression-list
.Op > Ns Ar expression
.Xc
.It Xo Ic printf Ar format
.Op Ar ... , expression-list
.Op > Ns Ar expression
.Xc
.It Ic return Op Ar expression
.It Xo Ic next
.No # skip remaining patterns on this input line
.Xc
.It Xo Ic nextfile
.No # skip rest of this file, open next, start at top
.Xc
.It Xo Ic delete
.Sm off
.Ar array Ic \&[ Ar expression Ic \&]
.Sm on
.No # delete an array element
.Xc
.It Xo Ic delete Ar array
.No # delete all elements of array
.Xc
.It Xo Ic exit
.Op Ar expression
.No # exit immediately; status is Ar expression
.Xc
.El
.Pp
Statements are terminated by
semicolons, newlines or right braces.
An empty
.Ar expression-list
stands for
.Ar $0 .
String constants are quoted
.Li \&"" ,
with the usual C escapes recognized within
(see
.Xr printf 1
for a complete list of these).
Expressions take on string or numeric values as appropriate,
and are built using the operators
.Ic + \- * / % ^
.Pq exponentiation ,
and concatenation
.Pq indicated by whitespace .
The operators
.Ic \&! ++ \-\- += \-= *= /= %= ^=
.Ic > >= < <= == != ?\&:
are also available in expressions.
Variables may be scalars, array elements
(denoted
.Li x[i] )
or fields.
Variables are initialized to the null string.
Array subscripts may be any string,
not necessarily numeric;
this allows for a form of associative memory.
Multiple subscripts such as
.Li [i,j,k]
are permitted; the constituents are concatenated,
separated by the value of
.Va SUBSEP
.Pq see the section on variables below .
.Pp
The
.Ic print
statement prints its arguments on the standard output
(or on a file if
.Pf > Ar file
or
.Pf >> Ar file
is present or on a pipe if
.Pf |\ \& Ar cmd
is present), separated by the current output field separator,
and terminated by the output record separator.
.Ar file
and
.Ar cmd
may be literal names or parenthesized expressions;
identical string values in different statements denote
the same open file.
The
.Ic printf
statement formats its expression list according to the format
(see
.Xr printf 1 ) .
.Pp
Patterns are arbitrary Boolean combinations
(with
.Ic "\&! || &&" )
of regular expressions and
relational expressions.
.Nm
supports extended regular expressions
.Pq EREs .
See
.Xr re_format 7
for more information on regular expressions.
Isolated regular expressions
in a pattern apply to the entire line.
Regular expressions may also occur in
relational expressions, using the operators
.Ic ~
and
.Ic !~ .
.Pf / Ar re Ns /
is a constant regular expression;
any string (constant or variable) may be used
as a regular expression, except in the position of an isolated regular expression
in a pattern.
.Pp
A pattern may consist of two patterns separated by a comma;
in this case, the action is performed for all lines
from an occurrence of the first pattern
through an occurrence of the second, inclusive.
.Pp
A relational expression is one of the following:
.Pp
.Bl -tag -width Ds -offset indent -compact
.It Ar expression matchop regular-expression
.It Ar expression relop expression
.It Ar expression Ic in Ar array-name
.It Xo Ic \&( Ns
.Ar expr , expr , \&... Ns Ic \&) in
.Ar array-name
.Xc
.El
.Pp
where a
.Ar relop
is any of the six relational operators in C,
and a
.Ar matchop
is either
.Ic ~
(matches)
or
.Ic !~
(does not match).
A conditional is an arithmetic expression,
a relational expression,
or a Boolean combination
of these.
.Pp
The special patterns
.Ic BEGIN
and
.Ic END
may be used to capture control before the first input line is read
and after the last.
.Ic BEGIN
and
.Ic END
do not combine with other patterns.
They may appear multiple times in a program and execute
in the order they are read by
.Nm
.Pp
Variable names with special meanings:
.Pp
.Bl -tag -width "FILENAME " -compact
.It Va ARGC
Argument count, assignable.
.It Va ARGV
Argument array, assignable;
non-null members are taken as filenames.
.It Va CONVFMT
Conversion format when converting numbers
(default
.Qq Li %.6g ) .
.It Va ENVIRON
Array of environment variables; subscripts are names.
.It Va FILENAME
The name of the current input file.
.It Va FNR
Ordinal number of the current record in the current file.
.It Va FS
Regular expression used to separate fields; also settable
by option
.Fl F Ar fs .
.It Va NF
Number of fields in the current record.
.Va $NF
can be used to obtain the value of the last field in the current record.
.It Va NR
Ordinal number of the current record.
.It Va OFMT
Output format for numbers (default
.Qq Li %.6g ) .
.It Va OFS
Output field separator (default blank).
.It Va ORS
Output record separator (default newline).
.It Va RLENGTH
The length of the string matched by the
.Fn match
function.
.It Va RS
Input record separator (default newline).
If empty, blank lines separate records.
If more than one character long,
.Va RS
is treated as a regular expression, and records are
separated by text matching the expression.
.It Va RSTART
The starting position of the string matched by the
.Fn match
function.
.It Va SUBSEP
Separates multiple subscripts (default 034).
.El
.Sh FUNCTIONS
The awk language has a variety of built-in functions:
arithmetic, string, input/output, general, and bit-operation.
.Pp
Functions may be defined (at the position of a pattern-action statement)
thusly:
.Pp
.Dl function foo(a, b, c) { ...; return x }
.Pp
Parameters are passed by value if scalar, and by reference if array name;
functions may be called recursively.
Parameters are local to the function; all other variables are global.
Thus local variables may be created by providing excess parameters in
the function definition.
.Ss Arithmetic Functions
.Bl -tag -width "atan2(y, x)"
.It Fn atan2 y x
Return the arctangent of
.Fa y Ns / Ns Fa x
in radians.
.It Fn cos x
Return the cosine of
.Fa x ,
where
.Fa x
is in radians.
.It Fn exp x
Return the exponential of
.Fa x .
.It Fn int x
Return
.Fa x
truncated to an integer value.
.It Fn log x
Return the natural logarithm of
.Fa x .
.It Fn rand
Return a random number,
.Fa n ,
such that
.Sm off
.Pf 0 \*(Le Fa n No \*(Lt 1 .
.Sm on
.It Fn sin x
Return the sine of
.Fa x ,
where
.Fa x
is in radians.
.It Fn sqrt x
Return the square root of
.Fa x .
.It Fn srand expr
Sets seed for
.Fn rand
to
.Fa expr
and returns the previous seed.
If
.Fa expr
is omitted, the time of day is used instead.
.El
.Ss String Functions
.Bl -tag -width "split(s, a, fs)"
.It Fn gsub r t s
The same as
.Fn sub
except that all occurrences of the regular expression are replaced.
.Fn gsub
returns the number of replacements.
.It Fn index s t
The position in
.Fa s
where the string
.Fa t
occurs, or 0 if it does not.
.It Fn length s
The length of
.Fa s
taken as a string,
number of elements in an array for an array argument,
or length of
.Va $0
if no argument is given.
.It Fn match s r
The position in
.Fa s
where the regular expression
.Fa r
occurs, or 0 if it does not.
The variable
.Va RSTART
is set to the starting position of the matched string
.Pq which is the same as the returned value
or zero if no match is found.
The variable
.Va RLENGTH
is set to the length of the matched string,
or \-1 if no match is found.
.It Fn split s a fs
Splits the string
.Fa s
into array elements
.Va a[1] , a[2] , ... , a[n]
and returns
.Va n .
The separation is done with the regular expression
.Ar fs
or with the field separator
.Va FS
if
.Ar fs
is not given.
An empty string as field separator splits the string
into one array element per character.
.It Fn sprintf fmt expr ...
The string resulting from formatting
.Fa expr , ...
according to the
.Xr printf 1
format
.Fa fmt .
.It Fn sub r t s
Substitutes
.Fa t
for the first occurrence of the regular expression
.Fa r
in the string
.Fa s .
If
.Fa s
is not given,
.Va $0
is used.
An ampersand
.Pq Sq &
in
.Fa t
is replaced in string
.Fa s
with regular expression
.Fa r .
A literal ampersand can be specified by preceding it with two backslashes
.Pq Sq \e\e .
A literal backslash can be specified by preceding it with another backslash
.Pq Sq \e\e .
.Fn sub
returns the number of replacements.
.It Fn substr s m n
Return at most the
.Fa n Ns -character
substring of
.Fa s
that begins at position
.Fa m
counted from 1.
If
.Fa n
is omitted, or if
.Fa n
specifies more characters than are left in the string,
the length of the substring is limited by the length of
.Fa s .
.It Fn tolower str
Returns a copy of
.Fa str
with all upper-case characters translated to their
corresponding lower-case equivalents.
.It Fn toupper str
Returns a copy of
.Fa str
with all lower-case characters translated to their
corresponding upper-case equivalents.
.El
.Ss Input/Output and General Functions
.Bl -tag -width "getline [var] < file"
.It Fn close expr
Closes the file or pipe
.Fa expr .
.Fa expr
should match the string that was used to open the file or pipe.
.It Ar cmd | Ic getline Op Va var
Read a record of input from a stream piped from the output of
.Ar cmd .
If
.Va var
is omitted, the variables
.Va $0
and
.Va NF
are set.
Otherwise
.Va var
is set.
If the stream is not open, it is opened.
As long as the stream remains open, subsequent calls
will read subsequent records from the stream.
The stream remains open until explicitly closed with a call to
.Fn close .
.Ic getline
returns 1 for a successful input, 0 for end of file, and \-1 for an error.
.It Fn fflush [expr]
Flushes any buffered output for the file or pipe
.Fa expr ,
or all open files or pipes if
.Fa expr
is omitted.
.Fa expr
should match the string that was used to open the file or pipe.
.It Ic getline
Sets
.Va $0
to the next input record from the current input file.
This form of
.Ic getline
sets the variables
.Va NF ,
.Va NR ,
and
.Va FNR .
.Ic getline
returns 1 for a successful input, 0 for end of file, and \-1 for an error.
.It Ic getline Va var
Sets
.Va $0
to variable
.Va var .
This form of
.Ic getline
sets the variables
.Va NR
and
.Va FNR .
.Ic getline
returns 1 for a successful input, 0 for end of file, and \-1 for an error.
.It Xo
.Ic getline Op Va var
.Pf \ \&< Ar file
.Xc
Sets
.Va $0
to the next record from
.Ar file .
If
.Va var
is omitted, the variables
.Va $0
and
.Va NF
are set.
Otherwise
.Va var
is set.
If
.Ar file
is not open, it is opened.
As long as the stream remains open, subsequent calls will read subsequent
records from
.Ar file .
.Ar file
remains open until explicitly closed with a call to
.Fn close .
.It Fn systime
returns the current date and time as a standard
.Dq seconds since the epoch
value.
.It Fn strftime fmt timestamp
formats
.Fa timestamp
(a value in seconds since the epoch)
according to
Fa fmt ,
which is a format string as supported by
.Xr strftime 3 .
Both
.Fa timestamp
and
.Fa fmt
may be omitted; if no
.Fa timestamp ,
the current time of day is used, and if no
.Fa fmt ,
a default format of
.Dq %a %b %e %H:%M:%S %Z %Y
is used.
.It Fn system cmd
Executes
.Fa cmd
and returns its exit status.
This will be -1 upon error,
.Fa cmd 's
exit status upon a normal exit,
256 +
.Va sig
upon death-by-signal, where
.Va sig
is the number of the murdering signal,
or 512 +
.Va sig
if there was a core dump.
.El
.Ss Bit-Operation Functions
.Bl -tag -width "lshift(a, b)"
.It Fn compl x
Returns the bitwise complement of integer argument x.
.It Fn and v1 v2 ...
Performs a bitwise AND on all arguments provided, as integers.
There must be at least two values.
.It Fn or v1 v2 ...
Performs a bitwise OR on all arguments provided, as integers.
There must be at least two values.
.It Fn xor v1 v2 ...
Performs a bitwise Exclusive-OR on all arguments provided, as integers.
There must be at least two values.
.It Fn lshift x n
Returns integer argument x shifted by n bits to the left.
.It Fn rshift x n
Returns integer argument x shifted by n bits to the right.
.El
.Sh EXIT STATUS
.Ex -std awk
.Pp
But note that the
.Ic exit
expression can modify the exit status.
.Sh ENVIRONMENT VARIABLES
If
.Va POSIXLY_CORRECT
is set in the environment, then
.Nm
follows the POSIX rules for
.Fn sub
and
.Fn gsub
with respect to consecutive backslashes and ampersands.
.Sh EXAMPLES
Print lines longer than 72 characters:
.Pp
.Dl length($0) > 72
.Pp
Print first two fields in opposite order:
.Pp
.Dl { print $2, $1 }
.Pp
Same, with input fields separated by comma and/or spaces and tabs:
.Bd -literal -offset indent
BEGIN { FS = ",[ \et]*|[ \et]+" }
      { print $2, $1 }
.Ed
.Pp
Add up first column, print sum and average:
.Bd -literal -offset indent
{ s += $1 }
END { print "sum is", s, " average is", s/NR }
.Ed
.Pp
Print all lines between start/stop pairs:
.Pp
.Dl /start/, /stop/
.Pp
Simulate echo(1):
.Bd -literal -offset indent
BEGIN { # Simulate echo(1)
        for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
        printf "\en"
        exit }
.Ed
.Pp
Print an error message to standard error:
.Bd -literal -offset indent
{ print "error!" > "/dev/stderr" }
.Ed
.Sh SEE ALSO
.Xr cut 1 ,
.Xr lex 1 ,
.Xr printf 1 ,
.Xr sed 1 ,
.Xr re_format 7
.Rs
.%A A. V. Aho
.%A B. W. Kernighan
.%A P. J. Weinberger
.%T The AWK Programming Language
.%I Addison-Wesley
.%D 1988
.%O ISBN 0-201-07981-X
.Re
.Sh STANDARDS
The
.Nm
utility is compliant with the
.St -p1003.1-2008
specification,
except
.Nm
does not support {n,m} pattern matching.
.Pp
The flags
.Fl d ,
.Fl safe ,
and
.Fl version
as well as the commands
.Cm fflush , compl , and , or ,
.Cm xor , lshift , rshift ,
are extensions to that specification.
.Sh HISTORY
An
.Nm
utility appeared in
.At v7 .
.Sh BUGS
There are no explicit conversions between numbers and strings.
To force an expression to be treated as a number add 0 to it;
to force it to be treated as a string concatenate
.Li \&""
to it.
.Pp
The scope rules for variables in functions are a botch;
the syntax is worse.
.Pp
Input is expected to be UTF-8 encoded.
Other multibyte character sets are not handled.
However, in eight-bit locales,
.Nm
treats each input byte as a separate character.
.Sh UNUSUAL FLOATING-POINT VALUES
.Nm
was designed before IEEE 754 arithmetic defined Not-A-Number (NaN)
and Infinity values, which are supported by all modern floating-point
hardware.
.Pp
Because
.Nm
uses
.Xr strtod 3
and
.Xr atof 3
to convert string values to double-precision floating-point values,
modern C libraries also convert strings starting with
.Va inf
and
.Va nan
into infinity and NaN values respectively.
This led to strange results,
with something like this:
.Bd -literal -offset indent
echo nancy | awk '{ print $1 + 0 }'
.Ed
.Pp
printing
.Dq nan
instead of zero.
.Pp
.Nm
now follows GNU AWK, and prefilters string values before attempting
to convert them to numbers, as follows:
.Bl -tag -width "Hexadecimal values"
.It Hexadecimal values
Hexadecimal values (allowed since C99) convert to zero, as they did
prior to C99.
.It NaN values
The two strings
.Dq +nan
and
.Dq -nan
(case independent) convert to NaN.
No others do.
(NaNs can have signs.)
.It Infinity values
The two strings
.Dq +inf
and
.Dq -inf
(case independent) convert to positive and negative infinity, respectively.
No others do.
.El
.Sh DEPRECATED BEHAVIOR
One True Awk has accepted
.Fl F Ar t
to mean the same as
.Fl F Ar <TAB>
to make it easier to specify tabs as the separator character.
Upstream One True Awk has deprecated this wart in the name of better
compatibility with other awk implementations like gawk and mawk.
.Pp
Historically,
.Nm
did not accept
.Dq 0x
as a hex string.
However, since One True Awk used strtod to convert strings to floats, and since
.Dq 0x12
is a valid hexadecimal representation of a floating point number,
On
.Fx ,
.Nm
has accepted this notation as an extension since One True Awk was imported in
.Fx 5.0 .
Upstream One True Awk has restored the historical behavior for better
compatibility between the different awk implementations.
Both gawk and mawk already behave similarly.
Starting with
.Fx 14.0
.Nm
will no longer accept this extension.
.Pp
The
.Fx
.Nm
sets the locale for many years to match the environment it was running in.
This lead to pattern ranges, like
.Dq "[A-Z]"
sometimes matching lower case characters in some locales.
This misbehavior was never in upstream One True Awk and has been removed as a
bug in
.Fx 12.3 ,
.Fx 13.1 ,
and
.Fx 14.0 .