GAWK
Section: Utility Commands (1)
Updated: Jun 09 2022
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NAME
gawk - pattern scanning and processing language
SYNOPSIS
gawk
[ POSIX or GNU style options ]
-f
program-file
[
--
] file ...
gawk
[ POSIX or GNU style options ]
[
--
]
program-text
file ...
DESCRIPTION
Gawk
is the GNU Project's implementation of the AWK programming language.
It conforms to the definition of the language in
the POSIX 1003.1 standard.
This version in turn is based on the description in
The AWK Programming Language,
by Aho, Kernighan, and Weinberger.
Gawk
provides the additional features found in the current version
of Brian Kernighan's
awk
and numerous GNU-specific extensions.
The command line consists of options to
gawk
itself, the AWK program text (if not supplied via the
-f
or
--include
options), and values to be made
available in the
ARGC
and
ARGV
pre-defined AWK variables.
PREFACE
This manual page is intentionally as terse as possible.
Full details are provided in GAWK: Effective AWK Programming, and you should look
there for the full story on any specific feature.
Where possible, links to the online version of the manual
are provided.
OPTION FORMAT
Gawk
options may be either traditional POSIX-style one letter options,
or GNU-style long options. POSIX options start with a single ``-'',
while long options start with ``--''.
Long options are provided for both GNU-specific features and
for POSIX-mandated features.
Gawk-specific
options are typically used in long-option form.
Arguments to long options are either joined with the option
by an
=
sign, with no intervening spaces, or they may be provided in the
next command line argument.
Long options may be abbreviated, as long as the abbreviation
remains unique.
Additionally, every long option has a corresponding short
option, so that the option's functionality may be used from
within
#!
executable scripts.
OPTIONS
Gawk
accepts the following options.
Standard options are listed first, followed by options for
gawk
extensions, listed alphabetically by short option.
- -f program-file,
-
--file program-file
Read the AWK program source from the file
program-file,
instead of from the first command line argument.
Multiple
-f
options may be used.
Files read with
-f
are treated as if they begin with an implicit @namespace "awk" statement.
- -F fs,
-
--field-separator fs
Use
fs
for the input field separator (the value of the
FS
predefined
variable).
- -v var=val, --assign var=val
-
Assign the value
val
to the variable
var,
before execution of the program begins.
Such variable values are available to the
BEGIN
rule of an AWK program.
- -b,
-
--characters-as-bytes
Treat all input data as single-byte characters.
The
--posix
option overrides this one.
- -c,
-
--traditional
Run in
compatibility
mode. In compatibility mode,
gawk
behaves identically to Brian Kernighan's
awk;
none of the GNU-specific extensions are recognized.
- -C,
-
--copyright
Print the short version of the GNU copyright information message on
the standard output and exit successfully.
- -d[file], --dump-variables[=file]
-
Print a sorted list of global variables, their types and final values to
file.
The default file is
awkvars.out
in the current directory.
- -D[file], --debug[=file]
-
Enable debugging of AWK programs.
By default, the debugger reads commands interactively from the keyboard
(standard input).
The optional
file
argument specifies a file with a list
of commands for the debugger to execute non-interactively.
In this mode of execution,
gawk
loads the
AWK source code and then prompts for debugging commands.
Gawk
can only debug AWK program source provided with the
-f
and
--include
options.
The debugger is documented in GAWK: Effective AWK Programming; see
https://www.gnu.org/software/gawk/manual/html_node/Debugger.html#Debugger.
- -e program-text,
-
--source program-text
Use
program-text
as AWK program source code.
Each argument supplied via
-e
is treated as if it begins with an implicit @namespace "awk" statement.
- -E file, --exec file
-
Similar to
-f,
however, this is option is the last one processed.
This should be used with
#!
scripts, particularly for CGI applications, to avoid
passing in options or source code (!) on the command line
from a URL.
This option disables command-line variable assignments.
- -g,
-
--gen-pot
Scan and parse the AWK program, and generate a GNU
.pot
(Portable Object Template)
format file on standard output with entries for all localizable
strings in the program. The program itself is not executed.
- -h,
-
--help
Print a relatively short summary of the available options on
the standard output.
Per the
GNU Coding Standards,
these options cause an immediate, successful exit.
- -i include-file, --include include-file
-
Load an awk source library.
This searches for the library using the
AWKPATH
environment variable. If the initial search fails, another attempt will
be made after appending the
.awk
suffix. The file will be loaded only
once (i.e., duplicates are eliminated), and the code does not constitute
the main program source.
Files read with
--include
are treated as if they begin with an implicit @namespace "awk" statement.
- -I,
-
--trace
Print the internal byte code names as they are executed when running
the program. The trace is printed to standard error. Each ``op code''
is preceded by a
+
sign in the output.
- -l lib,
-
--load lib
Load a
gawk
extension from the shared library
lib.
This searches for the library using the
AWKLIBPATH
environment variable. If the initial search fails, another attempt will
be made after appending the default shared library suffix for the platform.
The library initialization routine is expected to be named
dl_load().
- -L [value], --lint[=value]
-
Provide warnings about constructs that are
dubious or non-portable to other AWK implementations.
See
https://www.gnu.org/software/gawk/manual/html_node/Options.html#Options
for the list of possible values for
value.
- -M,
-
--bignum
Force arbitrary precision arithmetic on numbers. This option has
no effect if
gawk
is not compiled to use the GNU MPFR and GMP libraries.
(In such a case,
gawk
issues a warning.)
NOTE:
This feature is
on parole.
The primary
gawk
maintainer is no longer supporting it, although there is
a member of the development team who is. If this situation
changes, the feature
will be removed from
gawk.
- -n,
-
--non-decimal-data
Recognize octal and hexadecimal values in input data.
Use this option with great caution!
- -N,
-
--use-lc-numeric
Force
gawk
to use the locale's decimal point character when parsing input data.
- -o[file], --pretty-print[=file]
-
Output a pretty printed version of the program to
file.
The default file is
awkprof.out
in the current directory.
This option implies
--no-optimize.
- -O,
-
--optimize
Enable
gawk's
default optimizations upon the internal representation of the program.
This option is on by default.
- -p[prof-file], --profile[=prof-file]
-
Start a profiling session, and send the profiling data to
prof-file.
The default is
awkprof.out
in the current directory.
The profile contains execution counts of each statement in the program
in the left margin and function call counts for each user-defined function.
Gawk
runs more slowly in this mode.
This option implies
--no-optimize.
- -P,
-
--posix
This turns on
compatibility
mode, and disables a number of common extensions.
- -r,
-
--re-interval
Enable the use of
interval expressions
in regular expression matching.
Interval expressions
are enabled by default, but this option remains for backwards compatibility.
- -s,
-
--no-optimize
Disable
gawk's
default optimizations upon the internal representation of the program.
- -S,
-
--sandbox
Run
gawk
in sandbox mode, disabling the
system()
function, input redirection with
getline,
output redirection with
print and printf,
and loading dynamic extensions.
Command execution (through pipelines) is also disabled.
- -t,
-
--lint-old
Provide warnings about constructs that are
not portable to the original version of UNIX
awk.
- -V,
-
--version
Print version information for this particular copy of
gawk
on the standard output.
This is useful when reporting bugs.
Per the
GNU Coding Standards,
these options cause an immediate, successful exit.
- --
-
Signal the end of options. This is useful to allow further arguments to the
AWK program itself to start with a ``-''.
In compatibility mode,
any other options are flagged as invalid, but are otherwise ignored.
In normal operation, as long as program text has been supplied, unknown
options are passed on to the AWK program in the
ARGV
array for processing.
For POSIX compatibility, the
-W
option may be used, followed by the name of a long option.
AWK PROGRAM EXECUTION
An AWK program consists of a sequence of
optional directives,
pattern-action statements,
and optional function definitions.
-
@include "filename"
@load "filename"
@namespace "name"
pattern { action statements }
function name(parameter list) { statements }
Gawk
first reads the program source from the
program-file(s)
if specified,
from arguments to
--source,
or from the first non-option argument on the command line.
The
-f
and
--source
options may be used multiple times on the command line.
Gawk
reads the program text as if all the
program-files
and command line source texts
had been concatenated together.
In addition, lines beginning with
@include
may be used to include other source files into your program.
This is equivalent
to using the
--include
option.
Lines beginning with
@load
may be used to load extension functions into your program. This is equivalent
to using the
--load
option.
The environment variable
AWKPATH
specifies a search path to use when finding source files named with
the
-f
and
--include
options. If this variable does not exist, the default path is
".:/usr/local/share/awk".
(The actual directory may vary, depending upon how
gawk
was built and installed.)
If a file name given to the
-f
option contains a ``/'' character, no path search is performed.
The environment variable
AWKLIBPATH
specifies a search path to use when finding source files named with
the
--load
option. If this variable does not exist, the default path is
"/usr/local/lib/gawk".
(The actual directory may vary, depending upon how
gawk
was built and installed.)
Gawk
executes AWK programs in the following order.
First,
all variable assignments specified via the
-v
option are performed.
Next,
gawk
compiles the program into an internal form.
Then,
gawk
executes the code in the
BEGIN
rule(s) (if any),
and then proceeds to read
each file named in the
ARGV
array (up to
ARGV[ARGC-1]).
If there are no files named on the command line,
gawk
reads the standard input.
If a filename on the command line has the form
var=val
it is treated as a variable assignment. The variable
var
will be assigned the value
val.
(This happens after any
BEGIN
rule(s) have been run.)
If the value of a particular element of
ARGV
is empty (""),
gawk
skips over it.
For each input file,
if a
BEGINFILE
rule exists,
gawk
executes the associated code
before processing the contents of the file. Similarly,
gawk
executes
the code associated with
ENDFILE
rules
after processing the file.
For each record in the input,
gawk
tests to see if it matches any
pattern
in the AWK program.
For each pattern that the record matches,
gawk
executes the associated
action.
The patterns are tested in the order they occur in the program.
Finally, after all the input is exhausted,
gawk
executes the code in the
END
rule(s) (if any).
Command Line Directories
According to POSIX, files named on the
awk
command line must be
text files. The behavior is ``undefined'' if they are not. Most versions
of
awk
treat a directory on the command line as a fatal error.
For
gawk,
a directory on the command line
produces a warning, but is otherwise skipped. If either of the
--posix
or
--traditional
options is given, then
gawk
reverts to
treating directories on the command line as a fatal error.
VARIABLES, RECORDS AND FIELDS
AWK variables are dynamic; they come into existence when they are
first used. Their values are either floating-point numbers or strings,
or both,
depending upon how they are used.
Additionally,
gawk
allows variables to have regular-expression type.
AWK also has one dimensional
arrays; arrays with multiple dimensions may be simulated.
However,
gawk
provides true arrays of arrays.
Several pre-defined variables are set as a program
runs; these are described as needed and summarized below.
Records
Normally, records are separated by newline characters. You can control how
records are separated by assigning values to the built-in variable
RS.
See
https://www.gnu.org/software/gawk/manual/html_node/Records.html
for the details.
Fields
As each input record is read,
gawk
splits the record into
fields,
using the value of the
FS
variable as the field separator.
Additionally,
FIELDWIDTHS
and
FPAT
may be used to control input field splitting.
See the details, starting at
https://www.gnu.org/software/gawk/manual/html_node/Fields.html.
Each field in the input record may be referenced by its position:
$1,
$2,
and so on.
$0
is the whole record,
including leading and trailing whitespace.
The variable
NF
is set to the total number of fields in the input record.
References to non-existent fields (i.e., fields after
$NF)
produce the null string. However, assigning to a non-existent field
(e.g.,
$(NF+2) = 5)
increases the value of
NF,
creates any intervening fields with the null string as their values, and
causes the value of
$0
to be recomputed, with the fields being separated by the value of
OFS.
References to negative numbered fields cause a fatal error.
Decrementing
NF
causes the values of fields past the new value to be lost, and the value of
$0
to be recomputed, with the fields being separated by the value of
OFS.
Assigning a value to an existing field
causes the whole record to be rebuilt when
$0
is referenced.
Similarly, assigning a value to
$0
causes the record to be resplit, creating new
values for the fields.
Built-in Variables
Gawk's
built-in variables are listed below.
This list is purposely terse. For details, see
https://www.gnu.org/software/gawk/manual/html_node/Built_002din-Variables.
- ARGC
-
The number of command line arguments.
- ARGIND
-
The index in
ARGV
of the current file being processed.
- ARGV
-
Array of command line arguments. The array is indexed from
0 to
ARGC
- 1.
- BINMODE
-
On non-POSIX systems, specifies use of ``binary'' mode for all file I/O.
See
https://www.gnu.org/software/gawk/manual/html_node/PC-Using.html
for the details.
- CONVFMT
-
The conversion format for numbers, "%.6g", by default.
- ENVIRON
-
An array containing the values of the current environment.
The array is indexed by the environment variables, each element being
the value of that variable.
- ERRNO
-
If a system error occurs either doing a redirection for
getline,
during a read for
getline,
or during a
close(),
then
ERRNO
is set to
a string describing the error.
The value is subject to translation in non-English locales.
- FIELDWIDTHS
-
A whitespace-separated list of field widths. When set,
gawk
parses the input into fields of fixed width, instead of using the
value of the
FS
variable as the field separator.
Each field width may optionally be preceded by a colon-separated
value specifying the number of characters to skip before the field starts.
- FILENAME
-
The name of the current input file.
If no files are specified on the command line, the value of
FILENAME
is ``-''.
However,
FILENAME
is undefined inside the
BEGIN
rule
(unless set by
getline).
- FNR
-
The input record number in the current input file.
- FPAT
-
A regular expression describing the contents of the
fields in a record.
When set,
gawk
parses the input into fields, where the fields match the
regular expression, instead of using the
value of
FS
as the field separator.
- FS
-
The input field separator, a space by default.
See
https://www.gnu.org/software/gawk/manual/html_node/Field-Separators.html
for the details.
- FUNCTAB
-
An array whose indices and corresponding values
are the names of all the user-defined
or extension functions in the program.
NOTE:
You may not use the
delete
statement with the
FUNCTAB
array.
- IGNORECASE
-
Controls the case-sensitivity of all regular expression
and string operations.
See
https://www.gnu.org/software/gawk/manual/html_node/Case_002dsensitivity.html
for details.
- LINT
-
Provides dynamic control of the
--lint
option from within an AWK program.
- NF
-
The number of fields in the current input record.
- NR
-
The total number of input records seen so far.
- OFMT
-
The output format for numbers, "%.6g", by default.
- OFS
-
The output field separator, a space by default.
- ORS
-
The output record separator, by default a newline.
- PREC
-
The working precision of arbitrary precision floating-point
numbers, 53 by default.
- PROCINFO
-
The elements of this array provide access to information about the
running AWK program.
See
https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset
for the details.
- ROUNDMODE
-
The rounding mode to use for arbitrary precision arithmetic on
numbers, by default "N" (IEEE-754 roundTiesToEven mode).
See
https://www.gnu.org/software/gawk/manual/html_node/Setting-the-rounding-mode
for the details.
- RS
-
The input record separator, by default a newline.
- RT
-
The record terminator.
Gawk
sets
RT
to the input text that matched the character or regular expression
specified by
RS.
- RSTART
-
The index of the first character matched by
match();
0 if no match.
- RLENGTH
-
The length of the string matched by
match();
-1 if no match.
- SUBSEP
-
The string used to separate multiple subscripts in array
elements, by default "\034".
- SYMTAB
-
An array whose indices are the names of all currently defined
global variables and arrays in the program.
You may not use the
delete
statement with the
SYMTAB
array, nor assign to elements with an index that is
not a variable name.
- TEXTDOMAIN
-
The text domain of the AWK program; used to find the localized
translations for the program's strings.
Arrays
Arrays are subscripted with an expression between square brackets
([ and ]).
If the expression is an expression list
(expr, expr ...)
then the array subscript is a string consisting of the
concatenation of the (string) value of each expression,
separated by the value of the
SUBSEP
variable.
This facility is used to simulate multiply dimensioned
arrays. For example:
-
i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"
assigns the string "hello, world\n" to the element of the array
x
which is indexed by the string "A\034B\034C". All arrays in AWK
are associative, i.e., indexed by string values.
The special operator
in
may be used to test if an array has an index consisting of a particular
value:
-
if (val in array)
print array[val]
If the array has multiple subscripts, use
(i, j) in array.
The
in
construct may also be used in a
for
loop to iterate over all the elements of an array.
However, the
(i, j) in array
construct only works in tests, not in
for
loops.
An element may be deleted from an array using the
delete
statement.
The
delete
statement may also be used to delete the entire contents of an array,
just by specifying the array name without a subscript.
gawk
supports true multidimensional arrays. It does not require that
such arrays be ``rectangular'' as in C or C++.
See
https://www.gnu.org/software/gawk/manual/html_node/Arrays
for details.
Namespaces
Gawk
provides a simple
namespace
facility to help work around the fact that all variables in
AWK are global.
A
qualified name
consists of a two simple identifiers joined by a double colon
(::).
The left-hand identifier represents the namespace and the right-hand
identifier is the variable within it.
All simple (non-qualified) names are considered to be in the
``current'' namespace; the default namespace is
awk.
However, simple identifiers consisting solely of uppercase
letters are forced into the
awk
namespace, even if the current namespace is different.
You change the current namespace with an
@namespace "name"
directive.
The standard predefined builtin function names may not be used as
namespace names. The names of additional functions provided by
gawk
may be used as namespace names or as simple identifiers in other
namespaces.
For more details, see
https://www.gnu.org/software/gawk/manual/html_node/Namespaces.html#Namespaces.
Variable Typing And Conversion
Variables and fields
may be (floating point) numbers, or strings, or both.
They may also be regular expressions. How the
value of a variable is interpreted depends upon its context. If used in
a numeric expression, it will be treated as a number; if used as a string
it will be treated as a string.
To force a variable to be treated as a number, add zero to it; to force it
to be treated as a string, concatenate it with the null string.
Uninitialized variables have the numeric value zero and the string value ""
(the null, or empty, string).
When a string must be converted to a number, the conversion is accomplished
using
strtod(3).
A number is converted to a string by using the value of
CONVFMT
as a format string for
sprintf(3),
with the numeric value of the variable as the argument.
However, even though all numbers in AWK are floating-point,
integral values are
always
converted as integers.
Gawk
performs comparisons as follows:
If two variables are numeric, they are compared numerically.
If one value is numeric and the other has a string value that is a
``numeric string,'' then comparisons are also done numerically.
Otherwise, the numeric value is converted to a string and a string
comparison is performed.
Two strings are compared, of course, as strings.
Note that string constants, such as "57", are
not
numeric strings, they are string constants.
The idea of ``numeric string''
only applies to fields,
getline
input,
FILENAME,
ARGV
elements,
ENVIRON
elements and the elements of an array created by
split()
or
patsplit()
that are numeric strings.
The basic idea is that
user input,
and only user input, that looks numeric,
should be treated that way.
Octal and Hexadecimal Constants
You may use C-style octal and hexadecimal constants in your AWK
program source code.
For example, the octal value
011
is equal to decimal
9,
and the hexadecimal value
0x11
is equal to decimal 17.
String Constants
String constants in AWK are sequences of characters enclosed
between double quotes (like "value"). Within strings, certain
escape sequences
are recognized, as in C.
See
https://www.gnu.org/software/gawk/manual/html_node/Escape-Sequences
for the details.
Regexp Constants
A regular expression constant is a sequence of characters enclosed
between forward slashes (like
/value/).
The escape sequences described in the manual may also be used inside
constant regular expressions
(e.g.,
/[ \t\f\n\r\v]/
matches whitespace characters).
Gawk
provides
strongly typed
regular expression constants. These are written with a leading
@
symbol (like so:
@/value/).
Such constants may be assigned to scalars (variables, array elements)
and passed to user-defined functions. Variables that have been so
assigned have regular expression type.
PATTERNS AND ACTIONS
AWK is a line-oriented language. The pattern comes first, and then the
action. Action statements are enclosed in
{
and
}.
Either the pattern may be missing, or the action may be missing, but,
of course, not both. If the pattern is missing, the action
executes for every single record of input.
A missing action is equivalent to
-
{ print }
which prints the entire record.
Comments begin with the
#
character, and continue until the
end of the line.
Empty lines may be used to separate statements.
Normally, a statement ends with a newline, however, this is not the
case for lines ending in
a comma,
{,
?,
:,
&&,
or
||.
Lines ending in
do
or
else
also have their statements automatically continued on the following line.
In other cases, a line can be continued by ending it with a ``\'',
in which case the newline is ignored. However, a ``\'' after a
#
is not special.
Multiple statements may
be put on one line by separating them with a ``;''.
This applies to both the statements within the action part of a
pattern-action pair (the usual case),
and to the pattern-action statements themselves.
Patterns
AWK patterns may be one of the following:
-
BEGIN
END
BEGINFILE
ENDFILE
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN
and
END
are two special kinds of patterns which are not tested against
the input.
The action parts of all
BEGIN
patterns are merged as if all the statements had
been written in a single
BEGIN
rule. They are executed before any
of the input is read. Similarly, all the
END
rules are merged,
and executed when all the input is exhausted (or when an
exit
statement is executed).
BEGIN
and
END
patterns cannot be combined with other patterns in pattern expressions.
BEGIN
and
END
patterns cannot have missing action parts.
BEGINFILE
and
ENDFILE
are additional special patterns whose actions are executed
before reading the first record of each command-line input file
and after reading the last record of each file.
Inside the
BEGINFILE
rule, the value of
ERRNO
is the empty string if the file was opened successfully.
Otherwise, there is some problem with the file and the code should
use
nextfile
to skip it. If that is not done,
gawk
produces its usual fatal error for files that cannot be opened.
For
/regular expression/
patterns, the associated statement is executed for each input record that matches
the regular expression.
Regular expressions are essentially the same as those in
egrep(1).
See
https://www.gnu.org/software/gawk/manual/html_node/Regexp.html
for the details on regular expressions.
A
relational expression
may use any of the operators defined below in the section on actions.
These generally test whether certain fields match certain regular expressions.
The
&&,
||,
and
!
operators are logical AND, logical OR, and logical NOT, respectively, as in C.
They do short-circuit evaluation, also as in C, and are used for combining
more primitive pattern expressions. As in most languages, parentheses
may be used to change the order of evaluation.
The
?:
operator is like the same operator in C. If the first pattern is true
then the pattern used for testing is the second pattern, otherwise it is
the third. Only one of the second and third patterns is evaluated.
The
pattern1, pattern2
form of an expression is called a
range pattern.
It matches all input records starting with a record that matches
pattern1,
and continuing until a record that matches
pattern2,
inclusive. It does not combine with any other sort of pattern expression.
Actions
Action statements are enclosed in braces,
{
and
}.
Action statements consist of the usual assignment, conditional, and looping
statements found in most languages. The operators, control statements,
and input/output statements
available are patterned after those in C.
Operators
The operators in AWK, in order of decreasing precedence, are:
- (...)
-
Grouping
- $
-
Field reference.
- ++ --
-
Increment and decrement, both prefix and postfix.
- ^
-
Exponentiation.
- + - !
-
Unary plus, unary minus, and logical negation.
- * / %
-
Multiplication, division, and modulus.
- + -
-
Addition and subtraction.
- space
-
String concatenation.
- | |&
-
Piped I/O for
getline,
print,
and
printf.
- < > <= >= == !=
-
The regular relational operators.
- ~ !~
-
Regular expression match, negated match.
- in
-
Array membership.
- &&
-
Logical AND.
- ||
-
Logical OR.
- ?:
-
The C conditional expression. This has the form
expr1 ? expr2 : expr3.
If
expr1
is true, the value of the expression is
expr2,
otherwise it is
expr3.
Only one of
expr2
and
expr3
is evaluated.
- = += -= *= /= %= ^=
-
Assignment. Both absolute assignment
(var = value)
and operator-assignment (the other forms) are supported.
Control Statements
The control statements are
as follows:
-
if (condition) statement [ else statement ]
while (condition) statement
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }
switch (expression) {
case value|regex : statement
...
[ default: statement ]
}
I/O Statements
The input/output statements are as follows:
- close(file [, how])
-
Close an open file, pipe or coprocess.
The optional
how
should only be used when closing one end of a
two-way pipe to a coprocess.
It must be a string value, either
"to" or "from".
- getline
-
Set
$0
from the next input record; set
NF,
NR,
FNR,
RT.
- getline <file
-
Set
$0
from the next record of
file;
set
NF,
RT.
- getline var
-
Set
var
from the next input record; set
NR,
FNR,
RT.
- getline var <file
-
Set
var
from the next record of
file;
set
RT.
- command | getline [var]
-
Run
command,
piping the output either into
$0
or
var,
as above, and
RT.
- command |& getline [var]
-
Run
command
as a coprocess
piping the output either into
$0
or
var,
as above, and
RT.
(The command
can also be a socket. See the subsection
Special File Names,
below.)
- fflush([file])
-
Flush any buffers associated with the open output file or pipe
file.
If
file
is missing or if it
is the null string,
then flush all open output files and pipes.
- next
-
Stop processing the current input record.
Read the next input record
and start processing over with the first pattern in the
AWK program.
Upon reaching the end of the input data,
execute any
END
rule(s).
- nextfile
-
Stop processing the current input file. The next input record read
comes from the next input file.
Update
FILENAME
and
ARGIND,
reset
FNR
to 1, and start processing over with the first pattern in the
AWK program.
Upon reaching the end of the input data,
execute any
ENDFILE
and
END
rule(s).
- print
-
Print the current record.
The output record is terminated with the value of
ORS.
- print expr-list
-
Print expressions.
Each expression is separated by the value of
OFS.
The output record is terminated with the value of
ORS.
- print expr-list >file
-
Print expressions on
file.
Each expression is separated by the value of
OFS.
The output record is terminated with the value of
ORS.
- printf fmt, expr-list
-
Format and print.
- printf fmt, expr-list >file
-
Format and print on
file.
- system(cmd-line)
-
Execute the command
cmd-line,
and return the exit status.
(This may not be available on non-POSIX systems.)
See
https://www.gnu.org/software/gawk/manual/html_node/I_002fO-Functions.html#I_002fO-Functions
for the full details on the exit status.
Additional output redirections are allowed for
print
and
printf.
- print ... >> file
-
Append output to the
file.
- print ... | command
-
Write on a pipe.
- print ... |& command
-
Send data to a coprocess or socket.
(See also the subsection
Special File Names,
below.)
The
getline
command returns 1 on success, zero on end of file, and -1 on an error.
If the
errno(3)
value indicates that the I/O operation may be retried,
and PROCINFO["input", "RETRY"]
is set, then -2 is returned instead of -1, and further calls to
getline
may be attempted.
Upon an error,
ERRNO
is set to a string describing the problem.
NOTE:
Failure in opening a two-way socket results in a non-fatal error being
returned to the calling function. If using a pipe, coprocess, or socket to
getline,
or from
print
or
printf
within a loop, you
must
use
close()
to create new instances of the command or socket.
AWK does not automatically close pipes, sockets, or coprocesses when
they return EOF.
The AWK versions of the
printf
statement and
sprintf()
function
are similar to those of C. For details, see
https://www.gnu.org/software/gawk/manual/html_node/Printf.html.
Special File Names
When doing I/O redirection from either
print
or
printf
into a file,
or via
getline
from a file,
gawk
recognizes certain special filenames internally. These filenames
allow access to open file descriptors inherited from
gawk's
parent process (usually the shell).
These file names may also be used on the command line to name data files.
The filenames are:
- -
-
The standard input.
- /dev/stdin
-
The standard input.
- /dev/stdout
-
The standard output.
- /dev/stderr
-
The standard error output.
- /dev/fd/n
-
The file associated with the open file descriptor
n.
The following special filenames may be used with the
|&
coprocess operator for creating TCP/IP network connections:
-
-
/inet/tcp/lport/rhost/rport
-
-
/inet4/tcp/lport/rhost/rport
-
-
/inet6/tcp/lport/rhost/rport
Files for a TCP/IP connection on local port
lport
to
remote host
rhost
on remote port
rport.
Use a port of
0
to have the system pick a port.
Use
/inet4
to force an IPv4 connection,
and
/inet6
to force an IPv6 connection.
Plain
/inet
uses the system default (most likely IPv4).
Usable only with the
|&
two-way I/O operator.
-
-
/inet/udp/lport/rhost/rport
-
-
/inet4/udp/lport/rhost/rport
-
-
/inet6/udp/lport/rhost/rport
Similar, but use UDP/IP instead of TCP/IP.
Numeric Functions
AWK has the following built-in arithmetic functions:
- atan2(y, x)
-
Return the arctangent of
y/x
in radians.
- cos(expr)
-
Return the cosine of
expr,
which is in radians.
- exp(expr)
-
The exponential function.
- int(expr)
-
Truncate to integer.
- log(expr)
-
The natural logarithm function.
- rand()
-
Return a random number
N,
between zero and one,
such that 0 ≤ N < 1.
- sin(expr)
-
Return the sine of
expr,
which is in radians.
- sqrt(expr)
-
Return the square root of
expr.
- srand([expr])
-
Use
expr
as the new seed for the random number generator. If no
expr
is provided, use the time of day.
Return the previous seed for the random
number generator.
String Functions
Gawk
has the following built-in string functions; details are provided in
https://www.gnu.org/software/gawk/manual/html_node/String-Functions.
- asort(s [, d [, how] ])
-
Return the number of elements in the source
array
s.
Sort
the contents of
s
using
gawk's
normal rules for
comparing values, and replace the indices of the
sorted values
s
with sequential
integers starting with 1. If the optional
destination array
d
is specified,
first duplicate
s
into
d,
and then sort
d,
leaving the indices of the
source array
s
unchanged. The optional string
how
controls the direction and the comparison mode.
Valid values for
how
are
described in
https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html#String-Functions.
s and d
are allowed to be the same array; this only makes sense when
supplying the third argument as well.
- asorti(s [, d [, how] ])
-
Return the number of elements in the source
array
s.
The behavior is the same as that of
asort(),
except that the array
indices
are used for sorting, not the array values.
When done, the array is indexed numerically, and
the values are those of the original indices.
The original values are lost; thus provide
a second array if you wish to preserve the original.
The purpose of the optional string
how
is the same as for
asort().
Here too,
s and d
are allowed to be the same array; this only makes sense when
supplying the third argument as well.
- gensub(r, s, h [, t])
-
Search the target string
t
for matches of the regular expression
r.
If
h
is a string beginning with
g
or
G,
then replace all matches of
r
with
s.
Otherwise,
h
is a number indicating which match of
r
to replace.
If
t
is not supplied, use
$0
instead.
Within the replacement text
s,
the sequence
\n,
where
n
is a digit from 1 to 9, may be used to indicate just the text that
matched the
n'th
parenthesized subexpression. The sequence
\0
represents the entire matched text, as does the character
&.
Unlike
sub()
and
gsub(),
the modified string is returned as the result of the function,
and the original target string is
not
changed.
- gsub(r, s [, t])
-
For each substring matching the regular expression
r
in the string
t,
substitute the string
s,
and return the number of substitutions.
If
t
is not supplied, use
$0.
An
&
in the replacement text is replaced with the text that was actually matched.
Use
\&
to get a literal
&.
(This must be typed as "\\&"; see
https://www.gnu.org/software/gawk/manual/html_node/Gory-Details.html#Gory-Details
for a fuller discussion of the rules for ampersands
and backslashes in the replacement text of
sub(),
gsub(),
and
gensub().)
- index(s, t)
-
Return the index of the string
t
in the string
s,
or zero if
t
is not present.
(This implies that character indices start at one.)
- length([s])
-
Return the length of the string
s,
or the length of
$0
if
s
is not supplied.
With an array argument,
length()
returns the number of elements in the array.
- match(s, r [, a])
-
Return the position in
s
where the regular expression
r
occurs, or zero if
r
is not present, and set the values of
RSTART
and
RLENGTH.
Note that the argument order is the same as for the
~
operator:
str ~
re.
See
https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html#String-Functions
for a description of how the array
a
is filled if it is provided.
- patsplit(s, a [, r [, seps] ])
-
Split the string
s
into the array
a
and the separators array
seps
on the regular expression
r,
and return the number of fields.
Element values are the portions of
s
that matched
r.
The value of
seps[i]
is the possibly null separator that appeared after
a[i].
The value of
seps[0]
is the possibly null leading separator.
If
r
is omitted,
FPAT
is used instead.
The arrays
a
and
seps
are cleared first.
Splitting behaves identically to field splitting with
FPAT.
- split(s, a [, r [, seps] ])
-
Split the string
s
into the array
a
and the separators array
seps
on the regular expression
r,
and return the number of fields. If
r
is omitted,
FS
is used instead.
The arrays
a
and
seps
are cleared first.
seps[i]
is the field separator matched by
r
between
a[i]
and
a[i+1].
Splitting behaves identically to field splitting.
- sprintf(fmt, expr-list)
-
Print
expr-list
according to
fmt,
and return the resulting string.
- strtonum(str)
-
Examine
str,
and return its numeric value.
If
str
begins
with a leading
0,
treat it
as an octal number.
If
str
begins
with a leading
0x
or
0X,
treat it
as a hexadecimal number.
Otherwise, assume it is a decimal number.
- sub(r, s [, t])
-
Just like
gsub(),
but replace only the first matching substring.
Return either zero or one.
- substr(s, i [, n])
-
Return the at most
n-character
substring of
s
starting at
i.
If
n
is omitted, use the rest of
s.
- tolower(str)
-
Return a copy of the string
str,
with all the uppercase characters in
str
translated to their corresponding lowercase counterparts.
Non-alphabetic characters are left unchanged.
- toupper(str)
-
Return a copy of the string
str,
with all the lowercase characters in
str
translated to their corresponding uppercase counterparts.
Non-alphabetic characters are left unchanged.
Gawk
is multibyte aware. This means that
index(),
length(),
substr()
and
match()
all work in terms of characters, not bytes.
Time Functions
Gawk
provides the following functions for obtaining time stamps and
formatting them. Details are provided in
https://www.gnu.org/software/gawk/manual/html_node/Time-Functions.
- mktime(datespec [, utc-flag])
-
Turn
datespec
into a time stamp of the same form as returned by
systime(),
and return the result.
If
utc-flag
is present and is non-zero or non-null, the time is assumed to be in
the UTC time zone; otherwise, the
time is assumed to be in the local time zone.
If
datespec
does not contain enough elements or if the resulting time
is out of range,
mktime()
returns -1.
See
https://www.gnu.org/software/gawk/manual/html_node/Time-Functions.html#Time-Functions
for the details of
datespec.
- strftime([format [, timestamp[, utc-flag]]])
-
Format
timestamp
according to the specification in
format.
If
utc-flag
is present and is non-zero or non-null, the result
is in UTC, otherwise the result is in local time.
The
timestamp
should be of the same form as returned by
systime().
If
timestamp
is missing, the current time of day is used.
If
format
is missing, a default format equivalent to the output of
date(1)
is used.
The default format is available in
PROCINFO[strftime].
See the specification for the
strftime()
function in ISO C for the format conversions that are
guaranteed to be available.
- systime()
-
Return the current time of day as the number of seconds since the Epoch
(1970-01-01 00:00:00 UTC on POSIX systems).
Bit Manipulations Functions
Gawk
supplies the following bit manipulation functions.
They work by converting double-precision floating point
values to
uintmax_t
integers, doing the operation, and then converting the
result back to floating point.
Passing negative operands to any of these functions causes
a fatal error.
The functions are:
- and(v1, v2 [, ...])
-
Return the bitwise AND of the values provided in the argument list.
There must be at least two.
- compl(val)
-
Return the bitwise complement of
val.
- lshift(val, count)
-
Return the value of
val,
shifted left by
count
bits.
- or(v1, v2 [, ...])
-
Return the bitwise OR of the values provided in the argument list.
There must be at least two.
- rshift(val, count)
-
Return the value of
val,
shifted right by
count
bits.
- xor(v1, v2 [, ...])
-
Return the bitwise XOR of the values provided in the argument list.
There must be at least two.
Type Functions
The following functions provide type related information about
their arguments.
- isarray(x)
-
Return true if
x
is an array, false otherwise.
- typeof(x)
-
Return a string indicating the type of
x.
The string will be one of
"array",
"number",
"regexp",
"string",
"strnum",
"unassigned",
or
"undefined".
Internationalization Functions
The following functions may be used from within your AWK program for
translating strings at run-time.
For full details, see
https://www.gnu.org/software/gawk/manual/html_node/I18N-Functions.html#I18N-Functions.
- bindtextdomain(directory [, domain])
-
Specify the directory where
gawk
looks for the
.gmo
files, in case they
will not or cannot be placed in the ``standard'' locations.
It returns the directory where
domain
is ``bound.''
The default
domain
is the value of
TEXTDOMAIN.
If
directory
is the null string (""), then
bindtextdomain()
returns the current binding for the
given
domain.
- dcgettext(string [, domain [, category]])
-
Return the translation of
string
in text domain
domain
for locale category
category.
The default value for
domain
is the current value of
TEXTDOMAIN.
The default value for
category
is "LC_MESSAGES".
- dcngettext(string1, string2, number [, domain [, category]])
-
Return the plural form used for
number
of the translation of
string1
and
string2
in
text domain
domain
for locale category
category.
The default value for
domain
is the current value of
TEXTDOMAIN.
The default value for
category
is "LC_MESSAGES".
Boolean Valued Functions
You can create special Boolean-typed values; see the manual for how
they work and why they exist.
- mkbool(expression)
-
Based on the boolean value of
expression
return either a true value or a false value.
True values have numeric value one.
False values have numeric value zero.
USER-DEFINED FUNCTIONS
Functions in AWK are defined as follows:
-
function name(parameter list) { statements }
Functions execute when they are called from within expressions
in either patterns or actions. Actual parameters supplied in the function
call are used to instantiate the formal parameters declared in the function.
Arrays are passed by reference, other variables are passed by value.
Local variables are declared as extra parameters
in the parameter list. The convention is to separate local variables from
real parameters by extra spaces in the parameter list. For example:
-
function f(p, q, a, b) # a and b are local
{
...
}
/abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required
to immediately follow the function name,
without any intervening whitespace.
This restriction does not apply to the built-in functions listed above.
Functions may call each other and may be recursive.
Function parameters used as local variables are initialized
to the null string and the number zero upon function invocation.
Use
return expr
to return a value from a function. The return value is undefined if no
value is provided, or if the function returns by ``falling off'' the
end.
Functions may be called indirectly. To do this, assign
the name of the function to be called, as a string, to a variable.
Then use the variable as if it were the name of a function, prefixed with an
@
sign, like so:
-
function myfunc()
{
print "myfunc called"
...
}
{ ...
the_func = "myfunc"
@the_func() # call through the_func to myfunc
...
}
If
--lint
has been provided,
gawk
warns about calls to undefined functions at parse time,
instead of at run time.
Calling an undefined function at run time is a fatal error.
DYNAMICALLY LOADING NEW FUNCTIONS
You can dynamically add new functions written in C or C++ to the running
gawk
interpreter with the
@load
statement.
The full details are beyond the scope of this manual page;
see
https://www.gnu.org/software/gawk/manual/html_node/Dynamic-Extensions.html#Dynamic-Extensions.
SIGNALS
The
gawk
profiler accepts two signals.
SIGUSR1
causes it to dump a profile and function call stack to the
profile file, which is either
awkprof.out,
or whatever file was named with the
--profile
option. It then continues to run.
SIGHUP
causes
gawk
to dump the profile and function call stack and then exit.
INTERNATIONALIZATION
String constants are sequences of characters enclosed in double
quotes. In non-English speaking environments, it is possible to mark
strings in the AWK program as requiring translation to the local
natural language. Such strings are marked in the AWK program with
a leading underscore (``_''). For example,
-
gawk 'BEGIN { print "hello, world" }'
always prints
hello, world.
But,
-
gawk 'BEGIN { print _"hello, world" }'
might print
bonjour, monde
in France.
See
https://www.gnu.org/software/gawk/manual/html_node/Internationalization.html#Internationalization
for the steps involved in producing and running a localizable
AWK program.
GNU EXTENSIONS
Gawk
has a too-large number of extensions to POSIX
awk.
They are described in
https://www.gnu.org/software/gawk/manual/html_node/POSIX_002fGNU.html.
All the extensions
can be disabled by
invoking
gawk
with the
--traditional
or
--posix
options.
ENVIRONMENT VARIABLES
The
AWKPATH
environment variable can be used to provide a list of directories that
gawk
searches when looking for files named via the
-f,
--file,
-i
and
--include
options, and the
@include
directive. If the initial search fails, the path is searched again after
appending
.awk
to the filename.
The
AWKLIBPATH
environment variable can be used to provide a list of directories that
gawk
searches when looking for files named via the
-l
and
--load
options.
The
GAWK_PERSIST_FILE
environment variable, if present, specifies a file to use as
the backing store for persistent memory.
This is an experimental feature.
See GAWK: Effective AWK Programming for the details.
The
GAWK_READ_TIMEOUT
environment variable can be used to specify a timeout
in milliseconds for reading input from a terminal, pipe
or two-way communication including sockets.
For connection to a remote host via socket,
GAWK_SOCK_RETRIES
controls the number of retries, and
GAWK_MSEC_SLEEP
the interval between retries.
The interval is in milliseconds. On systems that do not support
usleep(3),
the value is rounded up to an integral number of seconds.
If
POSIXLY_CORRECT
exists in the environment, then
gawk
behaves exactly as if
--posix
had been specified on the command line.
If
--lint
has been specified,
gawk
issues a warning message to this effect.
EXIT STATUS
If the
exit
statement is used with a value,
then
gawk
exits with
the numeric value given to it.
Otherwise, if there were no problems during execution,
gawk
exits with the value of the C constant
EXIT_SUCCESS.
This is usually zero.
If an error occurs,
gawk
exits with the value of
the C constant
EXIT_FAILURE.
This is usually one.
If
gawk
exits because of a fatal error, the exit
status is 2. On non-POSIX systems, this value may be mapped to
EXIT_FAILURE.
VERSION INFORMATION
This man page documents
gawk,
version 5.2.
AUTHORS
The original version of UNIX
awk
was designed and implemented by Alfred Aho,
Peter Weinberger, and Brian Kernighan of Bell Laboratories. Brian Kernighan
continues to maintain and enhance it.
Paul Rubin and Jay Fenlason,
of the Free Software Foundation, wrote
gawk,
to be compatible with the original version of
awk
distributed in Seventh Edition UNIX.
John Woods contributed a number of bug fixes.
David Trueman, with contributions
from Arnold Robbins, made
gawk
compatible with the new version of UNIX
awk.
Arnold Robbins is the current maintainer.
See GAWK: Effective AWK Programming for a full list of the contributors to
gawk
and its documentation.
See the
README
file in the
gawk
distribution for up-to-date information about maintainers
and which ports are currently supported.
BUG REPORTS
If you find a bug in
gawk,
please use the
gawkbug(1)
program to report it.
Full instructions for reporting a bug are provided in
https://www.gnu.org/software/gawk/manual/html_node/Bugs.html.
Please
carefully read and follow the instructions given there.
This will make bug reporting and resolution much easier for everyone involved.
Really.
BUGS
The
-F
option is not necessary given the command line variable assignment feature;
it remains only for backwards compatibility.
This manual page is too long;
gawk
has too many features.
SEE ALSO
egrep(1),
sed(1),
gawkbug(1),
printf(3),
and
strftime(3).
The AWK Programming Language,
Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger,
Addison-Wesley, 1988. ISBN 0-201-07981-X.
GAWK: Effective AWK Programming,
Edition 5.2, shipped with the
gawk
source.
The current version of this document is available online at
https://www.gnu.org/software/gawk/manual.
The GNU
gettext
documentation, available online at
https://www.gnu.org/software/gettext.
EXAMPLES
Print and sort the login names of all users:
BEGIN { FS = ":" }
{ print $1 | "sort" }
Count lines in a file:
{ nlines++ }
END { print nlines }
Precede each line by its number in the file:
{ print FNR, $0 }
Concatenate and line number (a variation on a theme):
{ print NR, $0 }
Run an external command for particular lines of data:
tail -f access_log |
awk '/myhome.html/ { system("nmap " $1 ">> logdir/myhome.html") }'
COPYING PERMISSIONS
Copyright © 1989, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2001, 2002, 2003, 2004, 2005, 2007, 2009,
2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019,
2020, 2021, 2022
Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual page provided the copyright notice and this permission
notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual page under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual page into another language, under the above conditions for
modified versions, except that this permission notice may be stated in
a translation approved by the Foundation.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- PREFACE
-
- OPTION FORMAT
-
- OPTIONS
-
- AWK PROGRAM EXECUTION
-
- Command Line Directories
-
- VARIABLES, RECORDS AND FIELDS
-
- Records
-
- Fields
-
- Built-in Variables
-
- Arrays
-
- Namespaces
-
- Variable Typing And Conversion
-
- Octal and Hexadecimal Constants
-
- String Constants
-
- Regexp Constants
-
- PATTERNS AND ACTIONS
-
- Patterns
-
- Actions
-
- Operators
-
- Control Statements
-
- I/O Statements
-
- Special File Names
-
- Numeric Functions
-
- String Functions
-
- Time Functions
-
- Bit Manipulations Functions
-
- Type Functions
-
- Internationalization Functions
-
- Boolean Valued Functions
-
- USER-DEFINED FUNCTIONS
-
- DYNAMICALLY LOADING NEW FUNCTIONS
-
- SIGNALS
-
- INTERNATIONALIZATION
-
- GNU EXTENSIONS
-
- ENVIRONMENT VARIABLES
-
- EXIT STATUS
-
- VERSION INFORMATION
-
- AUTHORS
-
- BUG REPORTS
-
- BUGS
-
- SEE ALSO
-
- EXAMPLES
-
- COPYING PERMISSIONS
-
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Time: 19:46:02 GMT, April 19, 2024