GAWK(1) Free Software Foundation GAWK(1)
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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
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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 option is the last one processed.
This should be used with #! scripts, particularly for CGI
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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.
-k, --csv
Enable CSV special processing. See Comma Separated Values,
below, for more detail.
-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
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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
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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
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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.
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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.
Comma Separated Values
When invoked with either the -k or the --csv option, gawk does not use
regular record determination and field splitting as described above.
Instead, records are terminated by unquoted newlines, and fields are
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separated by commas. Double-quotes may be used to enclose fields
containing commas, newlines, or doubled double-quotes. See
https://www.gnu.org/software/gawk/manual/html_node/Comma-Separated-
Fields.html for more details.
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
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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
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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
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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
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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.
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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.
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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.
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| |& 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".
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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.
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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 function 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:
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- 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
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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
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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
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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.
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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.
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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.
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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
...
}
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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.
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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 5.3.
AUTHORS
The original version of UNIX awk was designed and implemented by
Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell
Laboratories. Ozan Yigit is the the current maintainer. Brian
Kernighan occasionally dabbles in its development.
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.
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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 AND QUESTIONS
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.
On the other hand, if you have a question as to how to accomplish a
particular task using awk or gawk, send an email to help-gawk@gnu.org
with your request for help.
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, second edition, Alfred V. Aho, Brian W.
Kernighan, Peter J. Weinberger, Addison-Wesley, 2023. ISBN 9-780138-
269722.
GAWK: Effective AWK Programming, Edition 5.3, 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" }
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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 c 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, 2023, 2024
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.
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