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GAWK(1) Utility Commands GAWK(1)
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-spe‐
cific extensions.
The command line consists of options to gawk itself, the AWK pro‐
gram 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.
When gawk is invoked with the --profile option, it starts gather‐
ing profiling statistics from the execution of the program. Gawk
runs more slowly in this mode, and automatically produces an exe‐
cution profile in the file awkprof.out when done. See the --pro‐
file option, below.
Gawk also has an integrated debugger. An interactive debugging
session can be started by supplying the --debug option to the com‐
mand line. 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.
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 alphabeti‐
cally 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 (or --file) 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. In other
words, don't pay any attention to the locale information
when attempting to process strings as multibyte 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. See GNU EXTEN‐
SIONS, below, for more information.
-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. If no file is provided, gawk uses a
file named awkvars.out in the current directory.
Having a list of all the global variables is a good way to
look for typographical errors in your programs. You would
also use this option if you have a large program with a lot
of functions, and you want to be sure that your functions
don't inadvertently use global variables that you meant to
be local. (This is a particularly easy mistake to make
with simple variable names like i, j, and so on.)
-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-interac‐
tively.
-e program-text
--source program-text
Use program-text as AWK program source code. This option
allows the easy intermixing of library functions (used via
the -f and --include options) with source code entered on
the command line. It is intended primarily for medium to
large AWK programs used in shell scripts. 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 pro‐
cessed. 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 dis‐
ables 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. See the GNU gettext
distribution for more information on .pot files.
-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 consti‐
tute the main program source. Files read with --include
are treated as if they begin with an implicit @namespace
"awk" statement.
-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 rou‐
tine 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. With an optional
argument of fatal, lint warnings become fatal errors. This
may be drastic, but its use will certainly encourage the
development of cleaner AWK programs. With an optional
argument of invalid, only warnings about things that are
actually invalid are issued. (This is not fully implemented
yet.) With an optional argument of no-ext, warnings about
gawk extensions are disabled.
-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.)
-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. Although the POSIX standard requires
this behavior, and gawk does so when --posix is in effect,
the default is to follow traditional behavior and use a
period as the decimal point, even in locales where the
period is not the decimal point character. This option
overrides the default behavior, without the full draconian
strictness of the --posix option.
-o[file]
--pretty-print[=file]
Output a pretty printed version of the program to file. If
no file is provided, gawk uses a file named awkprof.out in
the current directory. This option implies --no-optimize.
-O
--optimize
Enable gawk's default optimizations upon the internal rep‐
resentation of the program. Currently, this just includes
simple constant folding. 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. The profile con‐
tains execution counts of each statement in the program in
the left margin and function call counts for each user-
defined function. This option implies --no-optimize.
-P
--posix
This turns on compatibility mode, with the following addi‐
tional restrictions:
· \x escape sequences are not recognized.
· You cannot continue lines after ? and :.
· The synonym func for the keyword function is not recog‐
nized.
· The operators ** and **= cannot be used in place of ^ and
^=.
-r
--re-interval
Enable the use of interval expressions in regular expres‐
sion matching (see Regular Expressions, below). Interval
expressions were not traditionally available in the AWK
language. The POSIX standard added them, to make awk and
egrep consistent with each other. They are enabled by
default, but this option remains for use together with
--traditional.
-s
--no-optimize
Disable gawk's default optimizations upon the internal rep‐
resentation 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. This
effectively blocks a script from accessing local resources,
except for the files specified on the command line.
-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 mainly for knowing
if the current copy of gawk on your system is up to date
with respect to whatever the Free Software Foundation is
distributing. This is also useful when reporting bugs.
(Per the GNU Coding Standards, these options cause an imme‐
diate, successful exit.)
-- Signal the end of options. This is useful to allow further
arguments to the AWK program itself to start with a “-”.
This provides consistency with the argument parsing conven‐
tion used by most other POSIX programs.
In compatibility mode, any other options are flagged as invalid,
but are otherwise ignored. In normal operation, as long as pro‐
gram text has been supplied, unknown options are passed on to the
AWK program in the ARGV array for processing. This is particu‐
larly useful for running AWK programs via the #! executable
interpreter mechanism.
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, pat‐
tern-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. This is useful for building
libraries of AWK functions, without having to include them in each
new AWK program that uses them. It also provides the ability to
mix library functions with command line programs.
In addition, lines beginning with @include may be used to include
other source files into your program, making library use even eas‐
ier. 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, depend‐
ing upon how gawk was built and installed.) If a file name given
to the -f option contains a “/” character, no path search is per‐
formed.
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.) Command line variable assignment is most useful
for dynamically assigning values to the variables AWK uses to con‐
trol how input is broken into fields and records. It is also use‐
ful for controlling state if multiple passes are needed over a
single data file.
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. Simi‐
larly, gawk executes the code associated with ENDFILE after pro‐
cessing 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.
Starting with version 4.0 of 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. Gawk provides true arrays of arrays; see Arrays,
below. 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. If RS is any single character, that charac‐
ter separates records. Otherwise, RS is a regular expression.
Text in the input that matches this regular expression separates
the record. However, in compatibility mode, only the first char‐
acter of its string value is used for separating records. If RS
is set to the null string, then records are separated by empty
lines. When RS is set to the null string, the newline character
always acts as a field separator, in addition to whatever value FS
may have.
Fields
As each input record is read, gawk splits the record into fields,
using the value of the FS variable as the field separator. If FS
is a single character, fields are separated by that character. If
FS is the null string, then each individual character becomes a
separate field. Otherwise, FS is expected to be a full regular
expression. In the special case that FS is a single space, fields
are separated by runs of spaces and/or tabs and/or newlines.
NOTE: The value of IGNORECASE (see below) also affects how fields
are split when FS is a regular expression, and how records are
separated when RS is a regular expression.
If the FIELDWIDTHS variable is set to a space-separated list of
numbers, each field is expected to have fixed width, and gawk
splits up the record using the specified widths. Each field width
may optionally be preceded by a colon-separated value specifying
the number of characters to skip before the field starts. The
value of FS is ignored. Assigning a new value to FS or FPAT over‐
rides the use of FIELDWIDTHS.
Similarly, if the FPAT variable is set to a string representing a
regular expression, each field is made up of text that matches
that regular expression. In this case, the regular expression
describes the fields themselves, instead of the text that sepa‐
rates the fields. Assigning a new value to FS or FIELDWIDTHS
overrides the use of FPAT.
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. Fields need not be referenced by constants:
n = 5
print $n
prints the fifth field in the input record.
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 inter‐
vening 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:
ARGC The number of command line arguments (does not include
options to gawk, or the program source).
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. Dynamically changing the contents
of ARGV can control the files used for data.
BINMODE On non-POSIX systems, specifies use of “binary” mode
for all file I/O. Numeric values of 1, 2, or 3, spec‐
ify that input files, output files, or all files,
respectively, should use binary I/O. String values of
"r", or "w" specify that input files, or output files,
respectively, should use binary I/O. String values of
"rw" or "wr" specify that all files should use binary
I/O. Any other string value is treated as "rw", but
generates a warning message.
CONVFMT The conversion format for numbers, "%.6g", by default.
ENVIRON An array containing the values of the current environ‐
ment. The array is indexed by the environment vari‐
ables, each element being the value of that variable
(e.g., ENVIRON["HOME"] might be "/home/arnold").
In POSIX mode, changing this array does not affect the
environment seen by programs which gawk spawns via re‐
direction or the system() function. Otherwise, gawk
updates its real environment so that programs it
spawns see the changes.
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. If the string in ERRNO corresponds
to a system error in the errno(3) variable, then the
numeric value can be found in PROCINFO["errno"]. For
non-system errors, PROCINFO["errno"] will be zero.
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.
See Fields, above.
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. See Fields, above.
FS The input field separator, a space by default. See
Fields, above.
FUNCTAB An array whose indices and corresponding values are
the names of all the user-defined or extension func‐
tions in the program. NOTE: You may not use the
delete statement with the FUNCTAB array.
IGNORECASE Controls the case-sensitivity of all regular expres‐
sion and string operations. If IGNORECASE has a non-
zero value, then string comparisons and pattern match‐
ing in rules, field splitting with FS and FPAT, record
separating with RS, regular expression matching with ~
and !~, and the gensub(), gsub(), index(), match(),
patsplit(), split(), and sub() built-in functions all
ignore case when doing regular expression operations.
NOTE: Array subscripting is not affected. However,
the asort() and asorti() functions are affected.
Thus, if IGNORECASE is not equal to zero, /aB/ matches
all of the strings "ab", "aB", "Ab", and "AB". As
with all AWK variables, the initial value of IGNORE‐
CASE is zero, so all regular expression and string
operations are normally case-sensitive.
LINT Provides dynamic control of the --lint option from
within an AWK program. When true, gawk prints lint
warnings. When false, it does not. When assigned the
string value "fatal", lint warnings become fatal
errors, exactly like --lint=fatal. Any other true
value just prints warnings.
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 informa‐
tion about the running AWK program. On some systems,
there may be elements in the array, "group1" through
"groupn" for some n, which is the number of supplemen‐
tary groups that the process has. Use the in operator
to test for these elements. The following elements
are guaranteed to be available:
PROCINFO["argv"] The command line arguments as
received by gawk at the C-lan‐
guage level. The subscripts
start from zero.
PROCINFO["egid"] The value of the getegid(2) sys‐
tem call.
PROCINFO["errno"] The value of errno(3) when ERRNO
is set to the associated error
message.
PROCINFO["euid"] The value of the geteuid(2) sys‐
tem call.
PROCINFO["FS"] "FS" if field splitting with FS
is in effect, "FPAT" if field
splitting with FPAT is in effect,
"FIELDWIDTHS" if field splitting
with FIELDWIDTHS is in effect, or
"API" if API input parser field
splitting is in effect.
PROCINFO["gid"] The value of the getgid(2) system
call.
PROCINFO["identifiers"]
A subarray, indexed by the names
of all identifiers used in the
text of the AWK program. The
values indicate what gawk knows
about the identifiers after it
has finished parsing the program;
they are not updated while the
program runs. For each identi‐
fier, the value of the element is
one of the following:
"array" The identifier is an
array.
"builtin" The identifier is a
built-in function.
"extension" The identifier is an
extension function
loaded via @load or
--load.
"scalar" The identifier is a
scalar.
"untyped" The identifier is
untyped (could be
used as a scalar or
array, gawk doesn't
know yet).
"user" The identifier is a
user-defined func‐
tion.
PROCINFO["pgrpid"] The value of the getpgrp(2) sys‐
tem call.
PROCINFO["pid"] The value of the getpid(2) system
call.
PROCINFO["platform"] A string indicating the platform
for which gawk was compiled. It
is one of:
"djgpp", "mingw"
Microsoft Windows, using
either DJGPP, or MinGW,
respectively.
"os2" OS/2.
"posix"
GNU/Linux, Cygwin, Mac OS
X, and legacy Unix sys‐
tems.
"vms" OpenVMS or Vax/VMS.
PROCINFO["ppid"] The value of the getppid(2) sys‐
tem call.
PROCINFO["strftime"] The default time format string
for strftime(). Changing its
value affects how strftime() for‐
mats time values when called with
no arguments.
PROCINFO["uid"] The value of the getuid(2) system
call.
PROCINFO["version"] The version of gawk.
The following elements are present if loading dynamic
extensions is available:
PROCINFO["api_major"]
The major version of the extension API.
PROCINFO["api_minor"]
The minor version of the extension API.
The following elements are available if MPFR support
is compiled into gawk:
PROCINFO["gmp_version"]
The version of the GNU GMP library used for
arbitrary precision number support in gawk.
PROCINFO["mpfr_version"]
The version of the GNU MPFR library used for
arbitrary precision number support in gawk.
PROCINFO["prec_max"]
The maximum precision supported by the GNU MPFR
library for arbitrary precision floating-point
numbers.
PROCINFO["prec_min"]
The minimum precision allowed by the GNU MPFR
library for arbitrary precision floating-point
numbers.
The following elements may set by a program to change
gawk's behavior:
PROCINFO["NONFATAL"]
If this exists, then I/O errors for all redi‐
rections become nonfatal.
PROCINFO["name", "NONFATAL"]
Make I/O errors for name be nonfatal.
PROCINFO["command", "pty"]
Use a pseudo-tty for two-way communication with
command instead of setting up two one-way
pipes.
PROCINFO["input", "READ_TIMEOUT"]
The timeout in milliseconds for reading data
from input, where input is a redirection string
or a filename. A value of zero or less than
zero means no timeout.
PROCINFO["input", "RETRY"]
If an I/O error that may be retried occurs when
reading data from input, and this array entry
exists, then getline returns -2 instead of fol‐
lowing the default behavior of returning -1 and
configuring input to return no further data.
An I/O error that may be retried is one where
errno(3) has the value EAGAIN, EWOULDBLOCK,
EINTR, or ETIMEDOUT. This may be useful in
conjunction with PROCINFO["input", "READ_TIME‐
OUT"] or in situations where a file descriptor
has been configured to behave in a non-blocking
fashion.
PROCINFO["sorted_in"]
If this element exists in PROCINFO, then its
value controls the order in which array ele‐
ments are traversed in for loops. Supported
values are "@ind_str_asc", "@ind_num_asc",
"@val_type_asc", "@val_str_asc",
"@val_num_asc", "@ind_str_desc",
"@ind_num_desc", "@val_type_desc",
"@val_str_desc", "@val_num_desc", and
"@unsorted". The value can also be the name
(as a string) of any comparison function
defined as follows:
function cmp_func(i1, v1, i2, v2)
where i1 and i2 are the indices, and v1 and v2
are the corresponding values of the two ele‐
ments being compared. It should return a num‐
ber less than, equal to, or greater than 0,
depending on how the elements of the array are
to be ordered.
ROUNDMODE The rounding mode to use for arbitrary precision
arithmetic on numbers, by default "N" (IEEE-754
roundTiesToEven mode). The accepted values are:
"A" or "a"
for rounding away from zero. These are only
available if your version of the GNU MPFR
library supports rounding away from zero.
"D" or "d" for roundTowardNegative.
"N" or "n" for roundTiesToEven.
"U" or "u" for roundTowardPositive.
"Z" or "z" for roundTowardZero.
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 spec‐
ified by RS.
RSTART The index of the first character matched by match(); 0
if no match. (This implies that character indices
start at one.)
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.
The array may be used for indirect access to read or
write the value of a variable:
foo = 5
SYMTAB["foo"] = 4
print foo # prints 4
The typeof() function may be used to test if an ele‐
ment in SYMTAB is an array. 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 con‐
catenation of the (string) value of each expression, separated by
the value of the SUBSEP variable. This facility is used to simu‐
late 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 con‐
struct only works in tests, not in for loops.
An element may be deleted from an array using the delete state‐
ment. 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++. For example:
a[1] = 5
a[2][1] = 6
a[2][2] = 7
NOTE: You may need to tell gawk that an array element is really a
subarray in order to use it where gawk expects an array (such as
in the second argument to split()). You can do this by creating
an element in the subarray and then deleting it with the delete
statement.
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 names‐
pace and the right-hand identifier is the variable within it. All
simple (non-qualified) names are considered to be in the ``cur‐
rent'' 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" direc‐
tive.
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 GAWK: Effective AWK Pro‐
gramming.
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. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a string value of "12" and not "12.00".
NOTE: When operating in POSIX mode (such as with the --posix
option), beware that locale settings may interfere with the way
decimal numbers are treated: the decimal separator of the numbers
you are feeding to gawk must conform to what your locale would
expect, be it a comma (,) or a period (.).
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 per‐
formed. 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 pat‐
split() 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. These are:
\\ A literal backslash.
\a The “alert” character; usually the ASCII BEL character.
\b Backspace.
\f Form-feed.
\n Newline.
\r Carriage return.
\t Horizontal tab.
\v Vertical tab.
\xhex digits
The character represented by the string of hexadecimal digits
following the \x. Up to two following hexadecimal digits are
considered part of the escape sequence. E.g., "\x1B" is the
ASCII ESC (escape) character.
\ddd The character represented by the 1-, 2-, or 3-digit sequence
of octal digits. E.g., "\033" is the ASCII ESC (escape)
character.
\c The literal character c.
In compatibility mode, the characters represented by octal and
hexadecimal escape sequences are treated literally when used in
regular expression constants. Thus, /a\52b/ is equivalent to
/a\*b/.
Regexp Constants
A regular expression constant is a sequence of characters enclosed
between forward slashes (like /value/). Regular expression match‐
ing is described more fully below; see Regular Expressions.
The escape sequences described earlier may also be used inside
constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches white‐
space 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. Nor‐
mally, a statement ends with a newline, however, this is not the
case for lines ending in a comma, {, ?, :, &&, or ||. Lines end‐
ing in do or else also have their statements automatically contin‐
ued on the following line. In other cases, a line can be contin‐
ued 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 com‐
mand-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 expres‐
sion. Regular expressions are the same as those in egrep(1), and
are summarized below.
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 log‐
ical NOT, respectively, as in C. They do short-circuit evalua‐
tion, also as in C, and are used for combining more primitive pat‐
tern 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 pat‐
tern is true then the pattern used for testing is the second pat‐
tern, 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 pat‐
tern2, inclusive. It does not combine with any other sort of pat‐
tern expression.
Regular Expressions
Regular expressions are the extended kind found in egrep. They
are composed of characters as follows:
c Matches the non-metacharacter c.
\c Matches the literal character c.
. Matches any character including newline.
^ Matches the beginning of a string.
$ Matches the end of a string.
[abc...] A character list: matches any of the characters abc....
You may include a range of characters by separating
them with a dash. To include a literal dash in the
list, put it first or last.
[^abc...] A negated character list: matches any character except
abc....
r1|r2 Alternation: matches either r1 or r2.
r1r2 Concatenation: matches r1, and then r2.
r+ Matches one or more r's.
r* Matches zero or more r's.
r? Matches zero or one r's.
(r) Grouping: matches r.
r{n}
r{n,}
r{n,m} One or two numbers inside braces denote an interval
expression. If there is one number in the braces, the
preceding regular expression r is repeated n times. If
there are two numbers separated by a comma, r is
repeated n to m times. If there is one number followed
by a comma, then r is repeated at least n times.
\y Matches the empty string at either the beginning or the
end of a word.
\B Matches the empty string within a word.
\< Matches the empty string at the beginning of a word.
\> Matches the empty string at the end of a word.
\s Matches any whitespace character.
\S Matches any nonwhitespace character.
\w Matches any word-constituent character (letter, digit,
or underscore).
\W Matches any character that is not word-constituent.
\` Matches the empty string at the beginning of a buffer
(string).
\' Matches the empty string at the end of a buffer.
The escape sequences that are valid in string constants (see
String Constants) are also valid in regular expressions.
Character classes are a feature introduced in the POSIX standard.
A character class is a special notation for describing lists of
characters that have a specific attribute, but where the actual
characters themselves can vary from country to country and/or from
character set to character set. For example, the notion of what
is an alphabetic character differs in the USA and in France.
A character class is only valid in a regular expression inside the
brackets of a character list. Character classes consist of [:, a
keyword denoting the class, and :]. The character classes defined
by the POSIX standard are:
[:alnum:] Alphanumeric characters.
[:alpha:] Alphabetic characters.
[:blank:] Space or tab characters.
[:cntrl:] Control characters.
[:digit:] Numeric characters.
[:graph:] Characters that are both printable and visible. (A
space is printable, but not visible, while an a is
both.)
[:lower:] Lowercase alphabetic characters.
[:print:] Printable characters (characters that are not control
characters.)
[:punct:] Punctuation characters (characters that are not letter,
digits, control characters, or space characters).
[:space:] Space characters (such as space, tab, and formfeed, to
name a few).
[:upper:] Uppercase alphabetic characters.
[:xdigit:] Characters that are hexadecimal digits.
For example, before the POSIX standard, to match alphanumeric
characters, you would have had to write /[A-Za-z0-9]/. If your
character set had other alphabetic characters in it, this would
not match them, and if your character set collated differently
from ASCII, this might not even match the ASCII alphanumeric char‐
acters. With the POSIX character classes, you can write
/[[:alnum:]]/, and this matches the alphabetic and numeric charac‐
ters in your character set, no matter what it is.
Two additional special sequences can appear in character lists.
These apply to non-ASCII character sets, which can have single
symbols (called collating elements) that are represented with more
than one character, as well as several characters that are equiva‐
lent for collating, or sorting, purposes. (E.g., in French, a
plain “e” and a grave-accented “`” are equivalent.)
Collating Symbols
A collating symbol is a multi-character collating element
enclosed in [. and .]. For example, if ch is a collating
element, then [[.ch.]] is a regular expression that
matches this collating element, while [ch] is a regular
expression that matches either c or h.
Equivalence Classes
An equivalence class is a locale-specific name for a list
of characters that are equivalent. The name is enclosed in
[= and =]. For example, the name e might be used to repre‐
sent all of “e”, “´”, and “`”. In this case, [[=e=]] is a
regular expression that matches any of e, ´, or `.
These features are very valuable in non-English speaking locales.
The library functions that gawk uses for regular expression match‐
ing currently only recognize POSIX character classes; they do not
recognize collating symbols or equivalence classes.
The \y, \B, \<, \>, \s, \S, \w, \W, \`, and \' operators are spe‐
cific to gawk; they are extensions based on facilities in the GNU
regular expression libraries.
The various command line options control how gawk interprets char‐
acters in regular expressions.
No options
In the default case, gawk provides all the facilities of
POSIX regular expressions and the GNU regular expression
operators described above.
--posix
Only POSIX regular expressions are supported, the GNU oper‐
ators are not special. (E.g., \w matches a literal w).
--traditional
Traditional UNIX awk regular expressions are matched. The
GNU operators are not special, and interval expressions are
not available. Characters described by octal and hexadeci‐
mal escape sequences are treated literally, even if they
represent regular expression metacharacters.
--re-interval
Allow interval expressions in regular expressions, even if
--traditional has been provided.
Actions
Action statements are enclosed in braces, { and }. Action state‐
ments consist of the usual assignment, conditional, and looping
statements found in most languages. The operators, control state‐
ments, 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 (** may also be used, and **= for the
assignment operator).
+ - ! 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. NOTE: Do not
use a constant regular expression (/foo/) on the left-
hand side of a ~ or !~. Only use one on the right-
hand side. The expression /foo/ ~ exp has the same
meaning as (($0 ~ /foo/) ~ exp). This is usually not
what you want.
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 sup‐
ported.
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 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.
Coprocesses are a gawk extension. (The com‐
mand can also be a socket. See the subsec‐
tion Special File Names, below.)
next Stop processing the current input record.
Read the next input record and start pro‐
cessing 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 sepa‐
rated 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 out‐
put record is terminated with the value of
ORS.
printf fmt, expr-list Format and print. See The printf Statement,
below.
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 GAWK: Effective AWK
Programming for the full details on the exit
status.
fflush([file]) Flush any buffers associated with the open
output file or pipe file. If file is miss‐
ing or if it is the null string, then flush
all open output files and pipes.
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 subsec‐
tion 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 printf Statement
The AWK versions of the printf statement and sprintf() function
(see below) accept the following conversion specification formats:
%a, %A A floating point number of the form [-]0xh.hhhhp+-dd (C99
hexadecimal floating point format). For %A, uppercase
letters are used instead of lowercase ones.
%c A single character. If the argument used for %c is
numeric, it is treated as a character and printed. Other‐
wise, the argument is assumed to be a string, and the only
first character of that string is printed.
%d, %i A decimal number (the integer part).
%e, %E A floating point number of the form [-]d.dddddde[+-]dd.
The %E format uses E instead of e.
%f, %F A floating point number of the form [-]ddd.dddddd. If the
system library supports it, %F is available as well. This
is like %f, but uses capital letters for special “not a
number” and “infinity” values. If %F is not available,
gawk uses %f.
%g, %G Use %e or %f conversion, whichever is shorter, with non‐
significant zeros suppressed. The %G format uses %E
instead of %e.
%o An unsigned octal number (also an integer).
%u An unsigned decimal number (again, an integer).
%s A character string.
%x, %X An unsigned hexadecimal number (an integer). The %X for‐
mat uses ABCDEF instead of abcdef.
%% A single % character; no argument is converted.
Optional, additional parameters may lie between the % and the con‐
trol letter:
count$ Use the count'th argument at this point in the formatting.
This is called a positional specifier and is intended pri‐
marily for use in translated versions of format strings,
not in the original text of an AWK program. It is a gawk
extension.
- The expression should be left-justified within its field.
space For numeric conversions, prefix positive values with a
space, and negative values with a minus sign.
+ The plus sign, used before the width modifier (see below),
says to always supply a sign for numeric conversions, even
if the data to be formatted is positive. The + overrides
the space modifier.
# Use an “alternate form” for certain control letters. For
%o, supply a leading zero. For %x, and %X, supply a lead‐
ing 0x or 0X for a nonzero result. For %e, %E, %f and %F,
the result always contains a decimal point. For %g, and
%G, trailing zeros are not removed from the result.
0 A leading 0 (zero) acts as a flag, indicating that output
should be padded with zeroes instead of spaces. This
applies only to the numeric output formats. This flag only
has an effect when the field width is wider than the value
to be printed.
' A single quote character instructs gawk to insert the
locale's thousands-separator character into decimal num‐
bers, and to also use the locale's decimal point character
with floating point formats. This requires correct locale
support in the C library and in the definition of the cur‐
rent locale.
width The field should be padded to this width. The field is
normally padded with spaces. With the 0 flag, it is padded
with zeroes.
.prec A number that specifies the precision to use when printing.
For the %e, %E, %f and %F, formats, this specifies the num‐
ber of digits you want printed to the right of the decimal
point. For the %g, and %G formats, it specifies the maxi‐
mum number of significant digits. For the %d, %i, %o, %u,
%x, and %X formats, it specifies the minimum number of dig‐
its to print. For the %s format, it specifies the maximum
number of characters from the string that should be
printed.
The dynamic width and prec capabilities of the ISO C printf() rou‐
tines are supported. A * in place of either the width or prec
specifications causes their values to be taken from the argument
list to printf or sprintf(). To use a positional specifier with a
dynamic width or precision, supply the count$ after the * in the
format string. For example, "%3$*2$.*1$s".
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.
These are particularly useful for error messages. For example:
print "You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print "You blew it!" | "cat 1>&2"
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 con‐
nection, 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 gen‐
erator. 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:
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 destina‐
tion 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 any of the strings
valid for PROCINFO["sorted_in"]. It can
also be the name of a user-defined compar‐
ison function as described in
PROCINFO["sorted_in"].
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 origi‐
nal values are lost; thus provide a second
array if you wish to preserve the origi‐
nal. The purpose of the optional string
how is the same as described previously
for asort().
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 char‐
acter &. 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 GAWK:
Effective AWK Programming 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.) It is a fatal error to use a
regexp constant for t.
length([s]) Return the length of the string s, or the
length of $0 if s is not supplied. As a
non-standard extension, 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.
If array a is provided, a is cleared and
then elements 1 through n are filled with
the portions of s that match the corre‐
sponding parenthesized subexpression in r.
The zero'th element of a contains the por‐
tion of s matched by the entire regular
expression r. Subscripts a[n, "start"],
and a[n, "length"] provide the starting
index in the string and length respec‐
tively, of each matching substring.
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, described above.
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 sepa‐
rator matched by r between a[i] and
a[i+1]. If r is a single space, then
leading whitespace in s goes into the
extra array element seps[0] and trailing
whitespace goes into the extra array ele‐
ment seps[n], where n is the return value
of split(s, a, r, seps). Splitting
behaves identically to field splitting,
described above. In particular, if r is a
single-character string, that string acts
as the separator, even if it happens to be
a regular expression metacharacter.
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 hexadeci‐
mal 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 counter‐
parts. 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 counter‐
parts. Non-alphabetic characters are left
unchanged.
Gawk is multibyte aware. This means that index(), length(), sub‐
str() and match() all work in terms of characters, not bytes.
Time Functions
Since one of the primary uses of AWK programs is processing log
files that contain time stamp information, gawk provides the fol‐
lowing functions for obtaining time stamps and formatting them.
mktime(datespec [, utc-flag])
Turn datespec into a time stamp of the same form as
returned by systime(), and return the result. The date‐
spec is a string of the form YYYY MM DD HH MM SS[ DST].
The contents of the string are six or seven numbers rep‐
resenting respectively the full year including century,
the month from 1 to 12, the day of the month from 1 to
31, the hour of the day from 0 to 23, the minute from 0
to 59, the second from 0 to 60, and an optional daylight
saving flag. The values of these numbers need not be
within the ranges specified; for example, an hour of -1
means 1 hour before midnight. The origin-zero Gregorian
calendar is assumed, with year 0 preceding year 1 and
year -1 preceding year 0. 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 the DST daylight saving flag is
positive, the time is assumed to be daylight saving
time; if zero, the time is assumed to be standard time;
and if negative (the default), mktime() attempts to
determine whether daylight saving time is in effect for
the specified time. If datespec does not contain enough
elements or if the resulting time is out of range,
mktime() returns -1.
strftime([format [, timestamp[, utc-flag]]])
Format timestamp according to the specification in for‐
mat. 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 sys‐
tems).
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.
NOTE: 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. This
function is mainly for use with the elements of multi‐
dimensional arrays and with function parameters.
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 GAWK:
Effective AWK Programming.
bindtextdomain(directory [, domain])
Specify the directory where gawk looks for the .gmo files,
in case they will not or cannot be placed in the ``stan‐
dard'' locations (e.g., during testing). It returns the
directory where domain is ``bound.''
The default domain is the value of TEXTDOMAIN. If direc‐
tory 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".
If you supply a value for category, it must be a string
equal to one of the known locale categories described in
GAWK: Effective AWK Programming. You must also supply a
text domain. Use TEXTDOMAIN if you want to use the current
domain.
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 cate‐
gory is "LC_MESSAGES".
If you supply a value for category, it must be a string
equal to one of the known locale categories described in
GAWK: Effective AWK Programming. You must also supply a
text domain. Use TEXTDOMAIN if you want to use the current
domain.
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.
Since functions were not originally part of the AWK language, the
provision for local variables is rather clumsy: They 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 avoids a syntactic ambiguity with the concatenation operator.
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.
As a gawk extension, 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
...
}
As of version 4.1.2, this works with user-defined functions,
built-in functions, and extension functions.
If --lint has been provided, gawk warns about calls to undefined
functions at parse time, instead of at run time. Calling an unde‐
fined function at run time is a fatal error.
The word func may be used in place of function, although this is
deprecated.
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 GAWK: Effec‐
tive AWK Programming.
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.
There are several steps involved in producing and running a local‐
izable AWK program.
1. Add a BEGIN action to assign a value to the TEXTDOMAIN vari‐
able to set the text domain to a name associated with your
program:
BEGIN { TEXTDOMAIN = "myprog" }
This allows gawk to find the .gmo file associated with your
program. Without this step, gawk uses the messages text
domain, which likely does not contain translations for your
program.
2. Mark all strings that should be translated with leading under‐
scores.
3. If necessary, use the dcgettext() and/or bindtextdomain()
functions in your program, as appropriate.
4. Run gawk --gen-pot -f myprog.awk > myprog.pot to generate a
.pot file for your program.
5. Provide appropriate translations, and build and install the
corresponding .gmo files.
The internationalization features are described in full detail in
GAWK: Effective AWK Programming.
POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX standard,
as well as with the latest version of Brian Kernighan's awk. To
this end, gawk incorporates the following user visible features
which are not described in the AWK book, but are part of the Brian
Kernighan's version of awk, and are in the POSIX standard.
The book indicates that command line variable assignment happens
when awk would otherwise open the argument as a file, which is
after the BEGIN rule is executed. However, in earlier implementa‐
tions, when such an assignment appeared before any file names, the
assignment would happen before the BEGIN rule was run. Applica‐
tions came to depend on this “feature.” When awk was changed to
match its documentation, the -v option for assigning variables
before program execution was added to accommodate applications
that depended upon the old behavior. (This feature was agreed
upon by both the Bell Laboratories developers and the GNU develop‐
ers.)
When processing arguments, gawk uses the special option “--” to
signal the end of arguments. In compatibility mode, it warns
about but otherwise ignores undefined options. In normal opera‐
tion, such arguments are passed on to the AWK program for it to
process.
The AWK book does not define the return value of srand(). The
POSIX standard has it return the seed it was using, to allow keep‐
ing track of random number sequences. Therefore srand() in gawk
also returns its current seed.
Other features are: The use of multiple -f options (from MKS awk);
the ENVIRON array; the \a, and \v escape sequences (done origi‐
nally in gawk and fed back into the Bell Laboratories version);
the tolower() and toupper() built-in functions (from the Bell Lab‐
oratories version); and the ISO C conversion specifications in
printf (done first in the Bell Laboratories version).
HISTORICAL FEATURES
There is one feature of historical AWK implementations that gawk
supports: It is possible to call the length() built-in function
not only with no argument, but even without parentheses! Thus,
a = length # Holy Algol 60, Batman!
is the same as either of
a = length()
a = length($0)
Using this feature is poor practice, and gawk issues a warning
about its use if --lint is specified on the command line.
GNU EXTENSIONS
Gawk has a too-large number of extensions to POSIX awk. They are
described in this section. All the extensions described here can
be disabled by invoking gawk with the --traditional or --posix
options.
The following features of gawk are not available in POSIX awk.
· No path search is performed for files named via the -f option.
Therefore the AWKPATH environment variable is not special.
· There is no facility for doing file inclusion (gawk's @include
mechanism).
· There is no facility for dynamically adding new functions writ‐
ten in C (gawk's @load mechanism).
· The \x escape sequence.
· The ability to continue lines after ? and :.
· Octal and hexadecimal constants in AWK programs.
· The ARGIND, BINMODE, ERRNO, LINT, PREC, ROUNDMODE, RT and
TEXTDOMAIN variables are not special.
· The IGNORECASE variable and its side-effects are not available.
· The FIELDWIDTHS variable and fixed-width field splitting.
· The FPAT variable and field splitting based on field values.
· The FUNCTAB, SYMTAB, and PROCINFO arrays are not available.
· The use of RS as a regular expression.
· The special file names available for I/O redirection are not
recognized.
· The |& operator for creating coprocesses.
· The BEGINFILE and ENDFILE special patterns are not available.
· The ability to split out individual characters using the null
string as the value of FS, and as the third argument to split().
· An optional fourth argument to split() to receive the separator
texts.
· The optional second argument to the close() function.
· The optional third argument to the match() function.
· The ability to use positional specifiers with printf and
sprintf().
· The ability to pass an array to length().
· The and(), asort(), asorti(), bindtextdomain(), compl(), dcget‐
text(), dcngettext(), gensub(), lshift(), mktime(), or(), pat‐
split(), rshift(), strftime(), strtonum(), systime() and xor()
functions.
· Localizable strings.
· Non-fatal I/O.
· Retryable I/O.
The AWK book does not define the return value of the close() func‐
tion. Gawk's close() returns the value from fclose(3), or
pclose(3), when closing an output file or pipe, respectively. It
returns the process's exit status when closing an input pipe. The
return value is -1 if the named file, pipe or coprocess was not
opened with a redirection.
When gawk is invoked with the --traditional option, if the fs
argument to the -F option is “t”, then FS is set to the tab char‐
acter. Note that typing gawk -F\t ... simply causes the shell to
quote the “t,” and does not pass “\t” to the -F option. Since
this is a rather ugly special case, it is not the default behav‐
ior. This behavior also does not occur if --posix has been speci‐
fied. To really get a tab character as the field separator, it is
best to use single quotes: gawk -F'\t' ....
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 direc‐
tive. 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_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 con‐
trols 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 5.0.
AUTHORS
The original version of UNIX awk was designed and implemented by
Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell Labora‐
tories. 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 dis‐
tributed in Seventh Edition UNIX. John Woods contributed a number
of bug fixes. David Trueman, with contributions from Arnold Rob‐
bins, 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 con‐
tributors to gawk and its documentation.
See the README file in the gawk distribution for up-to-date infor‐
mation about maintainers and which ports are currently supported.
BUG REPORTS
If you find a bug in gawk, please send electronic mail to
bug-gawk@gnu.org. Please include your operating system and its
revision, the version of gawk (from gawk --version), which C com‐
piler you used to compile it, and a test program and data that are
as small as possible for reproducing the problem.
Before sending a bug report, please do the following things.
First, verify that you have the latest version of gawk. Many bugs
(usually subtle ones) are fixed at each release, and if yours is
out of date, the problem may already have been solved. Second,
please see if setting the environment variable LC_ALL to LC_ALL=C
causes things to behave as you expect. If so, it's a locale issue,
and may or may not really be a bug. Finally, please read this man
page and the reference manual carefully to be sure that what you
think is a bug really is, instead of just a quirk in the language.
Whatever you do, do NOT post a bug report in comp.lang.awk. While
the gawk developers occasionally read this newsgroup, posting bug
reports there is an unreliable way to report bugs. Similarly, do
NOT use a web forum (such as Stack Overflow) for reporting bugs.
Instead, please use the electronic mail addresses given above.
Really.
If you're using a GNU/Linux or BSD-based system, you may wish to
submit a bug report to the vendor of your distribution. That's
fine, but please send a copy to the official email address as
well, since there's no guarantee that the bug report will be for‐
warded to the gawk maintainer.
BUGS
The -F option is not necessary given the command line variable
assignment feature; it remains only for backwards compatibility.
SEE ALSO
egrep(1), sed(1), getpid(2), getppid(2), getpgrp(2), getuid(2),
geteuid(2), getgid(2), getegid(2), getgroups(2), printf(3), strf‐
time(3), usleep(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 5.0, 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") }'
ACKNOWLEDGEMENTS
Brian Kernighan provided valuable assistance during testing and
debugging. We thank him.
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, Free Software Founda‐
tion, 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, pro‐
vided 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.
Free Software Foundation May 22 2019 GAWK(1)