GAWK(1) Utility Commands GAWK(1)
gawk - pattern scanning and processing language
gawk [ POSIX or GNU style options ] -f program-file [ -- ] file
...
gawk [ POSIX or GNU style options ] [ -- ] program-text file ...
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.
When gawk is invoked with the --profile option, it starts
gathering profiling statistics from the execution of the program.
Gawk runs more slowly in this mode, and automatically produces an
execution profile in the file awkprof.out when done. See the
--profile option, below.
Gawk also has an integrated debugger. An interactive debugging
session can be started by supplying the --debug option to the
command 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.
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.
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 (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
EXTENSIONS, 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-interactively.
-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
processed. This should be used with #! scripts,
particularly for CGI applications, to avoid passing in
options or source code (!) on the command line from a URL.
This option disables command-line variable assignments.
-g
--gen-pot
Scan and parse the AWK program, and generate a GNU .pot
(Portable Object Template) format file on standard output
with entries for all localizable strings in the program.
The program itself is not executed. 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
constitute the main program source. Files read with
--include are treated as if they begin with an implicit
@namespace "awk" statement.
-I
--trace
Print the internal byte code names as they are executed
when running the program. The trace is printed to standard
error. Each ``op code'' is preceded by a + sign in the
output.
-l lib
--load lib
Load a gawk extension from the shared library lib. This
searches for the library using the AWKLIBPATH environment
variable. If the initial search fails, another attempt
will be made after appending the default shared library
suffix for the platform. The library initialization
routine is expected to be named dl_load().
-L [value]
--lint[=value]
Provide warnings about constructs that are dubious or non-
portable to other AWK implementations. 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
representation 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
contains 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
additional restrictions:
• \x escape sequences are not recognized.
• You cannot continue lines after ? and :.
• The synonym func for the keyword function is not
recognized.
• The operators ** and **= cannot be used in place of
^ and ^=.
-r
--re-interval
Enable the use of interval expressions in regular
expression 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
representation of the program.
-S
--sandbox
Run gawk in sandbox mode, disabling the system() function,
input redirection with getline, output redirection with
print and printf, and loading dynamic extensions. Command
execution (through pipelines) is also disabled. 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
immediate, 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
convention 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
program text has been supplied, unknown options are passed on to
the AWK program in the ARGV array for processing. This is
particularly 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.
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. 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
easier. This is equivalent to using the --include option.
Lines beginning with @load may be used to load extension
functions into your program. This is equivalent to using the
--load option.
The environment variable AWKPATH specifies a search path to use
when finding source files named with the -f and --include
options. If this variable does not exist, the default path is
".:/usr/local/share/awk". (The actual directory may vary,
depending upon how gawk was built and installed.) If a file name
given to the -f option contains a “/” character, no path search
is performed.
The environment variable AWKLIBPATH specifies a search path to
use when finding source files named with the --load option. If
this variable does not exist, the default path is
"/usr/local/lib/gawk". (The actual directory may vary, depending
upon how gawk was built and installed.)
Gawk executes AWK programs in the following order. First, all
variable assignments specified via the -v option are performed.
Next, gawk compiles the program into an internal form. Then,
gawk executes the code in the BEGIN rule(s) (if any), and then
proceeds to read each file named in the ARGV array (up to
ARGV[ARGC-1]). If there are no files named on the command line,
gawk reads the standard input.
If a filename on the command line has the form var=val it is
treated as a variable assignment. The variable var will be
assigned the value val. (This happens after any BEGIN rule(s)
have been run.) Command line variable assignment is most useful
for dynamically assigning values to the variables AWK uses to
control how input is broken into fields and records. It is also
useful 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.
Similarly, gawk executes the code associated with ENDFILE 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.
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.
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
character 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 character 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 overrides 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
separates 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
intervening fields with the null string as their values, and
causes the value of $0 to be recomputed, with the fields being
separated by the value of OFS. References to negative numbered
fields cause a fatal error. Decrementing NF causes the values of
fields past the new value to be lost, and the value of $0 to be
recomputed, with the fields being separated by the value of OFS.
Assigning a value to an existing field causes the whole record to
be rebuilt when $0 is referenced. Similarly, assigning a value
to $0 causes the record to be resplit, creating new values for
the fields.
Built-in Variables
Gawk's built-in variables are:
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, specify 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 environment.
The array is indexed by the environment variables, 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
redirection 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 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. If IGNORECASE has a non-zero
value, then string comparisons and pattern matching 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 IGNORECASE 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. The values allowed for the
--lint option may also be assigned to LINT, with the same
effects. 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 information
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 supplementary 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-language level. The subscripts start from
zero.
PROCINFO["egid"]
The value of the getegid(2) system 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) system 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
identifier, 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 function.
PROCINFO["pgrpid"]
The value of the getpgrp(2) system 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
systems.
"vms" OpenVMS or Vax/VMS.
PROCINFO["ppid"]
The value of the getppid(2) system call.
PROCINFO["strftime"]
The default time format string for strftime().
Changing its value affects how strftime() formats
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
redirections 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
following 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_TIMEOUT"] 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 elements 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 elements being
compared. It should return a number 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 specified
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 element 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
concatenation of the (string) value of each expression, separated
by the value of the SUBSEP variable. This facility is used to
simulate multiply dimensioned arrays. For example:
i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"
assigns the string "hello, world\n" to the element of the array x
which is indexed by the string "A\034B\034C". All arrays in AWK
are associative, i.e., indexed by string values.
The special operator in may be used to test if an array has an
index consisting of a particular value:
if (val in array)
print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct may also be used in a for loop to iterate over
all the elements of an array. However, the (i, j) in array
construct only works in tests, not in for loops.
An element may be deleted from an array using the delete
statement. The delete statement may also be used to delete the
entire contents of an array, just by specifying the array name
without a subscript.
gawk supports true multidimensional arrays. It does not require
that such arrays be ``rectangular'' as in C or C++. 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
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 GAWK: Effective AWK
Programming.
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
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. 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 matching 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
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.
AWK is a line-oriented language. The pattern comes first, and
then the action. Action statements are enclosed in { and }.
Either the pattern may be missing, or the action may be missing,
but, of course, not both. If the pattern is missing, the action
executes for every single record of input. A missing action is
equivalent to
{ print }
which prints the entire record.
Comments begin with the # character, and continue until the end
of the line. Empty lines may be used to separate statements.
Normally, a statement ends with a newline, however, this is not
the case for lines ending in a comma, {, ?, :, &&, or ||. Lines
ending in do or else also have their statements automatically
continued on the following line. In other cases, a line can be
continued by ending it with a “\”, in which case the newline is
ignored. However, a “\” after a # is not special.
Multiple statements may be put on one line by separating them
with a “;”. This applies to both the statements within the
action part of a pattern-action pair (the usual case), and to the
pattern-action statements themselves.
Patterns
AWK patterns may be one of the following:
BEGIN
END
BEGINFILE
ENDFILE
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN and END are two special kinds of patterns which are not
tested against the input. The action parts of all BEGIN patterns
are merged as if all the statements had been written in a single
BEGIN rule. They are executed before any of the input is read.
Similarly, all the END rules are merged, and executed when all
the input is exhausted (or when an exit statement is executed).
BEGIN and END patterns cannot be combined with other patterns in
pattern expressions. BEGIN and END patterns cannot have missing
action parts.
BEGINFILE and ENDFILE are additional special patterns whose
actions are executed before reading the first record of each
command-line input file and after reading the last record of each
file. Inside the BEGINFILE rule, the value of ERRNO is the empty
string if the file was opened successfully. Otherwise, there is
some problem with the file and the code should use nextfile to
skip it. If that is not done, gawk produces its usual fatal error
for files that cannot be opened.
For /regular expression/ patterns, the associated statement is
executed for each input record that matches the regular
expression. Regular expressions are 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
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.
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
characters. With the POSIX character classes, you can write
/[[:alnum:]]/, and this matches the alphabetic and numeric
characters 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
equivalent for collating, or sorting, purposes. (E.g., in
French, a plain “e” and a grave-accented “e`” 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
represent all of “e”, “e´”, and “e`”. In this case, [[=e=]]
is a regular expression that matches any of e, e´, or e`.
These features are very valuable in non-English speaking locales.
The library functions that gawk uses for regular expression
matching 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
specific to gawk; they are extensions based on facilities in the
GNU regular expression libraries.
The various command line options control how gawk interprets
characters 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
operators 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
hexadecimal 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
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 (** 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 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 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 command can also be a socket. See the
subsection Special File Names, below.)
next Stop processing the current input record. Read the next
input record and start processing over with the first
pattern in the AWK program. Upon reaching the end of the
input data, execute any END rule(s).
nextfile
Stop processing the current input file. The next input
record read comes from the next input file. Update
FILENAME and ARGIND, reset FNR to 1, and start processing
over with the first pattern in the AWK program. Upon
reaching the end of the input data, execute any ENDFILE
and END rule(s).
print Print the current record. The output record is terminated
with the value of ORS.
print expr-list
Print expressions. Each expression is separated by the
value of OFS. The output record is terminated with the
value of ORS.
print expr-list >file
Print expressions on file. Each expression is separated
by the value of OFS. The output record is terminated with
the value of ORS.
printf fmt, expr-list
Format and print. 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 missing 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
subsection Special File Names, below.)
The getline command returns 1 on success, zero on end of file,
and -1 on an error. If the errno(3) value indicates that the I/O
operation may be retried, and PROCINFO["input", "RETRY"] is set,
then -2 is returned instead of -1, and further calls to getline
may be attempted. Upon an error, ERRNO is set to a string
describing the problem.
NOTE: Failure in opening a two-way socket results in a non-fatal
error being returned to the calling function. If using a pipe,
coprocess, or socket to getline, or from print or printf within a
loop, you must use close() to create new instances of the command
or socket. AWK does not automatically close pipes, sockets, or
coprocesses when they return EOF.
The 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.
Otherwise, 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
nonsignificant 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
format uses ABCDEF instead of abcdef.
%% A single % character; no argument is converted.
Optional, additional parameters may lie between the % and the
control letter:
count$ Use the count'th argument at this point in the formatting.
This is called a positional specifier and is intended
primarily 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
leading 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
numbers, 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 current 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 number of digits you want printed to the
right of the decimal point. For the %g, and %G formats,
it specifies the maximum number of significant digits.
For the %d, %i, %o, %u, %x, and %X formats, it specifies
the minimum number of digits 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()
routines 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 connection, and /inet6 to force an IPv6 connection.
Plain /inet uses the system default (most likely IPv4).
Usable only with the |& two-way I/O operator.
/inet/udp/lport/rhost/rport
/inet4/udp/lport/rhost/rport
/inet6/udp/lport/rhost/rport
Similar, but use UDP/IP instead of TCP/IP.
Numeric Functions
AWK has the following built-in arithmetic functions:
atan2(y, x)
Return the arctangent of y/x in radians.
cos(expr)
Return the cosine of expr, which is in radians.
exp(expr)
The exponential function.
int(expr)
Truncate to integer.
log(expr)
The natural logarithm function.
rand() Return a random number N, between zero and one, such that
0 ≤ N < 1.
sin(expr)
Return the sine of expr, which is in radians.
sqrt(expr)
Return the square root of expr.
srand([expr])
Use expr as the new seed for the random number generator.
If no expr is provided, use the time of day. Return the
previous seed for the random number generator.
String Functions
Gawk has the following built-in string functions:
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 any of the strings valid for PROCINFO["sorted_in"].
It can also be the name of a user-defined comparison
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 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 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 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 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 corresponding
parenthesized subexpression in r. The zero'th element of
a contains the portion 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 respectively, 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 separator 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 element 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 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
Since one of the primary uses of AWK programs is processing log
files that contain time stamp information, gawk provides the
following 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
datespec is a string of the form YYYY MM DD HH MM SS[
DST]. The contents of the string are six or seven numbers
representing 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 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.
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
multidimensional 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
``standard'' locations (e.g., during testing). 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".
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
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.
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
undefined function at run time is a fatal error.
The word func may be used in place of function, although this is
deprecated.
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:
Effective AWK Programming.
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.
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
localizable AWK program.
1. Add a BEGIN action to assign a value to the TEXTDOMAIN
variable 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
underscores.
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.
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
implementations, when such an assignment appeared before any file
names, the assignment would happen before the BEGIN rule was run.
Applications 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 developers.)
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
operation, 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
keeping 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
originally in gawk and fed back into the Bell Laboratories
version); the tolower() and toupper() built-in functions (from
the Bell Laboratories version); and the ISO C conversion
specifications in printf (done first in the Bell Laboratories
version).
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.
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
written 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(),
dcgettext(), dcngettext(), gensub(), lshift(), mktime(),
or(), patsplit(), 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()
function. 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
character. 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
behavior. This behavior also does not occur if --posix has been
specified. To really get a tab character as the field separator,
it is best to use single quotes: gawk -F'\t' ....
The AWKPATH environment variable can be used to provide a list of
directories that gawk searches when looking for files named via
the -f, --file, -i and --include options, and the @include
directive. If the initial search fails, the path is searched
again after appending .awk to the filename.
The AWKLIBPATH environment variable can be used to provide a list
of directories that gawk searches when looking for files named
via the -l and --load options.
The GAWK_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.
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.
This man page documents gawk, version 5.1.
The original version of UNIX awk was designed and implemented by
Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell
Laboratories. Brian Kernighan continues to maintain and enhance
it.
Paul Rubin and Jay Fenlason, of the Free Software Foundation,
wrote gawk, to be compatible with the original version of awk
distributed in Seventh Edition UNIX. John Woods contributed a
number of bug fixes. David Trueman, with contributions from
Arnold Robbins, made gawk compatible with the new version of UNIX
awk. Arnold Robbins is the current maintainer.
See GAWK: Effective AWK Programming for a full list of the
contributors to gawk and its documentation.
See the README file in the gawk distribution for up-to-date
information about maintainers and which ports are currently
supported.
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
compiler 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
forwarded to the gawk maintainer.
The -F option is not necessary given the command line variable
assignment feature; it remains only for backwards compatibility.
egrep(1), sed(1), getpid(2), getppid(2), getpgrp(2), getuid(2),
geteuid(2), getgid(2), getegid(2), getgroups(2), printf(3),
strftime(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 5.1, 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 .
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") }'
Brian Kernighan provided valuable assistance during testing and
debugging. We thank him.
Copyright © 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2001, 2002, 2003, 2004, 2005, 2007, 2009, 2010, 2011, 2012,
2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 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.
This page is part of the gawk (GNU awk) project. Information
about the project can be found at
⟨http://www.gnu.org/software/gawk/⟩. If you have a bug report for
this manual page, see
⟨http://pkg-shadow.alioth.debian.org/getinvolved.php⟩. This page
was obtained from the project's upstream Git repository
⟨git://git.savannah.gnu.org/gawk.git⟩ on 2020-12-18. (At that
time, the date of the most recent commit that was found in the
repository was 2020-12-07.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Free Software Foundation Aug 31 2020 GAWK(1)
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