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.
This manual page is intentionally as terse as possible. Full details are provided in GAWK: Effective AWK Programming, and you should look there for the full story on any specific feature. Where possible, links to the online version of the manual are provided.
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.
In this mode of execution, gawk loads the AWK source code and then prompts for debugging commands. Gawk can only debug AWK program source provided with the -f and --include options. The debugger is documented in GAWK: Effective AWK Programming; see https://www.gnu.org/software/gawk/manual/html_node/Debugger.html#Debugger.
NOTE: This feature is on parole. The primary gawk maintainer is no longer supporting it, although there is a member of the development team who is. If this situation changes, the feature will be removed from gawk.
In compatibility mode, any other options are flagged as invalid, but are otherwise ignored. In normal operation, as long as program text has been supplied, unknown options are passed on to the AWK program in the ARGV array for processing.
For POSIX compatibility, the -W option may be used, followed by the name of a long option.
An AWK program consists of a sequence of optional directives, pattern-action statements, and optional function definitions.
@include "filename"
@load "filename"
@namespace "name"
pattern { action statements }
function name(parameter list) {
statements }
Gawk first reads the program source from the program-file(s) if specified, from arguments to --source, or from the first non-option argument on the command line. The -f and --source options may be used multiple times on the command line. Gawk reads the program text as if all the program-files and command line source texts had been concatenated together.
In addition, lines beginning with @include may be used to include other source files into your program. This is equivalent to using the --include option.
Lines beginning with @load may be used to load extension functions into your program. This is equivalent to using the --load option.
The environment variable AWKPATH specifies a search path to use when finding source files named with the -f and --include options. If this variable does not exist, the default path is ".:/usr/local/share/awk". (The actual directory may vary, depending upon how gawk was built and installed.) If a file name given to the -f option contains a “/” character, no path search is performed.
The environment variable AWKLIBPATH specifies a search path to use when finding source files named with the --load option. If this variable does not exist, the default path is "/usr/local/lib/gawk". (The actual directory may vary, depending upon how gawk was built and installed.)
Gawk executes AWK programs in the following order. First, all variable assignments specified via the -v option are performed. Next, gawk compiles the program into an internal form. Then, gawk executes the code in the BEGIN rule(s) (if any), and then proceeds to read each file named in the ARGV array (up to ARGV[ARGC-1]). If there are no files named on the command line, gawk reads the standard input.
If a filename on the command line has the form var=val it is treated as a variable assignment. The variable var will be assigned the value val. (This happens after any BEGIN rule(s) have been run.)
If the value of a particular element of ARGV is empty (""), gawk skips over it.
For each input file, if a BEGINFILE rule exists, gawk executes the associated code before processing the contents of the file. Similarly, gawk executes the code associated with ENDFILE rules after processing the file.
For each record in the input, gawk tests to see if it matches any pattern in the AWK program. For each pattern that the record matches, gawk executes the associated action. The patterns are tested in the order they occur in the program.
Finally, after all the input is exhausted, gawk executes the code in the END rule(s) (if any).
According to POSIX, files named on the awk command line must be text files. The behavior is ``undefined'' if they are not. Most versions of awk treat a directory on the command line as a fatal error.
For gawk, a directory on the command line produces a warning, but is otherwise skipped. If either of the --posix or --traditional options is given, then gawk reverts to treating directories on the command line as a fatal error.
AWK variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings, or both, depending upon how they are used. Additionally, gawk allows variables to have regular-expression type. AWK also has one dimensional arrays; arrays with multiple dimensions may be simulated. However, gawk provides true arrays of arrays. Several pre-defined variables are set as a program runs; these are described as needed and summarized below.
Normally, records are separated by newline characters. You can control how records are separated by assigning values to the built-in variable RS. See https://www.gnu.org/software/gawk/manual/html_node/Records.html for the details.
As each input record is read, gawk splits the record into fields, using the value of the FS variable as the field separator. Additionally, FIELDWIDTHS and FPAT may be used to control input field splitting. See the details, starting at https://www.gnu.org/software/gawk/manual/html_node/Fields.html.
Each field in the input record may be referenced by its position: $1, $2, and so on. $0 is the whole record, including leading and trailing whitespace.
The variable NF is set to the total number of fields in the input record.
References to non-existent fields (i.e., fields after $NF) produce the null string. However, assigning to a non-existent field (e.g., $(NF+2) = 5) increases the value of NF, creates any intervening fields with the null string as their values, and causes the value of $0 to be recomputed, with the fields being separated by the value of OFS. References to negative numbered fields cause a fatal error. Decrementing NF causes the values of fields past the new value to be lost, and the value of $0 to be recomputed, with the fields being separated by the value of OFS.
Assigning a value to an existing field causes the whole record to be rebuilt when $0 is referenced. Similarly, assigning a value to $0 causes the record to be resplit, creating new values for the fields.
When invoked with ether the -k or the --csv option, gawk does not use regular record determination and field splitting as described above. Instead, records are terminated by unquoted newlines, and fields are separated by commas. Double-quotes may be used to enclose fields containing commas, newlines, or doubled double-quotes. See https://www.gnu.org/software/gawk/manual/html_node/Comma-Separated-Fields.html for more details.
Gawk's built-in variables are listed below. This list is purposely terse. For details, see https://www.gnu.org/software/gawk/manual/html_node/Built_002din-Variables.
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:
assigns the string "hello, world\n" to the element of the array x which is indexed by the string "A\034B\034C". All arrays in AWK are associative, i.e., indexed by string values.
The special operator in may be used to test if an array has an index consisting of a particular value:
if (val in array) print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct may also be used in a for loop to iterate over all the elements of an array. However, the (i, j) in array construct only works in tests, not in for loops.
An element may be deleted from an array using the delete statement. The delete statement may also be used to delete the entire contents of an array, just by specifying the array name without a subscript.
gawk supports true multidimensional arrays. It does not require that such arrays be ``rectangular'' as in C or C++. See https://www.gnu.org/software/gawk/manual/html_node/Arrays for details.
Gawk provides a simple namespace facility to help work around the fact that all variables in AWK are global.
A qualified name consists of a two simple identifiers joined by a double colon (::). The left-hand identifier represents the namespace and the right-hand identifier is the variable within it. All simple (non-qualified) names are considered to be in the ``current'' namespace; the default namespace is awk. However, simple identifiers consisting solely of uppercase letters are forced into the awk namespace, even if the current namespace is different.
You change the current namespace with an @namespace "name" directive.
The standard predefined builtin function names may not be used as namespace names. The names of additional functions provided by gawk may be used as namespace names or as simple identifiers in other namespaces. For more details, see https://www.gnu.org/software/gawk/manual/html_node/Namespaces.html#Namespaces.
Variables and fields may be (floating point) numbers, or strings, or both. They may also be regular expressions. How the value of a variable is interpreted depends upon its context. If used in a numeric expression, it will be treated as a number; if used as a string it will be treated as a string.
To force a variable to be treated as a number, add zero to it; to force it to be treated as a string, concatenate it with the null string.
Uninitialized variables have the numeric value zero and the string value "" (the null, or empty, string).
When a string must be converted to a number, the conversion is accomplished using strtod(3). A number is converted to a string by using the value of CONVFMT as a format string for sprintf(3), with the numeric value of the variable as the argument. However, even though all numbers in AWK are floating-point, integral values are always converted as integers.
Gawk performs comparisons as follows: If two variables are numeric, they are compared numerically. If one value is numeric and the other has a string value that is a “numeric string,” then comparisons are also done numerically. Otherwise, the numeric value is converted to a string and a string comparison is performed. Two strings are compared, of course, as strings.
Note that string constants, such as "57", are not numeric strings, they are string constants. The idea of “numeric string” only applies to fields, getline input, FILENAME, ARGV elements, ENVIRON elements and the elements of an array created by split() or patsplit() that are numeric strings. The basic idea is that user input, and only user input, that looks numeric, should be treated that way.
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 in AWK are sequences of characters enclosed between double quotes (like "value"). Within strings, certain escape sequences are recognized, as in C. See https://www.gnu.org/software/gawk/manual/html_node/Escape-Sequences for the details.
A regular expression constant is a sequence of characters enclosed between forward slashes (like /value/).
The escape sequences described in the manual may also be used inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).
Gawk provides strongly typed regular expression constants. These are written with a leading @ symbol (like so: @/value/). Such constants may be assigned to scalars (variables, array elements) and passed to user-defined functions. Variables that have been so assigned have regular expression type.
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.
AWK patterns may be one of the following:
BEGIN END BEGINFILE ENDFILE /regular expression/ relational expression pattern && pattern pattern || pattern pattern ? pattern : pattern (pattern) ! pattern pattern1, pattern2
BEGIN and END are two special kinds of patterns which are not tested against the input. The action parts of all BEGIN patterns are merged as if all the statements had been written in a single BEGIN rule. They are executed before any of the input is read. Similarly, all the END rules are merged, and executed when all the input is exhausted (or when an exit statement is executed). BEGIN and END patterns cannot be combined with other patterns in pattern expressions. BEGIN and END patterns cannot have missing action parts.
BEGINFILE and ENDFILE are additional special patterns whose actions are executed before reading the first record of each command-line input file and after reading the last record of each file. Inside the BEGINFILE rule, the value of ERRNO is the empty string if the file was opened successfully. Otherwise, there is some problem with the file and the code should use nextfile to skip it. If that is not done, gawk produces its usual fatal error for files that cannot be opened.
For /regular expression/ patterns, the associated statement is executed for each input record that matches the regular expression. Regular expressions are essentially the same as those in egrep(1). See https://www.gnu.org/software/gawk/manual/html_node/Regexp.html for the details on regular expressions.
A relational expression may use any of the operators defined below in the section on actions. These generally test whether certain fields match certain regular expressions.
The &&, ||, and ! operators are logical AND, logical OR, and logical NOT, respectively, as in C. They do short-circuit evaluation, also as in C, and are used for combining more primitive pattern expressions. As in most languages, parentheses may be used to change the order of evaluation.
The ?: operator is like the same operator in C. If the first pattern is true then the pattern used for testing is the second pattern, otherwise it is the third. Only one of the second and third patterns is evaluated.
The pattern1, pattern2 form of an expression is called a range pattern. It matches all input records starting with a record that matches pattern1, and continuing until a record that matches pattern2, inclusive. It does not combine with any other sort of pattern expression.
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.
The operators in AWK, in order of decreasing precedence, are:
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 ] }
The input/output statements are as follows:
Additional output redirections are allowed for print and printf.
The getline command returns 1 on success, zero on end of file, and -1 on an error. If the errno(3) value indicates that the I/O operation may be retried, and PROCINFO["input", "RETRY"] is set, then -2 is returned instead of -1, and further calls to getline may be attempted. Upon an error, ERRNO is set to a string describing the problem.
NOTE: Failure in opening a two-way socket results in a non-fatal error being returned to the calling function. If using a pipe, coprocess, or socket to getline, or from print or printf within a loop, you must use close() to create new instances of the command or socket. AWK does not automatically close pipes, sockets, or coprocesses when they return EOF.
The AWK versions of the printf statement and sprintf() function are similar to those of C. For details, see https://www.gnu.org/software/gawk/manual/html_node/Printf.html.
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 following special filenames may be used with the |& coprocess operator for creating TCP/IP network connections:
AWK has the following built-in arithmetic functions:
Gawk has the following built-in string functions; details are provided in https://www.gnu.org/software/gawk/manual/html_node/String-Functions.
Gawk is multibyte aware. This means that index(), length(), substr() and match() all work in terms of characters, not bytes.
Gawk provides the following functions for obtaining time stamps and formatting them. Details are provided in https://www.gnu.org/software/gawk/manual/html_node/Time-Functions.
Gawk supplies the following bit manipulation functions. They work by converting double-precision floating point values to uintmax_t integers, doing the operation, and then converting the result back to floating point. Passing negative operands to any of these functions causes a fatal error.
The functions are:
The following functions provide type related information about their arguments.
The following functions may be used from within your AWK program for translating strings at run-time. For full details, see https://www.gnu.org/software/gawk/manual/html_node/I18N-Functions.html#I18N-Functions.
The default domain is the value of TEXTDOMAIN. If directory is the null string (""), then bindtextdomain() returns the current binding for the given domain.
You can create special Boolean-typed values; see the manual for how they work and why they exist.
Functions in AWK are defined as follows:
Functions execute when they are called from within expressions in either patterns or actions. Actual parameters supplied in the function call are used to instantiate the formal parameters declared in the function. Arrays are passed by reference, other variables are passed by value.
Local variables are declared as extra parameters in the parameter list. The convention is to separate local variables from real parameters by extra spaces in the parameter list. For example:
function f(p, q, a, b) # a and b are local { ... } /abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required to immediately follow the function name, without any intervening whitespace. This restriction does not apply to the built-in functions listed above.
Functions may call each other and may be recursive. Function parameters used as local variables are initialized to the null string and the number zero upon function invocation.
Use return expr to return a value from a function. The return value is undefined if no value is provided, or if the function returns by “falling off” the end.
Functions may be called indirectly. To do this, assign the name of the function to be called, as a string, to a variable. Then use the variable as if it were the name of a function, prefixed with an @ sign, like so:
function myfunc() { print "myfunc called" ... } { ... the_func = "myfunc" @the_func() # call through the_func to myfunc ... }
If --lint has been provided, gawk warns about calls to undefined functions at parse time, instead of at run time. Calling an undefined function at run time is a fatal error.
You can dynamically add new functions written in C or C++ to the running gawk interpreter with the @load statement. The full details are beyond the scope of this manual page; see https://www.gnu.org/software/gawk/manual/html_node/Dynamic-Extensions.html#Dynamic-Extensions.
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,
always prints hello, world. But,
might print bonjour, monde in France. See https://www.gnu.org/software/gawk/manual/html_node/Internationalization.html#Internationalization for the steps involved in producing and running a localizable AWK program.
Gawk has a too-large number of extensions to POSIX awk. They are described in https://www.gnu.org/software/gawk/manual/html_node/POSIX_002fGNU.html. All the extensions can be disabled by invoking gawk with the --traditional or --posix options.
The AWKPATH environment variable can be used to provide a list of directories that gawk searches when looking for files named via the -f, --file, -i and --include options, and the @include directive. If the initial search fails, the path is searched again after appending .awk to the filename.
The AWKLIBPATH environment variable can be used to provide a list of directories that gawk searches when looking for files named via the -l and --load options.
The GAWK_PERSIST_FILE environment variable, if present, specifies a file to use as the backing store for persistent memory. This is an experimental feature. See GAWK: Effective AWK Programming for the details.
The GAWK_READ_TIMEOUT environment variable can be used to specify a timeout in milliseconds for reading input from a terminal, pipe or two-way communication including sockets.
For connection to a remote host via socket, GAWK_SOCK_RETRIES controls the number of retries, and GAWK_MSEC_SLEEP the interval between retries. The interval is in milliseconds. On systems that do not support usleep(3), the value is rounded up to an integral number of seconds.
If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly as if --posix had been specified on the command line. If --lint has been specified, gawk issues a warning message to this effect.
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 5.3.
The original version of UNIX awk was designed and implemented by Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell Laboratories. Ozan Yigit is the the current maintainer. Brian Kernighan occasionally dabbles in its development.
Paul Rubin and Jay Fenlason, of the Free Software Foundation, wrote gawk, to be compatible with the original version of awk distributed in Seventh Edition UNIX. John Woods contributed a number of bug fixes. 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 use the gawkbug(1) program to report it.
Full instructions for reporting a bug are provided in https://www.gnu.org/software/gawk/manual/html_node/Bugs.html. Please carefully read and follow the instructions given there. This will make bug reporting and resolution much easier for everyone involved. Really.
The -F option is not necessary given the command line variable assignment feature; it remains only for backwards compatibility.
This manual page is too long; gawk has too many features.
egrep(1), sed(1), gawkbug(1), printf(3), and strftime(3).
The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X.
GAWK: Effective AWK Programming, Edition 5.2, shipped with the gawk source. The current version of this document is available online at https://www.gnu.org/software/gawk/manual.
The GNU gettext documentation, available online at https://www.gnu.org/software/gettext.
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") }'
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Nov 02 2023 | Free Software Foundation |