NAME
gawk − pattern scanning and processing language
SYNOPSIS
gawk [
POSIX or GNU style options ]
−f program-file [ −−
] file ...
gawk [ POSIX or GNU style
options ] [ −− ] program-text file
...
DESCRIPTION
Gawk is the GNU Project’s implementation of the AWK programming language. It conforms to the definition of the language in the POSIX 1003.1 Standard. This version in turn is based on the description in The AWK Programming Language, by Aho, Kernighan, and Weinberger. Gawk provides the additional features found in the current version of Brian Kernighan’s awk and a number of GNU -specific extensions.
The command line consists of options to gawk itself, the AWK program text (if not supplied via the −f or −−file 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 option. The debugger is documented in GAWK: Effective AWK Programming.
OPTION FORMAT
Gawk options may be either traditional POSIX -style one letter options, or GNU -style long options. POSIX options start with a single “−”, while long options start with “−−”. Long options are provided for both GNU -specific features and for POSIX -mandated features.
Gawk-specific options are typically used in long-option form. Arguments to long options are either joined with the option by an = sign, with no intervening spaces, or they may be provided in the next command line argument. Long options may be abbreviated, as long as the abbreviation remains unique.
Additionally, every long option has a corresponding short option, so that the option’s functionality may be used from within #! executable scripts.
OPTIONS
Gawk
accepts the following options. Standard options are listed
first, followed by options for gawk extensions,
listed 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.
−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 terminal. 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 −−file options) with source code entered on the command line. It is intended primarily for medium to large AWK programs used in shell scripts.
−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.
−l lib |
−−load lib
Load a 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.)
−M |
−−bignum
Force arbitrary precision arithmetic on numbers. This option has no effect if gawk is not compiled to use the GNU MPFR and MP libraries.
−n |
−−non−decimal−data
Recognize octal and hexadecimal values in input data. Use this option with great caution!
−N |
−−use−lc−numeric
This forces 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.
−O |
−−optimize
Enable optimizations upon the internal representation of the program. Currently, this includes simple constant-folding, and tail call elimination for recursive functions. The gawk maintainer hopes to add additional optimizations over time.
−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.
−P |
−−posix
This turns on compatibility mode, with the following additional restrictions:
• |
\x escape sequences are not recognized. | ||
• |
Only space and tab act as field separators when FS is set to a single space, newline does not. | ||
• |
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 with −−traditional.
−S |
−−sandbox
Runs 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.
AWK PROGRAM EXECUTION
An AWK program consists of a sequence of pattern-action statements and optional function definitions.
@include
"filename"
@load "filename"
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 −i option.
Lines beginning with @load may be used to load shared libraries into your program. This is equivalent to using the −l option.
The environment variable AWKPATH specifies a search path to use when finding source files named with the −f and −i 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 −l 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]). 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.
VARIABLES, RECORDS AND FIELDS
AWK variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings, or both, depending upon how they are used. 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 blank 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. (But see the section
POSIX COMPATIBILITY, below). 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. 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. 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"). Changing this array does not affect the environment seen by programs which gawk spawns via redirection or the system() function. | ||
ERRNO |
If a system error occurs either doing a redirection for getline, during a read for getline, or during a close(), then ERRNO will contain a string describing the error. The value is subject to translation in non-English locales. | ||
FIELDWIDTHS |
A whitespace separated list of field widths. When set, gawk parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator. 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 the FS variable 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. When assigned the string value "fatal", lint warnings become fatal errors, exactly like −−lint=fatal. Any other true value just prints warnings. | ||
NF |
The number of fields in the current input record. | ||
NR |
The total number of input records seen so far. | ||
OFMT |
The output format for numbers, "%.6g", by default. | ||
OFS |
The output field separator, a space by default. | ||
ORS |
The output record separator, by default a newline. | ||
PREC |
The working precision of arbitrary precision floating-point numbers, 53 by default. | ||
PROCINFO |
The elements of this array provide access to 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["egid"]
The value of the getegid(2) system call. |
PROCINFO["strftime"]
The default time format string for strftime().
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, or "FIELDWIDTHS" if field splitting with FIELDWIDTHS is in effect. |
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.
"extension"
The identifier is an extension function loaded via @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["gid"] |
The value of the getgid(2) system call.
PROCINFO["pgrpid"] |
The process group ID of the current process. | ||
PROCINFO["pid"] |
The process ID of the current process. | ||
PROCINFO["ppid"] |
The parent process ID of the current process. | ||
PROCINFO["uid"] |
The value of the getuid(2) system call. |
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 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.
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["mpfr_version"]
The version of the GNU MPFR library used for arbitrary precision number support in gawk. This entry is not present if MPFR support is not compiled into gawk.
PROCINFO["gmp_version"]
The version of the GNU MP library used for arbitrary precision number support in gawk. This entry is not present if MPFR support is not compiled into gawk.
PROCINFO["prec_max"]
The maximum precision supported by the GNU MPFR library for arbitrary precision floating-point numbers. This entry is not present if MPFR support is not compiled into gawk.
PROCINFO["prec_min"]
The minimum precision allowed by the GNU MPFR library for arbitrary precision floating-point numbers. This entry is not present if MPFR support is not compiled into gawk.
PROCINFO["api_major"]
The major version of the extension API. This entry is not present if loading dynamic extensions is not available.
PROCINFO["api_minor"]
The minor version of the extension API. This entry is not present if loading
dynamic extensions is not available.
PROCINFO["version"] |
the version of gawk. |
|||
ROUNDMODE |
The rounding mode to use for arbitrary precision arithmetic on numbers, by default "N" (IEEE-754 roundTiesToEven mode). The accepted values are "N" or "n" for roundTiesToEven, "U" or "u" for roundTowardPositive, "D" or "d" for roundTowardNegative, "Z" or "z" for roundTowardZero, and if your version of GNU MPFR library supports it, "A" or "a" for roundTiesToAway.
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 character 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 isarray() 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.
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.
Variable
Typing And Conversion
Variables and fields may be (floating point) numbers, or
strings, or both. 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 0 to it; to force it to be treated as a string, concatenate it with the null string.
Uninitialized variables have the numeric value 0 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. As in ISO C, all following hexadecimal digits are considered part of the escape sequence. (This feature should tell us something about language design by committee.) 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.
The escape sequences may also be used inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters). 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/. PATTERNS AND ACTIONSAWK 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 is executed 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. Blank 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. 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
BEGIN 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 bodies 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 will be 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
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:
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 “`” are equivalent.) 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,” “´,” and “`.” In this case, [[=e=]] is a regular expression that matches any of e, ´, or `. These features are very valuable in non-English speaking locales. The library functions that gawk uses for regular expression 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. 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
Operators
< > <= >= != == The regular relational operators.
= += −= *= /= %= ^= Assignment. Both absolute assignment (var = value) and operator-assignment (the other forms) are supported. Control
Statements if
(condition) statement [ else
statement ] I/O
Statements
command | getline [var] Run command piping the output either into $0 or var, as above, and RT. command |& getline [var] Run command as a co-process piping the output either into $0 or var, as above, and RT. Co-processes are a gawk extension. (command can also be a socket. See the subsection Special File Names, below.)
printf fmt, expr-list >file Format and print on file.
Additional
output redirections are allowed for print and
printf. Appends output to the file. print ... | command Writes on a pipe. print ... |& command Sends data to a co-process or socket. (See also the subsection Special File Names, below.) The getline command returns 1 on success, 0 on end of file, and −1 on an error. Upon an error, ERRNO is set to a string describing the problem. NOTE: Failure in opening a two-way socket results in a non-fatal error being returned to the calling function. If using a pipe, co-process, or socket to getline, or from print or printf within a loop, you must use close() to create new instances of the command or socket. AWK does not automatically close pipes, sockets, or co-processes when they return EOF. The printf
Statement
Optional, additional parameters may lie between the % and the control letter:
The dynamic width and prec capabilities of the ISO C printf() routines are supported. A * in place of either the width or prec specifications causes their values to be taken from the argument list to printf or sprintf() . To use a positional specifier with a dynamic width or precision, supply the count$ after the * in the format string. For example, "%3$*2$.*1$s". Special File
Names
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 |&
co-process operator for creating TCP/IP network connections:
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). /inet/udp/lport/rhost/rport
Similar, but use UDP/IP instead of TCP/IP. Numeric
Functions
String
Functions
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