GAWK(1)

NAME
gawk - pattern scanning and processing language

SYNOPSIS
gawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
gawk [ POSIX or GNU style options ] [ -- ] program-text file ...

DESCRIPTION
Gawk is the GNU Project's implementation of the AWK programming language. It conforms to the definition of the language in the POSIX 1003.2 Command Language And Utilities Standard. This version in turn is based on the description in The AWK Programming Language, by Aho, Kernighan, and Weinberger, with the additional features found in the System V Release 4 version of UNIX awk. Gawk also provides more recent Bell Labs awk extensions, and some 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.

OPTION FORMAT

Gawk options may be either the traditional POSIX one letter options, or the 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.

Following the POSIX standard, gawk-specific options are supplied via arguments to the -W option. Multiple -W options may be supplied Each -W option has a corresponding long option, as detailed below. 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.

OPTIONS

Gawk accepts the following options.

-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 block of an AWK program.

-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.

-mf NNN
-mr NNN
    Set various memory limits to the value NNN. The f flag sets the maximum number of fields, and the r flag sets the maximum record size. These two flags and the -m option are from the Bell Labs research version of UNIX awk. They are ignored by gawk, since gawk has no pre-defined limits.

-W traditional
-W compat
--traditional
--compat
    Run in compatibility mode. In compatibility mode, gawk behaves identically to UNIX awk; none of the GNU-specific extensions are recognized. The use of --traditional is preferred over the other forms of this option. See GNU EXTENSIONS, below, for more information.

-W copyleft
-W copyright
--copyleft
--copyright
    Print the short version of the GNU copyright information message on the standard output, and exits successfully.

-W help
-W usage
--help
--usage
    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.)

-W lint
--lint
    Provide warnings about constructs that are dubious or non-portable to other AWK implementations.

-W lint-old
--lint-old
    Provide warnings about constructs that are not portable to the original version of Unix awk.

-W posix
--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.

    •   The synonym func for the keyword function is not recognized.

    •   The operators ** and **= cannot be used in place of ^ and ^=.

    •   The fflush() function is not available.

-W re-interval
--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. However, their use is likely to break old AWK programs, so gawk only provides them if they are requested with this option, or when --posix is specified.

-W source 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.

-W version
--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 is mainly for consistency with the argument parsing convention used by most other POSIX programs.

In compatibility mode, any other options are flagged as illegal, 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.

AWK PROGRAM EXECUTION

An AWK program consists of a sequence of pattern-action statements and optional function definitions.

    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 will read 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.

The environment variable AWKPATH specifies a search path to use when finding source files named with the -f option. 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.

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 block(s) (if any), and then proceeds to read each file named in the ARGV array. 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 block(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 record in the input, gawk tests to see if it matches any pattern in the AWK program. For each pattern that the record matches, the associated action is executed. 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 block(s) (if any).

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. Several pre-defined variables are set as a program runs; these will be 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 will separate 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 discussion of --posix, below). Note that the value of IGNORECASE (see below) will also affect 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 will split up the record using the specified widths. The value of FS is ignored. Assigning a new value to FS overrides the use of FIELDWIDTHS, and restores the default behavior.

Each field in the input record may be referenced by its position, $1, $2, and so on. $0 is the whole record. The value of a field may be assigned to as well. 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) will increase the value of NF, create any intervening fields with the null string as their value, and cause 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.

  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.

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. (This may change in a future version of gawk.)

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.

FIELDWIDTHS
    A white-space separated list of fieldwidths. When set, gawk parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator. The fixed field width facility is still experimental; the semantics may change as gawk evolves over time.

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 block.

FNR The input record number in the current input file.

FS  The input field separator, a space by default. See Fields, above.

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, record separating with RS, regular expression matching with ~ and !~, and the gensub(), gsub(), index(), match(), split(), and sub() pre-defined functions will all ignore case when doing regular expression operations. 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. Under Unix, the full ISO 8859-1 Latin-1 character set is used when ignoring case. NOTE: In versions of gawk prior to 3.0, IGNORECASE only affected regular expression operations. It now affects string comparisons as well.

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.

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.

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".

  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 in an if or while statement to see 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.

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.

  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.

When a string must be converted to a number, the conversion is accomplished using atof(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".

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. According to the POSIX standard, even if two strings are numeric strings, a numeric comparison is performed. However, this is clearly incorrect, and gawk does not do this.

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() that are numeric strings. The basic idea is that user input, and only user input, that looks numeric, should be treated that way.

Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string).

PATTERNS AND ACTIONS
AWK is a line-oriented language. The pattern comes first, and then the action. Action statements are enclosed in { and }. Either the pattern may be missing, or the action may be missing, but, of course, not both. If the pattern is missing, the action will be 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 “,”, {, ?, :, &&, 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 will be 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
AWK patterns may be one of the following:

    BEGIN
    END
    /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 block. They are executed before any of the input is read. Similarly, all the END blocks 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.

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...]
    character list, matches any of the characters abc....

[^abc...]
    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 regexp 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.

    Interval expressions are only available if either --posix or --re-interval is specified on the command line.

\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.

\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 below) are also legal in regular expressions.

Character classes are a new 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 regexp inside the brackets of a character list. Character classes consist of [:, a keyword denoting the class, and :]. Here are the character classes defined by the POSIX standard.

[: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:]
    Lower-case 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:]
    Upper-case 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. With the POSIX character classes, you can write /[[:alnum:]]/, and this will match all the alphabetic and numeric characters in your character set.

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 symbols is a multi-character collating element enclosed in [. and .]. For example, if ch is a collating element, then [[.ch.]] is a regexp that matches this collating element, while [ch] is a regexp 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 regexp that matches any of
     .BR e ,
     .BR e' , or
     .BR 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, \<, \>, \w, \W, \`, and \' operators are specific to gawk; they are extensions based on facilities in the GNU regexp libraries.

The various command line options control how gawk interprets characters in regexps.

No options
    In the default case, gawk provide all the facilities of POSIX regexps and the GNU regexp operators described above. However, interval expressions are not supported.

--posix
    Only POSIX regexps are supported, the GNU operators are not special. (E.g., \w matches a literal w). Interval expressions are allowed.

--traditional
    Traditional Unix awk regexps are matched. The GNU operators are not special, interval expressions are not available, and neither are the POSIX character classes ([[:alnum:]] and so on). Characters described by octal and hexadecimal escape sequences are treated literally, even if they represent regexp metacharacters.

--re-interval
    Allow interval expressions in regexps, 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.

< >
<= >=
!= ==
    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 was intended.

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 }

  I/O Statements

The input/output statements are as follows:

close(file)
    Close file (or pipe, see below).

getline
    Set $0 from next input record; set NF, NR, FNR.

getline <file
    Set $0 from next record of file; set NF.

getline var
    Set var from next input record; set NR, FNR.

getline var <file
    Set var from next record of file.

next
    Stop processing the current input record. The next input record is read and processing starts over with the first pattern in the AWK program. If the end of the input data is reached, the END block(s), if any, are executed.

nextfile
    Stop processing the current input file. The next input record read comes from the next input file. FILENAME and ARGIND are updated, FNR is reset to 1, and processing starts over with the first pattern in the AWK program. If the end of the input data is reached, the END block(s), if any, are executed. NOTE: Earlier versions of gawk used next file, as two words. While this usage is still recognized, it generates a warning message and will eventually be removed.

print
    Prints the current record. The output record is terminated with the value of the ORS variable.

print expr-list
    Prints expressions. Each expression is separated by the value of the OFS variable. The output record is terminated with the value of the ORS variable.

print expr-list >file
    Prints expressions on file. Each expression is separated by the value of the OFS variable. The output record is terminated with the value of the ORS variable.

printf fmt, expr-list
    Format and print.

printf fmt, expr-list >file
    Format and print on file.

system(cmd-line)
    Execute the command cmd-line, and return the exit status. (This may not be available on non-POSIX systems.)

fflush([file])
    Flush any buffers associated with the open output file or pipe file. If file is missing, then standard output is flushed. If file is the null string, then all open output files and pipes have their buffers flushed.

Other input/output redirections are also allowed. For print and printf, >> file appends output to the file, while | command writes on a pipe. In a similar fashion, command | getline pipes into getline. The getline command will return 0 on end of file, and -1 on an error.

NOTE: If using a pipe to getline, or from print or printf within a loop, you must use close() to create new instances of the command. AWK does not automatically close pipes 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:

%c  An ASCII 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  A floating point number of the form [-]ddd.dddddd.

%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.

There are optional, additional parameters that may lie between the % and the control letter:

-   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, and %f, the result will always contain a decimal point. For %g, and %G, trailing zeros are not removed from the result.

0   A leading 0 (zero) acts as a flag, that indicates output should be padded with zeroes instead of spaces. This applies even to non-numeric output formats. This flag only has an effect when the field width is wider than the value to be printed.

width
    The field should be padded to this width. The field is normally padded with spaces. If the 0 flag has been used, it is padded with zeroes.

.prec
    A number that specifies the precision to use when printing. For the %e, %E, 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, %o, %i, %u, %x, and %X formats, it specifies the minimum number of digits to print. For a string, it specifies the maximum number of characters from the string that should be printed.

The dynamic width and prec capabilities of the ANSI C printf() routines are supported. A * in place of either the width or prec specifications will cause their values to be taken from the argument list to printf or sprintf().

  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). Other special filenames provide access to information about the running gawk process. The filenames are:

/dev/pid
    Reading this file returns the process ID of the current process, in decimal, terminated with a newline.

/dev/ppid
    Reading this file returns the parent process ID of the current process, in decimal, terminated with a newline.

/dev/pgrpid
    Reading this file returns the process group ID of the current process, in decimal, terminated with a newline.

/dev/user
    Reading this file returns a single record terminated with a newline. The fields are separated with spaces. $1 is the value of the getuid(2) system call, $2 is the value of the geteuid(2) system call, $3 is the value of the getgid(2) system call, and $4 is the value of the getegid(2) system call. If there are any additional fields, they are the group IDs returned by getgroups(2). Multiple groups may not be supported on all systems.

/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"

These file names may also be used on the command line to name data files.

  Numeric Functions

AWK has the following pre-defined arithmetic functions:

atan2(y, x)
    returns the arctangent of y/x in radians.

cos(expr)
    returns the cosine of expr, which is in radians.

exp(expr)
    the exponential function.

int(expr)
    truncates to integer.

log(expr)
    the natural logarithm function.

rand()
    returns a random number between 0 and 1.

sin(expr)
    returns the sine of expr, which is in radians.

sqrt(expr)
    the square root function.

srand([expr])
    uses expr as a new seed for the random number generator. If no expr is provided, the time of day will be used. The return value is the previous seed for the random number generator.

  String Functions

Gawk has the following pre-defined string functions:

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 no t is supplied, $0 is used 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 &. See Effective AWK Programming for a fuller discussion of the rules for &'s and backslashes in the replacement text of sub(), gsub(), and gensub().

index(s, t)
    returns the index of the string t in the string s, or 0 if t is not present.

length([s])
    returns the length of the string s, or the length of $0 if s is not supplied.

match(s, r)
    returns the position in s where the regular expression r occurs, or 0 if r is not present, and sets the values of RSTART and RLENGTH.

split(s, a [, r])
    splits the string s into the array a on the regular expression r, and returns the number of fields. If r is omitted, FS is used instead. The array a is cleared first. Splitting behaves identically to field splitting, described above.

sprintf(fmt, expr-list)
    prints expr-list according to fmt, and returns the resulting string.

sub(r, s [, t])
    just like gsub(), but only the first matching substring is replaced.

substr(s, i [, n])
    returns the at most n-character substring of s starting at i. If n is omitted, the rest of s is used.

tolower(str)
    returns a copy of the string str, with all the upper-case characters in str translated to their corresponding lower-case counterparts. Non-alphabetic characters are left unchanged.

toupper(str)
    returns a copy of the string str, with all the lower-case characters in str translated to their corresponding upper-case counterparts. Non-alphabetic characters are left unchanged.

  Time Functions

Since one of the primary uses of AWK programs is processing log files that contain time stamp information, gawk provides the following two functions for obtaining time stamps and formatting them.

systime()
    returns the current time of day as the number of seconds since the Epoch (Midnight UTC, January 1, 1970 on POSIX systems).

strftime([format [, timestamp]])
    formats timestamp according to the specification in format. 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) will be used. See the specification for the strftime() function in ANSI C for the format conversions that are guaranteed to be available. A public-domain version of strftime(3) and a man page for it come with gawk; if that version was used to build gawk, then all of the conversions described in that man page are available to gawk.

  String Constants

String constants in AWK are sequences of characters enclosed between double quotes ("). 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 ANSI 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.

\c  The literal character c.

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 regexp constants. Thus, /a\52b/ is equivalent to /a\*b/.

FUNCTIONS
Functions in AWK are defined as follows:

    function name(parameter list) { statements }

Functions are executed 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 & 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 white space. This is to avoid 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.

If --lint has been provided, gawk will warn 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.

EXAMPLES
Print and sort the login names of all users:
	BEGIN	{ FS = ":" }
		{ print $1 | "sort" }
Count lines in a file:
		{ nlines++ }
	END	{ print nlines }
Precede each line by its number in the file:
	{ print FNR, $0 }
Concatenate and line number (a variation on a theme):
	{ print NR, $0 }

SEE ALSO
egrep(1), getpid(2), getppid(2), getpgrp(2), getuid(2), geteuid(2), getgid(2), getegid(2), getgroups(2)

The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X.

Effective AWK Programming, Edition 1.0, published by the Free Software Foundation, 1995.

POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX standard, as well as with the latest version of UNIX awk. To this end, gawk incorporates the following user visible features which are not described in the AWK book, but are part of the Bell Labs version of awk, and are in the POSIX standard.

The -v option for assigning variables before program execution starts is new. The book indicates that command line variable assignment happens when awk would otherwise open the argument as a file, which is after the BEGIN block is executed. However, in earlier implementations, when such an assignment appeared before any file names, the assignment would happen before the BEGIN block was run. Applications came to depend on this “feature.” When awk was changed to match its documentation, this option was added to accommodate applications that depended upon the old behavior. (This feature was agreed upon by both the AT&T and GNU developers.)

The -W option for implementation specific features is from the POSIX standard.

When processing arguments, gawk uses the special option “--” to signal the end of arguments. In compatibility mode, it will warn about, but otherwise ignore, 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 new 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 AT&T's); the tolower() and toupper() built-in functions (from AT&T); and the ANSI C conversion specifications in printf (done first in AT&T's version).

GNU EXTENSIONS
Gawk has a 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 option.

The following features of gawk are not available in POSIX awk.

    •   The \x escape sequence. (Disabled with --posix.)

    •   The fflush() function. (Disabled with --posix.)

    •   The systime(), strftime(), and gensub() functions.

    •   The special file names available for I/O redirection are not recognized.

    •   The ARGIND, ERRNO, and RT variables are not special.

    •   The IGNORECASE variable and its side-effects are not available.

    •   The FIELDWIDTHS variable and fixed-width field splitting.

    •   The use of RS as a regular expression.

    •   The ability to split out individual characters using the null string as the value of FS, and as the third argument to split().

    •   No path search is performed for files named via the -f option. Therefore the AWKPATH environment variable is not special.

    •   The use of nextfile to abandon processing of the current input file.

    •   The use of delete array to delete the entire contents of an array.

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 a file or pipe, respectively.

When gawk is invoked with the --traditional option, if the fs argument to the -F option is “t”, then FS will be 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 quotes: gawk -F'\t' ....

HISTORICAL FEATURES
There are two features of historical AWK implementations that gawk supports. First, 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)

This feature is marked as “deprecated” in the POSIX standard, and gawk will issue a warning about its use if --lint is specified on the command line.

The other feature is the use of either the continue or the break statements outside the body of a while, for, or do loop. Traditional AWK implementations have treated such usage as equivalent to the next statement. Gawk will support this usage if --traditional has been specified.

ENVIRONMENT VARIABLES
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 will issue a warning message to this effect.

The AWKPATH environment variable can be used to provide a list of directories that gawk will search when looking for files named via the -f and --file options.

BUGS
The -F option is not necessary given the command line variable assignment feature; it remains only for backwards compatibility.

If your system actually has support for /dev/fd and the associated /dev/stdin, /dev/stdout, and /dev/stderr files, you may get different output from gawk than you would get on a system without those files. When gawk interprets these files internally, it synchronizes output to the standard output with output to /dev/stdout, while on a system with those files, the output is actually to different open files. Caveat Emptor.

Syntactically invalid single character programs tend to overflow the parse stack, generating a rather unhelpful message. Such programs are surprisingly difficult to diagnose in the completely general case, and the effort to do so really is not worth it.

VERSION INFORMATION
This man page documents gawk, version 3.0.6.

AUTHORS
The original version of UNIX awk was designed and implemented by Alfred Aho, Peter Weinberger, and Brian Kernighan of AT&T Bell Labs. 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.

The initial DOS port was done by Conrad Kwok and Scott Garfinkle. Scott Deifik is the current DOS maintainer. Pat Rankin did the port to VMS, and Michal Jaegermann did the port to the Atari ST. The port to OS/2 was done by Kai Uwe Rommel, with contributions and help from Darrel Hankerson. Fred Fish supplied support for the Amiga.

BUG REPORTS
If you find a bug in gawk, please send electronic mail to bug-gawk@gnu.org. Please include your operating system and its revision, the version of gawk, what 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 two 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 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. Instead, please use the electronic mail addresses given above.

ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs provided valuable assistance during testing and debugging. We thank him.

COPYING PERMISSIONS
Copyright © 1996-2000 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.