GAWK(1)

NAME
gawk - pattern scanning and processing language

SYNOPSIS
gawk [ -Ffs ] -f program-file [ -f program-file ... ] [ -- ] file ...
gawk [ -Ffs ] [ -- ] program-text file ...

DESCRIPTION
Gawk is the GNU Project's implementation of the AWK programming language. It conforms to the definition and description of the language in The AWK Programming Language, by Aho, Kernighan, and Weinberger, with the additional features defined in the System V Release 4 version of UNIX awk.

The command line consists of options to gawk itself, the AWK program text (if not supplied via the -f option), and values to be made available in the ARGC and ARGV pre-defined AWK variables.

The options that gawk accepts are:

-Ffs
    Use fs for the input field separator (the value of the FS predefined variable).

-f program-file
    Read the AWK program source from the file program-file, instead of from the first command line argument.

--  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 System V programs.

Any other options are flagged as illegal, but are otherwise ignored. (However, see the GNU EXTENSIONS section, below.)

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, or from the first non-option argument on the command line. The -f option may be used multiple times on the command line. Gawk will read the program text as if all the program-files 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. To use a library function in a file from a program typed in on the command line, specify /dev/tty as one of the program-files, type your program, and end it with a ^D (control-d).

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/lib/awk:/usr/local/lib/awk". If a file name given to the -f option contains a ``/'' character, no path search is performed.

Gawk compiles the program into an internal form, 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 ``file'' named 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 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.

For each line in the input, gawk tests to see if it matches any pattern in the AWK program. For each pattern that the line matches, the associated action is executed.

VARIABLES AND FIELDS
AWK variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings, depending upon how they are used. AWK also has single dimension arrays; multiply dimensioned arrays may be simulated. There are several pre-defined variables that AWK sets as a program runs; these will be described as needed and summarized below.

As each input line is read, gawk splits the line 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. Otherwise, FS is expected to be a full regular expression. In the special case that FS is a single blank, fields are separated by runs of blanks and/or tabs. Note that the value of IGNORECASE (see below) will also affect how fields are split when FS is a regular expression.

Each field in the input line may be referenced by its position, $1, $2, and so on. $0 is the whole line. 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 line. The variable NF is set to the total number of fields in the input line.

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.

  Built-in Variables

AWK's built-in variables are:

    ARGC
        the number of command line arguments (does not include options to gawk, or the program source).

    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.

    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 /u/arnold). Changing this array does not affect the environment seen by programs which gawk spawns via redirection or the system function.

    FILENAME
        the name of the current input file. If no files are specified on the command line, the value of FILENAME is ``-''.

    FNR the input record number in the current input file.

    FS  the input field separator, a blank by default.

    IGNORECASE
        Controls the case-sensitivity of all regular expression operations. If IGNORECASE has a non-zero value, then pattern matching in rules, field splitting with FS, regular expression matching with ~ and !~, and the gsub(), 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 operations are normally case-sensitive.

    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 blank by default.

    ORS the output record separator, by default a newline.

    RS  the input record separator, by default a newline. RS is exceptional in that 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, then the newline character always acts as a field separator, in addition to whatever value FS may have.

    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.

  Variable Typing

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.

The AWK language defines comparisons as being done numerically if possible, otherwise one or both operands are converted to strings and a string comparison is performed.

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 line of input. A missing action is equivalent to

    { print }

which prints the entire line.

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 line 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 lines starting with a line that matches pattern1, and continuing until a line 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 except newline.

    ^   matches the beginning of a line or a string.

    $   matches the end of a line or a string.

    [abc...]
        character class, matches any of the characters abc....

    [^abc...]
        negated character class, matches any character except abc... and newline.

    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.

  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.

The operators in AWK, in order of increasing precedence, are

    = += -= *= /= %= ^=
        Assignment. Both absolute assignment (var = value) and operator-assignment (the other forms) are supported.

    ?:  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.

    ||  logical OR.

    &&  logical AND.

    ~ !~
        regular expression match, negated match.

    < <= > >= != ==
        the regular relational operators.

    blank
        string concatenation.

    + - addition and subtraction.

    * / %
        multiplication, division, and modulus.

    + - !
        unary plus, unary minus, and logical negation.

    ^   exponentiation (** may also be used, and **= for the assignment operator).

    ++ --
        increment and decrement, both prefix and postfix.

    $   field reference.

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]
    exit [ expression ]
    { statements }

The input/output statements are as follows:

    close(filename)
        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 NF, 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.

    print
        prints the current record.

    print expr-list
        prints expressions.

    print expr-list >file
        prints expressions on file.

    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 systems besides UNIX and GNU.)

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. Getline will return 0 on end of file, and -1 on an error.

The AWK versions of the printf and sprintf (see below) functions 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  A decimal number (the integer part).

    %e  A floating point number of the form [-]d.ddddddE[+-]dd.

    %f  A floating point number of the form [-]ddd.dddddd.

    %g  Use e or f conversion, whichever is shorter, with nonsignificant zeros suppressed.

    %o  An unsigned octal number (again, an integer).

    %s  A character string.

    %x  An unsigned hexadecimal number (an integer).

    %%  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.

    width
        The field should be padded to this width. If the number has a leading zero, then the field will be padded with zeros. Otherwise it is padded with blanks.

    .prec
        A number indicating the maximum width of strings or digits to the right of the decimal point.

The dynamic width and prec capabilities of the C library printf routines are not supported. However, they may be simulated by using the AWK concatenation operation to build up a format specification dynamically.

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). The filenames are:

    /dev/stdin
        The standard input.

    /dev/stdout
        The standard output.

    /dev/stderr
        The standard error output.

    /dev/fd/n
        The file denoted by 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.

AWK has the following pre-defined arithmetic functions:

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

    cos(expr)
        returns the cosine 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 in radians.

    sqrt(expr)
        the square root function.

    srand(expr)
        use 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.

AWK has the following pre-defined string functions:

    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.

    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.

    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.

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

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

    substr(s, i, n)
        returns the 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.

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  new line.

    \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).

FUNCTIONS
Functions in AWK are defined as follows:

    function name(parameter list) { statements }

Functions are executed when called from within the action parts of regular pattern-action statements. 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.

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

SYSTEM V RELEASE 4 COMPATIBILITY
A primary goal for gawk is compatibility 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 awk in System V Release 4.

When processing arguments, gawk uses the special option ``--'' to signal the end of arguments, and warns about, but otherwise ignores, undefined options.

The AWK book does not define the return value of srand(). The System V Release 4 version of UNIX awk 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.

The use of multiple -f options is a new feature, as is the ENVIRON array.

GNU EXTENSIONS
Gawk has some extensions to System V awk. They are described in this section. All the extensions described here can be disabled by compiling gawk with -DSTRICT, or by invoking gawk with the name awk. If the underlying operating system supports the /dev/fd directory and corresponding files, then gawk can be compiled with -DNO_DEV_FD to disable the special filename processing.

The following features of gawk are not available in System V awk.

    •   The \a, \v, or \x escape sequences are not recognized.

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

    •   The tolower and toupper built-in string functions are not available.

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

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

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 as awk, if the fs argument to the -F option is ``t'', then FS will be set to the tab character. Since this is a rather ugly special case, it is not the default behavior.

The rest of the features described in this section may change at some time in the future, or may go away entirely. You should not write programs that depend upon them.

Gawk accepts the following additional options:

-v  Print version information for this particular copy of gawk on the error 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.

-V  Print the GNU copyright information message on the error output.

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

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 of Dalhousie University, with contributions from Arnold Robbins at Emory University, made gawk compatible with the new version of UNIX awk.

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