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.
Gawk options may be either the traditional [PX] one letter options, or the GNU style long options. [PX] options start with a single ``-'', while long options start with ``--''. Long options are provided for both GNU-specific features and for [PX] mandated features.
Following the [PX] 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.
Gawk accepts the following options.
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.
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).
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.
Gawk's built-in variables are:
Arrays are subscripted with an expression between square brackets ([ and ]). If the expression is an expression list (expr, expr ...) then the array subscript is a string consisting of the concatenation of the (string) value of each expression, separated by the value of the SUBSEP variable. This facility is used to simulate multiply dimensioned arrays. For example:
assigns the string "hello, world\n" to the element of the array x which is indexed by the string "A\034B\034C". All arrays in AWK are associative, i.e. indexed by string values.
The special operator in may be used 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.
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 [PX] 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).
{ 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.
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.
Interval expressions are only available if either --posix or --re-interval is specified on the command line.
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.
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.)
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.
The operators in AWK, in order of decreasing precedence, are
The control statements are as follows:
if (condition) statement [ else statement ]
while (condition) statement
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }
The input/output statements are as follows:
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.
The AWK versions of the printf statement and sprintf() function (see below) accept the following conversion specification formats:
There are optional, additional parameters that may lie between the % and the control letter:
The dynamic width and prec capabilities of the [AN] 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().
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:
These are particularly useful for error messages. For example:
whereas you would otherwise have to use
These file names may also be used on the command line to name data files.
AWK has the following pre-defined arithmetic functions:
Gawk has the following pre-defined string 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.
String constants in AWK are sequences of characters enclosed between double quotes ("). Within strings, certain escape sequences are recognized, as in C. These are:
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 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.
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.
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 }
The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X.
AWK Language Programming, Edition 1.0, published by the Free Software Foundation, 1995.
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 [PX] 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 [PX] 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 [AN] C conversion specifications in printf (done first in AT&T's version).
The following features of gawk are not available in [PX] awk.
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' ....
If gawk was compiled for debugging, it will accept the following additional options:
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 [PX] 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.
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.
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.
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 [UX]. John Woods contributed a number of bug fixes. David Trueman, with contributions from Arnold Robbins, made gawk compatible with the new version of [UX] 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.
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.
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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.
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