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.
Gawk accepts the following options, which should be available on any implementation of the AWK language.
Following the POSIX standard, gawk-specific options are supplied via arguments to the -W option. Multiple -W options may be supplied, or multiple arguments may be supplied together if they are separated by commas, or enclosed in quotes and separated by white space. Case is ignored in arguments to the -W option. Gawk recognizes the following arguments to -W:
Any other options are flagged as illegal, but are otherwise ignored.
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 executes AWK programs in the following order. First, gawk compiles the program into an internal form. Next, all variable assignments specified via the -v option are performed. 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 ``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.
If the value of a particular element of ARGV is empty (""), gawk skips over it.
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. 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 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.
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.
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.
AWK'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.
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.
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 (draft 11), even if two strings are numeric strings, a numeric comparison is performed. However, this is clearly incorrect, and gawk does not do this.
Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string).
{ 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.
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 records starting with a line that matches pattern1, and continuing until a record that matches pattern2, inclusive. It does not combine with any other sort of pattern expression.
The escape sequences that are valid in string constants (see below) are also legal in regular expressions.
The operators in AWK, in order of increasing 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]
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. 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:
There are optional, additional parameters that may lie between the % and the control letter:
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.
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:
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:
AWK has the following pre-defined string functions:
Since one of the primary uses of AWK programs in 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).
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.
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.
The GAWK Manual, published by the Free Software Foundation, 1991.
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 accomodate applications that depended upon the old behaviour. (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, 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 (and 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).
The following features of gawk are not available in POSIX 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 -W compat option, 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.
For the 2.12 version of gawk, the -c, -v, -V, -a, and -e options of the 2.11 version are recognized. However, gawk will print a warning message, and these options will go away in the 2.13 version.
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 and AudioFAX, made gawk compatible with the new version of UNIX awk.