AWK programming language

Awk is an example of a programming language that extensively uses the string datatype, associative arrays (that is, arrays indexed by key strings), and regular expressions. The power, terseness, and limitations of awk programs and sed scripts inspired Larry Wall to write Perl.

Awk is one of the early tools to appear in Version 7 UNIX and gained popularity as a way to add computational features to a UNIX pipeline.
A version of awk is a standard feature of nearly every modern Unix-like operating system available today. Implementations of awk exist as installed software for almost all other operating systems.

Structure of awk programs

/pattern/ { action } where pattern is a regular expression and action is a command. Awk looks through the input file; when it finds a line that matches pattern, it executes the command(s) specified in action. Alternate line forms include:

BEGIN { action } -
: Executes action commands at the beginning of the script execution, i.e., before any of the lines are processed.

END { action } -
: Similar to the previous form, but executes action after the script execution.

/pattern/ -
: Prints any lines matching pattern.

{ action } -
: Executes action for each line in the input.

Each of these forms can be included multiple times in the command file. Lines in the command file are executed in order, so if there are two "BEGIN" statements, the first is executed, then the second, and then the rest of the lines. BEGIN and END statements do not have to be located before and after (respectively) the other lines in the command file.

Awk commands

For brevity, the enclosing curly braces ( { } ) will be omitted from these examples.

The print command

print This displays the contents of the current line. In awk, lines are broken down into fields, and these can be displayed separately:

print $1 -
: Displays the first field of the current line

print $1, $3 -
: Displays the first and third fields of the current line, separated by a predefined string called the output field separator (OFS) whose default value is a single space character

Although these fields ($X) may bear resemblance to variables (the $ symbol indicates variables in perl), they actually refer to the fields of the current line. A special case, $0, refers to the entire line. In fact, the commands "print" and "print $0" are identical in functionality.

The print command can also display the results of calculations and/or function calls:

print 3+2 print foobar(3) print foobar(variable) print sin(3-2)

Variables, et cetera

User-defined functions

print add_three(36) # prints 39 Functions can have variables that are in the local scope. The names of these are added to the end of the argument list, though values for these should be omitted when calling the function. It is convention to add some whitespace in the argument list before the local variables, in order to indicate where the parameters end and the local variables begin.

Hello world program

Here is the ubiquitous "Hello world program" program written in AWK:

BEGIN { print "Hello, world!" }

Awk versions and implementations

GNU awk, or gawk, is another free software implementation. It was written before the orginal implementation became freely available, and is still widely used.

Downloads and further information about these versions are available from the sites listed below.

Digression