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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.

Awk is one of the early tools to appear in Version 3 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.

1 Structure of awk programs

Generally speaking, two pieces of data are given to awk: a command file and a primary input file. A command file (which can be an actual file, or can be included in the command line invocation of awk) contains a series of commands which tell awk how to process the input file. The primary input file is typically text that is formatted in some way; it can be an actual file, or it can be read by awk from the standard input. A typical awk program consists of a series of lines, each of the form

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.

2 Awk commands

Awk commands are the statement that is substituted for action in the examples above. Awk commands can include function calls, variable assignments, calculations, or any combination thereof. Awk contains built-in support for many functions; many more are provided by the various flavors of awk. Also, some flavors support the inclusion of dynamically linked libraries, which can also provide more functions.

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

2.1 The print command

The print command is used to display text to the output device (usually a monitor, though often a file or

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

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: