7.3 Actions

An awk program or script consists of a series of rules and function definitions interspersed. (Functions are described later. See section User-Defined Functions.) A rule contains a pattern and an action, either of which (but not both) may be omitted. The purpose of the action is to tell awk what to do once a match for the pattern is found. Thus, in outline, an awk program generally looks like this:

[pattern]  { action }
 pattern  [{ action }]
…
function name(args) { … }
…

An action consists of one or more awk statements, enclosed in curly braces (‘{…}’). Each statement specifies one thing to do. The statements are separated by newlines or semicolons. The curly braces around an action must be used even if the action contains only one statement, or if it contains no statements at all. However, if you omit the action entirely, omit the curly braces as well. An omitted action is equivalent to ‘{ print $0 }’:

/foo/  { }     match foo, do nothing --- empty action
/foo/          match foo, print the record --- omitted action

The following types of statements are supported in awk:

Expressions

Call functions or assign values to variables (see section Expressions). Executing this kind of statement simply computes the value of the expression. This is useful when the expression has side effects (see section Assignment Expressions).

Control statements

Specify the control flow of awk programs. The awk language gives you C-like constructs (if, for, while, and do) as well as a few special ones (see section Control Statements in Actions).

Compound statements

Consist of one or more statements enclosed in curly braces. A compound statement is used in order to put several statements together in the body of an if, while, do, or for statement.

Input statements

Use the getline command (see section Explicit Input with getline). Also supplied in awk are the next statement (see section The next Statement), and the nextfile statement (see section Using gawk’s nextfile Statement).

Output statements

Such as print and printf. See section Printing Output.

Deletion statements

For deleting array elements. See section The delete Statement.