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UNIX in a Nutshell: System V Edition

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11. The Awk Scripting Language

This section presents the following topics:

  • Command-line syntax

  • Conceptual overview

  • Patterns and procedures

  • System variables

  • Operators

  • Variable and array assignment

  • Group listing of commands

  • Alphabetical summary of commands

For more information, see the Nutshell Handbook sed & awk .

11.1 Command-line Syntax

The syntax for invoking awk has two forms:

awk [ options ] ' script ' var = value file(s)

awk [ options ] -f scriptfile var = value file(s)

You can specify a script directly on the command line, or you can store a script in a scriptfile and specify it with -f . Variables can be assigned a value on the command line. The value can be a literal, a shell variable ( $ name ), or a command substitution ( `cmd` ), but the value is available only after a line of input is read (i.e., after the BEGIN statement). Awk operates on one or more files . If none is specified (or if - is specified), awk reads from the standard input.

The recognized options are:

-F c

Set the field separator to character c . This is the same as setting the system variable FS . Nawk allows c to be a regular expression. Each input line, or record, is divided into fields by white space (blanks or tabs) or by some other user-definable record separator. Fields are referred to by the variables $1 , $2 ,..., $ n . $0 refers to the entire record.

-v var = value

Assign a value to variable var . This allows assignment before the script begins execution. (Available in nawk only.)

For example, to print the first three (colon-separated) fields on a separate line:



awk -F: '{print $1; print $2; print $3}' /etc/passwd

Numerous examples are shown later in this section under "Patterns and Procedures."


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