Linux in a Nutshell, Fourth Edition

Patterns and Procedures

gawk scripts consist of patterns and procedures:

            pattern {procedure}

Both are optional. If pattern is missing, { procedure } is applied to all records. If { procedure } is missing, the matched record is printed. By default, each line of input is a record, but you can specify a different record separator through the RS variable.

Patterns

A pattern can be any of the following:

/regular expression/
relational expression
               pattern-matching expression
               pattern,pattern
               BEGIN
               END

Some rules regarding patterns include:

• Expressions can be composed of quoted strings, numbers, operators, functions, defined variables, or any of the predefined variables described later under “gawk System Variables.”

• Regular expressions use the extended set of metacharacters and are described in Chapter 9.

• In addition, ^ and $ can be used to refer to the beginning and end of a field, respectively, rather than the beginning and end of a record.

• Relational expressions use the relational operators listed under “Operators” later in this chapter. Comparisons can be either string or numeric. For example, $2 > $1 selects lines for which the second field is greater than the first.

• Pattern-matching expressions use the operators ~ (match) and !~ (don’t match). See “Operators” later in this chapter.

• The BEGIN pattern lets you specify procedures that take place before the first input record is processed. (Generally, you set global variables here.)

• The END pattern lets you specify procedures that ...