sed & awk, 2nd Edition

Language Summary for awk

This section summarizes how awk processes input records and describes the various syntactic elements that make up an awk program.

Records and Fields

Each line of input is split into fields. By default, the field delimiter is one or more spaces and/or tabs. You can change the field separator by using the -F command-line option. Doing so also sets the value of FS. The following command-line changes the field separator to a colon:

awk -F: -f awkscr /etc/passwd

You can also assign the delimiter to the system variable FS. This is typically done in the BEGIN procedure, but can also be passed as a parameter on the command line.

awk -f awkscr FS=: /etc/passwd

Each input line forms a record containing any number of fields. Each field can be referenced by its position in the record. “$1” refers to the value of the first field; “$2” to the second field, and so on. “$0” refers to the entire record. The following action prints the first field of each input line:

{ print $1 }

The default record separator is a newline. The following procedure sets FS and RS so that awk interprets an input record as any number of lines up to a blank line, with each line being a separate field.

BEGIN { FS = "\n"; RS = "" }

It is important to know that when RS is set to the empty string, newline always separates fields, in addition to whatever value FS may have. This is discussed in more detail in both The AWK Programming Language and Effective AWK Programming.

Format of a Script

An awk script is ...