In UNIX and UNIX-like operating systems, a filter is program that gets most of its data from standard input and writes its main results to standard output. UNIX filters are often used as elements of pipelines.

The classic filter would be grep; here's an example:

 cut -d : -f 1 </etc/passwd | grep foo

This finds all registered users that have "foo" as part of their username.

Common UNIX filter programs are: cat, cut, grep, head, sort, uniq and tail. Programs like awk and sed can be used to build quite complex filters because they are fully programmable.

A more complete list of UNIX filter programs:

• awk
• cat
• comm
• cut
• expand
• compress
• fold
• grep
• head
• nl
• pr
• sed
• sh
• sort
• split
• strings
• tail
• tee
• tr
• uniq
• wc

See also: http://wiki.debian.net/index.cgi?filter