Filter program

A filter program is classically a UNIX program that is designed to be used as part of a pipeline of two or more UNIX utilities. Generally a filter program will read its standard input and write to its standard output and do little else. Conventionally a filter program distinguishes itself by being fairly simple and performing essentially one operation, usually some sort of simple transformation of its input data.

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