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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

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