Filter (Unix)

In Unix and Unix-like operating systems, a filter is a program that gets most of its data from its standard input (the main input stream) and writes its main results to its standard output (the main output stream). Unix filters are often used as elements of pipelines. The pipe operator ("|") on a command line signifies that the main output of the command to the left is passed as main input to the command on the right.

The classic filter would be grep, which at it simplest prints to its output any lines containing a character string. The following is an example:

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

This finds all registered users that have "foo" as part of their username by using the cut command to take the first field (username) of each line of the Unix system password file and passing them all as input to grep, which searches its input for lines containing the character string "foo" and prints them on its output.

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.

v t e Unix command-line interface programs and shell builtins
File system	cat chattr chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste patch printf read sed sort strings tail tr troff uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List

Filter (Unix)

List of Unix filter programs

See also