id	author	title	date	pages	extension	mime	words	sentences	flesch	summary	cache	txt
en-wikipedia-org-9575		Filter (software) - Wikipedia			.html	text/html	1030	126	70	Some operating systems such as Unix are rich with filter programs. 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). The classic filter in Unix is Ken Thompson's grep, which Doug McIlroy cites as what "ingrained the tools outlook irrevocably" in the operating system, with later tools imitating it.[1] 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. List of Unix filter programs[edit]	./cache/en-wikipedia-org-9575.html	./txt/en-wikipedia-org-9575.txt