Learning the UNIX Operating System, Fourth Edition

Pipes and Filters

In addition to redirecting input/output to a named file, you can connect two commands together so that the output from one program becomes the input of the next program. Two or more commands connected in this way form a pipe. To make a pipe, put a vertical bar (|) on the command line between two commands. When a pipe is set up between two commands, the standard output of the command to the left of the pipe symbol becomes the standard input of the command to the right of the pipe symbol. Any two programs can form a pipe as long as the first program writes to standard output and the second program reads from standard input.

When a program takes its input from another program, performs some operation on that input, and writes the result to the standard output (which may be piped to yet another program), it is referred to as a filter. One of the most common uses of filters is to modify output. Just as a common filter culls unwanted items, the UNIX filters can be used to restructure output.

Almost all UNIX commands can be used to form pipes. Some programs that are commonly used as filters are described below. Note that these programs aren’t used only as filters or parts of pipes. They’re also useful on their own.

grep

The grep program searches a file or files for lines that have a certain pattern. The syntax is:

grep pattern file(s)

The name “grep” derives from the ed (a UNIX line editor) command g/re/p which means "globally search for a regular expression and ...