Learning Unix for Mac OS X Panther

Pipes and Filters

We’ve seen how to redirect input from a file and output to a file. You can also connect two programs together so that the output from one program becomes the input of the next program. Two or more programs 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 commands 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. A common use of filters is to modify output. Just as a common filter culls unwanted items, Unix filters can restructure output.

Most Unix programs can be used to form pipes. Some programs that are commonly used as filters are described in the next sections. 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 the contents of files for lines that have a certain pattern. The syntax is:

grep "pattern" file(s)

The name “grep” is derived from the ed (a Unix line editor) command g/ re/p, which means "globally search for a regular expression ...