Using Pipes

Pipes are connectors that join two or more programs or commands together. A pipe allows you to take the output of one program and use it as input to another program without the use of intermediate files.

The metacharacter for the pipe is the vertical bar (|). For example, suppose you want to list all the current users logged into the system and then alphabetically sort them and print them out. The command line reads:

who | sort

In the following example, a list of people logged into a system is produced by the who command. That output is sent as input into the sort command which outputs the sorted list of people on the system to the display. For example:

$ who
michael    tty02        Oct  4 14:49
dave       tty03        Oct  4 14:49
mary       tty00        Oct  4 13:34
george     tty04        Oct  4 14:49
keith      tty05        Oct  4 14:49
$ who | sort
dave       tty03        Oct  4 14:49
george     tty04        Oct  4 14:49
keith      tty05        Oct  4 14:49
mary       tty01        Oct  4 13:34
michael    tty02        Oct  4 14:49

Two-Way Pipes

Two-way pipes or coprocesses can be established between the shell and a job. The parent process is the original shell and the child process (or subprocess) is the job, the command or shell spawned from the parent shell.

The standard input and output of the spawned command can be written to and read from the parent shell in a two-way pipe. A two-way pipe is created by placing the |& metacharacter after the command to be executed. See Chapter 23 “Advanced Concepts and Commands” for details on two-way pipes.