The Linux Programming Interface

PIPES AND FIFOS

This chapter describes pipes and FIFOs. Pipes are the oldest method of IPC on the

UNIX system, having appeared in Third Edition UNIX in the early 1970s. Pipes

provide an elegant solution to a frequent requirement: having created two pro-

cesses to run different programs (commands), how can the shell allow the output

produced by one process to be used as the input to the other process? Pipes can be

used to pass data between related processes (the meaning of related will become

clear later). FIFOs are a variation on the pipe concept. The important difference is

that FIFOs can be used for communication between any processes.

44.1 Overview

Every user of the shell is familiar with the use of pipes in commands such as the

following, which counts the number of files in a directory:

$ ls | wc -l

In order to execute the above command, the shell creates two processes, executing

ls and wc, respectively. (This is done using fork() and exec(), which are described in

Chapters 24 and 27.) Figure 44-1 shows how the two processes employ the pipe.

Among other things, Figure 44-1 is intended to illustrate how pipes got their

name. We can think of a pipe as a piece of plumbing that allows data to flow from

one process to another.