Redirection and Pipes

Steven J Zeil

Old Dominion University Dept. of Computer Science

May. 26, 2006

Contents:

1 Redirection

2 Pipes

(A printable version of this document is available here.)

One of the interesting ideas that pervades Unix is that many, if not most, programs can be viewed as “filters” or “transforms” that take a stream of text as input and produce an altered stream of text as output. Many Unix commands are designed to perform relatively trivial tasks, perhaps not very useful by themselves, that can be chained together in interesting and useful ways.

The practical consequence of this is that Unix shells devote special attention to a standard input stream that forms the main input to most programs/commands, and to a standard output stream that forms the main output from most programs/commands.1

There is actually a second output stream supported by many programs, the standard error stream, used for writing error/debugging messages.

The shell attempts to make it easy either to redirect one of these standard streams to a file or to pipe the standard output stream of one program into the standard input of another.