In the context of Unix operating systems, as well as other Operating Systems like MS-DOS, a pipe signifies that the output of one program feeds directly as input to another program. The Unix shell uses the pipe character (|) to join programs together. A sequence of commands joined together by pipes is known as a pipeline. Often filter programs form the constituent programs in a pipeline -- see Pipes and filters.
An example of a pipeline, which should print the numbers from 1 to 10:
while : ; do echo ; done | head | nl -b a
See pipe for other uses of the word.
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