Pipes and filters

In the context of Unix operating systems, 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. For creating this mechanism, all Unix tools have access to three distinct files:

• stdin the standard input file
• stdout the standard output file
• stderr the standard error file

By joining one tool's stdout to another tool's stdin, a pipeline is formed. Errors are sent to a side track and accumulated.

Often filter programs form the constituent programs in a pipeline.

An example of a pipeline:

cat * | grep "alice" | grep -v "wonderland" | wc -l

will print out the number of lines in all files in the current directory which contain the text "alice", but not the text "wonderland".

The pipeline has four parts:

• cat * concatenates the text of all files to its stdout;
• grep "alice" reads its stdin as lines, and prints on its stdout only those lines which contain the word "alice";
• grep -v "wonderland" reads its stdin and prints on its stdout only those remaining lines which do not contain the word "wonderland";
• wc -l counts the lines on its stdin, and prints a line count on its stdout.

Pipes and filters can be viewed as a form of functional programming, using byte streams as data objects.

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