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Bash Reference Manual

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3.2.2 Pipelines

A pipeline is a sequence of simple commands separated by `|'.

The format for a pipeline is
 
[time [-p]] [!] command1 [| command2 ...]

The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output.

The reserved word time causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. The `-p' option changes the output format to that specified by POSIX. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed. See section 5.2 Bash Variables, for a description of the available formats. The use of time as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external time command cannot time these easily.

If the pipeline is not executed asynchronously (see section 3.2.3 Lists of Commands), the shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell (see section 3.7.3 Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status of the last command.


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  Copyright 2003   by The Free Software Foundation     Updated Jun 2003