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3.2.2 Pipelines

A pipeline is a sequence of simple commands separated by one of the control operators ‘|’ or ‘|&’.

The format for a pipeline is

[time [-p]] [!] command1 [ [| or |&] 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. This connection is performed before any redirections specified by the command.

If ‘|&’ is used, the standard error of command1 is connected to command2’s standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error is performed after any redirections specified by the command.

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. When the shell is in POSIX mode (see Bash POSIX Mode), it does not recognize time as a reserved word if the next token begins with a ‘-’. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed. See 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.

When the shell is in POSIX mode (see Bash POSIX Mode), time may be followed by a newline. In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT variable may be used to specify the format of the time information.

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

Each command in a pipeline is executed in its own subshell (see Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled (see The Set Builtin). If pipefail is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ‘!’ precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.


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