3.2.3 Pipelines

A pipeline is a sequence of one or more 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 command1.

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

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 of Commands), the shell waits for all commands in the pipeline to complete.

Each command in a multi-command pipeline, where pipes are created, is executed in its own subshell, which is a separate process (see Command Execution Environment). If the lastpipe option is enabled using the shopt builtin (see The Shopt Builtin), the last element of a pipeline may be run by the shell process when job control is not active.

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.