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bash: Pipelines

 
 3.2.2 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 the command.
 
    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 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 (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.  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 (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 (Lists), the
 shell waits for all commands in the pipeline to complete.
 
    Each command in a pipeline is executed in its own subshell (
 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 (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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