GNU parallel

GNU parallel is a command-line utility for Linux and other Unix-like operating systems which allows the user to execute shell scripts or commands in parallel. GNU parallel is free software, written by Ole Tange in Perl. It is available under the terms of GPLv3.

Parallel

Developer(s)	GNU Project
Stable release	20230522 / 22 May 2023 (2023-05-22)
Written in	Perl
Operating system	GNU
Type	Utility
License	GPLv3

Usage

Introduction video, Part 1

Introduction video, Part 2

The most common usage is to replace the shell loop, for example <syntaxhighlight lang="bash"> while read x; do

   do_something "$x"

done < list </syntaxhighlight> to the form of <syntaxhighlight lang="bash"> cat list | parallel do_something </syntaxhighlight> where the file list contains arguments for do_something and where process_output may be empty.

Scripts using parallel are often easier to read than scripts using pexec.

The program parallel features also

• grouping of standard output and standard error so the output of the parallel running jobs do not run together;
• retaining the order of output to remain the same order as input;
• dealing nicely with filenames containing special characters such as space, single quote, double quote, ampersand, and UTF-8 encoded characters;

By default, parallel runs as many jobs in parallel as there are CPU cores.

Examples

<syntaxhighlight lang="bash"> find . -name "*.foo" | parallel grep bar </syntaxhighlight>

The above is the parallel equivalent to:

<syntaxhighlight lang="bash"> find . -name "*.foo" -exec grep bar {} + </syntaxhighlight>

This searches in all files in the current directory and its subdirectories whose name end in .foo for occurrences of the string bar. The parallel command will work as expected unless a file name contains a newline. In order to avoid this limitation one may use:

<syntaxhighlight lang="bash"> find . -name "*.foo" -print0 | parallel -0 grep bar </syntaxhighlight>

The above command uses the null character to delimit file names.

<syntaxhighlight lang="bash"> find . -name "*.foo" | parallel -X mv {} /tmp/trash </syntaxhighlight>

The above command expands {} with as many arguments as the command line length permits, distributing them evenly among parallel jobs if required. This can lower process overhead for short-lived commands that take less time to finish than they do to launch.

<syntaxhighlight lang="bash"> find . -maxdepth 1 -type f -name "*.ogg" | parallel -X -r cp -v -p {} /home/media </syntaxhighlight>

The command above does the same as:

<syntaxhighlight lang="bash"> cp -v -p *.ogg /home/media </syntaxhighlight>

However, the former command which uses find/parallel/cp is more resource efficient and will not halt with an error if the expansion of *.ogg is too large for the shell.

See also

• xargs
• pexec
• GNU Queue

External links

• Project page and manual page of GNU parallel