# GNU Parallel #

GNU Parallel lets us run shell commands in parallel.

	# apt-get install parallel
	$ mkdir -p ~/.parallel && touch ~/.parallel/will-cite

`Parallel` accepts a list of arguments, which we provide after `:::`:

	$ parallel echo ::: alpha bravo charlie
	alpha
	bravo
	charlie

If we supply multiple lists of arguments, `parallel` combines them:

	$ parallel echo ::: a b ::: c d
	a c
	a d
	b c
	b d

The output order my vary.

The tool provides a number of replacement string options. The default string `{}` represents the input:

	$ parallel echo {} ::: /tmp
	/tmp

(i.e. {} is replaced by a line of input, STDIN by default or else the arguments following `:::`.)

The replacement string `{/}` removes everything up to and including the last forward slash:

	$ parallel echo {/} ::: /tmp/test.txt
	test.txt

If you want to return the path only, use the `{//}` string:

	$ parallel echo {//} ::: /tmp/test.txt
	/tmp

The string `{.}` removes any filename extension:

	$ parallel echo {.} ::: /tmp/test.txt
	/tmp/test

See the man page for more substitution and argument options (e.g. `::::` to specify input read from files).

To run the same command multiple times for the same argument:

	$ seq 5 | parallel -n0 echo "Hello!"

Test with `--dry-run`:

	$ parallel --dry-run -k sleep {}\; echo {} ::: 5 2 1 4 3
	sleep 5; echo 5
	sleep 2; echo 2
	sleep 1; echo 1
	sleep 4; echo 4
	sleep 3; echo 3

(`-k` forces the output order to match the arguments; I'm not certain that implies any guarantee about the order of execution.)

## References ##

- https://www.gnu.org/software/parallel/man.html
- https://www.gnu.org/software/parallel/parallel_tutorial.html
- http://www.shakthimaan.com/posts/2014/11/27/gnu-parallel/news.html
- https://www.msi.umn.edu/support/faq/how-can-i-use-gnu-parallel-run-lot-commands-parallel
- https://www.gnu.org/software/parallel/