by Ciro Santilli (@cirosantilli, 34)
The author Ole Tange answers every question about it on Stack Exchange. What a legend!
This program makes you respect GNU make a bit more. Good old make with -j can not only parallelize, but also take in account a dependency graph.
Some examples under: 
man parallel_exampes
To get the input argument explicitly job number use the magic string {}, e.g.: 
printf 'a\nb\nc\n' | parallel echo '{}'
sample output: 
a
b
c
To get the job number use {#} as in: 
printf 'a\nb\nc\n' | parallel echo '{} {#}'
sample output: 
a 1
b 2
c 3
c 3
{%} contains which thread the job running in, e.g. if we limit it to 2 threads with -j2: 
printf 'a\nb\nc\nd\n' | parallel -j2 echo '{} {#} {%}'
sample output: 
a 1 1
b 2 1
c 3 2
d 4 1
The percent must be a reference to "split the inputs module the number of workers", and modulo uses the % symbol in many programming languages such as C.
To pass multiple CLI argments per command you can use -X e.g.: 
printf 'a\nb\nc\nd\n' | parallel -j2 -X echo '{} {#} {%}'
sample output: 
a b 1 1
c d 2 2
Way too few people know about this. Spread the word.
https://web.archive.org/web/20221208132133if_/https://i.stack.imgur.com/lHXP3.png

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