# Joys of gnu parallel

## 2015/08/26

I often have to perform a task of this form: given some paired input files left_input.1left_input.2, ... and right_input.1, right_input.2, ..., I need to make some output files output.1, output.2, ....

I used to do this by writing a bash loop: 

for i in 1 2 3 4 5
do
script --left=left.$i --right=right.$i --output=output.\$i
done

which I hated for a number of reasons: I need to type in the 1 2 3 4 5 by hand, the loop runs one-by-one, and it takes up multiple lines.

I found a solution that involves more keystrokes but which I think is more elegant: parallel --xapply script --left={1} --right={2} --output={3} ::: left.* ::: right.* ::: output.* This only works if the files output.* already exist, but I can use a little sed command to create the names I want. Just replace the output.* with ls left.* | sed 's/left/output/'!

Edit: Many thanks to Ole, whose comment is below. You can put a Perl regex inside the curly braces and instead write parallel script --left={} --right={=s/left/right/=} --output={=s/left/output/=} ::: left.*