GNU parallel with variable sequence?

7
votes

I want to run a program prog in parallel using GNU's parallel, with an argument that takes a value in a sequence. For example:

parallel prog ::: {1..100}

However, I don't know the upper bound of the sequence in advance, so I would like to be able to do something like:

parallel prog ::: {1..$x}

where $x is a number that I'll compute somewhere. How can I achieve this?

1
bashgnu-parallel
You will need eval. - jordanm
eval can be used, but is not necessary or recommended. - chepner

1 Answers

8
votes

Assuming the seq program (or something like it) is available, 

parallel prog ::: $(seq 1 $x)

If not, you can fake it:

parallel prog ::: $(for ((i=1; i < x; i++)) do; echo $i; done)

As Ole points out, if $x is large, then the resulting sequence of numbers may be too large to fit on the command line. In that case, use either of the two methods above to feed the arguments to parallel via standard input:

seq 1 "$x" | parallel prog
for ((i=1; i<x; i++)) do; echo "$i"; done | parallel prog