So I have a while loop:

cat live_hosts | while read host; do \
    sortstuff.sh -a "$host" > sortedstuff-"$host"; done

But this can take a long time. How would I use GNU Parallel for this while loop?


You don't use a while loop.

parallel "sortstuff.sh -a {} > sortedstuff-{}" <live_hosts

Note that this won't work if you have paths in your live_hosts (e.g. /some/dir/file) as it would expand to sortstuff.sh -a /some/dir/file > sortedstuff-/some/dir/file (resulting in no such file or directory); for those cases use {//} and {/} (see gnu-parallel manual for details):

parallel "sortstuff.sh -a {} > {//}/sortedstuff-{/}" <live_hosts
  • Is it possible to use tee with parallel when putting the output into sortedstuff? So I can see the output as it goes. – Proletariat Sep 22 '15 at 14:16
  • 1
    @Proletariat - you want to output to terminal too ? Just replace > with | tee e.g. the first command becomes parallel "sortstuff.sh -a {} | tee sortedstuff-{}" <live_hosts – don_crissti Sep 22 '15 at 15:10

As an old-school "do one thing and do it well" Unix guy, I'd put the string substitution stuff into a wrapper script:

sortstuff.sh -a "$1" > sortedstuff-"$1"

If you call it wrapper.sh, the parallel command to call it would be:

parallel wrapper.sh < live_hosts

Note that you don't need cat for this kind of thing, which saves an external program invocation.


You don't need parallel, since the body of the loop doesn't depend on previous iterations. Just start a new background process for each host.

while read host; do
    sortstuff.sh -a "$host" > sortedstuff-"$host" &
done < live_hosts
wait    # Optional, to block until the background tasks are done

parallel does make it easier to manage certain aspects, though; you can limit the number of jobs running in parallel more easily.

  • 3
    If wc -l live_hosts is more than the number of disk spindles or CPU cores — depending on whether the task is I/O or CPU-bound — you're going to eat up a lot of the advantage you get from parallelism with a solution like that. The ability of parallel to limit the number of jobs isn't just nice, it's near-essential, if processing speed is your goal. – Warren Young Sep 11 '15 at 20:47

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