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Asked: 2010-06-20 07:44:17 +0800 CST

How do I run two different long running commands at the same time in a bash script?

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I have to chmod and chown hundreds of thousands of files as part of a migration script. Each command takes about an hour and a half to complete. I realized these two operations can be run at the same time which cuts down on running time, which I confirmed by testing in the shell.

I know the trick of pushing commands into the background with '&', but I need to make sure both processes finish before proceeding with the rest of the script.

Thanks

linux scripting bash

3 Answers

  1. Use the wait command.

    This demo:

    #!/bin/bash
echo $SECONDS
sleep 12&
sleep 15&
jobs
wait
echo $SECONDS
echo "jobs are done"

    Produces this output:

    0
[1]-  Running                 sleep 12 &
[2]+  Running                 sleep 15 &
15
jobs are done

    There's a fifteen second pause before the last two lines are output.

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  2. You can capture the pid and use wait.

    chmod options
CHMODPID=$!
chown options
CHOWNPID=$!
wait $CHMODPID
wait $CHOWNPID

    (should work with bash; details may differ on other shell types)

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  3. If you need to make sure that both processes are finished before proceeding to the next step, a simple solution would be to write something to a file and check it. Just do a head or cat on the file (with default value 0) and add 1 to the first line. If you detect 2, then both processes are completed. Of course the addition should be done at the very bottom of your scripts.

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