Shell: how to run two scripts A and B in parallel and stop B when A is over

Question

Suppose I have a shell script A.sh whose content is:

sleep 2
echo "A: 2s"
sleep 2
echo "A: 4s"
sleep 2
echo "A: 6s"

I also have a shell script B.sh whose content is:

sleep 2
echo "B: 2s"
sleep 2
echo "B: 4s"
sleep 2
echo "B: 6s"
sleep 2
echo "B: 8s"
sleep 2
echo "B: 10s"

When I hope to run A and B in parallel, I created a script C.sh whose content is:

sh A.sh &
sh B.sh &
wait

The output is:

B: 2s
A: 2s
A: 4s
B: 4s
A: 6s
B: 6s
B: 8s
B: 10s

In this way A and B can run in parallel. Now I have one more requirement that is I hope to stop B when A is over. I followed the steps mentioned here and modified C.sh to:

parallel -j2 --halt now,success=1 ::: 'sh A.sh' 'sh B.sh'

The output is:

A: 2s
A: 4s
A: 6s
parallel: This job succeeded:
sh A.sh

It seems B is not running in this way. Can anyone advise me what is the correct way to run two scripts A and B in parallel and stop B when A is over?

I think it is running, but output is grouped by job by default, try `--linebuffer` Maybe also add `--tag` to see what comes from where. — Mark Setchell, Jan 12 '21 at 09:22
@MarkSetchell Hi I think you are right. I can see the outputs if they are redirected to files. — Richard Hu, Jan 12 '21 at 09:30
@MarkSetchell adding `--linebuffer` also helps. Thanks for your answer! — Richard Hu, Jan 12 '21 at 09:36
I wasn't at a proper machine earlier, but have put it as a normal answer now so folks can find it. — Mark Setchell, Jan 12 '21 at 10:18

score 5 · Answer 1 · answered Jan 12 '21 at 09:23

5

to run two scripts A and B in parallel and stop B when A is over?

So do just that.

sh B.sh &
b_pid=$!
{
    sh A.sh 
    kill "$b_pid"
} &
wait

answered Jan 12 '21 at 09:23

KamilCuk

120,984
8
59
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score 1 · Accepted Answer · answered Jan 12 '21 at 10:18

1

I think it is running, but output is grouped by job by default, so you won't see the output from B till A is finished but when A finishes, the whole GNU Parallel process exits.

Try switching to line-buffered output:

parallel --linebuffer sh ::: [AB].sh

answered Jan 12 '21 at 10:18

Mark Setchell

191,897
31
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score 0 · Answer 3 · answered Jan 12 '21 at 10:43

If you're using a sufficiently modern bash (eg, this does not work in 3.2.5.7, but it does in 5.0.16, so it was introduced sometime between those), you can do:

a.sh &
b.sh &
wait -n
kill -- -$$

This waits until either process terminates and then sends a SIGTERM to all the jobs in the process group. Note, this will send the SIGTERM to all processes in the group, including the main script, so you may need to handle that. If you want to restrict the jobs to which the signal is sent, run them in their own process group and send the signal appropriately.

Shell: how to run two scripts A and B in parallel and stop B when A is over

3 Answers3