How to assign the output of a background job into a bash variable?

Question

I would like to run a background job in bash and assign its result to a variable.

I prefer not to use temporary files and I would love to run multiple similar background tasks at the same time.

root@root:/# var=$(echo "hello world")
root@root:/# echo $var
hello world
root@root:/# back_var=$(sleep 2s && echo "hello world back") &
[1] 2102
root@root:/# wait
root@root:/#jobs
[1]+  Done                    back_var=$(sleep 2s && echo "hello world back")
root@root:/# echo $back_var

root@root:/#

I prefer not to use gnu-parallel or temp files.

To be even clearer that's not a trivial question IMHO:

root@root:/# back_var_1=$(sleep 4s && echo "Don't waste my time" &) &
[1] 26584
root@root:/# wait
[1]+  Done                    back_var_1=$(sleep 4s && echo "Don't waste my time" &)
root@root:/# echo $back_var_1

root@root:/#

I don't understand why you are catching sleep. Why don't you just say sleep 2 && var=$(echo "hello world") &? — fedorqui
– fedorqui, Commented Jun 16, 2016 at 9:05
@fedorqui, it's just a toy mode example in order to demonstrate what I want to achieve. Consider a case where the in parenthesis is an async operation (IO) with long execution time command. — 0x90
– 0x90, Commented Jun 16, 2016 at 9:09
OK, I see. Probably worth reading Bash: Capture output of command run in background. I haven't gone through it thoroughly, but may contain juicy info. — fedorqui
– fedorqui, Commented Jun 16, 2016 at 9:11
@0x90 : I've got my mistake. Have you considered using named pipes ? — blackSmith
– blackSmith, Commented Jun 16, 2016 at 14:01
Uh, don't be root, especially if you don't know exactly what you are doing. — tripleee
– tripleee, Commented Jun 21, 2016 at 5:12

blackSmith · Accepted Answer · 2016-06-16 14:29:38Z

3

With named pipes it's possible. However, I don't know whether it'll be considered as a temp file :

 $ mkfifo pipo
 $ sleep 4s && echo "this is not fair" > pipo & 
 [1] 25356

 $ back_var=$(cat pipo)
 [1]+  Done                    sleep 4s && echo "this is not fair" > pipo

 $ echo $back_var
 this is not fair

Hope it helps.

answered Jun 16, 2016 at 14:29

blackSmith

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1 Comment

0x90 Over a year ago

It's a nice idea. Fifos are blocking too, so one can use it as a sync mechanism as well.

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