Unable to execute a bash script in background from another bash script

Question

I have a shell script which I want to invoke in background, from another shell script. Both the scripts are bash scripts.
First script(a.sh) is something like:

read a
echo 'something'
read b
echo 'something else'
# some if else conditions
nohup bash b.sh 2>&1 > /tmp/log &

When I try to execute the above script as: ./a.sh arg1 arg2 | tee log , very strangely it stucks at the nohup line for the second script b.sh to finish whereas it should not.

But when I just have the following line in the script, it works as expected:
nohup bash b.sh 2>&1 > /tmp/log &

Please help. I can also share the exact scripts if required.

No. It's stuck. By stuck I mean it waits for the second script to finish. Obviously pressing enter would insert a new line at the stdout, but it is not out of the first script. — Kartikeya Sinha
– Kartikeya Sinha, Commented May 7, 2014 at 14:31
It worked ! Thanks a lot. But I am still unable to understand why it was happening. And when same line was there in a separate script(which had no other line) why it worked there and not here. Any suggestions? — Kartikeya Sinha
– Kartikeya Sinha, Commented May 7, 2014 at 14:38
Not sure from which script you tool it but I have provided you right syntax for it — anubhava
– anubhava, Commented May 7, 2014 at 15:26

Community · Accepted Answer · 2017-05-23 11:50:22Z

2

This is what you wanted to happen:

The script runs and output things
The script runs a command in the background
The script exits
The pipe closes
tee exits

This is what's happening:

The script runs and outputs things
The script runs a command in the background, with the pipe as stderr
The script exits
The pipe is still held open by the backgrounded command
tee never exits.

You fixed it by pointing stderr to the log file rather than the pipe, so that the backgrounded process no longer keeps the pipe open.

The meaning of the order of 2>&1 and the redirect has been explained in other answers, and shellcheck would have automatically pointed out this problem in your code.

edited May 23, 2017 at 11:50

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answered May 7, 2014 at 15:32

that other guy

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1 Comment

Kartikeya Sinha Over a year ago

Thanks! That's exactly what I needed. Would take care of the order in future.

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