How to I prevent exec from keeping the script running?

Question

I'm using exec like this:

#!/bin/bash
exec > >(tee /tmp/mylog.log) 2>&1
bash -c 'sleep 12312' &

I want the output of the commands to be in a log file, and then I want to start a long running script, and completely detach it, without doing anything to its output.

But I've noticed that running a script with exec to redirect output to a file makes it continue running with the long-running script. For example, save the above script to a file /tmp/start.sh, run it, ps aux will show that /bin/bash /tmp/start.sh is still running!

If I remove the exec line, however, it's working as expected.

So how do I "detach" exec stdout redirection and have the script die once it's over?

Answer 1

The bash process that keeps running is the parent of tee, not the original script. You can see that by adding traces that display the process IDs:

#!/bin/bash
echo original=$$
exec > >(echo substitution=$BASHPID; tee /tmp/mylog.log) 2>&1
bash -c 'echo sleeper=$$; sleep 12312' &

This makes an additional bash process appear, the parent of sleep. To get rid of these bash processes, use exec throughout:

#!/bin/bash
echo original=$$
exec > >(echo substitution=$BASHPID; exec tee /tmp/mylog.log) 2>&1
bash -c 'echo sleeper=$$; exec sleep 12312' &

I tested this answer on bash 4.3. Future versions of bash may become smarter about not tail calls (like ksh).