Looping until background process is done (with `jobs`) doesn't work in script

Question

I'm trying to loop until a background process (launched earlier in the script) is complete. An easily reproducible test case is:

ping -c 10 localhost &>/dev/null &

On the command line, I can loop while [[ -n $(jobs) ]] (while $(jobs) is not null).

$ ping -c 10 localhost &>/dev/null &
[1] 19078
$ while [[ -n $(jobs) ]]; do echo -n .; sleep 1; done
.........[1]+  Done                    ping -c 5 localhost &> /dev/null

However, the same two lines in a script will keep printing .s until I hit Ctrl-C.

Weirdly, if I call jobs inside the loop, the script ends as expected.

$ ./background-ping.sh
.[1]+  Running                 ping -c 5 localhost &> /dev/null &
.[1]+  Running                 ping -c 5 localhost &> /dev/null &
.[1]+  Running                 ping -c 5 localhost &> /dev/null &
.[1]+  Running                 ping -c 5 localhost &> /dev/null &
.[1]+  Done                    ping -c 5 localhost &> /dev/null

I am aware that there are other ways to check if the background job is complete (e.g. checking /proc), but I want to know why checking jobs doesn't work as expected.

Answer 1

If I understand your problem, your 2 lines won't stop in a script.

That's what I made in mine:

ping -c 5 google.com &>/dev/null &
while [[ -n $(jobs -r) ]]; do echo -n "."; sleep 1; done

jobs -r checks running processes, and calling my script works as expected, the script stops when ping is done.

EDIT: I think that in a script the parent process is known as a running process and so that's why jobs continue to think that there is a running process. That's an hypothesis

Is that the answer ? (or maybe I really don't understand what you mean, my poor english can be the problem...)

Answer 2

Whenever you have issues like this, you should always try printing your variables so you understand what's going on. In this case, jobs also returns finished jobs. If you run this:

ping -c 5 localhost &>/dev/null &
while [[ -n $(jobs | tee -a temp) ]]; do
    echo -n .; 
    sleep 1; 
done

You will see the following output in temp:

[1]+  Running                 ping -c 5 localhost &> /dev/null &
[1]+  Running                 ping -c 5 localhost &> /dev/null &
[1]+  Running                 ping -c 5 localhost &> /dev/null &
[1]+  Running                 ping -c 5 localhost &> /dev/null &
[1]+  Done                    ping -c 5 localhost &> /dev/null
[1]+  Done                    ping -c 5 localhost &> /dev/null
[1]+  Done                    ping -c 5 localhost &> /dev/null
[1]+  Done                    ping -c 5 localhost &> /dev/null
[1]+  Done                    ping -c 5 localhost &> /dev/null
[...]

So, the output of jobs here is never empty. Even when the job is finished, jobs still returns the Done message. That's why Metal3d's solution using jobs -r worked.

What is more confusing is why running jobs within the loop causes it to work correctly. The answer will be something to do with the fact that while [[ -n $(jobs) ]] runs jobs in a separate subshell but I'm not sure about the details. I have posted a question about this on U&L if anyone is interested.

Answer 3

You can use wait command that waits for background jobs to be stopped

Example:

$ ping -c 10 localhost &>/dev/null &
[29787]
$ wait
[1]+  Done ping -c 10 localhost &>/dev/null

The wait command blocks and when ping has finished, prompt is released then prints a message.

You can use is with several commands:

$ ping -c 5 localhost &>/dev/null & ping -c 5 facebook.com  &>/dev/null &
[1] 29867
[2] 29868
$ wait
[1]-  Done                    ping -c 5 localhost &>/dev/null
[2]+  Done                    ping -c 5 facebook.com &>/dev/null