Bash wait for all subprocesses of script

Question

I have a script with

for i in 1 2 3 4; do
    do_something $i &
done

And when I call it, it terminates before all do_something terminated. I found this question with many different answers.

Edit: help wait tells me that

If ID is not given, waits for all currently active child processes, and the return status is zero.

Is it not sufficient to just add a single wait at the end?

No you need to wait for each of the process to complete. There is an answer suggesting how to wait for all the bg processes in your link — Inian, Sep 12 '19 at 07:08
What is your question? wait is a shell builtin. Use help wait instead of man wait — pLumo, Sep 12 '19 at 07:08
Why don't you just try? E.g. for i in 1 2 3 4; do sleep $i & done; wait. Spoiler: Yes it is sufficient. — pLumo, Sep 12 '19 at 07:10
@pLumo Because it's critical that it work for the real world use case and I would rather rely on knowledge than a single example :) — fabian789, Sep 12 '19 at 07:13
anyways, you might want to use parallel do_something ::: 1 2 3 4; — pLumo, Sep 12 '19 at 07:15
Notice that wait only works with direct children of your script. If you use (do_something $i &) (ie in a subshell) your wait will not wait for it, because the actual do_something process will be reparented to pid 1 (init). — , Sep 12 '19 at 07:25

Kusalananda · Accepted Answer · 2021-04-13T10:14:46.223

14

Yes, it's enough to use a single wait with no arguments at the end to wait for all background jobs to terminate.

Note that background jobs started in a subshell would need to be waited for in the same subshell that they were started in. You have no instance of this in the code that you show.

Note also that the question that you link to asks about checking the exit status of the background jobs. This would require wait to be run once for each background job (with the PID of that job as an argument).

edited Apr 13 '21 at 10:14

answered Sep 12 '19 at 07:50

Kusalananda

333,661

This did not work for me. A single line of wait at the end with no parameters, did not help me. I loop over git repositories and pull them. Coming from this question. – Saeed Neamati Jan 26 '22 at 11:50
@SaeedNeamati That question and the question linked to in the question above both start background jobs in a subshell but call wait outside that subshell. – Kusalananda Aug 13 '22 at 09:50

score -1 · Answer 2 · answered Sep 12 '19 at 08:41

-1

In my opinion, even though a single wait without parameters should be sufficient, this is not a good practice (relay on a default behaviour). I would collect the pid returned from each call, and wait for these pids explicitly.

answered Sep 12 '19 at 08:41

Eran Ben-Natan

568
2
7

6

Why would relying on default behaviour be bad practice? The wait command is a standard command with well defined semantics. – Kusalananda Sep 12 '19 at 09:11
This answer would be seriously improved if it included any reasons for wait without arguments being not a good practice. – Neinstein Aug 16 '23 at 12:45

Bash wait for all subprocesses of script

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