Run a background script inside a docker container

dockerprocess

In my Dockerfile, I run a script:

RUN /bin/sh -c scripts/init.sh

Inside init.sh, all commands ending with & are not executed: I cannot run background processes.
Any idea why?

Best Answer

I had the similar issue and something like the following helped me.

RUN nohup bash -c "scripts/init.sh &" && sleep 4

In many cases the server you started isn’t yet fully ready. To allow the server a bit more time to get ready add a sleep command. How large the argument sleep needs to depend on the service you start and you probably need to tweak it.

Related Solutions

Linux – What’s a reliable technique for killing background processes on script termination

The super simple solution is to add this at the end of the script:

kill -- -$$

Explanation:

$$ gives us the PID of the running shell. So, kill $$ would send a SIGTERM to the shell process. However, if we negate the PID, kill sends a SIGTERM to every process in the process group. We need the -- beforehand so kill knows that -$$ is a process group ID and not a flag.

Note that this relies on the running shell being a process group leader! Otherwise, $$ (the PID) will not match the process group ID, and you end up sending a signal to who knows where (well, probably nowhere as there is unlikely to be a process group with a matching ID if we're not a group leader).

When the shell starts, it creates a new process group[1]. Every forked process becomes a member of that process group, unless they explicitly change their process group via a syscall (setpgid).

The easiest way to guarantee a particular script runs as a process group leader is to launch it using setsid. For example, I have a few of these status scripts which I launch from a parent script:

#!/bin/sh
wifi_status &
bat_status &

Written like this, both the wifi and battery scripts run with the same process group as the parent script, and kill -- -$$ doesn't work. The fix is:

#!/bin/sh
setsid wifi_status &
setsid bat_status &

I found pstree -p -g useful to visualise process & process group IDs.

Thanks to everyone who contributed and made me dig a little deeper, I learnt stuff! :)

[1] is there other circumstances where the shell creates a process group? eg. on starting a subshell? i don't know...

How to get list of all child process spawned by a script

To answer your question directly, the command

jobs -p

gives you the list of all child processes.

Alternative #1

But in your case it might be easier to just use the command wait without any params:

first_program &
second_program &
wait

This will wait until ALL child processes have finished.

Alternative #2

Another alternative is using $! to get the PID of the last program and perhaps accumulate in a variable, like so:

pids=""
first_program &
pids="$pids $!"
second_program &
pids="$pids $!"

and then use wait with that (this is in case you only want to wait for a subset of you child processes):

wait $pids

Alternative #3

OR, if you want to wait only until ANY process has finished, you can use

wait -n $pids

Bonus info

If you want a sigterm to your bash script to close your child processes as well, you will need to propagate the signal with something like this (put this somewhere at the top, before starting any process):

trap 'kill $(jobs -p)' SIGINT SIGTERM EXIT