Question

using bash in linux is it possible to spawn parallel processes in the foreground? For example the following :

top.sh

#!/bin/bash

./myscript1.sh &
./myscript2.sh &

will spawn two processes in parallel as background threads. However is it possible to spawn these as foreground threads? The aim is to automatically kill myscript1.sh and myscript2.sh, when top.sh is killed. Thanks

Answer 1

You can only have one job in the foreground. You can make your script react to any signal that reaches it and forward the signal to other jobs. You need to make sure your script sticks around, if you want to have a central way of killing the subprocesses: call wait so that your script will not exit until all the jobs have died or the script itself is killed.

#!/bin/bash
jobs=
trap 'kill -HUP $jobs' INT TERM HUP
myscript1.sh & jobs="$jobs $!"
myscript2.sh & jobs="$jobs $!"
wait

You can still kill only the wrapper script by sending it a signal that it doesn’t catch, such as SIGQUIT (which I purposefully left out) or SIGKILL (which can’t be caught).

There’s a way to have all the processes in the foreground: connect them through pipes. Ignore SIGPIPE so that the death of a process doesn’t kill the previous one. Save and restore stdin and stdout through other file descriptors if you need them. This way the script and the background tasks will be in the same process group, so pressing Ctrl+C will kill both the wrapper script and the subprocesses. If you kill the wrapper script directly, that won’t affect the subprocesses; you can kill the process group instead by passing the negative of the PID of the wrapper script (e.g. kill -TERM -1234).

trap '' PIPE
{
  myscript1.sh <&3 >&4 |
  myscript2.sh <&3 >&4
} 3<&0 4>&1