bash shutdown hook; or, kill all background processes when main process is killed

Question

I have a bash that runs endless commands as background processes:

#!/bin/bash

function xyz() {
  # some awk command
}

endlesscommand "param 1" | xyz &  # async
pids=$!
endlesscommand "param 2" | xyz &  # async
pids="$pids "$!
endlesscommand "param 3" | xyz    # sync so the script doesn't leave

The only way to stop this script is (must be) Ctrl-C or kill and when that happens, I need to kill all the background processes listed in the $pids variable.

How do I do that?

If it was possible to catch the kill signal on the main process and execute a function when that happens (shutdown hook), I would do something like:

for $pid in $pids; do kill $pid; done;

But I can't find how to do this...

Answer 1

Here's a trap that doesn't need you to track pids:

trap 'jobs -p | xargs kill' EXIT

EDIT: @Barmar asked if this works within non-sourced scripts, where job control isn't usually available. It does. Consider this script:

$ cat no-job-control
#! /bin/bash

set -e -o pipefail

# Prove job control is off
if suspend
then
  echo suspended
else
  echo suspension failed, job control must be off
fi

echo

# Set up the trap
trap 'jobs -p | xargs kill' EXIT

# Make some work
(echo '=> Starting 0'; sleep 5; echo '=> Finishing 0') &
(echo '=> Starting 1'; sleep 5; echo '=> Finishing 1') &
(echo '=> Starting 2'; sleep 5; echo '=> Finishing 2') &

echo "What's in jobs -p?"
echo

jobs -p

echo
echo "Ok, exiting now"
echo

When run we see the pids of the three group leaders, and then see them killed:

$ ./no-job-control
./no-job-control: line 6: suspend: cannot suspend: no job control
suspension failed, job control must be off

=> Starting 0
What's in jobs -p?
=> Starting 1

54098
54099
54100

Ok, exiting now

=> Starting 2
./no-job-control: line 31: 54098 Terminated: 15          ( echo '=> Starting 0'; sleep 5; echo '=> Finishing 0' )
./no-job-control: line 31: 54099 Terminated: 15          ( echo '=> Starting 1'; sleep 5; echo '=> Finishing 1' )
./no-job-control: line 31: 54100 Terminated: 15          ( echo '=> Starting 2'; sleep 5; echo '=> Finishing 2' )

If we instead comment out the trap line and re-run, the three jobs do not die and in fact print out their final messages a few seconds later. Notice the returned prompt interleaved with the final outputs.

$ ./no-job-control
./no-job-control: line 6: suspend: cannot suspend: no job control
suspension failed, job control must be off

=> Starting 0
What's in jobs -p?

54110
54111
54112
=> Starting 1

Ok, exiting now

=> Starting 2
$ => Finishing 0
=> Finishing 2
=> Finishing 1

Answer 2

You can make use of pgrep and a function to kill all processes created under the main process like this. This would not only kill the direct child processes but also those created under it.

#!/bin/bash

function killchildren {
    local LIST=() IFS=$'\n' A
    read -a LIST -d '' < <(exec pgrep -P "$1")
    local A SIGNAL="${2:-SIGTERM}"
    for A in "${LIST[@]}"; do
        killchildren_ "$A" "$SIGNAL"
    done
}

function killchildren_ {
    local LIST=()
    read -a LIST -d '' < <(exec pgrep -P "$1")
    kill -s "$2" "$1"
    if [[ ${#LIST[@]} -gt 0 ]]; then
        local A
        for A in "${LIST[@]}"; do
            killchildren_ "$A" "$2"
        done
    fi
}

trap 'killchildren "$BASHPID"' EXIT

endlesscommand "param 1" &
endlesscommand "param 2" &
endlesscommand "param 3" &

while pgrep -P "$BASHPID" >/dev/null; do
    wait
done

As for your original code, it would be better to just use arrays, and you also don't need to use a for loop:

#!/bin/bash

trap 'kill "${pids[@]}"' EXIT

pids=()
endlesscommand "param 1" &  # async
pids+=("$!")
endlesscommand "param 2" &  # async
pids+=("$!")
endlesscommand "param 3" &  # syncing this is not a good idea since if the main process would end along with it if it ends earlier.
pids+=("$!")

while pgrep -P "$BASHPID" >/dev/null; do
    wait
done

Original function reference: http://www.linuxquestions.org/questions/blog/konsolebox-210384/bash-functions-to-list-and-kill-or-send-signals-to-process-trees-34624/