Terminating a shell script bar from a script foo

Question

I have a script foo which, if provided an argument start, starts, among other things a script bar in the background and exits - bar contains an infinite loop.

At a later stage, I want to call foo with argument stop and I would like that script bar, which still runs in the background stops running.

What is the text book way of achieving this?

Answer 1

In case multiple bar instances can run simultaneously, and foo stop should stop/kill them all, use pkill:

$ pkill bar

to kill all processes named bar.

In case only one bar instance is allowed to run, a solution with a "pidfile" would be viable.

In foo:

pidfile=/var/run/bar.pid

if ((start)); then
    if [ -e "$pidfile" ]; then
        echo "$pidfile exists."
        # clean-up, or simply abort...
        exit 1
    fi
    bar &
    echo $! >"$pidfile"
fi

if ((stop)); then
    if [ ! -e "$pidfile" ]; then
        echo "$pidfile not found."
        exit 1
    fi
    kill "$(<"$pidfile")"
    rm -f "$pidfile"
fi