Question

Getting a background process ID is easy to do from the prompt by going:

$ my_daemon &
$ echo $!

But what if I want to run it as a different user like:

su - joe -c "/path/to/my_daemon &;"

Now how can I capture the PID of my_daemon?

Answer 1

Succinctly – with a good deal of difficulty.

You have to arrange for the su’d shell to write the child PID to a file and then pick the output. Given that it will be ‘joe’ creating the file and not ‘dex’, that adds another layer of complexity.

The simplest solution is probably:

su - joe -c "/path/to/my_daemon & echo \$! > /tmp/su.joe.$$"
bg=$(</tmp/su.joe.$$)
rm -f /tmp/su.joe.$$   # Probably fails - joe owns it, dex does not

The next solution involves using a spare file descriptor – number 3.

su - joe -c "/path/to/my_daemon 3>&- & echo \$! 1>&3" 3>/tmp/su.joe.$$
bg=$(</tmp/su.joe.$$)
rm -f /tmp/su.joe.$$

If you’re worried about interrupts etc (and you probably should be), then you trap things too:

tmp=/tmp/su.joe.$$
trap "rm -f $tmp; exit 1" 0 1 2 3 13 15
su - joe -c "/path/to/my_daemon 3>&- & echo \$! 1>&3" 3>$tmp
bg=$(<$tmp)
rm -f $tmp
trap 0 1 2 3 13 15

(The caught signals are HUP, INT, QUIT, PIPE and TERM – plus 0 for shell exit.)

Warning: nice theory – untested code…