Best way to make a shell script daemon?

Question

I'm wondering if there is a better way to make a daemon that waits for something using only sh than:

#! /bin/sh
trap processUserSig SIGUSR1
processUserSig() {
  echo "doing stuff"
}

while true; do
  sleep 1000
done

In particular, I'm wondering if there's any way to get rid of the loop and still have the thing listen for the signals.

user_926057 · Answer

Like many answers this one is not a "real" daemonization but rather an alternative to nohup approach.

echo "script.sh" | at now

There are obviously differences from using nohup. For one there is no detaching from the parent in the first place. Also "script.sh" doesn't inherit parent's environment.

By no means this is a better alternative. It is simply a different (and somewhat lazy) way of launching processes in background.

P.S. I personally upvoted carlo's answer as it seems to be the most elegant and works both from terminal and inside scripts

user_6634796 · Answer

Here is the minimal change to the original proposal to create a valid daemon in Bourne shell (or Bash):

#!/bin/sh
if [ "$1" != "__forked__" ]; then
    setsid "$0" __forked__ "$@" &
    exit
else
    shift
fi

trap 'siguser1=true' SIGUSR1
trap 'echo "Clean up and exit"; kill $sleep_pid; exit' SIGTERM
exec > outfile
exec 2> errfile
exec 0< /dev/null

while true; do
    (sleep 30000000 &>/dev/null) &
    sleep_pid=$!
    wait
    kill $sleep_pid &>/dev/null
    if [ -n "$siguser1" ]; then
        siguser1=''
        echo "Wait was interrupted by SIGUSR1, do things here."
    fi
done

Explanation:

Line 2-7: A daemon must be forked so it doesn't have a parent. Using an artificial argument to prevent endless forking. "setsid" detaches from starting process and terminal.
Line 9: Our desired signal needs to be differentiated from other signals.
Line 10: Cleanup is required to get rid of dangling "sleep" processes.
Line 11-13: Redirect stdout, stderr and stdin of the script.
Line 16: sleep in the background
Line 18: wait waits for end of sleep, but gets interrupted by (some) signals.
Line 19: Kill sleep process, because that is still running when signal is caught.
Line 22: Do the work if SIGUSR1 has been caught.

Guess it does not get any simpler than that.

Best way to make a shell script daemon?

12 Answers