Run cron job only if it isn't already running

Question

I\'m trying to set up a cron job as a sort of watchdog for a daemon that I\'ve created. If the daemon errors out and fails, I want the cron job to periodically restart it...

Answer 1

As others have stated, writing and checking a PID file is a good solution. Here's my bash implementation:

#!/bin/bash

mkdir -p "$HOME/tmp"
PIDFILE="$HOME/tmp/myprogram.pid"

if [ -e "${PIDFILE}" ] && (ps -u $(whoami) -opid= |
                           grep -P "^\s*$(cat ${PIDFILE})$" &> /dev/null); then
  echo "Already running."
  exit 99
fi

/path/to/myprogram > $HOME/tmp/myprogram.log &

echo $! > "${PIDFILE}"
chmod 644 "${PIDFILE}"