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

# one instance only (works unless your cmd has 'grep' in it)
ALREADY_RUNNING_EXIT_STATUS=0
bn=`basename $0`
proc=`ps -ef | grep -v grep | grep "$bn" | grep -v " $$ "`
[ $? -eq 0 ] && {
    pid=`echo $proc | awk '{print $2}'`
    echo "$bn already running with pid $pid"
    exit $ALREADY_RUNNING_EXIT_STATUS
}

UPDATE .. better way using flock:

/usr/bin/flock -n /tmp/your-app.lock /path/your-app args