A couple of Bash tips

1. Single instance
   # make use of bash `set -C'
   (set -C; : > $lockfile) 2>/dev/null
   if [ $? -ne 0 ]; then
     log_msg "$0 already running, exit"
     exit 0
   fi
   
   
2. Daemonize script
   # `-d' option indicates to daemonize
   if [ x"$1" = "x-d" ]; then
     $0 2>&1 >/dev/null &
     exit 0
   fi