Atomic locking for bash scripts

gistfile1.txt

#!/bin/bash
lockfile=/var/lock/$(basename $0)

#Redirects output of file descriptor 5 to lockfile while this script runs
exec 5>"${lockfile}"

# flock file descriptor 5, otherwise fail
if ! flock -n 5  ; then
   echo "Exiting: Another instance of $0 is already running";
   exit 1
fi
# We now have the lockfile until this script exits