Simple bash lock file example using lockfile-progs.

locktest.sh

#!/bin/bash
# Requires: apt-get install lockfile-progs
# Test: /root/locktest.sh& sleep 2; /root/locktest.sh

lock_file='/var/run/locker'
# 0 for immidiate fail, see 'man lockfile-progs' for details
lock_retry=0

lock(){
        lockfile-create -r $lock_retry -p $lock_file && return 0
        echo "ERROR: Can't get lock"
        exit $?
}

unlock(){ lockfile-remove $lock_file; }

lock
sleep 10
unlock
echo done