UNIX for Beginners Questions & Answers

#!/bin/bash

# Use a lock file.  Lock released on exit unless earlier.
mylockfile="/var/lock/whatever"

[ ! -f $mylockfile ] && touch $mylockfile

exec 77>> $mylockfile       # Direct file descriptor 77 (or whatever) to the lock file

flock -w 30 -x 77           # Wait for up to 30 seconds to acquire the lock on fd 77

if [ $? -ne 0 ] 
then
   logger "$0 bypass due to lock"
   exit 99
else
   logger "$0 has the lock"
fi

# Carry on with the rest of the script.....