How to write a shell script to ensure only one instance of the script runs for each user.

In a BASH Script:

if [ -e ${LOCKFILE} ] && kill -0 `cat ${LOCKFILE}`; then
    echo "Already running!"
    exit 1
trap "rm -f ${LOCKFILE}; exit" INT TERM EXIT
echo $$ > ${LOCKFILE}

Start by determining a name for the lock file. In this case, the lock file is generated by suffixing a common name with the username of the current user. Then, check if the lock file exists and if the PID contained within the lock file is running. If it is, exit with a message.

Create a trap to remove the lock file on a clean exit, or unclean exits (any exit with the signal INT or TERM). Finally, if the script has not exited yet, create the lock file, and store the PID of the current process ($$) in it.