Run cron job only if it isn't already running

Use flock. It's new. It's better.

Now you don't have to write the code yourself. Check out more reasons here: https://serverfault.com/a/82863

/usr/bin/flock -n /tmp/my.lockfile /usr/local/bin/my_script

I do this for a print spooler program that I wrote, it's just a shell script:

#!/bin/shif ps -ef | grep -v grep | grep doctype.php ; then        exit 0else        /home/user/bin/doctype.php >> /home/user/bin/spooler.log &        #mailing program        /home/user/bin/simplemail.php "Print spooler was not running...  Restarted."         exit 0fi

It runs every two minutes and is quite effective. I have it email me with special information if for some reason the process is not running.

As others have stated, writing and checking a PID file is a good solution. Here's my bash implementation:

#!/bin/bashmkdir -p "$HOME/tmp"PIDFILE="$HOME/tmp/myprogram.pid"if [ -e "${PIDFILE}" ] && (ps -u $(whoami) -opid= |                           grep -P "^\s*$(cat ${PIDFILE})$" &> /dev/null); then  echo "Already running."  exit 99fi/path/to/myprogram > $HOME/tmp/myprogram.log &echo $! > "${PIDFILE}"chmod 644 "${PIDFILE}"