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

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/bash

mkdir -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 99
fi

/path/to/myprogram > $HOME/tmp/myprogram.log &

echo $! > "${PIDFILE}"
chmod 644 "${PIDFILE}"