Cron Jobs on OS X

Since OS X Tiger, cron has been replaced by a utility called launchd, and three separate launch daemons. Rather than having to run crontab  to manipulate scripts, they are run by launchd according to three separate directories inside of /etc/periodic.
The scripts contained in these folders are run at specified intervals by three preference files, in XML format, found in the /System/Library/LaunchDaemons folder. You can manipulate the .plist files with a text editor or Apple’s Property List Editor (if you’ve installed Developer Tools.) They are named, simply enough, com.apple.periodic-daily.plist, com.apple.periodic-weekly.plist, andcom.apple.periodic-monthly.plist. They are, by default, set to run at the same time as the old cron jobs, in the middle of the night. If you happen to shut down your Mac, it might be a good idea to change these intervals to a time when you’re sure the computer will not be shut down, as these jobs are important.

You will find a script in each of these folders called 999.local.  This file is set to read-ony by default, and is for  ”backwards compatibility  with the old /etc/daily.local” according to the comments in the file.  (I’m running Snow Leopard.)  I’d recommend not modifying the scripts in the /etc/periodic folders, and creating a /etc/daily.local, /etc/weekly.local, and /etc/monthly.local file for your cron jobs, as you have the 999.local script in each of the daily, weekly, and monthly sub-directories of /etc/periodic to tell your scripts to run.  Any future system updates could change the default files in those three directories, so if you modify those you may end up losing your scripts. Avoid that by setting up your own.

If you are running aTiger, the 999.local file may not exist, and you will have 500.daily, 500.weekly, and 500.monthly files inside their appropriate /etc/periodic directories.  You should still create a daily.local, weekly.local, and monthly.local script file and place them in the /etc directory, and they will be called  from the respective 500.* file.