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ISP-In-A-Box: The $500 Mac mini (Chapter XIII, Crontab Scheduling)

Well, our plan for today was to get down in the weeds of BSD and actually show you how to wrestle with crontabs to schedule tasks on your Mac or Mac mini. But, as luck would have it, our friends at iMagicWeb had a better solution that they wrote us about. So let’s shift gears and wrestle with crontab in the least painless way possible … which is to use a terrific piece of free software called CronniX. It was written specifically to manage cron jobs using the traditional Mac OS X Aqua interface. Before we get to CronniX, let’s briefly address what crontab is and what it does for you. Think of crontab as the ultimate scheduler. It can run almost any BSD application, script, or command as well as any Mac OS X application or Applescript at any time or combination of times that you care to dream up. Stated another way, you use crontab to schedule background jobs, i.e. tasks that you want your computer to execute without bothering you or your computer’s display. We already are using cron jobs to handle our Email Reminders, and our Webalizer statistics, and our daily Web Calendars. But we’ve barely scratched the surface of the possibilities. Later this week we’ll add automatic backups of your main hard drive to any external USB or firewire hard disk. And there will be specific tasks that you come up with where a cron job is the obvious solution.

There are three main things you do with cron jobs: you create them, you change them, and you delete them. To change or delete an existing job, just highlight the job and click Edit or Delete. To create a new entry, you need to be aware that cron jobs can be run as any user including the root user, and they inherit the privileges of that user. The moral here is, for security’s sake and the overall health of your system, don’t run jobs as the root user unless you have to. In creating cron jobs, you can schedule them to run at a particular time or times or at timed intervals, e.g. every 15 minutes. And you can schedule cron jobs to only run on a certain day of the week or month or on an assortment of days chosen by you. You can also quickly bring your server to its knees by running disk-intensive cron jobs every minute of every hour of every day of the year. The choice is yours. Aside from not running cron jobs as root unless you have to, the only other bit of advice I would offer is this. Test whatever you plan to schedule as a background job in the foreground first. When run as a foreground task (i.e. from a Terminal window), you can watch it to make certain it does what you wanted it to do. If it doesn’t, you can kill the job by pressing Ctrl-C. It is extremely difficult to diagnose scripting bugs in applications you have running in the background particularly when all you hear is your hard disk beating its brains out. Before you can kill a background job, it usually has plenty of opportunity to wreak enormous havoc on your computer. Having said all that, what makes the Mac such an incredibly powerful server platform is due at least in part to the almost limitless capacity to schedule and execute background tasks of every variety imaginable.

Up to now we’ve been setting up cron jobs using Webmin which is every bit as efficient as swatting a fly with a sledge hammer. For one thing, you have to crank up the Webmin server just to be able to access Webmin using a web browser. For some tasks, this is a great approach. But for scheduling cron jobs, it’s a little silly. So, as I mentioned, our original plan for today was to cover the intricate details of using crontab and managing cron jobs from the Terminal command prompt. Lo and behold, it turns out there is a terrific little Mac OS X application called CronniX which makes the drudgery of crontabs all but disappear. As mentioned, the application is free although the author does accept donations. His web site is http://h5197.serverkompetenz.net/cronnix/. So go to the site, click on the download link and download the latest and greatest version (3.0b3.dmg). It will decompress into a folder on your desktop. Open the folder and drag the CronniX application to your Applications folder.

When you start up CronniX, it will default to the crontab for your current login ID. You can switch crontabs by pressing Command-O and entering the desired username including root and system. Existing crontab entries will be displayed for the username selected. An asterisk (*) in crontab lingo means all while an asterisk followed by a slash and a number means every so many minutes or hours or days. For example, */15 in the minutes column would mean every 15 minutes while */4 in the hours column would mean every 4 hours. To create a new crontab entry, you simply click the New button and fill in the blanks. Checking the checkbox beside a field sets that field to * or all. The syntax for commands is exactly what you would type in a Terminal window to execute the command. And, for Mac OS X applications, you can execute them by checking "Prepend /usr/bin/open" and then clicking the Browse button and choosing the desired Application. Once you’ve made your entry, click on the Save button to actually post your new entries to the selected crontab. Then press Command-Q to close CronniX. It’s as simple as that.