Fixing Mac OS X’s “periodic” command

Here’s the command:

These entries are read, left-to-right, as minute, hour, day, month, day-of-month, so you can see that my daily cron job is run at 3:15am every night, my weekly job at 4:30am on the 6th day of each week (Sunday, because Unix starts counting at day = 0), and the monthly job is run at 5:30am on the first day of each month…

If you don’t have your computer running at 3:15am every night – and you probably don’t because either you shut it off or you have it “sleeping” – then an easy way to run the daily job is to duplicate the command:

But that’s not a good solution! Why? Because the periodic script needs to run as the root user, and it’s not smart enough to check and ensure that’s the case. Instead, you get lots of weird errors about permissions being incorrect. To fix it, you need to actually run the command:

But why not instead fix periodic to ensure that you’re running as root? Well, one reason is that periodic can ostensibly be run as other users, but if you check out the man page, you’ll see it’s described: “The periodic program is intended to be called by cron(8) to execute shell
scripts located in the specified directory.”

How do you fix a system script to run the way you want? There are two approaches. One is that you can actually edit the periodic script itself, which involves you finding out where it lives:

You can see it’s a shell script (as I suspected) and that it lives in the system directory /usr/sbin (I’m on Panther, btw, for all of this). The first few lines of the script is where we could add a simple test to ensure that it’s being run as root, as the highlighted lines below show:

If you’re not comfortable hacking into system shell scripts – which I can totally understand – then an alternative is to create a short wrapper script that you ensure is earlier in your path. It might look like this:

Short and sweet. One more step needed: check your path to ensure that it’s in a directory that shows up prior to /usr/sbin or, even better, create an alias that ensures it’s the one executed when you type in periodic at the command line:

Drop this into your .bashrc or .cshrc (depending on your login shell), log out, log in again, and next time you’re on the ball and remember to run your periodic commands, but forget to use sudo, you’ll get the friendly error message:

There’s a lot more we can do with this idea, too, some of which is discussed – at length, with plenty of fun examples – in my new book Wicked Cool Shell Scripts.

I hope this article was helpful reading!