I'm looking to add a crontab entry to execute a script every 30 minutes, on the hour and 30 minutes past the hour or something close. I have the following, but it doesn't seem to run on 0.
*/30 * * * *
What string do I need to use?
The cron is running on OSX.
Try this:
0,30 * * * * your command goes here
According to the official Mac OS X crontab(5) manpage, the / syntax is supported. Thus, to figure out why it wasn't working for you, you'll need to look at the logs for cron. In those logs, you should find a clear failure message.
Note: Mac OS X appears to use Vixie Cron, the same as Linux and the BSDs.
If your cron job is running on Mac OS X only, you may want to use launchd instead.
From Scheduling Timed Jobs (official Apple docs):
Note: Although it is still supported, cron is not a recommended solution. It has been deprecated in favor of launchd.
You can find additional information (such as the launchd Wikipedia page) with a simple web search.
You mention you are using OS X- I have used cronnix in the past. It's not as geeky as editing it yourself, but it helped me learn what the columns are in a jiffy. Just a thought.