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Instead of a range of values, you can also use the asterisk operator. To specify a job to be run every 20 minutes, you can use “*/20”.
Slash (/) – Can be used to assign step values (or incremental values). For example, you can use */10 in the minute's field to specify that you want the job to run every 10 minutes, or */5 in the hour field means that it will run every 5 hours.
For example, 0-23/2 can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours just use */2. In this example, */10 in the minutes field to specify command execution every 10 minute.
Schedule Crontab at 30 Seconds In the above configuration, we have scheduled the script twice. The first cron runs every 1 minute and the second cron also starts at the same time but waits for 30 seconds before execution. So the entire cycle runs every 30 seconds.
Do:
0,30 * * * * your_command
crontab does not understand "intervals", it only understands "schedule"
valid hours: 0-23 -- valid minutes: 0-59
example #1
30 * * * * your_command
this means "run when the minute of each hour is 30" (would run at: 1:30, 2:30, 3:30, etc)
example #2
*/30 * * * * your_command
this means "run when the minute of each hour is evenly divisible by 30" (would run at: 1:30, 2:00, 2:30, 3:00, etc)
example #3
0,30 * * * * your_command
this means "run when the minute of each hour is 0 or 30" (would run at: 1:30, 2:00, 2:30, 3:00, etc)
it's another way to accomplish the same results as example #2
example #4
19 * * * * your_command
this means "run when the minute of each hour is 19" (would run at: 1:19, 2:19, 3:19, etc)
example #5
*/19 * * * * your_command
this means "run when the minute of each hour is evenly divisible by 19" (would run at: 1:19, 1:38, 1:57, 2:19, 2:38, 2:57 etc)
note: several refinements have been made to this post by various users including the author
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.
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