How would I get a cron job to run every 30 minutes?

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.

6 Answers

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, 2:00, 2:19, 2:38, 2:57, 3:00 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 can use both of ',' OR divide '/' symbols.
But, '/' is better.
Suppose the case of 'every 5 minutes'. If you use ',', you have to write the cron job as following:

0,5,10,15,20,25,30,35,.... * * * * your_command 

It means run your_command in every hour in all of defined minutes: 0,5,10,...

However, if you use '/', you can write the following simple and short job:

*/5 * * * * your_command 

It means run your_command in the minutes that are dividable by 5 or in the simpler words, '0,5,10,...'

So, dividable symbol '/' is the best choice always;

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.