执行：

  0,30 * * * * your_command 
 

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.

Do:

0,30 * * * * your_command