Run a shell script at specific times

• Use launchd instead of cron to schedule tasks. With launchd, if a task is skipped because the computer is shut off or in sleep, the task is added to the queue when the computer comes back online.

  To use launchd to schedule timer-based jobs, use the StartCalendarInterval or StartInterval key.

  For more information about launchd, see its man page.

The cron daemon executes scheduled commands defined in crontab files. The cron daemon searches the /usr/lib/cron/tabs/ folder for crontab files named after accounts in /etc/passwd, and loads the files into memory. The cron daemon also searches the /etc/crontab/ folder for crontab files, which are in a different format. cron then cycles every minute, examining stored crontab files and checking each command to see if it should be run in the current minute.

When commands execute, output is mailed to the owner of the crontab file or to the user named in the optional MAILTO environment variable in the crontab file.

You use the crontab tool to install, uninstall, or list the tables used to drive the cron daemon. Users can have their own crontab file.

• To configure your crontab file, use the crontab -e command. This opens an empty crontab file in edit mode.

  Here’s an example of a crontab file:

  SHELL=/bin/shPATH=/bin:/sbin:/usr/bin:/usr/sbinHOME=/var/log#min hour mday month wday command30 18 * * 1-5 diskutil repairPermissions /Volumes/MacHD50 23 * * 0 diskutil repairVolume /Volumes/MacHD

  In this example, the first entry repairs disk permissions for the MacHD volume at 18:30 every day, Monday through Friday:

  30 18 * * 1-5 diskutil repairPermissions /Volumes/MacHD

  The second entry schedules a repair volume operation to run at 23:50 every Sunday:

  50 23 * * 0 diskutil repairVolume /Volumes/MacHD