There are times when I need to create a clock that will run a script at a certain time or interval. Here's how to do it using crontab.

This diagram explains how to set a timer.

*    *     *    *     *  command to be executed
-    -    -    -     -
|     |     |     |     |
|     |     |     |     +----- day of week (0 - 6) (Sunday=0)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of month      (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

When you're ready to start your timer, run crontab.

crontab -e

This will trigger every 6 hours.

0    6    *    *    *    /path/to/shell/script