cron is a Linux system daemon that will execute a program at a preset time. To use cron, log in as the user that cron needs to run as, and execute the following command in the terminal:
Here is the format of a cron job file:
where each field is defined as
Minutes that program should be executed on. 0-59. Do not set as * or the program will be run once a minute.
Hour that program should be executed on. 0-23. * for every hour.
Day of the month that process should be executed on. 1-31. * for every day.
Month that program whould be executed on. 1-12 * for every month.
Day of the week. 0-6 where Sunday = 0, Monday = 1, …., Saturday = 6. * for every day of the week.
Program to be executed. Include full path information.
Here are some examples:
Will run /usr/bin/foo every 15 minutes on every hour, day-of-month, month, and day-of-week. In other words, it will run every 15 minutes for as long as the machine it running.
Will run /usr/bin/foo at 3:10am on every day.
Will run /usr/bin/foo at 12:10am on the first day of the month.
Will run /usr/bin/foo at 12:10am on the first month of the year.
Will run /usr/bin/foo at 2:10pm on every Monday.
There are more options for these. See man man crontab -S 5.
You must use crontab to load cron jobs into cron. First create a text file that uses the above rule to describe the cron job that you want to load into cron. But before you load it, type crontab -l to list any jobs that are currently loaded in crontab.
If none are listed, then it is safe to load your job. Example. If you wanted to run /usr/local/bin/foo once a day at 3:10am, then create a text file