Cron

Crontab
Before we can use, you first need to understand the crontab itself. Each line in a crontab needs to specify five time fields in the following order: the minutes (0-59), hours (0-23), days of the month (1-31), months (1-12), and days of the week (0-7, Monday is 1, Sunday is 0 and 7). The days of the weeks and months can be specified by three-letter abbreviations like mon, tue, jan, feb, etc. Each field can also specify a range of values (e.g. 1-5 or mon-fri), a comma separated list of values (e.g. 1,2,3 or mon,tue,wed) or a range of values with a step (e.g. 1-6/2 as 1,3,5).

Use
For system cron, edit /etc/crontab file and run: # crontab /etc/crontab. To see the actual crontab of the current user (or root) run: crontab -l. For a user create a file with the cron rules and run: $ crontab file. The crontab can be edited directly trough crontab -e.