Understanding Cron

Here, here and here.

Useful commands

$ sudo /sbin/service crond on
$ sudo /sbin/service crond start
$ ls /var/spool/cron/
$ sudo ls /var/spool/cron/crontab
$ crontab -e
$ crontab -l

Example of job definition

# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed