Merke mir hier nur ein paar Links zu cron:
Command scheduling with cron
cron auf Debian Administration.
cron-apt
Debian Cron
Cron and Crontab usage and examples
NTP Server and Client Configuration in debian
CRON(8)
CRON(5)

A crontab file contains instructions to the cron(8) daemon of the general form: „run this command at this time on this date“. Each user has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab. Uucp and News will usually have their own crontabs, eliminating the need for explicitly running su(1) as part of a cron command. ….