cron - daemon to execute scheduled commands

Cron reads the file /etc/crontab and schedules the listed jobs.

/etc/crontab: List of periodic jobs read by crond.

/etc/crontab consists of single line definitions:

time user executable

time pattern: minute, hour, day of the month, month, weekday 

Example:
 
50 13,14 * * 5,6,7 root /usr/bin/play /etc/some_music.wav

will play a WAV file (as user root) at 
13:50:00 and at 14:50:00 
on Friday, Saturday, and Sunday

The cron-installation itself uses a cron-job that executes scripts the following directories:

cron.hourly
cron.daily
cron.weekly
cron.monthly

You may (as root) put your own scripts these directories.

Cron for Regular Users

Commonly only the root user has the right to change the /etc/crontab file.
Crontab files for regular users are kept separately under /var: Cron searches /var/spool/cron/tabs for crontab files which are named after user accounts and loads them into memory.
These file are not intended to be edited directly. The crontab command should be used to maintain crontab files for individual users:

 crontab [ -u user ] { -l | -r | -e }

Options:
         -l  display current crontab. 
         -r  remove current crontab.
	 -e  edit current  crontab  using the  
	     editor specified by the VISUAL or 
	     EDITOR environment variables (Vi).

The user crontab table has six columns (not seven as the global one):
minute, hour, day-of-month, month, day-of-week and the command to be executed.