Restrictions:

if /etc/cron.allow exists only users listed within can access crontab

if it does not exists everybody can use crontab, as far as its name is not listed in /etc/cron.deny (1 user per line)

if this does not exist neither everybody can use crontab

Commands:

to specify the editor to be used
export EDITOR=pico

to edit the crontab list for the current user
crontab -e

to display the crontab list for the current user
crontab -l

to remove the crontab list for the current user
crontab -r

Syntax:

consists in 5 fields for specifying the period and the command to be run

* * * * * command args

[minutes 0-59] [hours 0-23] [day of month 1-31] [month 1-12] [day of week 0-6 sunday=0] [command] [args for the command]

each field is represented by an * and can be a:

- number
- number, number
- number-number (which means an interval)

*/n means “every n times” according to the position of the * (mins, hours, days…)

eg.

*  * */1 * * updatedb

means “run updatedb every day”

email:

by default cron jobs send an email to the user account executing the cronjob. If not needed add at the end of the crontab command
> /dev/null 2>&1

logging:

if you need to log the program output simply add at the end of the command line
> /home/user/program.log
which is the path to the file log.