Cron is a daemon that runs periodic tasks. crontab is name of textfile that is used to control cron. And crontab is also the name of program used to modify the file called crontab. Every user could conceivably have a crontab file. These are often stored in /var/spool/cron/crontabs/. If my user name is perderabo, then my real crontab is /var/spool/cron/crontabs/perderabo. When cron was first written there was a single crontab called /etc/crontab and only root could modify it. Now that everyone can use cron, each crontab gets the name of the user who owns it. Because there are so many possible crontabs, cron can't monitor them all for changes. That's why you must use the crontab program. In addition to modifying the crontab file, it also lets cron know about the change. If you simply edit the file in /var/spool/cron/crontabs/, cron will not notice the change.

The crontab command:-root can always use the crontab command. Other users may be locked out. A file, usually at /usr/lib/cron/cron.deny contains a list of users who are prohibited from using cron. If that file doesn't exist, /usr/lib/cron/cron.allow may list users who can use cron. If neither file exists, only root can use cron. To let everyone use cron, create an empty cron.deny file.

But be careful with that crontab command! We must have a dozen threads from folks who accidently did a "crontab -r" which removes your crontab completely. There is no easy way to recover from that. Rather than simply using "crontab -e" to edit your file, some of our members suggest
crontab -l > mycrontab
vi mycrontab
crontab < mycrontab

The Format of a crontab entry

A typically crontab entry might be:
15 18 * * 1-5 /some/script
This says to run /some/script at 18:15 on Monday through Friday.

The first five fields are:
minute (0-59)
hour (0-23)
day of the month(1-31)
month of the year (1-12)
day of the week (0-6 with 0 = Sunday)

Each field can be an asterisk meaning all values, or a single integer, several integers separated by commas, or two integers separated by a hypen to indicate a range.

With some versions of cron, day of the week is 1-7 with 7 = Sunday. Many versions of cron accept either 0 or 7 as Sunday.