CRON
Cron is a scheduling daemon process on Linux and other Unix variants, it enables unix users to execute commands or scripts (groups of commands) automatically at a specified time/date. Cron is very useful to run backup scripts.

Cron Files/Directory
/etc/crontab
/etc/cron.d
/var/spool/cron/(username)

/etc/cron.hourly
/etc/cron.daily
/etc/cron.weekly
/etc/cron.monthly

Crontab Restrictions
Cron has a built in feature of allowing you to specify who may and who may not use it. It can done by using “/etc/cron.allow” and “/etc/cron.deny” files.
If the username exist in the file “/etc/cron.allow“, that user is authorize to execute cron. If “/etc/cron.allow” file does not exist, you can use crontab if username does not appear in the file “/etc/cron.deny“. If only “cron.deny” exists and its empty, than all users can use crontab. If neither file exists, than only the root user can use crontab. The “allow/deny” files consist of one user name per line.

Crontab Options and Explanation
A crontab file has five fields for specifying time, day and date followed by the command to be run at that interval. Below is Crontab syntax,

*  *  *  *  * command to be executed
-  -  -  -  -
| | | | |
| | | | +—– day of week
| | | +——– month
| | +———– day of month
| +————— hour
+—————— minute
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).

* Is treated as a wild card. Meaning all possible value.
*/5 is treated as ever 5 minutes, hours, days, or months. Replacing the 5 with another numerical value will change this option.
2,8,12 “,” Treated as an “OR“, so if placed in the hours, this could mean at 2, 8 or 12 o-clock. Or if put in days it would be on 2,8 and12. “,” is used to define more options [date, time, day]
6-10 Treats for any value between 6 and 10. So, if placed in day of month this would be days 6 through 16. Or if put in hours it would be between 6 and 10. “-” is used to define the range [date, time]

Crontab Commands
crontab -e Edit your crontab file, or create one if it doesn’t already exist.
crontab -l Display your crontab entries.
crontab -r Remove your current crontab entry..
crontab -u username -e This option allows you to modify or view the crontab file of a user, only super user can use this option.

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