If your host is running under the Linux (Unix) operating system, you can perform tasks in the server on a regular basis. For example, sending mails at a certain time of the day, backing up database at 6 in the morning and so on.

"Cron" is a program that runs in the background of the Linux server. The program runs certain commands defined by you at fixed time intervals. A tyical cron file will look something like this:

# My Sample Cron File
12 01 02 * * /usr/bin/du /var > /home/test/diskSpace.txt

Cron command lines consist of 6 whitespace separated fields. The first 5 fields are used to specify when to run the command, and the last field determines what command to run. The first field(12 in this case) represents minute, the second field represents hour, third field represents day of month, fourth field represents month(1=Jan, 2=Feb and so on), fifth field represents day of week(0=sunday, 1=monday and so on).

Note: * means every time and # means comments.

To edit the crontab file, we use the crontab command like so:

"crontab [ -e | -l -r ]"

The switch -e means to edit the current cron file. -l means to list the current file, and -r means to remove the current cron file. Try typing "crontab -l" and you will see the cron file contents. To access the crontab command, you need to have SSH(Secure Shell) access to the server. Basically, SSH allows you to type commands in the command prompt. If you have limited access to SSH, your host might provide some other ways to access cron via the control panel.

For example, if I want to run a php script at 12 midnight everyday, I will type "crontab -e", then write my script as follows:

0 0 * * * php /home/username/public_html/www.sitecritic.net.php

The command that I run is "php". It is also recommended that you use full path reference to your script. In this case, it is /home/username/public_html/www.sitecritic.net.php.

In Conclusion, cron can save you alot of time and effort in doing daily repetitive tasks. By running a customised php script, you can automate the task of sending mass mails, backing up database, creating RSS feeds...etc.