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.
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