Cron is a unix utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon.

Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

How to use Cron (Dreamhost.com ) 

• You'll need to enable shell access (SSH) before you can modify your crontab
• Crontab file: Create and Edit
• Generate log file
  
• Example
• Run php from cron

Enable shell access (Dreamhost.com )

1. Visit the Manage Users (https://panel.dreamhost.com/index.cgi?tree=users.users& ) page of the control panel.
2. Click the [ edit ] link located next to the user you'd like to enable shell access for.
3. Check the 'Enable ssh/telnet?' box for that user.
4. Click 'Save Changes' to save the change.
5. Wait about 20 minutes for this change to take effect.

Crontab file: Create and Edit

crontab -e     Edit your crontab file, or create one if it doesn't already exist.
crontab -l      Display your crontab file.
crontab -r      Remove your crontab file.

MAILTO=
 This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
 
# 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).
# |      |       |       |       |       commands
3       2       *       *       0,6     /some/command/to/run
3       2       *       *       1-5     /another/command/to/run

 DreamHost cron does NOT have an environment. The following will not work!

* * * * * perl myscript.pl

* * * * * perl /home/myuser/mydomain.com/myscript.pl

Generate log file

 The output from commands is determined by the last entries on the line. If you don't specify anything at all, regular and error output will be e-mailed to you.

• MAILTO environment variable in the crontab If MAILTO is defined (and non-empty), mail is sent to the user so named. If MAILTO is defined but empty (MAILTO=""), no mail will be sent:
  

  MAILTO=
   This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
     

• Email you the regular and error output:
  

  8 2 * * * /home/you/bin/coolscript.pl

• Only email you errors: 

  8 2 * * * /home/you/bin/coolscript.pl >/dev/null

• Not tell you anything (+ disable E-mail):
  

  8 2 * * * /home/you/bin/coolscript.pl >/dev/null 2>&1

• log in file:
  

  8 2 * * * /home/you/bin/coolscript.pl > /home/you/cronlogs/you_log_cron.log

Example

•  Run a script every 20 minutes:

*/20 * * * * /usr/local/bin/perl /home/you/mydomain.com/myscript.pl

• Run a php page called cron.php every hour (use wget):

0 * * * * wget http://www.somedomain.com/cron.php

• Run a command at 2:08am 
  

8 2 * * * /home/someone/bin/coolscript.pl

• Run a command at 2:00am on saturday

0 2 * * 6 /home/you/weekly/weekly.pl

Execute php using cron

• To execute a PHP script using crontabs, you must add this line to the top of the PHP script you want to execute for it to work:

#!/usr/local/bin/php -q

• Alternately, you can set up your cron entry like this:

* * * * * /dh/cgi-system/php.cgi /home/youruser/yourdomain.com/path-to-php4-script.php
* * * * * /dh/cgi-system/php5.cgi /home/youruser/yourdomain.com/path-to-php5-script.php