Cron

For a general description of cron or crontab, we recommendCrontab on Wikipedia.

Reference

# (Use to post in the top of your crontab)
# ----------------- minute (0 - 59)
# |  -------------- hour (0 - 23)
# |  |  ----------- day of month (1 - 31)
# |  |  |  -------- month (1 - 12)
# |  |  |  |  ----- day of week (0 - 7) (Sunday=0 or 7)
# |  |  |  |  |
# *  *  *  *  *  command to be executed

Setting up cronjob

file, ' ezpublish.cron'

# This must be set to the directory where eZ publish is installed.
 EZPUBLISHROOT=/path/to/the/ez/publish/directory
 
 # Location of the PHP Command Line Interface binary.
 PHP=/usr/local/bin/php
 
 # Executes the runcronjobs.php script every 15th minute.
 0,15,30,45 * * * *      cd $EZPUBLISHROOT; $PHP runcronjobs.php -q 2>&1

External reference

• Crontab on Wikipedia
• Cronjob on eZpedia