Cron

man 5 crontab

This will show you the layout of the crontab file for your system. Modern cron for example take schedules like @daily and @monthly now as well as the traditional 5 space delimited digit fields for date and time. Typically:

field         allowed values -         --              minute         0-59 hour          0-23 day of month  1-31 month         1-12 (or names, see below) day of week   0-7 (0 or 7 is Sun, or use names)

and you follow that with the command you want to run. Remember, the output will be mailed to the user@ so redirect the output to /dev/null if you don't want this, however this is a risk if something goes awry. Let's see if we have anything in our cron yet:

$ crontab -l $

It is still empty, so we'll create one. Do note that it will open cron in your EDITOR field.

$ env | grep EDITOR

Let's say we want to download a file every 6 hours, and have the stdout go to /dev/null

$ crontab -e

and add this line: * 0,6,12,18 * * * /usr/bin/wget -O ~/slashdot.xml http://slashdot.org/slashdot.xml 1>/dev/null

This requires /usr/bin/wget of course.