cron
Idiots guide to Cron Jobs
First create a script that one wants to run and note it absolute file location
Decide who you want to run as and run of the following commands
#Run as root
sudo crontab -e
#Run as logged in user
crontab -e
Use this to figure out how to get the cron job to run at the correct times
Just have a couple spaces and the path to the absolute file location of the script like here
0 0,6,12,18 * * * /path/to/mycommand