I find the easiest way to add in the command one want to run at boot up of machine into /etc/init.d/rc.local

For example, I want to have network start at boot up, i can just added 'start-network' command which is actually a script to bring up network interface/dhcp ,etc. All I have to do is to do `nano /etc/init.d/rc.local` and add the command at the bottom of the page. At boot up, the BT will automatically run start-network and allow you to have network even before you login .