From Tech Solutions Documentation

So you have a script of your own that you want to run at bootup, each time you boot up your Linux OS. This will tell you how to do that.

Put your script in the /etc/init.d/ directory. Lets say you called it FOO. You then run

    update-rc.d FOO defaults

You also have to make the file you created, FOO, executable, using

    $chmod +x FOO

You can check out % man update-rc.d for more information. It is a Debian utility to install scripts. The option “defaults” puts a link to start FOO in run levels 2, 3, 4 and 5. (and puts a link to stop FOO into 0, 1 and 6.)

Also, to know which runlevel you are in, use the runlevel command.