Ok, this is pretty trivial but yet very important.
You're working under run-level 3 and you wish to execute a a super duper startup script - located under:/usr/local/bin/startup.sh, each time system enters run-level 3.
Let's see how it's done:
1.First of all chmod 700 /usr/local/bin/startup.sh (security reasons..).
2.Next, change directory to:
3.From here we will create a symbolic link to the real location of the script:
ln -s /usr/local/bin/startup.sh S99startup.sh
The S99 prefix, S=start ,99=startup sequence (last one).
From now every time user enters run-level 3 the startup.sh script will run.