1. Go to /etc/init.d and run sudo vim nginx-passenger.sh, then paste in this code:

#!/bin/bash
# this script starts the nginx process attached to passenger
sudo /opt/nginx/sbin/nginx

save and exit.

2. Make the file executable by typing sudo chmod +x /etc/init.d/nginx-passenger.sh

You can test to see if the script works by typing sudo /etc/init.d/nginx-passenger.sh this will run all the code in the script.

3. Go to /etc/init.d directory. Then run sudo update-rc.d nginx-passenger.sh defaults

Once all of this is in place, reboot your server and ngnix should now be automatically spawned on startup