Keep ubuntu server running when computer shut down
Now I've installed ubuntu server on my raspberry pi, so it's still running as I want. thanks
Ubuntu server in VirtualBox runs on your computer. When you switch off your computer, VirtualBox and anything inside go down as well, together with everything else on your computer. Also if you stop VirtualBox, everything in it (in this case Ubuntu Server) go down. What you are trying to do is not possible.