How to find out unused port on Linux server
netstat -an | grep LISTEN | grep 5444 will show if there's a process listening on that port. 

netstat -an | grep LISTEN will show you all ports with a process listening on them. You can then use lsof to see which processes are actually using a given port. 

There's no such concept of "find out the application port even the application was shutdown", as anything can listen on any port (with certain restrictions if not running as root). The closest thing to this is the /etc/services file, which documents ports that are "well known"/assigned by IANA. Also see http://www.iana.org/assignments/port-numbers. But, as I said, there's nothing to stop anything from running on one of these ports. 

To be honest, you should know what's running on a production box that you have root access to?