How to see which process is running on a specific port

netstat -lnp | grep :80
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        0      0 ::ffff:127.0.0.1:8005       :::*                        LISTEN      29693/java         
tcp        0      0 :::8009                     :::*                        LISTEN      29693/java         
tcp        0      0 :::8080                     :::*                        LISTEN      29693/java 

df - report file system disk space usage

df displays the amount of disk space available on the file system containing each file name argument. 
If no file name is given, the space available on all currently mounted file systems is shown.

Implement and extend relationship in UML

This is the meaning of the following picture:

Pan, Spoon, Pot extend AbstractUtensil
AbstractUtensil implements FactoryElement

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

Let's deal with the following error:

$ ssh username@servername  

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@   @ 

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! 

@   @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 

IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! 

Someone could be eavesdropping on you right now (man-in-the-middle attack)! 

It is also possible that the RSA host key has just been changed. 

The fingerprint for the RSA key sent by the remote host is 

23:00:20:83:de:02:95:f1:e3:34:be:57:3f:cf:2c:e7. 

Please contact your system administrator. 

Add correct host key in /home/xahria/.ssh/known_hosts to get rid of this message. 

Offending key in /home/xahria/.ssh/known_hosts:8 

RSA host key for localhost has changed and you have requested strict checking. 

Host key verification failed. 

to solve this problem go to /.ssh

$ cd /.ssh

$ nedit known_host

Delete all occurencies of servername and save the file.

That's it. Try again 
$ ssh username@servername