Root is the user name or account that by default has access to all commands and files on a Linux or other Unix-like operating system. It is also referred to as the root account, root user and the superuser.
What If I Forget Root Password? Check The below
Restart The Server and you must see this screen Pause it click arrow :
Second Press “a” then add “1” like the below screen after finish Press enter :
Now Black Screen :
and Reset Root Password like the below by type “passwd” command :
Note : This Way works with Redhat , Fedora and CentOs i didn’t try it on Ubuntu