su (substitute user) is a command to change a login session’s owner without the owner having to first log out of the session. (as specified in the su Command). In other words, it is called the switch user command

Note: It is most commonly employed to change the ownership from an ordinary user to the administrative user. Therefore it is often refered as the superuser command.

$sudo su root


Let’s suppose there is a user named alex. By default, the root account is disabled and doesn’t have any password. To access root, we can run:

$ sudo su root
[sudo] password for alex: 

Another way to login as root user:

$ sudo -s
[sudo] password for alex:

Here it asks for alex’s password not root password.

Useful Options / Examples

There’s some good commands you can run with su!

su -c will run the command that directly follows it on the same line

$ su -c 'ls /home/alex'

su - will change the current directory and environment to what would be expected if the new user had logged on to a new session.

If you are a superuser on the box and would like to masquerade as user alex, you can run:

$ sudo su - alex