`su` - run a command with substitute user and group ID

su [-] [user]

Options you will use

-, -l, --login: Start a login shell: clears the environment and cds to the target home.

Gotchas

On Ubuntu the root account has no password by default, so su - fails and sudo -i is what you want.

Taught in

4.3 Users, groups, and who you are right now

Try su in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.