su - temporarily log in as superuser or another user
su # Become superuser
su ast # Become ast
Su can be used to temporarily login as another user. It prompts for the
superuser password. If the correct password is entered, su creates a
shell with the desired uid. If no name is specified, root is assumed.
To exit the temporary shell, type CTRL-D. When memory is tight, it is
better to become superuser by logging out and then logging in again as
root, rather than using su since the latter creates an extra shell in
memory. Any user with a zero group id need not type a password. This
makes life easier for the system administrator.