Su -l

From OLPC

su - is a Linux command for substitute user and group rights while running a command within the Terminal Activity or a command console. In its default form, su -, it enables the root or admin or super user's permissions to modify the system (see root user).

[edit] Overview

It gives you a login shell session that affects the commands that are available.

Various instructions will direct you to type su -l (l being a lower-case letter L, for login) or just su. On some laptops, the next prompt will be "Password:". If this happens, and you do not have a root password, type sudo su instead.

Enter the Ctrl D key combination (or enter the logout command) to exit the root user session.

See reference wiki, http://man-wiki.net/index.php/1:su