The majority of Mac OS X users are never going to access the built-in Unix Terminal and never directly access the Darwin core of the operating system. Instead, they'll work within the GUI. However, Mac OS X developers and superusers will operate directly with Darwin quite extensively, and at times, they'll need to have root access.
By default, root access in Darwin is disabled. Trying to use su to change to root within the Terminal will fail. You have to enable root first using NetInfo.
To enable root within Mac OS X, access the Go menu option of Finder, and double-click on Applications. When the Applications window opens, double-click on the Utilities folder. In this folder, select and open NetInfo.
When NetInfo opens, select the Domain menu item and then Security. You'll need to authenticate yourself to the system first by selecting the Authenticate submenu option. Once you provide a password (and the system determines you have the authority to enable or disable root), accessing the Security menu again will show a newly enabled option: Enable Root User. Clicking on this enables root. However, you'll need to reauthenticate one more time to ensure the change goes through.
Once root is enabled for the system, it stays enabled until you disable it again. With root enabled, you'll be able to use su to login as root.
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