Running Mac OS X Tiger

Chapter 6. Users and Groups

Thanks to its Unix heritage, Mac OS X is a multiuser operating system through and through. This simple fact means there can be more than one user of your system. You can have accounts for every member of a household on one machine, and everyone’s stuff will remain independent and safe. Even better, with Fast User Switching , multiple users can be logged into the same machine at the same time. While only one user at a time can use the screen, with a quick click of a menu, you can switch effortlessly between user sessions.

What isn’t obvious at first glance is that the concept of users runs quite deep in Mac OS X. Not only are human users treated as separate entities by the system, but many nonhuman users exist on the system as well. This means different tasks can be performed safely and in isolation from other tasks. Users can also be associated with groups , allowing the system to treat many users the same way.

What Is a User Anyway?

From the operating system’s point of view, a user isn’t necessarily a real person who taps away at the keyboard. A user is simply an entity that can own files and execute programs. A user is defined in terms of an account that has a set of properties including a numeric user ID, such as 501, and a username. Internally the system uses the user ID to keep track of the files and processes that belong to a user. The username is a more human-readable form that is used heavily throughout the system so that you don’t have to think ...