User accounts and authentication are two of the most important areas for which a system administrator is responsible. User accounts are the means by which users present themselves to the system, prove that they are who they claim to be, and are granted or denied access to the information and resources on a system. Accordingly, properly setting up and managing user accounts is one of the administrator's chief tasks.
In this chapter we consider Unix user accounts, groups, and user authentication (the means by which the system verifies a user's identity). We will begin by spending a fair amount of time looking at the process of adding a new user. Later sections of the chapter will consider passwords and other aspects of user authentication in detail.