While computer intrusions over the Internet make headlines, a system administrator’s greatest security threats often come from a system’s own users. Maybe they won’t ship your data to a crime syndicate, but disgruntled and incompetent users will crash your servers given the chance—sometimes out of malice, but more often out of ignorance. Think about security as the combination of confidentiality, integrity, and availability, and it will immediately become clear how users with unrestricted system access can damage security.

Despite what you might have learned from the Bastard Operator from Hell, the system exists for the users, and proper management of those users’ accounts is absolutely necessary. In this chapter, we’ll cover one of the systems administrator’s most common tasks: managing users by adding, removing, configuring, and modifying user accounts.