What is a user?
A user, in the context of a Unix system, is simply a named entity that can do things on the system. Users can launch and own processes, own files and directories and have various permissions on them, and be allowed or prevented from doing things or using resources on the system. Practically, a user is who you log in as, what your processes run as, or who owns your files.
The word “user” is obviously a metaphor for a real person with a user account, a password, and so on. But most “users” on real systems don’t actually represent specific humans. They’re machine accounts, meant to group resources like processes and files for the purposes of security or organization.
But there’s a much more important distinction than whether or not an account is intended to be used interactively by a human operator. There are exactly two types of users, and before we jump into practical user-management skills, we need to talk about...
