UIDs and GIDs

User names and group names are ideal for us to work with, but are less useful to the system—it prefers to deal in numbers. For this reason, when users are created they are allocated an identity number, known as a UID (User Identifier). The system uses this to manage users, only displaying their names for our convenience. Similarly, each group is allocated a GID (Group Identifier), which is used in the same manner by the system.

Each user should have a unique UID. However, it's possible to share one and there are valid reasons for doing so, as we explained for UID 0 in Chapter 1.

All the information used to identify a user is stored within a series of files: the password file, the shadow file, and the group file. Let's look at these now before we start to add any users.