Users, groups, and permissions

Users and groups, together with permissions, are the main entities that are used in Unix operating systems to control access to resources.

Users and groups

Authorization to files and other resources are provided by users and groups. Users have unique usernames that are human-friendly identifiers, but from the operating system side, each user is represent by a unique positive integer: the User ID (UID). Groups are the other authorization mechanism and, as users, they have a name and a Group ID (GID). In the operating system, each process is associated with a user and each file and directory belongs to both a user and a group.

The /etc/passwd file contains all this information and...