uidgid

UID and GID are numeric identifiers used by Unix-like operating systems to represent users and groups. A user account has a user identifier (UID) and is associated with a primary group through a group identifier (GID). Groups also have a GID. The information is stored in account databases such as /etc/passwd for user data and /etc/group for group data, and is resolved through local or network name service backends. The superuser account typically uses UID 0 and GID 0, which grants unrestricted privileges.

The kernel uses IDs rather than names to enforce access control. File ownership and permissions are defined by a file’s owner UID and its group GID. When a process accesses a file, the kernel checks the process’s effective UID and GID, along with any supplementary group memberships, against the file’s permission bits or access control lists. The id command can display a user's UID, GID, and group memberships, while getent queries name service backends. Administrative tools such as chown and chgrp change ownership and group, and chmod adjusts permission bits.

Primary versus supplementary groups: each user has a primary GID from /etc/passwd, and additional group memberships are listed in /etc/group and used for access decisions. Users are added to groups using tools like useradd, usermod, or groupmod; when a user creates a file, the ownership is typically set to the creating process’s UID and GID.

In modern environments, UID/GID are also relevant in containers and user namespaces, where IDs can be mapped to provide isolation. Numeric IDs remain in file metadata even if usernames are unavailable, ensuring consistent permission handling.