Unix-like operating systems identify a user by a value called a User Identifier (UID) and Identify group by a group identifier (GID), are used to determine which system resources a user or group can access.
A UID (user identifier) is a number assigned by Linux to each user on the system. This number is used to identify the user to the system and to determine which system resources the user can access.
- UID 0 (zero) is reserved for the root.
- UIDs 1–99 are reserved for other predefined accounts.
- UID 100–999 are reserved by system for administrative and system accounts/groups.
- UID 1000–10000 are occupied by applications account.
- UID 10000+ are used for user accounts.
Groups in Linux are defined by GIDs (group IDs).
- GID 0 (zero) is reserved for the root group.
- GID 1–99 are reserved for the system and application use.
- GID 100+ allocated for the user’s group.