A computer is, by its very nature, a number-oriented machine. It identifies users and groups by numbers known as the user ID (UID) and group ID (GID). The alphabetic names display on your screen just for the your ease of use.
As previously mentioned, the root user is UID 0. Numbers from 1 through 499 and 65,534 are the system, or logical, users. Regular users have UIDs beginning with 1,000; Ubuntu assigns them sequentially beginning with this number.
With only a few exceptions, the GID is the same as the UID. Those exceptions are system users who need to act with root permissions: sync, shutdown, halt, and operator.
Ubuntu creates a private GID for every UID of 1,000 and greater. The system administrator can add other users to a GID or create a totally new group and add users to it. Unlike Windows NT and some UNIX variants, a group cannot be a member of another group in Linux.
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