Managing public and private groups

The Red Hat and, therefore, the CentOS user management systems deploy a private user group system. Each user created will also belong to an eponymous primary group; in other words, creating a user bob will also create a group bob, to which the user will be the only member.

Linux groups

Firstly, we have to understand a little about Linux groups. A user has both a primary group and secondary groups.

User ID and group ID (UID/GID) are used with the permission management structure in Linux. Every file in any filesystem will be owned by a user and a group by means of storing the UID and GID in the files metadata. Permissions can be assigned to the user, group, or others.

Each user has one UID and GID but belongs to just one group, which is a little restrictive, so users additionally have secondary groups. Users can change their current GID to one from their secondary groups using the /usr/bin/newgrp command, effectively switching their GID. In practice, this is...