A unique user UID
root uid 0 administrators are distinguished by uid=0
System users 1-499 (before 6) 1-999 (after 7)
Users 500+ 1000+
Group GID
Administrator group: root, 0 is not necessarily an administrator
System user group 1-499 (before 6) 1-999 (after 7)
User group 500+ 1000+
Primary group: The user must belong to one and only one primary group. When creating a user account, create a group with the same name, also called a private group
Additional groups: users can have zero or more additional groups
Whether the security context program can run depends on the identity of the user
User and group related files
/etc/passwd user information, attributes seven fields
getent passwd wang
/etc/shadow user password
getent shadow wang
/etc/group group attributes
/etc/gshadow group password! Login is not allowed