UNIX and Linux System Administration Handbook, 5th Edition

8	User Management

Modern computing environments span physical hardware, cloud systems, and virtual hosts. Along with the flexibility of this hybrid infrastructure comes an increasing need for centralized and structured account management. System administrators must understand both the traditional account model used by UNIX and Linux and the ways in which this model has been extended to integrate with directory services such as LDAP and Microsoft’s Active Directory.

Account hygiene is a key determinant of system security. Infrequently used accounts are prime targets for attackers, as are accounts with easily guessed passwords. ...