/etc/nologin
Explanation:
The /etc/nologin file is used to prevent all users except root from logging into the system. This file is usually created by the system administrator when the system is going down for maintenance or reboot. The file can contain a message that is displayed to the users who try to log in, explaining the reason for the system shutdown. The file is automatically removed by the system when it boots up again. References:
* LPI 102-500 Exam Objectives, Topic 104: Administrative Tasks, 104.5 Manage user accounts
* LPI 102-500 Study Guide, Chapter 4: User and Group Management, Section 4.3: Preventing Users from Logging In