Prevent non-root users from logging
Imagine that for some reason (i.e. maintenance tasks) you want to prevent non-root users from logging into the system. The next tip is a very simple way to achieve this goal.
If a file called /etc/nologin exists login will disable the begin of a session in this system. If you put some text into the file, users will be shown this text and their login attempts will be refused.
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