If the file /etc/nologin exists and is readable, login(1) will
allow access only to root. Other users will be shown the contents of this file
and their logins will be refused. This provides a simple way of temporarily
disabling all unprivileged logins.
This page is part of release 5.02 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at https://www.kernel.org/doc/man-pages/.