NOLOGIN(5) Linux Programmer's Manual NOLOGIN(5)
nologin - prevent non-root users from logging into the system
If the file /etc/nologin exists, login(1) will allow access only to root. Other users will be shown the con- tents of this file and their logins refused.
/etc/nologin
login(1), shutdown(8) Linux December 29 1992 1