Forum Debian Users Gang

man shutdown
(...)
       If  shutdown  is  called with a delay, it will create the advisory file
       /etc/nologin which causes programs such as login(1) to  not  allow  new
       user  logins.  This  file  is  created five minutes before the shutdown
       sequence starts. Shutdown removes this file if it is stopped before  it
       can  signal  init  (i.e.  it is cancelled or something goes wrong).  It
       also removes it before calling init to change the runlevel.
(...)