ACCESS CONTROL
       shutdown  can  be  called from init(8) when the magic keys CTRL-ALT-DEL are pressed, by creating an appropriate entry in /etc/inittab.
       This means that everyone who has physical access to the console keyboard can shut the system down. To prevent this, shutdown can check
       to  see if an authorized user is logged in on one of the virtual consoles. If shutdown is called with the -a argument (add this to the
       invocation of shutdown in /etc/inittab), it checks to see if the file /etc/shutdown.allow is present.   It  then  compares  the  login
       names  in  that file with the list of people that are logged in on a virtual console (from /var/run/utmp). Only if one of those autho-
       rized users or root is logged in, it will proceed. Otherwise it will write the message

       shutdown: no authorized users logged in

       to the (physical) system console. The format of /etc/shutdown.allow is one user name per line. Empty lines and comment lines (prefixed
       by a #) are allowed. Currently there is a limit of 32 users in this file.

       Note that if /etc/shutdown.allow is not present, the -a argument is ignored.