You wish to notify all users that you have to take down a service on which they rely. What command
will allow you to send a message to all currently logged on users? Enter only the command, not the
path.

Answer: wall

Explanation:
from the man pages:
wall – write a message to users
Wall displays the contents of file or, by default, its standard input, on the terminals of all currently
logged in users. Only the super-user can write on the terminals of users who have chosen to deny
messages or are using a program which automatically denies messages.