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, without path or options)

Answer: wall
Section: 107.1 Manage user and group accounts and related system files

Explanation/Reference:
From the man pages:
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. Reading from a file is refused when the invoker is not superuser and the program is suid or sgid.