wall - broadcast a message to all users

wall displays the contents of a file and broadcasts it to all terminals, of all users currently logged in.

Only a superuser can write to a terminal of users who have chosen to deny messages.

EXAMPLE:

# echo "Please note the server will be rebooted in 30 minutes" > message.txt
# wall < message.txt

To protect your terminal from wall/write messages from ordinary users, use the command "mesg n" . To accept these messages again use "mesg y". As mentioned above if the message is coming from root there is no hope - root can do pretty much anything.