Is there a command that I could use, as root, to message all the users connected to my system? I know that if you are going to perform a shutdown (shutdown -r now message), you can message all the users telling them "hey, I am going to shutdown now". But I simply just want to be able to send a message to everyone that says "hey, I am going to turn off the internet in 15 minutes".

Yeah. I searched google, but it was all trash. Any help? Thanks.