Any good system administrator needs to be able to send authoritative messages to any online users. Fortunately, with BSD variants such as FreeBSD and OpenBSD, it is very easy with a tool called “wall”.
To send a general message to all users logged into the system, follow this format:
your message here
more of your message
At the end of your message, press CTRL+D to send.
For example, if you want to inform all users that the system will be shutting down for maintenance, you would type:
The system will reboot to complete maintenance in 15 minutes. Please save your work
— Your friendly system admin
You can also send messages to specific group members with the “-g” option. For example, to send messages only to those in the “wheel” group (for administrators), enter:
# wall -g wheel
Wall is a helpful tool for communicating with users logged into your server, and it is available on most Unix-like operating systems, including BSD variants and Linux distributions.