Unixcop published a tutorial about how to use the wall command in linux.

How to use wall command in linux

wall (an abbreviation of write to all) is a Unix command-line utility that displays the contents of a computer file or standard input to all logged-in users. It is typically used by root to send out shutting down message to all users just before poweroff. 

How to use wall command in linux - Unixcop the Unix / Linux the admins deams