wall (an abbreviation of write to all) is a Unix command-line utility that displays the contents of a 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.
Invocation
wall reads the message from standard input by default when the filename is omitted. This is done by piping the output of the echo command:
The message may also be typed in much the same way cat is used; invoking wall by typing wall and pressing ↵ Enter followed by a message, pressing ↵ Enter and Ctrl+D:
Using a here-string:
Reading from a file is also supported:
All the commands above should display the following output on terminals that users allow write access to (see mesg(1)):
References
Wall (Unix) Wikipedia(Text) CC BY-SA