Wall (Unix)

 (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. 
Invocation.
 reads the message from standard input by default when the filename is omitted. This is done by piping the output of the command:
alice@sleipnir:~$ # `tty` to show the current terminal name
alice@sleipnir:~$ tty
/dev/pts/7
alice@sleipnir:~$ echo Remember to brush your teeth! | wall
The message may also be typed in much the same way is used: invoking by typing and pressing followed by a message, pressing and +:
alice@sleipnir:~$ wall
Remember to brush your teeth!
^D
Using a here-string:
alice@sleipnir:~$ wall «< 'Remember to brush your teeth!'
Reading from a file is also supported:
alice@sleipnir:~$ cat .important_announcement
Remember to brush your teeth!
alice@sleipnir:~$ wall .important_announcement # same as `wall !$`
All the commands above should display the following output on terminals that users allow write access to (see codice_1):
Broadcast Message from alice@sleipnir
 (/dev/pts/7) at 16:15 ...
Remember to brush your teeth!