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.

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:

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 cat is used: invoking wall by typing wall and pressing ↵ Enter followed by a message, pressing ↵ Enter and Ctrl+D:

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 mesg(1)):

Broadcast Message from alice@sleipnir
  (/dev/pts/7) at 16:15 ...

Remember to brush your teeth!

• Jordan Hubbard § rwall incident

• wall(1) – Version 7 Unix Programmer's Manual
• wall(1) – Linux User Commands Manual
• wall(1) – FreeBSD General Commands Manual
• wall(8) – Solaris 11.4 System Administration Commands Reference Manual