32v 1m wall
From Computer History Wiki
WALL(1M) UNIX Programmer's Manual WALL(1M)
wall - write to all users
_W_a_l_l reads its standard input until an end-of-file. It then sends this message, preceded by `Broadcast Message ...', to all logged in users.
The sender should be super-user to override any protections the users may have invoked.
`Cannot send to ...' when the open on a user's tty file fails.