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WALL(1M) WALL(1M)

wall - write to all users

/etc/wall

Wall 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.

/dev/tty?
/etc/utmp

mesg(1), write(1)

`Cannot send to ...' when the open on a user's tty file fails.

UNIX-7