mesg (Unix)
The Unix command that decides whether other users may write straight into your terminal — the do-not-disturb sign of the shared machine.
On early Unix a terminal was a file under /dev, and write, talk and wall worked by simply writing to that file. mesg n removed group write permission from your own terminal device; mesg y put it back. Run without arguments, it just told you which state you were in. The mechanism wasn't a protocol at all, only ordinary file permissions, which is about as Unix as anything gets.
The shield had a deliberate hole: root ignored permissions, so a wall announcing that the machine was going down got through regardless. The command is specified by POSIX and still ships with every Linux distribution, where it mostly shows up as a warning from login scripts run without a terminal attached.