@Edent I think it was CSI 3i in SCO Xenix that worked as a "print screen" command. But for a terminal, so it would send the content of the screen back to the tty, which was supposed to capture it and then do something with it, for example printing. So if somebody had a root shell open, and was mesg y, you could send them 'clear screen', some command, 'csi 3i' and the screen content - the command - would be echoed back to the tty, with the shell listening. Very quickly my admin had 'mesg n'