"write" was used to DM a person. And "mesg" was used to enable/disable messages from being displayed if you didn't want to be disturbed. I used this a lot earlier in my career. Later came "talk" which gave a fancy split screen chat session.
Notably, it did so by literally opening up the tty associated with the user's session and spitting bytes directly at it. It was a different world back then.
`stty 0 </dev/ttyxx` was briefly popular when I was in school (although setting an incorrect but nonzero baud rate would probably have been more annoying).
> Strange you don't believe Jon Hall, but willing to believe some random stranger on social media.
I'm not aware that Jon Hall would claim any particular expertise or interest in legal history, whereas there may well be "some random strangers on social media" who do.
Also, this is a matter which can be established by citing sources which Jon Hall might be unfamiliar with (for the simple reason that the topic might never have interested him sufficiently for him to research it.)
If someone buys the Kindle version, can you let us know how the formatting looks with all of its diagrams and code?
To get the PDF version I tried buying the e-book collection from InformIT, but Pearson (that runs the InformIT and Microsoft stores) no longer takes orders from people using a "@fastmail.com" e-mail.