[JdeBP]

You did, however, proffer a discussion of the Linux boot process, and Linux operating systems do not and did not have AT&T System 5 init. They have/had something resembling it, written by Miquel van Smoorenburg in the 1990s. But it isn't AT&T System 5 init, and it actively diverged from it early on.

Ironically, 20 years ago was almost three years after van Smoorenburg init+rc had diverged from the old idea of one "single-user" mode and had instead taken the route of two modes, emergency and rescue. In fact, we only need to wait a year or so until it has been 20 years since those names can be found in widespread use. It has already been more than 20 years since the name "emergency" gained traction.

* http://jdebp.info./FGA/emergency-and-rescue-mode-bootstrap.h...

So whilst things are like they were 20 years ago, how things were 20 years ago isn't in fact the old AT&T System 5 world that people nowadays tell one another it was. That was the 1980s, and it wasn't Linux-based. Indeed, by 25 years ago the AT&T world itself had already introduced ideas changing the original model of multi-user login, such as the Service Access Facility.

[siebenmann]

Belatedly: that was a very interesting read on the history of this in both Linux and Unix more generally. Thank you for the link.