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Both Mk and Rc started out with Research Unix, some years before Plan 9.

I've been seeing people, never ones who were actually there, make this claim in the last five to ten years. Rc was on tenth edition Unix. I've seen no evidence that it originated on Unix. The manual calls it the Plan 9 shell.

I was going by the fact that tenth edition Unix was released on 1989, while Plan 9 was first released in 1992: https://en.wikipedia.org/wiki/Research_Unix

Pehaps this is misleading.

Some clues:

The mk paper (and Unix man page) doesn't mention rc.

Plan 9 existed since the mid-80s.

This sentence in the Unix rc(1) sure makes it seem like rc was Plan 9-first:

Environment entries for variables are kludgy for UNIX compatibility.

The first paragraph of https://archive.org/details/rc-shell/

So rc was made for Plan 9. The examples in that paper are definitely on tenth edition Unix, I think this would be because

A fairly complete version of Plan 9 was built in 1987 and 1988, but development was abandoned. In May of 1989 work was begun on a completely new system

from https://doc.cat-v.org/plan_9/1st_edition/designing_plan_9 (1990).

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