I suspect it was Steve’s personal preference on the Bourne shell.   There’s next to no comments in the thing and the document on the shell just mentions the defaults and that you can change it.

Just for completeness, the PWBSH appears to allow the prompt to be changed with a command line argument.

We used to have a shell called “uu” at Hopkins which was sort of a prehistoric “sudo” that ran certain commands (icheck/dcheck/etc…) as root while setting back to the regular uid for others.  It used “@ “ for a prompt.   

From: TUHS <tuhs-bounces@minnie.tuhs.org> On Behalf Of Henry Bent
Sent: Monday, August 6, 2018 5:33 PM
To: TUHS main list <tuhs@minnie.tuhs.org>
Subject: Re: [TUHS] Origins of shell prompt suffixes % $ > #

On 6 August 2018 at 17:16, <ron@ronnatalie.com> wrote:

The early shells (Thompson, Mashey)  used "% " for regular user (and # for root).   The Thompson shell didn't have a setable prompt.
The Bourne shell (V7) had setable PS1 (start of command) and PS2 (continuation prompts) and set the to "$ " and "> " respectively.    Again # was used for root

Okay, but why did Bourne switch from "%" to "$"?  Was it to inform the user that they were using the new shell as opposed to the old one, or was there some other reasoning behind the switch?

-Henry