bug#75308: 30.0.93; man.el assume shell is bash

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Fri, 3 Jan 2025 03:26:42 -0600
> Cc: phelps@gnusto.com, 75308@debbugs.gnu.org, michael.albinus@gmx.de
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Cc: Michael Albinus <michael.albinus@gmx.de>
> >> From: Stefan Kangas <stefankangas@gmail.com>
> >> Date: Fri, 3 Jan 2025 02:46:28 -0600
> >>
> >> Ted Phelps <phelps@gnusto.com> writes:
> >>
> >> > Steps to reproduce:
> >> > * env SHELL=/bin/tcsh emacs -Q
> >> > * invoke M-x man
> >> > * request a man page (e.g. emacs)
> >> >
> >> > This will produce output like the following instead of the man page:
> >> >
> >> >     Unmatched '''.
> >> >     BEGIN: Command not found.
> >> >     anonblank=0: Command not found.
> >> >     }: Command not found.
> >> >     Illegal variable name.
> >> >     {: Command not found.
> >> >     }: Command not found.
> >> >     Illegal variable name.
> >> >     Badly placed ()'s.
> >> >     Unmatched '''.
> >> >
> >> >     process exited abnormally with code 1
> >> >
> >> > Replacing /bin/tcsh with /bin/bash in the above command produces the
> >> > expected man page.
> >> >
> >> > It looks like the behavior was broken by the following commit:
> >> >
> >> >     commit 861ac933dd8aed1028edc4b9142400e3702874d5
> >> >     Author: Michael Albinus <michael.albinus@gmx.de>
> >> >     Date:   Wed Nov 1 16:54:31 2023 +0100
> >> >
> >> > Many thanks,
> >> > -Ted
> >>
> >> Thanks for the bug report.  For emacs-30, we should look into a some
> >> small workaround.  I'm copying in Michae.
> >
> > I think we should force SHELL to be 'sh' (except on Windows) before
> > calling shell-command etc.
> 
> I was first thinking /bin/bash but if we don't expect to need it, then
> sh is better.

If we do need Bash and not just Bourne-compatible shell, we have a
serious bug there.  I hope that is not the case.