bug#68732: 29.1; shell-command-default-error-buffer is not interactive

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 26 Jan 2024 16:16:21 +0100
> From: Krzysztof Żelechowski <giecrilj@stegny.2a.pl>
> Cc: 68732@debbugs.gnu.org
> 
> The point is, the function shell-command does not use this variable because it only affects interactive
> use.  So indeed, writing a command that invokes shell-command is an option, but it does make a use
> case for the existence of this variable.
> 
> My POV is splitting standard output and standard error (maybe on demand) is a basic usability
> requirement and should not require writing custom commands.
> 
> The problem is not with using setq by users, it is with getting non-developers to write and execute
> Lisp scripts to accomplish basic tasks, including scripts that just invoke setq.  The latter obviously is
> not a problem when you are a scientist, but I imagine that the front end would be much cleaner if
> operable cleanly without scripting (again, this concept limited to basic tasks obviously).

We cannot possibly make every variable a defcustom, just because
someone somewhere will need to set a variable while not being a
programmer, can we?

So, while I understand your motivation, it still doesn't convince me
to make the variable a user option.  Splitting stdout and stderr might
be a basic usability requirement in your eyes, but IME users rarely
send stderr to a separate place, which is why this variable is not a
defcustom.