[External] : configure shell game

[Nico Williams]

On Tue, Jun 02, 2026 at 09:49:25AM -0600, Theo de Raadt wrote:
> > The warning message is somewhat useful: $() is POSIX now, and $() is
> > much better than ``.
> 
> Unix has no way to emit a warning message without scripts observing it,
> because warnings go to fd 2 stderr, which can and will be redirected
> and parsed by something else.   As such, the warning message is not
> useful because the only thing it does is break compatibliity.

It's usually true that messages written to stderr are a) not intended to
be written in stable formats nor to be parsed by software, and b) users
don't heed (a) and complain when the message formats change.

Solaris used to treat such messages as not-an-interface.  Maybe that's
changed, but I wouldn't know (Jan clearly can tell us).  Even if they
treated warning message formats as stable, they might not treat the set
of warning messages as closed -- I think it's quite difficult to do
that.

But Solaris might not be the only supported OS that takes that stance,
so while OpenBSD might take the opposite stance, OpenSSH might not be
able to either support such systems or insist that no unexpected
warnings go to stderr.  "When in Rome...".

I'm not stating an opinion as to whether messages sent to stderr are
stable interfaces.  I'm also not stating an opinion as to whether
introducing new warnings is a good idea.

Nico
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