Re: [PATCH v2] test_cmp: diagnose incorrect arguments

From: Eric Sunshine <sunshine@sunshineco.com>
Date: Aug 12, 2020, 16:15 UTC
Message-ID: <CAPig+cQNvJ02fm82oBtyyxHUqfNk3oAZJW__Lu2Kn0Qt3VaDWw@mail.gmail.com>
In-Reply-To: <20200812153705.GC33189@coredump.intra.peff.net>

On Wed, Aug 12, 2020 at 11:37 AM Jeff King <peff@peff.net> wrote:

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> On Tue, Aug 11, 2020 at 03:25:03PM -0400, Eric Sunshine wrote:
> >     test -e = -
> > Rather than comparing literal string "-e" to literal string "-", it's
> > instead (almost) asking if the file named "=" exists; I say "almost"
> > because it's actually an error since switch -e only accepts one
> > argument, but it's being given two arguments, "=" and "-".
>
> I don't think this is an error. The program can tell which you meant by
> the number of arguments. POSIX lays out some rules here (from "man
> 1posix test" on my system, but I'm sure you can find it online):

I intentionally didn't focus on or mention POSIX in my response because I wanted to represent the Real World reason why "x$var" is such a common idiom. As a person who regularly uses ancient operating systems and old computers (it's common for me to be stuck on computers 10-25 years old; my most up-to-date computer is 9.5 years old), Real World is something I focus on more than POSIX. Okay, I may be an outlier, but still...

Show 13 quoted lines

> > which may or may not be an error in a particular implementation of
> > 'test'. Some implementations may understand that "-" is not a valid
> > switch, thus infer that you're actually asking for an equality
> > comparison between arguments, but other implementations may complain
> > either that there is no switch named "-" or that those arguments
> > simply make no sense.
>
> Yeah, historically I think there were shells that were not quite so
> clever. I have no idea which ones, or whether any are still around. I
> don't think we've generally been that concerned with this case in Git,
> and nobody has complained. I'd be totally unsurprised to hear that SunOS
> /bin/sh doesn't get this right, but we've already written it off as
> unusable (it doesn't even support $() expansion).

I'm probably showing my age, but having had to deal with the many inconsistencies of implementation over the years, it's hard to _not_ worry about such things even now. Adding the "x" to "$1" or to making other minor portability concessions is reflex at this point. These portability concerns are also always on mind when dealing with Mac OS since so many of its utilities are ancient and retain the old behaviors.