Re: [PATCH v2] test_cmp: diagnose incorrect arguments

[Taylor Blau <me@ttaylorr.com>]

On Tue, Aug 11, 2020 at 03:25:03PM -0400, Eric Sunshine wrote:
> On Tue, Aug 11, 2020 at 2:33 PM Taylor Blau <me@ttaylorr.com> wrote:
> > On Sun, Aug 09, 2020 at 01:42:09PM -0400, Eric Sunshine wrote:
> > > +             test "x$2" = x- || test -e "$2" || BUG "test_cmp '$2' missing"
> >
> > Not related to your patch, but I've seen this style of "x$1" in a few
> > places in test-lib-functions.sh. Why can't this be written as 'test "$1"
> > = -'?
>
> Short answer: To prevent 'test' from thinking that the argument is a switch.

Makes sense. Now I feel silly for asking :).

> Longer answer:
>
> 'test' can accept both switches (i.e. "-e") and non-switch arguments.
> Keep in mind, too, that all the quoting is stripped by the shell
> _before_ 'test' ever sees its arguments. Let's say that the caller has
> a filename whose name actually is "-e" and passes that in as $1. So,
> what does 'test' see?
>
>     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 "-".
>
> You might say that having a file named "-e" (or similar) is unlikely,
> however, what is not unlikely is a caller passing "-" for
> standard-input as $1. In this case, 'test' sees:
>
>     test - = -
>
> 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.
>
> This is why it's a very common idiom in shell programming with 'test'
> to see "x" prepended, thus ensuring that the argument can't be
> confused with a switch.

Thanks for a careful and helpful explanation, as always :-). Makes sense
to me.

Thanks,
Taylor