Re: [PATCH v2] test_cmp: diagnose incorrect arguments

[Eric Sunshine <sunshine@sunshineco.com>]

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.

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.