Path: csiph.com!goblin3!goblin1!goblin.stu.neva.ru!usenet.stanford.edu!not-for-mail
From: konsolebox <konsolebox@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: Error on arithmetic evaluation of `~0`.
Date: Fri, 28 Dec 2018 03:33:30 +0800
Lines: 82
Approved: bug-bash@gnu.org
Message-ID: <mailman.6407.1545939228.1284.bug-bash@gnu.org>
References: <CAFra36gW8Csu-84oRLV7QsR7PbiMYq6g7WnMktjfnrSdTys38g@mail.gmail.com> <25e8adf0-96d5-1e38-faea-e7620ba3f846@case.edu> <CAFra36gzzbJLjqt9OYtJUuJara3KR2+ew-Gj3y2mA2L4X8CRyw@mail.gmail.com>
NNTP-Posting-Host: lists.gnu.org
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Cc: Chester Ramey <chet.ramey@case.edu>, bug-bash <bug-bash@gnu.org>
To: Bize Ma <binaryzebra@gmail.com>
Envelope-to: bug-bash@gnu.org
In-Reply-To: <CAFra36gzzbJLjqt9OYtJUuJara3KR2+ew-Gj3y2mA2L4X8CRyw@mail.gmail.com>
Precedence: list
Xref: csiph.com gnu.bash.bug:14970

Simple variables convert to array variables dynamically, but that doesn't
mean they should be interpreted exactly as if they are. I see that more of
just a convenient feature.

On Mon, Dec 24, 2018, 1:02 AM Bize Ma <binaryzebra@gmail.com wrote:

> Chet Ramey (<chet.ramey@case.edu>) wrote:
>
> > >
> > > While this works:
> > >
> > > var=(hello); echo "${var[ ~0]}"
> > > hello
> >
> > Because negative array subscripts count backwards from the end of the
> > array.
> >
>
> Doh!, yes. And, because of that: "${var[-1]}"
> should give the *last* element of array "var", shouldn't it?
>
> Consequently,  this happens:
>
>     $ unset var; var[0]=77; echo "${var[0]}"; echo "${var[-1]}"
>     77
>     77
>
> The only value in var is at index 0, which means it is also the *last*
> value.
>
>
> > > It is also interesting that this fails:
> > >
> > > var=hello; echo "${var[ ~0]}"
> > > bash: var: bad array subscript
> > >
> > > Isn't `var[0]` valid and equivalent to `var` ?
> >
> > Yes, but ~0 (-1) is not the same as 0.
> >
>
> Doh!, yes, of course, "0" is not equal to "~0" (-1). But if you were to
> compare the two last command lines *as posted:*
>
> > var=(hello); echo "${var[ ~0]}"
> > var=hello  ; echo "${var[ ~0]}"
>
> You will notice that the only difference is that one explicitly creates an
> array (and works) while the other creates an scalar (and fails).
>
> The point being that a variable which has an scalar value "var=hello"
> should act (for most practical cases) as an array for which only the
> value at address 0 has been defined.
>
> Both command line above should have printed "hello".
>
> But it should be more clear to you written as this:
>
>     unset var; var=(hello); echo "${var[0]}:${var[ -1]}"
>     unset var;  var=hello ; echo "${var[0]}:${var[ -1]}"
>
> The first line works, the second fails on the negative index.
>
> It seems that bash asserts that the variable is an array when the
> index is negative (and emits an error if var is not an array).
>
>
> > > This was "supposed" to be resolved in a dev version,
> > > but is still present on bash 5.
> >
> > The other arithmetic contexts you reported (the "pure" arithmetic
> contexts
> > the comment above references) were changed; this was left for backwards
> > compatibility. Like I said above, it looks like it's time to deemphasize
> > that.
> >
>
> In other words: you solved this for *some* arithmetic contexts, but not
> all.
>
> Thanks for the confirmation.
>