Path: csiph.com!news.uzoreto.com!newsreader4.netcologne.de!news.netcologne.de!fu-berlin.de!usenet.stanford.edu!not-for-mail
From: Andrew Neff <andrew.neff@visionsystemsinc.com>
Newsgroups: gnu.bash.bug
Subject: Re: Bash parameter transforamtion on empty array triggers unset variable.
Date: Tue, 11 Aug 2020 11:45:41 -0400
Lines: 72
Approved: bug-bash@gnu.org
Message-ID: <mailman.1576.1597160759.2739.bug-bash@gnu.org>
References: <CAJjXSE9b8G0aXyc+t58cbYRwFbiN5eX2rpKnyFwg728x5ZQhKQ@mail.gmail.com> <3ddb8a56-c7d0-b87e-8bb4-520acecc90d5@case.edu> <CAJjXSE8vfCpx7r4owo5V0tA=G1qi5nr5oDwqJwyQoM41oC2W3Q@mail.gmail.com>
NNTP-Posting-Host: lists.gnu.org
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Cc: chet@cwru.edu, bash-testers@cwru.edu, bug-bash@gnu.org
To: chet.ramey@case.edu
Envelope-to: bug-bash@gnu.org
In-Reply-To: <3ddb8a56-c7d0-b87e-8bb4-520acecc90d5@case.edu>
Received-SPF: none client-ip=2607:f8b0:4864:20::32b; envelope-from=andrew.neff@visionsystemsinc.com; helo=mail-ot1-x32b.google.com
X-Spam_score_int: -18
X-Spam_score: -1.9
X-Spam_bar: -
X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_NONE=0.001, URIBL_BLOCKED=0.001 autolearn=ham autolearn_force=no
X-Spam_action: no action
X-Content-Filtered-By: Mailman/MimeDel 2.1.23
X-BeenThere: bug-bash@gnu.org
X-Mailman-Version: 2.1.23
Precedence: list
List-Id: Bug reports for the GNU Bourne Again SHell <bug-bash.gnu.org>
List-Unsubscribe: <https://lists.gnu.org/mailman/options/bug-bash>, <mailto:bug-bash-request@gnu.org?subject=unsubscribe>
List-Archive: <https://lists.gnu.org/archive/html/bug-bash>
List-Post: <mailto:bug-bash@gnu.org>
List-Help: <mailto:bug-bash-request@gnu.org?subject=help>
List-Subscribe: <https://lists.gnu.org/mailman/listinfo/bug-bash>, <mailto:bug-bash-request@gnu.org?subject=subscribe>
X-Mailman-Original-Message-ID: <CAJjXSE8vfCpx7r4owo5V0tA=G1qi5nr5oDwqJwyQoM41oC2W3Q@mail.gmail.com>
X-Mailman-Original-References: <CAJjXSE9b8G0aXyc+t58cbYRwFbiN5eX2rpKnyFwg728x5ZQhKQ@mail.gmail.com> <3ddb8a56-c7d0-b87e-8bb4-520acecc90d5@case.edu>
Xref: csiph.com gnu.bash.bug:16740

Ah, I see the confusion.

The issue you pointed out, "@Q breaks set -o nounset" refers to quote
parameter expansion, as does the line in CHANGES-5.1, 1.q, which makes
sense to call this a bug that was allowed in bash 4.4 and 5.0.

I should have specified, the focus of this issue is the "@a" expansion. It
makes sense that @Q/E/P/A expansion should not work on unset variables with
nounset enabled. However, @a is uniquely different, in that it does not
have to do with the value of the variable, but rather the variable type. My
"set -eu" compatible library uses the @a expansion (for bash 4.4 and newer)
because it had previously worked on unset values with set -u enabled, which
was a very useful feature.

Here are 3 specific details I would like to address:

1. @a expansion should work on unset variables with "set -u" in bash 5.1.
It seems like the correct thing to do. Only @a expansion. This has been a
very useful feature in bash 4.4 and 5.0.
Should fail: (set -eu; declare -a x; echo "${x@Q}")
Should not fail:  (set -eu; declare -a x; echo "${x@a}")

2. With "set -u", the following works in bash 4.4 or newer (and makes sense
that it works): (set -eu; x=(); echo "${x[@]}")
Here x is not unset, it is set to an empty array. This expansion make sense
with nounset turned on because x is not unset, it is set to ()
However, this fails: (set -eu; x=(); echo "${x@a}")
This is an inconsistent behavior, and it seems to me the ${x@a} should
definitely work here, with nounset turns on

3. The same as #2, but for associative arrays
Works: (set -eu; declare -A x=(); echo "${x[@]}")
Does not work, but should: (set -eu; declare -A x=(); echo "${x@a}")

Thanks


On Tue, Aug 11, 2020 at 9:51 AM Chet Ramey <chet.ramey@case.edu> wrote:

> On 8/10/20 5:52 PM, Andrew Neff wrote:
>
> > Bash Version: 5.1
> > Patch Level: 0
> > Release Status: alpha
> >
> > Description:
> >         I do not know if this is related to bash 5.1 erroneously being
> > "a little aggressive about skipping over empty strings" mentioned
> > in "Declaring arrays with empty string in one line is bugged", but using
> > parameter transformation on an empty array, throws an error if "set -u"
> is
> > turned on. This was not how bash 4.4 and 5.0 worked. This bug makes it
> > impossible to check if an empty variable is an array using parameter
> > transformation while "set -u" is turned on
>
> This was a bug in bash-5.0 (and 4.4) and was fixed in early March 2019 as
> the result of
>
> https://lists.gnu.org/archive/html/bug-bash/2019-03/msg00010.html
>
> It's in CHANGES.
>
> There are some other changes in how bash displays attributes of unset
> variables when `nounset' is not enabled, but unset variables used in word
> expansions should trigger an error -- with the usual @/* exceptions -- when
> set -u is enabled.
>
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/
>