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Re: Bash parameter transforamtion on empty array triggers unset variable

From: Chet Ramey
Subject: Re: Bash parameter transforamtion on empty array triggers unset variable.
Date: Wed, 12 Aug 2020 09:49:05 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.11.0

On 8/11/20 11:45 AM, Andrew Neff wrote:
> 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.

Not quite. The report revealed that none of the parameter transformations
obeyed `set -u'. The fix was to make that happen, so that parameter
transformations were aligned with the other word expansions.

> I should have specified, the focus of this issue is the "@a" expansion. 

Sure, you want an exception.

> 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.

This is why it works on unset variables when `set -u' isn't enabled. The
question is whether that makes it special enough to be an exception.

> 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.

OK, you found it useful in something you wanted to do. You're restating the

> 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 ()

That's not why. The variable is indeed unset. The reason it doesn't fail
is because there's a special POSIX carveout for $@ with no positional
parameters and I extended it to ${var[@]}. The expansions of $@ and ${A[@]}
parallel each other in just about every other way, so it made sense.

> 3. The same as #2, but for associative arrays
> Works: (set -eu; declare -A x=(); echo "${x[@]}")

The same carveout for ${A[@]}.

So I guess the question is whether or not @A and @a are exceptional enough.

``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/

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