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From: Chet Ramey <chet.ramey@case.edu>
Newsgroups: gnu.bash.bug
Subject: Re: Bash parameter transforamtion on empty array triggers unset variable.
Date: Tue, 11 Aug 2020 09:51:15 -0400
Organization: ITS, Case Western Reserve University
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Xref: csiph.com gnu.bash.bug:16739

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/