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Re: [Bash 4.4.5] Variable indirection, error on empty variable


From: Chet Ramey
Subject: Re: [Bash 4.4.5] Variable indirection, error on empty variable
Date: Tue, 29 Nov 2016 09:19:39 -0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.5.0

On 11/27/16 5:07 AM, Otenba wrote:

> Bash Version: 4.4
> Patch Level: 5
> Release Status: release
> 
> Description:
> Performing variable indirection using an empty value causes an error. It
> didn't in Bash 4.3.

It's pretty clear that indirect expansion that attempts to reference
through an unset variable or a variable whose value is null should be an
error. Bash-4.3 didn't have the greatest error checking, and bash-4.4
catches the null value case.  I'll fix it to catch the unset variable
case, too.

> Repeat-By:
> $ reference=1
> $ echo ${!indirect} #variable indirection using an unset variable

If you think of indirect expansion as just a simple string replacement,
though it's more complicated than that, it's clear that this is logically
equivalent to echo ${}, which should be an error.


> $ indirect=
> $ echo ${!indirect} #variable indirection using an empty variable
> -bash: : bad substitution

Using the same logic as above, this expands to the equivalent of
echo ${" "}, which is another obvious error, though the message could
be a little better.

Chet

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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