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Re: 'local -x VARIABLE' does not clear variable for subprocesses

From: Chet Ramey
Subject: Re: 'local -x VARIABLE' does not clear variable for subprocesses
Date: Wed, 07 May 2014 10:04:11 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 5/7/14, 2:10 AM, Dan Douglas wrote:
> On Monday, May 05, 2014 11:18:50 AM Chet Ramey wrote:
>> The idea behind the bash-4.3 behavior is that the placeholder local
>> variable isn't set, and doesn't really exist.  It doesn't shadow a
>> global variable with the same name until it gets a value.  The bash-4.2
>> behavior was inconsistent: variables with attributes but without values
>> weren't quite set (value == 0x0, but still visible) and weren't quite
>> unset.  I tightened up the some of the consistency starting with bash-4.3
>> beta.  I'm sure there are still inconsistencies there (and, in fact, I
>> found one while looking at this).
> By "doesn't shadow" you mean that it _does_ hide the global right? Localizing 
> a variable should cover up globals and variables in parent scopes even if 
> they 
> aren't given a value. That seems true in bash 4.3 and nearly every shell 
> (except dash).

I mean that a local variable that has not been given a value, and is
technically unset, should not shadow a global variable with the same
name.  Run the following script, after accounting for the local/typeset
differences between shells.  The results are less conclusive than you think.

export VAR1=abc VAR2=abc

        local VAR1; local VAR2
        echo local: ${VAR1-unset1} ${VAR2-unset2}
        printenv VAR1 ; printenv VAR2


Shells that shadow print `local: unset1 unset2'.  Shells that don't shadow
print `local: abc abc'.  Everyone except bash-4.2 seems to print
`abc\nabc' with the printenv calls.

Shadow: bash-4.3, mksh, posh
Doesn't shadow: ksh93, dash, freebsd sh

zsh is off doing its own thing and just prints `local:'.  I presume that
means it silently assigns a null value.

> If you want to set an attribute without localizing the variable then you've 
> always been able to use "export" or "readonly" in place of "local -x".

Sure, but the OP wants to do exactly the opposite, and more.

> I also wouldn't want this example to change so that ref always appears set. 
> The localized x in g should hide f's x even with no value.

namerefs are their own special beasts.

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

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