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Re: Unset array doesn't work


From: Chet Ramey
Subject: Re: Unset array doesn't work
Date: Fri, 2 Mar 2018 14:43:02 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 2/28/18 2:00 PM, Robert Elz wrote:
>     Date:        Wed, 28 Feb 2018 10:27:23 -0500
>     From:        Chet Ramey <address@hidden>
>     Message-ID:  <address@hidden>
> 
>   | These are two different cases -- same context vs. a previous context. Your
>   | example is not the same as the original poster's.
> 
> OK, though I am not sure why that should make a difference.

It does.


>   | The global var is left unchanged, and satisfies references to $var with 
> v1.
> 
> The second clause of that is what is brain dead.   There should be a simple
> invariant in all shell contexts, which is
> 
>       unset X
>       Y=$X
> 
> assigns the null string to Y (except in the case where X is read only, which
> is not a case of concern here.)

Perhaps. But bash has never done this. Not from day one. That's 30 years.

> Always.
> 
> No (other) exceptions.
> 
> None.

Never done it that way. Ever. Not once. (We can do this all day. But levity
aside.)

> 
> Similarly the expression ${X-unset} should always produce "unset" if X
> has been unset and not subsequently given a value.
> 
> Whatever else is believed about how variables should work, that part should
> remain true - if we cannot rely upon unset working to create an unset 
> variable,
> then we're lost.

This is how bash dynamic scoping works. The exception for the declaration/
unset at the current scope was added 16 years ago, and the existing
behavior was already entrenched.


>   | You can mark any local instances in previous
>   | scopes (or the nearest previous scope that has a local instance of
>   | that variable) with a special `unset' attribute that will cause
>   | references to it to behave as if it were unset, leaving the global
>   | value unchanged. 
> 
> Yes.   That's the correct way I believe.

I can see doing this and allowing it to be toggled by a shell option.

> Lastly, where does the notion of "remove" come from?

As a way to describe the historical bash behavior, it works.  An instance
of a variable with a given name at a previous scope is removed, and another
instance of that variable at another scope is revealed.

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



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