On 2/28/18 2:00 PM, Robert Elz wrote:
> Date: Wed, 28 Feb 2018 10:27:23 -0500
> From: Chet Ramey <chet.ra...@case.edu>
> Message-ID: <ba594d01-b9c4-5cd8-9fa0-2f20fd1d2...@case.edu>
>
> | 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 c...@case.edu http://tiswww.cwru.edu/~chet/
