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Re: Error on arithmetic evaluation of `~0`.


From: Chet Ramey
Subject: Re: Error on arithmetic evaluation of `~0`.
Date: Sat, 22 Dec 2018 12:00:18 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 12/19/18 10:31 PM, Bize Ma wrote:
> This is the third time I am reporting this issue.

Not really, but let's go on.

> 
> This fails:
> 
> var=(hello); echo "${var[~0]}"
> syntax error: operand expected ...

Yes. The comment in the code says:

"Right now, the code
 suppresses tilde expansion when expanding in a pure arithmetic
 context, but allows it when expanding an array subscript.  This is
 for backwards compatibility, but I figure nobody's relying on it"

I suppose backwards compatibility here is less important at this point.

> 
> While this works:
> 
> var=(hello); echo "${var[ ~0]}"
> hello

Because negative array subscripts count backwards from the end of the
array.

> It is also interesting that this fails:
> 
> var=hello; echo "${var[ ~0]}"
> bash: var: bad array subscript
> 
> Isn't `var[0]` valid and equivalent to `var` ?

Yes, but ~0 (-1) is not the same as 0.

> This was "supposed" to be resolved in a dev version,
> but is still present on bash 5.

The other arithmetic contexts you reported (the "pure" arithmetic contexts
the comment above references) were changed; this was left for backwards
compatibility. Like I said above, it looks like it's time to deemphasize
that.

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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