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Re: Curious case of arithmetic expansion


From: Chet Ramey
Subject: Re: Curious case of arithmetic expansion
Date: Sun, 23 Apr 2017 14:02:16 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.0.1

On 4/23/17 8:28 AM, Florian Mayer wrote:
> What I’m saying is, that if bash does recursively apply expansion
> mechanisms on the identifiers until it can retrieve a number, 
> it should do it symmetrically. 

That's not a reasonable expectation.

Here's how it works. When bash reads a token, such as `foo', it reads
it as a string.  In contexts where it needs a variable name, such as
the left side of an assignment, it remains a string. When it needs to
be treated as a value, such as on the right side of an assignment, its
value is expanded as an expression until it results in a number (unset
variables evaluate to 0).

So, foo++ is essentially equivalent to

        ofoo=foo, foo=ofoo+1, ofoo

When foo's value is needed, it is evaluated. When foo needs to be assigned
a value, it is assigned a value.  You're arguing for non-determinism when
assigning a value based on whether or not a variable happens to contain a
string. Unless you specifically indicate you want something like that, like
when using namerefs, it's not reasonable.


> That is,
> it should remember what chain of expansion had been necessary for
> a particular number to appear at the end of the expansion.
> 
> So instead of 
> 124 moo 123
> The echo command should produce
> bar moo 124
> 
> (The expansion chain here was foo->bar->moo->123)
> 
>> It's because it's not really indirection, rather the content of the
>> variable is evaluated:
> No it is really indirection. Bash even has a special (and very limited)
> syntax for that.

It's not indirection, and I am not sure why you show the completely
different variable indirection syntax.  The mental model you're using
may equate the two, but they are not the same.

-- 
``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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