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From: Chet Ramey <chet.ramey@case.edu>
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Subject: Re: Inconsistent arithmetic evaluation of parameters
Date: Wed, 2 Sep 2015 10:23:33 -0400
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On 9/1/15 5:05 PM, Dennis Williamson wrote:

> It's the line above those two where I demonstrate the failure in the
> indirection and equivalency.
> 
> $ echo $((foo))  # expansion succeeds, indirection fails
> dash: 4: Illegal number: bar

Because `foo' is expanded, but the value is not treated as an expression.
The dash arithmetic evaluator never treats values resulting from expansion
as expressions, only as constants.

> $ echo $(($foo))  # both expansion and indirection succeed
> 5

Because the arithmetic evaluator sees $(( bar )).  The word expansion that
happens before arithmetic evaluation expands $foo.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04

is the Posix specification for arithmetic evaluation.
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/