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From: Greg Wooledge <wooledg@eeg.ccf.org>
Newsgroups: gnu.bash.bug
Subject: Re: Inconsistent arithmetic evaluation of parameters
Date: Tue, 1 Sep 2015 16:23:08 -0400
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On Tue, Sep 01, 2015 at 03:13:57PM -0500, Dennis Williamson wrote:
> The version of dash I have handy (0.5.7) has math support which IMHO is
> broken:
> 
> $ foo=bar
> $ bar=5
> $ echo $foo
> bar
> $ echo $((foo))
> dash: 4: Illegal number: bar
> $ echo $(($foo))
> 5
> $ echo $((bar))
> 5
> $ echo $(($bar))
> 5
> 
> Note the inconsistency in support of omitting the inner dollar sign.

$foo is expanded to bar, so the following two lines are always going to
be equivalent:

echo $(($foo))
echo $((bar))

POSIX also specifies (vaguely!!) that $((x)) and $(($x)) are equivalent.