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From: Greg Wooledge <wooledg@eeg.ccf.org>
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Subject: Re: expression evaluation problem
Date: Wed, 24 Jul 2019 13:51:03 -0400
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Xref: csiph.com gnu.bash.bug:15233

On Wed, Jul 24, 2019 at 09:39:46AM -0700, L A Walsh wrote:
> str='cf80'
> v=960  uxtra=1  c=0

Irrelevant alias shenanigans omitted.  These are your variables.

> # In evaluating this expression:
>   ((v = v | ( uxtra>=++c ? ((0x${str:2*c:2}) & 63) << (6*(uxtra-c)) : 0 )))
> 
> 
> I get 985 and not 960 as expected
> 
> Which only happens when 'c' is 0 in the middle 'str' expression,
> but the ++c should increment 'c' to '1' before it is tested to be
> less than or equal to 'uxtra'(=1).

The ${str:2*c:2} part is performed first, while c is still 0, and it
expands to "cf".

Only then does the arithmetic evaluation begin.

wooledg:~$ str='cf80'
wooledg:~$ v=960  uxtra=1  c=0
wooledg:~$ ((v = v | ( uxtra>=++c ? ((0xcf) & 63) << (6*(uxtra-c)) : 0 )))
wooledg:~$ declare -p v
declare -- v="975"

Even with that parameter expansion out of the way, this arithmetic
command is still ridiculously over-complicated.  I can't even guess
what it's supposed to do.

Have you considered performing your calculation in steps, with
intermediate values stored in temporary variables with clear names?
That greatly improves readability.

Isolating the ++c into its own step would also remove all questions
about whether the increment is performed before or after other
calculations (or in this case, parameter expansions).  In-lining ++c
inside a larger calculation can be OK in very simple situations, but
a nightmare to read/understand/debug in more complex cases.