Re: Airthmetic confusion...

[Meino Christian Cramer <Meino.Cramer@xxxxxx>]

From: Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>
Subject: Re: Airthmetic confusion...
Date: Tue, 17 May 2005 03:48:52 +0000

Thanks a lot for your explicit explanations.

Meino


> On May 17,  5:13am, Meino Christian Cramer wrote:
> }
> }  I see that "0" is false and !0 is true...but...we
> }  are doing arithmetic and not logical evaluation.
> 
> No, that's not true.
> 
> First of all, the closest thing to "logical" evaluation that the
> shell does is the &&, ||, etc., subexpressions WITHIN arithmetic
> expressions.  The && and || constructs that test process success
> or failure are NOT "logical" operators -- for one thing, they do
> not obey the usual Boolean precedence rules.  "set -e" is not a
> "logical" comparison by any stretch.
> 
> Second, when you write (( expression )) then you are explicitly
> instructing the shell to do BOTH arithmetic evaluation AND shell
> expression success/failure.  That's what the double parens MEAN,
> when not preceded by a dollar sign.
> 
> If you want ONLY arithmetic evaluation, use $(( expression )),
> and apply the colon command if necessary, e.g.
> 
>     : $(( x = a - b ))
> 
> or
> 
>     x=$(( a - b ))
> 
> Those are both successful shell expressions that result in zero-
> value assignments to x.
> 
> As with any other programming language, you have to write exactly
> what you mean, and you have to follow the language's semantic rules,
> not the rules as you think they should be.
> 
> }  As a sideffect a previously existant variable become inexistant by
> }  assigning a "0"?
> 
> What makes you believe that's happening?  Assignments can only create
> variables, they can't unset them.
>