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Date: Wed, 31 Aug 2005 13:42:05 -0500
From: Dan Nelson <dnelson@allantgroup.com>
To: Gary Kline <kline@tao.thought.org>
Message-ID: <20050831184205.GC16354@dan.emsphone.com>
References: <20050831181545.GA57907@thought.org>
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Cc: FreeBSD Mailing List <freebsd-questions@freebsd.org>
Subject: Re: Doing a modulo in /bin/sh??
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In the last episode (Aug 31), Gary Kline said:
> 	I can grab the results of "w=$date+%U)"; in C an use the modulo
> 	operator; is there a way to do this is /bin/sh?  ot zsh?
> 
> #/bin/sh
> w=$(date +%U)
> echo "w is $w";
> (even=$(w % 2 ));       ## flubs.
> echo "even is $even";       ## flubs.
> 
> if [ $even -eq 0 ]       ## flubs, obv'ly.
> then
>         echo "week is even";
> else
>         echo "week is odd";
> fi

For the simple even/odd case, you can AND with 1:

even=$(( w & 1 ))

For the general case:

xmodn=$(( x - ((x / n) * n) ))

which works since sh's arithmetic evaluator is integer-only.

zsh has the % modulo operator, so xmod=$(( x % n )) .

-- 
	Dan Nelson
	dnelson@allantgroup.com