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Date:      Tue, 18 Feb 2003 11:14:48 -0500
From:      Chet Ramey <chet@nike.ins.cwru.edu>
To:        phk@phk.freebsd.dk
Cc:        current@freebsd.org
Subject:   Re: Shell programming 101:  Is this an expr(1) bug ?
Message-ID:  <030218161448.AA93484.SM@nike.ins.cwru.edu>
In-Reply-To: <"Message from phk"@phk.freebsd.dk>
References:  <57374.1045568362@critter.freebsd.dk>

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> Running:
> 
> 	#!/bin/sh
> 	set -ex
> 
> 	for p in ad2 ad0 ad1
> 	do
> 		a0=`expr $p : '^ad\([0-9]\)$'`
> 	done
> 
> I get:
> 
> 	syv# sh _
> 	+ expr ad2 : ^ad\([0-9]\)$
> 	+ a0=2
> 	+ expr ad0 : ^ad\([0-9]\)$
> 	+ a0=0
> 	syv# echo $?
> 	1

The `set -e' says to exit the shell if a simple command fails.  POSIX.2
says that the exit status of an assignment statement without an accompanying
command is either 0 or the exit status of the last command substitution
performed while expanding the rhs.  `expr' returns 1 when the result is
`0'.  Thus the assignment statement fails and the shell exits.

Chet

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
( ``Discere est Dolere'' -- chet )

Chet Ramey, ITS, CWRU    chet@po.CWRU.Edu    http://cnswww.cns.cwru.edu/~chet/

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