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From: wcs@homxb.UUCP (Bill Stewart HO 4K-437 x0705)
Newsgroups: net.unix
Subject: Re: Why do people use:  if [ "x$FOO" = "x" ] .... ?
Message-ID: <582@homxb.UUCP>
Date: Thu, 18-Apr-85 19:32:04 EST
Article-I.D.: homxb.582
Posted: Thu Apr 18 19:32:04 1985
Date-Received: Fri, 19-Apr-85 01:37:03 EST
References: <361@ho95b.UUCP>
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 27

Last week I posted an article asking why people use constructs
like the above instead of if [ -z "$FOO" ] or if [ "Known" = "$FOO" ]
Yeah, I should have worried about the case where $FOO has a value
special to test, such as "-z" or "=".  The most interesting reply
pointed out that the answers were different on 4.2BSD.

For both ksh and SysV /bin/sh, the = operator has higher precedence
than -z or -n.  For 4.2 BSD it doesn't.
	if [ -z = -z ] ; then echo true ; else echo false ; fi
On SysV and ksh, this prints "true"; on 4.2BSD it prints "false".
On the other hand,
	if [ -z = ] ; then echo true ; else echo false; fi
On 4.2BSD this prints "false"; on ksh and SysV it prints
	"sh: test: argument expected"

So, those leftover-from-v6 constructs with the "x$FOO" are
still useful.
		Bill Stewart, ho95c!wcs
-- 
"The {first,last} major program written in ADA will be a COBOL interpreter."
					Stewart, 1984
Bill Stewart
AT&T Bell Labs, Holmdel NJ
HO 4K-435 x0705   (201-949-0705)
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