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From: rpw3@redwood.UUCP (Rob Warnock)
Newsgroups: net.unix
Subject: Re: Why do people use:  if [ "x$FOO" = "x" ] .... ?
Message-ID: <201@redwood.UUCP>
Date: Wed, 17-Apr-85 04:57:59 EST
Article-I.D.: redwood.201
Posted: Wed Apr 17 04:57:59 1985
Date-Received: Sat, 20-Apr-85 03:06:46 EST
References: <361@ho95b.UUCP> <5046@ukc.UUCP>
Organization: [Consultant], Foster City, CA
Lines: 41

There is one case... uh, situation... in which the x$FOO construct (NO quotes!)
is useful, and that's when you are trying to allow partial matching. (But see
the caveat that follows.)

Suppose you are asking for confirmation and you want any of "y", "ye",
or "yes", or "yo" or anything else starting with "y" to mean "yes",
and anything starting with "n" to mean "no". (Yes, this is not good
human engineering, but gimme a break, for the example's sake!). Then
you would use:

	echo -n 'Do it? '	# echo 'Do it? \c' for System V
	read answer
	case X$answer in
	Xy*)	... yes ... ;;
	Xn*)	... no ... ;;
	*)	echo 'Please type yes or no"... ;;
	esac

Using "$answer" doesn't work unless you explicitly list ALL of the choices,
since "*" isn't expanded inside strings, so you have to do this:

	case "$answer" in
	"y"|"ye"|"yes"|"yep"|"yea"|"yeah"|"yo")	... yes ...;;
	"n"|"no"|"nope"|"nada"|"nah")		... no ...;;
	*)					echo 'I do not understand...' ;;
	esac

Be that as it may, I have started using the latter form myself, as the
X$answer form does not protect against (even accidental) typing of multiple
words in the answer. The "$answer" form does not blow up in this case,
and so is preferred (by fumble-fingers such as myself). I generally allow
only "y" and "yes" for YES and "n" and "no" for NO; I have found I get few
complaints from "nay"-sayers... ;-}


Rob Warnock
Systems Architecture Consultant

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