Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.unix.questions
Subject: Re: Bourne Shell: Variable defined?
Keywords: Bourne shell, elementary question
Message-ID: <9899@smoke.BRL.MIL>
Date: 22 Mar 89 04:38:30 GMT
References: <1819@umbc3.UMBC.EDU>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 18

In article <1819@umbc3.UMBC.EDU> rostamia@umbc3.UMBC.EDU (Rouben Rostamian) writes:
>In C Shell the variable $?foo returns true or false (0 of 1, really)
>depending on whether or not the variable "foo" has been already defined.
>What is the equivalent construction in Bourne Shell?  In other words,
>how can I tell, in Bourne Shell, whether the variable "foo" exits.

Usually all you need is
	if [ X"$foo" = X ]
	then	:	# doesn't exist or is an empty string
	else	:	# exists as a nonempty string
	fi
The X is to avoid problems if $foo starts with a dash etc.
If that's not a problem then
	if [ "$foo" ]
	then	:	# nonempty
	else	:	# empty or nonexistent
	fi
is a simpler test.