Re: [PLUG] Distinguishing between environment variable with null value and one that is not set

[bergman@merctech.com]

In the message dated: Tue, 13 Oct 2009 15:52:24 EDT,
The pithy ruminations from "K.S. Bhaskar" on 
<[PLUG] Distinguishing between environment variable with null value and one tha
t is not set> were:
=> It appears that POSIX shell (and bash) have no built-in way to
=> distinguish between the case where an environment variable exists with
=> a null value and the case where it simply does not exist.  The

Yes, that seems to be the case.

Hmmm....I know that a null value is not the same as the variable not being set, 
but I'm having tough time thinking of a scenario where that difference is 
important, especially since the shell doesn't distinguish between the two 
states.

=> simplest way I have been able to come up with is to grep the output of
=> env and test for zero length, e.g.:

=> 
=> $ if [ -z "`env | grep foo`" ] ; then echo It doesn\'t exist\! ; fi
=> It doesn't exist!
=> $ if [ -z "`env | grep PWD`" ] ; then echo It doesn\'t exist\! ; fi
=> $
=> 
=> Is there a better way?  Thanks in advance.



I'd do something very similar:


	env | grep -q FOO
	if [ $? != 0 ] ; then
		echo "FOO is not set"
	else
		if [ -z $FOO ] ; then
			echo "FOO is set to NULL"
		else
			echo "FOO is \"$FOO\""
		fi
	fi


Is this really "better"? I don't know....but I find the use of "grep -q" to be 
"cleaner" and easier to examine visually than embedding a sub-shell and commands
within the test ([]) operator.

Mark


=> 
=> Regards
=> -- Bhaskar

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