#!/bin/bash

old-way() {
	local OX=$X SX=${X-y} X=$X

	X="blah"

	if [ "$SX" = y ]; then
		X=$OX
	else
		unset X
	fi
}


new-way() {
	local OX=$X SX=${X-n}

	X="blah"

	if [ "$SX" = n ]; then
		unset X
	else
		X=$OX
	fi
}


baseline() {
	X="blah"
}


echo "X unset - using old way"
unset X
old-way
ANSWER=${X+"X is set"}
[[ -z ${ANSWER} ]] && echo "X is not set" || echo "X is set to $X"

echo
echo

echo "X set - using old way"
X="A"	
old-way
ANSWER=${X+"X is set"}
[[ -z ${ANSWER} ]] && echo "X is not set" || echo "X is set to $X"

echo
echo

echo "X unset - using new way"
unset X
new-way
ANSWER=${X+"X is set"}
[[ -z ${ANSWER} ]] && echo "X is not set" || echo "X is set to $X"

echo
echo

echo "X set - using new way"
X="A"	
new-way
ANSWER=${X+"X is set"}
[[ -z ${ANSWER} ]] && echo "X is not set" || echo "X is set to $X"

echo
echo

echo "X unset - baseline"
unset X
baseline
ANSWER=${X+"X is set"}
[[ -z ${ANSWER} ]] && echo "X is not set" || echo "X is set to $X"

echo
echo

echo "X set - baseline"
X="A"	
baseline
ANSWER=${X+"X is set"}
[[ -z ${ANSWER} ]] && echo "X is not set" || echo "X is set to $X"