[bash] unset 변수와 empty 값 변수 구별

if [ -z "${VAR}" ]; then
    echo "VAR is unset or set to the empty string"
fi
if [ -z "${VAR+set}" ]; then
    echo "VAR is unset"
fi
if [ -n "${VAR+set}" ]; then
    echo "VAR is set, possibly to the empty string"
fi
if [ -n "${VAR+set}" ] && [ -z "${VAR}" ]; then
    echo "VAR is set to the empty string"
fi
if [ -n "${VAR}" ]; then
    echo "VAR is set to a non-empty string"
fi