#!/bin/bash

VAL="111"
echo $VAL

if [ -z ${VAL+x} ] ; then
  echo not exists
else
  echo exists
fi

if test "$VAL" = "" ; then
  echo empty
else
  echo exists
fi

unset VAL
echo $VAL

if [ -z ${VAL+x} ] ; then
  echo not exists
else
  echo exists
fi

if test "$VAL" = "" ; then
  echo empty
else
  echo exists
fi

$ sh ./sample1.sh
111
exists
exists

not exists
empty