env_test.rb

#!/usr/bin/env ruby

script = <<-BASH
if [ ! -n "${foobar+x}" ]; then \
  echo "foobar is unset"; \
elif [[ ${!foobar[@]} ]]; then \
  if [[ $foobar ]]; then \
    echo "foobar is set"; \
  else \
    echo "foobar is set, but empty"; \
  fi \
fi
BASH

ENV['foobar'] = nil
system('/bin/bash', '-c', script)

ENV['foobar'] = ''
system('/bin/bash', '-c', script)

ENV['foobar'] = 'foobar'
system('/bin/bash', '-c', script)