title: Unbound variable not causing exit from subshell when set -eu
body: <pre><code>$ cat test.sh
  set -eu
  echo "`wc -l &lt; $DNE`"
  echo should not get here
  
  $ /bin/bash test.sh
  test.sh: line 2: DNE: unbound variable
  
  should not get here
  </code></pre>
  
  <p>I'm running bash version 4.1.2. Is there a way to make sure all such usage of unbound variables in subshells cause the script to exit without having to modify each call involving a subshell?</p>