title: how to smart append LD_LIBRARY_PATH in shell when nounset
body: <p>In following shell, error shows <strong>LD_LIBRARY_PATH: unbound variable</strong> if the LD_LIBRARY_PATH not set.</p>
  
  <p>Can I use similar usage like ${xxx:-yyy} to simplified it.</p>
  
  <pre><code>#!/bin/bash
  set -o nounset
  export LD_LIBRARY_PATH=/mypath:$LD_LIBRARY_PATH
  </code></pre>