title: What is this bash parameter expansion syntax?
body: <p>I've never seen this <code>+x}</code> syntax before and I can't find docs about it. What does it do?</p>
  <pre class="lang-sh prettyprint-override"><code>if [[ -z &quot;${EMPLOYEE_CLUSTER+x}&quot; ]]; then
    export EMPLOYEE_CLUSTER=shared
  fi
  </code></pre>