title: What does [[ -z "path" ]] mean in bash
body: <p>I found this line in a bash script:</p>
  
  <pre><code>if [[ -z "$VIRTUAL_ENV" ]]; then
      source ./env/bin/activate
  fi
  </code></pre>
  
  <p>I was wondering what it meant?</p>
  
  <p>(I hope that's not a duplicate question, but it's very hard to google bash commands as they're single letters)</p>