title: What does this shell script do?
body: <pre><code>FROM_PROJECT=false
  if [[ -z $INV_NAME ]]; then
      if [[ -z $PRO_NAME ]]; then
              echo "error: variable not set" &gt;&amp;2
              exit 1
      elif [[ -z $INV_FILE ]]; then
              echo "error: variable INV not set" &gt;&amp;2
              exit 1
      fi
      FROM_PRO=true
  elif [[ -n $PRO_NAME ]] || [[ -n $INV_FILE ]]; then
      echo "error: variable INV_NAME is not compatible with PRO_NAME and             INVENTORY_FILE" &gt;&amp;2
      exit 1
  </code></pre>
  
  <p>i am not sure what these "-z" and "-n" doing here?</p>