title: How do I access a task output variable in Azure Dev Ops
body: <p>I am trying to use the new multistage yaml pipelines to download an artifact from a specific build.  The <code>DownloadBuildArtifacts@0</code> task has an output variable for <code>BuildNumber</code> that I would like to reference further on, but I can't figure out how to access it.</p>
  
  <p>In the GUI for the task there is a box that let's you set a reference name
  <a href="https://i.stack.imgur.com/TDPZX.png" rel="noreferrer"><img src="https://i.stack.imgur.com/TDPZX.png" alt="enter image description here"></a></p>
  
  <p>But from the yaml pipeline definition I can't figure out how access this variable.</p>
  
  <pre><code>- task: DownloadBuildArtifacts@0
    inputs:
      buildType: 'specific'
      project: 'Sandbox'
      pipeline: 'bash-testing'
      buildVersionToDownload: 'latest'
      downloadType: 'single'
      artifactName: 'someArtifactName'
      referenceNames: 'blahblah'
  - bash: echo $BLAHBLAH_BUILDNUMBER
  </code></pre>
  
  <p>Simply outputs</p>
  
  <p><a href="https://i.stack.imgur.com/8SWjl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/8SWjl.png" alt="enter image description here"></a></p>