title: How to run a task when variable is undefined in ansible?
body: <p>I am looking for a way to perform a task when ansible variable is not registers /undefined e.g </p>
  
  <pre><code>-- name: some task
     command:  sed -n '5p' "{{app.dirs.includes}}/BUILD.info" | awk '{print  $2}'
     when: (! deployed_revision) AND ( !deployed_revision.stdout )
     register: deployed_revision
  </code></pre>