title: Default value for a $PROPERTY in Gradle
body: <p>How can I specify a default value for this simple <code>build.gradle</code> script:</p>
  
  <pre><code>println "Hello $build_version" 
  </code></pre>
  
  <p>So that I don't get the error:</p>
  
  <pre><code>A problem occurred evaluating root project 'hello_gradle'.
  &gt; Could not find property '$build_version' on root project 'hello_gradle'.
  </code></pre>
  
  <p>I tried some of the operators, checking for nulls etc, but I think just the reference to the property makes it fail. I could fix that by always providing the property, but that's less than ideal. </p>
  
  <pre><code>gradle -Pbuild_version=World
  </code></pre>