title: create tar.gz file and name it using a variable
body: <p>I'm trying to create a tar.gz file of an XGBoost model by running a shell command using python. The following code works: </p>
  
  <pre><code>ts = strftime("%Y-%m-%d-%H-%M-%S", gmtime())
  model_name = "model-" + ts
  xgb._Booster.save_model(model_name)
  !tar czvf model.tar.gz $model_name
  </code></pre>
  
  <p>However, I want to use <code>model_name</code> in the naming of the tar.gz file, but can't figure out how to do that. The following does NOT work: </p>
  
  <pre><code>!tar czvf $model_name.tar.gz $model_name
  </code></pre>
  
  <p>How can I do this so that the value stored in <code>model_name</code> is the name of the tar.gz file I create?</p>