title: Difference between gcloud auth activate-service-account --key-file and GOOGLE_APPLICATION_CREDENTIALS
body: <p>I'm creating a shell script to handle automation for some of our workflows, 
  This workflow include accessing Google Buckets via Apache Beam GCP.
  I'm using a .json file with my service account, in which situations do i need to use:</p>
  
  <pre><code> gcloud auth activate-service-account --key-file myfile.json
  </code></pre>
  
  <p>vs</p>
  
  <pre><code>export GOOGLE_APPLICATION_CREDENTIALS=myfile.json
  </code></pre>