title: how to implement airflow DAG in a loop
body: <p>I just started with Airflow. I want to set up a DAG in a loop, where the next DAG starts when the previous DAG is completed. Here is the work flow that I want to achieve:</p>
  
  <pre><code>list_of_files = [......]
  for file in list_of_files:
     dag = DAG('pipeline', default_args=default_args, schedule_interval=None)
     t1 = BashOperator('copy_this_file', ....)
     t2 = BashOperator('process_this_file', ...)
     t1.set_downstream(t2)
  </code></pre>
  
  <p>If I run <code>airflow backfill pipeline -s 2019-05-01</code>, all the DAGs are started simultaneously. </p>