title: Airflow: Simple DAG with one task never finishes
body: <p>I have made a very simple DAG that looks like this:</p>
  
  <pre><code>from datetime import datetime
  from airflow import DAG
  from airflow.operators.bash_operator import BashOperator
  
  cleanup_command = "/home/ubuntu/airflow/dags/scripts/log_cleanup/log_cleanup.sh "
  
  dag = DAG(
  'log_cleanup',
  description='DAG for deleting old logs',
  schedule_interval='10 13 * * *',
  start_date=datetime(2018, 3, 30),
  catchup=False,
  )
  
  t1 = BashOperator(task_id='cleanup_task', bash_command=cleanup_command, dag=dag)
  </code></pre>
  
  <p>The task finishes successfully but despite of this the DAG remains in "running" status. Any idea what could cause this. The screenshot below show the issue with the DAG remaining running. The earlier runs are only finished because I manually mark status as success. [Edit: I had originally written: "The earlier runs are only finished because I manually set status to running."] </p>
  
  <p><a href="https://i.stack.imgur.com/B6shD.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/B6shD.jpg" alt="Screenshot showing that status of task is finished but DAG is still running"></a></p>