title: Can't call a variable inside os.system method in Python
body: <pre><code>ENV=raw_input("Enter Environment (QA/Prod):")
  print(ENV) 
  os.system('aws ec2 describe-instances --filters "Name=tag:Environment,Values=ENV" "Name=instance-state-code, Values=16" &gt; FilteredOP')
  </code></pre>
  
  <p>Hi,
  I am quite noob to Python. Here,
  I cannot able to call ENV variable in os.system command. Is there anything wrong with the syntax.  </p>