title: subprocess.check_output(): OSError file not found in Python
body: <p>Executing following command and its variations always results in an error, which I just cannot figure out:</p>
  
  <pre><code>command = "/bin/dd if=/dev/sda8 count=100 skip=$(expr 19868431049 / 512)"
  
  print subprocess.check_output([command])
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/lib/python2.7/subprocess.py", line 566, in check_output
      process = Popen(stdout=PIPE, *popenargs, **kwargs)
    File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
      errread, errwrite)
    File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
      raise child_exception
  OSError: [Errno 2] No such file or directory
  </code></pre>
  
  <p>WHich file it is referring to ? other commands like ls,wc are running correctly though, the command is also running well on terminal but not python script.</p>