title: Problem using os.system() with sed command
body: <p>I'm writing a small method to replace some text in a file.
  The only argument I need is the new text, as it is always the same file and text to be replaced.</p>
  
  <p>I'm having a problem using the os.system() call, when I try to use the argument of the method</p>
  
  <p>If I use a string like below, everything runs ok:</p>
  
  <pre><code>stringId = "GRRRRRRRRR"
  cmd="sed '1,$s/MANAGER_ID=[0-9]*/MANAGER_ID=" + stringId + "/g' path/file.old &gt; path/file.new"
  os.system(cmd)
  </code></pre>
  
  <p>Now, if i try to give a string as a parameter like below, the command is not executed.
  I do a print to see if the command is correct, and it is. I can even execute it with success if I copy / paste to my shell</p>
  
  <pre><code>import os
  def updateExportConfigId(id):
  stringId = "%s" % id
  cmd= "sed '1,$s/MANAGER_ID=[0-9]*/MANAGER_ID=" + stringId + "/g' path/file.old &gt; path/file.new"
  print "command is " + cmd
  os.system(cmd)
  </code></pre>
  
  <p>Does anyone knows what is wrong?</p>
  
  <p>Thanks</p>