python - Problem using os.system() with sed command

Question

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.

I'm having a problem using the os.system() call, when I try to use the argument of the method

If I use a string like below, everything runs ok:

stringId = "GRRRRRRRRR"
cmd="sed '1,$s/MANAGER_ID=[0-9]*/MANAGER_ID=" + stringId + "/g' path/file.old > path/file.new"
os.system(cmd)

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

import os
def updateExportConfigId(id):
stringId = "%s" % id
cmd= "sed '1,$s/MANAGER_ID=[0-9]*/MANAGER_ID=" + stringId + "/g' path/file.old > path/file.new"
print "command is " + cmd
os.system(cmd)

Does anyone knows what is wrong?

Thanks

Answer 1

Obligatory: don't use os.system - use the subprocess module:

import subprocess

def updateExportConfigId(m_id, source='path/file.old', 
                             destination='path/file.new'):
    if isinstance(m_id, unicode):
        m_id = m_id.encode('utf-8')
    cmd= [
          "sed",
          ",$s/MANAGER_ID=[0-9]*/MANAGER_ID=%s/g" % m_id,  
          source,
         ]
    subprocess.call(cmd, stdout=open(destination, 'w'))

with this code you can pass the manager id, it can have spaces, quote chars, etc. The file names can also be passed to the function, and can also contain spaces and some other special chars. That's because your shell is not unnecessarly invoked, so one less process is started on your OS, and you don't have to worry on escaping special shell characters.

Another option: Don't launch sed. Use python's re module.

import re
def updateExportConfigID(m_id, source, destination):
    if isinstance(m_id, unicode):
        m_id = m_id.encode('utf-8')
    for line in source:
        new_line = re.sub(r'MANAGER_ID=d*', 
                          r'MANAGER_ID=' + re.escape(m_id), 
                          line)
        destination.write(new_line)

and call it like this:

updateExportConfigID('GRRRR', open('path/file.old'), open('path/file.new', 'w'))

No new processes needed.