Avoiding shell metacharacters in os.popen

Nick Craig-Wood nick at craig-wood.com
Wed Sep 29 17:30:21 CEST 2004

I'm trying to avoid using shell metacharacters in os.popen in a portable

os.popen() only seems to take a string as the command which would need
tricky quoting.

os.popen2() can take a string or a list - the relevant code in Unix
python (in popen2.py) being...

    def _run_child(self, cmd):
        if isinstance(cmd, basestring):
            cmd = ['/bin/sh', '-c', cmd]
        for i in range(3, MAXFD):
            except OSError:
            os.execvp(cmd[0], cmd)

This is perfect behaviour as far as I'm concerned - if you pass a list
it doesn't go through the shell and if you pass a string it does.


  w, r = os.popen2(["ls", "-l"])

This leads on to my questions :-

1) is this behaviour (string vs list) of popen2 intentional?  Its not
documented anywhere and it doesn't work under Windows (gives
"TypeError: popen2() argument 1 must be string, not list")

2) is there anything similar for os.popen() planned?

3) is there an equivalent to the perl quotemeta() command.  This
quotes meta-characters in a string for use in the shell.  Here is a
suitable definition for Unix, but I don't think that \ quoting works
in Windows

   cmd = re.sub(r"(\W)", r"\\\1", cmd)

Avoiding shell metacharacter attacks is a must for secure programs.
Python does pretty well with its os.exec* and os.spawn* functions, but
seems to be lacking in the os.popen* department!

Any insights appreciated!

Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick

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