overwrite-sysctl.py

#!/usr/bin/env python

import os
import re
import sys

if 2 != len(sys.argv):
    print "Usage: %s <filename>" % sys.argv[0]
    exit(1)

want_to_set = {'net.ipv4.ip_forward': '1'}


# get currently set variables
def get_custom_variables(filename):
    custom_var = {}
    f = open(filename)
    while True:
        line = f.readline()
        if not line:
            break
        line = line.strip()
        if re.match('^# eub', line) or re.match('^#.*', line):
            continue
        splitted = line.split('=')
        if 2 == len(splitted):
            (key, value) = [i.strip() for i in splitted]
            custom_var[key] = value
    f.close()
    return custom_var


# add currently set variables
for key, value in get_custom_variables(sys.argv[1]).iteritems():
    if key in want_to_set:
        continue
    else:
        want_to_set[key] = value


f = open(sys.argv[1])
while True:
    line = f.readline()
    if not line:
        break
    line = line.strip()
    # when there is a newline only
    if 0 == len(line):
        print
        continue
    if re.match('^# eub', line):
        break
    # do not modify comments
    if re.match('^#.*', line):
        print line
    else:
        splitted = line.split('=')
        if 2 == len(splitted):
            key = splitted[0].strip()
            if key in want_to_set:
                print "#%s" % line
            else:
                print line

f.close()
print "# eub"
for k, v in want_to_set.iteritems():
    print "%s=%s" % (k, v)

The reason why I don't read a file reversely is that the "# eub" comment(delimiter) may not exist.