How to change my script to replace and add new variable in a configuration?

Question

Well this is my script. It is to configure my systems sysctl.conf.

infile = open('sysctl.conf')
outfile = open('sysctl.conf.new', 'w')

replacements = {'Net.ipv4.icmp_echo_ignore_all' :'1',
            'Net.ipv4.icmp_echo_ignore_broadcasts' :'1',
            'Net.ipv4.ip_forward' : '0',
            'Net.ipv4.tcp_syncookies':'1',
            'Net.ipv4.conf.all.rp_filter': '1',
            'Net.ipv4.conf.all.Log.martiansd':'1',
            'Net.ipv4.conf.all.Secure_redirects' : '1',
            'Net.ipv4.conf.all.Send_redirects' : '0',
            'Net.ipv4.conf.all.Accept_Source_Route':  '0',
            'Net.ipv4.conf.all.Accept_redirects':'0',
            'Net.ipv4.tcp_max_syn_backlog': '4096',
            }

for line in infile:
    if '#' in line:
        pass
    elif '=' in line:
        w = line.split('=')
        for var, value in replacements.iteritems():
            if var in w[0]:
                line=line.replace(w[1],value)
    outfile.write(line)
infile.close()
outfile.close()

This script works fine but there is one problem. If any of the parameters in replacement is not present in sysctl.conf then it is not going to add it in the new configuration file.It only modifies the parameters present with my values. I want to add all parameters in the configuration or change if they are already present. How to do it?

I know it should be easy but I am stuck here.

Answer 1

I would probably do something like this:

testing = True

if testing: ##################################################################

    infile = '''
key0=0
key1=1
 key1 = 1
key2=2 # comment1
#key3=3
  #key4=4
#key5=5 # comment
  #key6=6 # comment
key7=7

key8 = 8
    '''

    infilelines = infile.split('\n')


    class of():
        def write(self, s):
            print s
        def close(self):
            pass
    outfile = of()

    replacements = {
        'key1' :'11repl',
        'key2' :'22repl',
        'key3' :'33repl',
        'key4' :'44repl',
        'key5' :'55repl',
        'key6' :'66repl',
        }


else: #########################################################################

    # as proposed by csny, only open file quickly
    # (file is closed after with statement)
    with open('sysctl.conf') as infile:
        infilelines = infile.readlines()

    outfile = open('sysctl.conf.new', 'w')

    replacements = {'Net.ipv4.icmp_echo_ignore_all' :'1',
        'Net.ipv4.icmp_echo_ignore_broadcasts' :'1',
        'Net.ipv4.ip_forward' : '0',
        'Net.ipv4.tcp_syncookies':'1',
        'Net.ipv4.conf.all.rp_filter': '1',
        'Net.ipv4.conf.all.Log.martiansd':'1',
        'Net.ipv4.conf.all.Secure_redirects' : '1',
        'Net.ipv4.conf.all.Send_redirects' : '0',
        'Net.ipv4.conf.all.Accept_Source_Route':  '0',
        'Net.ipv4.conf.all.Accept_redirects':'0',
        'Net.ipv4.tcp_max_syn_backlog': '4096',
        }



for line in infilelines:

    # if # at the beginning (neglecting whitespaces): its only a line comment
    # write it directly to outfile and continue with next line
    if len(line.strip())==0 or line.strip()[0] == '#':
        outfile.write(line.strip())
        continue

    # try if this is a properly formated line like: key=val
    try:
        key, val = line.split('=')
        key = key.strip()
        val = val.strip()

    # something stange happend: It was not a proper key=val line
    # dont modify anything, just write the line to the new file
    except ValueError:
        # or comment out outfile.write to delete the strange line
        # from the output config file
        outfile.write(line)
        continue

    # maybe you want to allow line end comments like: key=val # comment?
    # lets try if the value actually contains a comment
    try:
        val, comment = val.split('#')
        comment = '# ' + comment.strip()
        val = val.strip()

    # there is no comment at the end of the line
    # (the val.split() returns only one value and thus the unpacking fails with:
    # ValueError: need more values to unpack)
    except ValueError:
        comment = ''


    # replace the val if the according key in the `replacements` dict
    # with the value stored in the key
    # otherwise don't change anything
    if key in replacements.keys():
        val = replacements[key]

    # put together the new line for the output file
    line = '%s=%s   %s' % (key, val, comment)
    outfile.write(line)

outfile.close()

See comments in the code. This takes the config line apart, changes the value if the key exists in the dict, and finally reassenbles the line to print to the out file. This also allows to have comments at the line endings.

Answer 2

Few comments about your code:

1. If you want to easily insert configuration from "replacements" if it doesn't exist, consider iterating each "replacement" over the lines of the file instead of each line of the file over "replacements". This way, if you don't find any replacement key in the lines, construct and write a new configuration line.
2. I would rather use "readlines()" and close sysctl.conf in order to free the sysctl.conf file instead of holding it open. If other processes use sysctl, while you're running this, it might harm your configurations.
3. You're parsing sysctl.conf file, which contains lines either starting with a "#", or as key = value. no comments after values exist.