Config file parsing tutorial?

tombmbdil has asked for the wisdom of the Perl Monks concerning the following question:

Wise Monks,

I spend much of my time mucking about with dhcp data. I'm also in the process of trying to learn OO perl. So, I figured I would practice my class-building skills by creating a DHCP module for myself which will gather various lease/network information.

I find parsing the dhcpd.leases file to be pretty easy because the data is nice and predictable. The human-generated dhcpd.conf file, on the other hand, is a nightmare. I have figured out a way to gather some of the information I need, but I know there must be some established algorithm that is much more elegant. I've browsed a few of the configuration file parsing modules on cpan to search for ideas, but either they aren't sufficiently commented, or they deal with simpler configuration schemes (as far as I can tell).

Does anyone know of an online tutorial somewhere that covers configuration file parsing algorithms?

The current bit of code I have works for what I need right now, but I know it's extremely oversimplified and prone to break under various conditions.

Here's a simplified snippet of what I have:

sub new
{
    my $invocant = shift;
    my $class = ref($invocant) || $invocant;
    
    my $self =
    {
        # Paths for the leases and conf files
        lease_file => '/var/state/dhcp/dhcpd.leases',
        conf_file => '/etc/dhcpd.conf',
            
        # Hash keyed by dhcpd.conf configuration option
        # Values contain references to subroutines for handling
        # each option
        conf_fields =>
        {
            host => \&_host,    # Static hosts
        },
        
        # Hash keyed by MAC address
        # Values contain array of lease arrays
        leases => { },
        
        @_,            
    };
    
    # Load static hosts
    &_init_conf($self);
    
    bless ($self, $class);    
    return $self;
    
}

# Initialize static hosts
sub _init_conf
{
    my $self = shift;
    open CONF, $self->{'conf_file'} or die "Couldn't open ",
                                        $self->{'conf_file'}, " $!";  
+                                      
    my $option;

    while (<CONF>)
    {
        # If the current line contains an open bracket grab the first 
+word
        # as the option key
        $option = $1 if $_ !~ /^\s*#/ && /^\s*(\w+).*{/;
        
        # Call the subroutine associated with the current option
        # Must eval this call in case no such subroutine exists.
        eval { $self->{'conf_fields'}{$option}->($self, $_) } if $opti
+on;
        
        # Clear option key if closing bracket
        $option = undef and eval { $self->{'conf_fields'}{$option}->($
+self, $_) }
                 if $_ !~ /^\s*#/  && /}/;    
    }
    close CONF;

}

# Method to handle static host declarations
{
my $mac, $ip;

    sub _host
    {
        my $self = shift;
        
        # Current line of dhcpd.conf
        $_ = shift; 
        
        # Grab MAC address
        $mac = mac_normal($1) if /\s*hardware ethernet (\w+:\w+:\w+:\w
++:\w+:\w+)/;
        
        # Grab IP
        $ip = $1 if /\s*fixed-address (\d+.\d+.\d+.\d+)/;
        
        # Push entry onto array of leases for host's MAC
        # and clear $mac, $ip variables
        push @{$self->{'leases'}{$mac}}, [ $ip, 'static'] and ($ip, $m
+ac) = ('', '')
             if /}/ and $ip && $mac;
    }

}
[download]

--------------------------
Sample Static Host:

host camel {
    hardware ethernet 00:AA:12:BB:A1:34;    
    filename "/default.cfg";
     fixed-address 10.1.2.3;
}
[download]

Comment on Config file parsing tutorial? Select or Download Code

Replies are listed 'Best First'.
Re: Config file parsing tutorial? by steves (Curate) on Feb 17, 2003 at 21:54 UTC
You might be able to get the data you need directly from the DHCP server instead of parsing config. files by using Net::DHCPClient.	[reply]
Re: Config file parsing tutorial? by Koschei (Monk) on Feb 18, 2003 at 10:48 UTC
I was doing some parsing in a similar domain the other year. I was parsing a file of info from which I wanted to create a dhcpd.conf file =) I found Parse::RecDescent to be an excellent parser. I thoroughly recommend it. It may not be the swiftest module on the block, but it's certainly one of the most powerful (well, until Damian releases his perl6-style parser). I found the speed acceptable since changing my conf wasn't something I did every month. -- Iain, aka Spoon.	[reply]
Re: Config file parsing tutorial? by traveler (Parson) on Feb 17, 2003 at 23:43 UTC
The file dhcpd-lib.pl that comes with Webmin (Webmin is BSD License) is a perl library for parsing the dhcpd.conf file. Also I found this on google. That should get you started. --traveler	[reply]
Re: Re: Config file parsing tutorial? by tombmbdil (Beadle) on Feb 18, 2003 at 06:18 UTC
Those were good links, thanks. The code in the google link isn't really generic enough for what I need, but I've been looking at the Webmin code and hopefully I can pull out some of the concepts if I stare at it long enough.	[reply]


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