#!/usr/bin/perl -w
use English;
use strict;
use vars qw( $INET_ADDR_FILE );

$INET_ADDR_FILE = "/home/adamm/.current_inet_addr";
die "only currently implemented for linux." unless ($OSNAME eq 'linux');


### MAIN

my ($old_inet_addr, $found_old_addr_file) = read_saved_inet_addr();
my $new_inet_addr = read_new_inet_addr();

if (!$found_old_addr_file)
{
  write_new_inet_addr($new_inet_addr);
  print "Current IP address ($new_inet_addr) stored to disk.\n";
}
elsif ($old_inet_addr ne $new_inet_addr)
{
  write_new_inet_addr($new_inet_addr);
  print <<OutputEnd;
    Change in IP address detected. New IP stored to disk.
    Old IP: $old_inet_addr
    New IP: $new_inet_addr
OutputEnd
}


### SUBS

sub read_saved_inet_addr {
  my ($fh, $oldfile_found, $old_addr);

  open($fh, "< $INET_ADDR_FILE")
    ? $oldfile_found = 1
    : warn "couldn't open $INET_ADDR_FILE. Hopefully this is the first run.\n";

  if ($oldfile_found)
  {
    $found_old_addr_file = 1;
    local $INPUT_RECORD_SEPARATOR = undef;
    $old_addr = <$fh>;
    $old_addr =~ s/\r|\n//;
    close($fh) or die "couldn't close $INET_ADDR_FILE";
  }

  return ($old_addr, $oldfile_found);
}

sub read_new_inet_addr {
  local $INPUT_RECORD_SEPARATOR = undef;
  my $ifconfig_output = `/sbin/ifconfig eth0`;
  my ($new_inet_addr) = ($ifconfig_output =~ m/inet\saddr:(.+?)\s/);
  return $new_inet_addr;
}

sub write_new_inet_addr {
  my $new_addr = shift;
  my $fh;
  open($fh, "> $INET_ADDR_FILE") or die "couldn't create $INET_ADDR_FILE";
  print $fh $new_addr,"\n";
  close($fh) or die "couldn't close $INET_ADDR_FILE";
}