#!/usr/bin/perl

use strict;

my $filename = "/proc/net/dev";

sub file_to_string ($)
{
        my $filename = shift;
        local $/;
        undef $/;
        open FILE, "< $filename" or return undef;
        my $string = <FILE>;
        $string = "" if ( not defined $string );
        close FILE;
        return $string;
}

sub get_samp
{
	my $stuff = file_to_string $filename;
	$stuff =~ /^\s*eth0:\s*(.*?)$/m or die "could not parse stuff ($stuff)";
	my @info = split(/\s+/, $1);
	return ($info[0], $info[8]);
}

my $sec = 5;

$sec = shift if ( $ARGV[0] =~ /^\d+$/ );

print "Sample period: $sec seconds\n";

my @this_samp;
my @last_samp;

@this_samp = get_samp();

while (1) {

	sleep $sec;

	@last_samp = @this_samp;
	@this_samp = get_samp();

	my $recei_bytes = $this_samp[0] - $last_samp[0]; # unit: bytes
	my $trans_bytes = $this_samp[1] - $last_samp[1]; # unit: bytes

	my $recei_rate = $recei_bytes / $sec * (8/(1024)); # unit: k bits / sec
	my $trans_rate = $trans_bytes / $sec * (8/(1024)); # unit: k bits / sec

	print sprintf(
		"Sec: %2d    Receive: %8.1f kbytes, %8.2f kbits/sec    Trans: %8.1f kbytes, %8.2f kbits/sec\n",
		$sec,
		$recei_bytes/1024, $recei_rate,
		$trans_bytes/1024, $trans_rate,
	);

	
}