During some QoS tests on Linux I needed to measure the traffic of the system in realtime without being able to compile any new software on it. The system had already perl installed so I googled to find a script that could monitor in/out traffic of an interface. The first script I found was this: http://perlmonks.org/?node_id=635792
While it’s actually doing what it says, it only runs just once. I wanted the script to run for a period of time. So I changed it a bit.
Here’s the outcome:
#!/usr/bin/perl
my $dev=$ARGV[0];
sub get_measures {
my $data = `cat /proc/net/dev | grep "$dev" | head -n1`;
$data =~ /$dev\:(\d+)\D+\d+\D+\d+\D+\d+\D+\d+\D+\d+\D+\d+\D+\d+\D+(\d+)\D+/;
my $recv = int($1/1024);
my $sent= int($2/1024);
return ($recv,$sent);
}
my @m1 = get_measures;
while(1) {
sleep 1;
my @m2 = get_measures;
my @rates = ($m2[0] - $m1[0], $m2[1]-$m1[1]);
foreach ('received' , ' transmit') {
printf "$_ rate:%sKB",shift @rates;
}
print "\n";
@m1=@m2;
}
I’ve changed it so that it’s:
a) running continuously until someone presses ctrl+c to stop it,
b) parsing the /proc/net/dev output instead of the ifconfig output. I think this is more efficient/fast than parsing the ifconfig output.
Sample output:

$iftraffic.pl eth0
received rate:1564KB transmit rate:71KB
received rate:1316KB transmit rate:44KB
received rate:1415KB transmit rate:48KB
received rate:1579KB transmit rate:76KB

I am sure that someone with more insight into perl than me can make it even more efficient.
You can also download a version with comments that I made so that one can make the script run for X number of repetitions instead of running until someone stops it.
http://www.void.gr/kargig/blog/wp-content/iftraffic.pl

Source : http://www.void.gr/kargig/blog/2009/05/04/iftraffic-perl-script-to-measure-inout-traffic-in-realtime/