今現在、サーバがどのくらいの回線帯域を使用しているか確認できるスクリプトを作りました。
ただしPerlですが参考まで。。

#! /usr/bin/perl

$interval = 2;

$line = `/sbin/ifconfig eth0 | grep "TX bytes"`;
$line =~ s/^.*TX bytes\:(\d+) .*$/$1/ ;
$previous = $line;
print "sampling interval is $interval sec.\n";
sleep $interval;

while (1){
  $line = `/sbin/ifconfig eth0 | grep "TX bytes"`;
  $line =~ s/^.*TX bytes\:(\d+) .*$/$1/ ;
  $bytes = $line - $previous;
  $previous = $line;

  $Mbps = int ($bytes * 8 / 1024 / 1024 / $interval *100 ) / 100;

  printf ("%12d bytes in %ssec / %3.2f Mbps\n", $bytes , $interval , $Mbps);
  sleep $interval;
}

コンソールで実行するとMbps単位で出力します。

% .traffic.pl
sampling interval is 2 sec.
     3827959 bytes in 2sec / 14.60 Mbps
     3695406 bytes in 2sec / 14.09 Mbps
     2831445 bytes in 2sec / 10.80 Mbps
     3737917 bytes in 2sec / 14.25 Mbps
     4167232 bytes in 2sec / 15.89 Mbps
     3855147 bytes in 2sec / 14.70 Mbps
     3276326 bytes in 2sec / 12.49 Mbps
     3629611 bytes in 2sec / 13.84 Mbps
     3169168 bytes in 2sec / 12.08 Mbps
     1673286 bytes in 2sec / 6.38 Mbps
     2655460 bytes in 2sec / 10.12 Mbps
     4125430 bytes in 2sec / 15.73 Mbps
     3399426 bytes in 2sec / 12.96 Mbps
     3068414 bytes in 2sec / 11.70 Mbps
     1149160 bytes in 2sec / 4.38 Mbps
         138 bytes in 2sec / 0.00 Mbps
         138 bytes in 2sec / 0.00 Mbps
         138 bytes in 2sec / 0.00 Mbps
         487 bytes in 2sec / 0.00 Mbps
        1719 bytes in 2sec / 0.00 Mbps
         204 bytes in 2sec / 0.00 Mbps
         138 bytes in 2sec / 0.00 Mbps
         138 bytes in 2sec / 0.00 Mbps
         138 bytes in 2sec / 0.00 Mbps
       33776 bytes in 2sec / 0.12 Mbps
         138 bytes in 2sec / 0.00 Mbps