统计网络接口的流量

#!/bin/sh
#
#
exnum=131072
speed=Mbit/s
if [ $2 = "m" ];then
        exnum=131072   #exnum=1024*1024/8
        speed=Mbit/s
fi

if [ $2 = "k" ];then
        exnum=1024
        speed=KByte/s
fi

while true
do
        firnum=`ifconfig $1|awk 'NR==7{print $2;exit}'|cut -c7-`
        echo $firnum
        sleep 1
        sednum=`ifconfig $1|awk 'NR==7{print $2;exit}'|cut -c7-`
        echo $sednum
        getnum=$(($sednum-$firnum))
        echo $getnum
        tra=`echo "scale=4; $getnum/$exnum"|bc`
        echo "The speed is ${tra} ${speed}"
done