Get upload/download speed of network interface

speed.sh

LANG=""
while true
do
    up_time1=`ifconfig $1 | grep "bytes" | awk '{print $6}' | awk -F: '{print $2}'`
    down_time1=`ifconfig $1 | grep "bytes" | awk '{print $2}' | awk -F: '{print $2}'`
sleep 1
clear
up_time2=`ifconfig $1 | grep "bytes" | awk '{print $6}' | awk -F: '{print $2}'`
down_time2=`ifconfig $1 | grep "bytes" | awk '{print $2}' | awk -F: '{print $2}'`
up_time1=${up_time1}
up_time2=${up_time2}
down_time1=${down_time1}
down_time2=${down_time2}
up_time=`expr $up_time2 - $up_time1`
down_time=`expr $down_time2 - $down_time1`
up_time=`expr $up_time / 1024`
down_time=`expr $down_time / 1024`
echo Upload speed: $up_time kb/s
echo Download speed: $down_time kb/s
done 

Usage:
bash speed.sh eth0