我看了下，linux下的/proc/net/dev记录了每块网卡发送和接受的包和字节数。因此萌生想法，写了一个。运行效果：
[root@74-82-173-217 ~]# ./net.sh
Current Ip: inet addr:74.82.173.217 Bcast:74.82.173.223 Mask:255.255.255.224
Summry info: RX bytes:203692709 (194.2 MiB) TX bytes:93525930 (89.1 MiB)
eth0 Receive Bytes: 573 Packets: 3
eth0 Send Bytes: 3086 Packets: 3
eth0 Receive Bytes: 378 Packets: 7
eth0 Send Bytes: 11236 Packets: 7
eth0 Receive Bytes: 324 Packets: 6
eth0 Send Bytes: 444 Packets: 2
eth0 Receive Bytes: 54 Packets: 1
eth0 Send Bytes: 0 Packets: 0
具体脚本的内容如下，几乎不需要修改，就可以拿到任何机器上去使用了。
[root@74-82-173-217 ~]# cat net.sh
#! /bin/bash
#Author: Vogts WangTao 2008-12-18
#Get summry info
echo "Current Ip: "`/sbin/ifconfig eth0 | grep inet`
echo "Summry info: "`/sbin/ifconfig eth0 | grep bytes`
#sleep 1 second ,monitor eth0
while true
do
receive1=`cat /proc/net/dev|grep eth0 | awk '{print$1}'|sed -s 's/eth0://g'`
receive_pack1=`cat /proc/net/dev|grep eth0 | awk '{print$2}'`
send1=`cat /proc/net/dev|grep eth0 | awk '{print$9}'`
send_pack1=`cat /proc/net/dev|grep eth0 | awk '{print$10}'`
sleep 1
receive2=`cat /proc/net/dev|grep eth0 | awk '{print$1}'|sed -s 's/eth0://g'`
receive_pack2=`cat /proc/net/dev|grep eth0 | awk '{print$2}'`
receive_cnt=`expr $receive2 - $receive1`
receive_pack_cnt=`expr $receive_pack2 - $receive_pack1`
send2=`cat /proc/net/dev|grep eth0 | awk '{print$9}'`
send_pack2=`cat /proc/net/dev|grep eth0 | awk '{print$10}'`
send_cnt=`expr $send2 - $send1`
send_pack_cnt=`expr $send_pack2 - $send_pack1`
echo 'eth0 Receive Bytes:' $receive_cnt ' Packets:' $receive_pack_cnt
echo 'eth0 Send Bytes:' $send_cnt ' Packets:' $send_pack_cnt
done