Automatic Shutdown

I need to find a way to shut down my Linux PC (Ubuntu 8.04 Server) after it has up & downloaded 100MB.
Anyone has any idea on how to do this?

#!/bin/bash
RX=`ifconfig eth0 | grep "RX bytes" | sed -e "s/\s*RX bytes:\([0-9]*\).*/\1/g"`
TX=`ifconfig eth0 | grep "TX bytes" | sed -e "s/.*TX bytes:\([0-9]*\).*/\1/g"`
echo RX: $RX
echo TX: $TX
Total=$(($RX + $TX))
echo Total: $Total

Gives me as output:

RX: 42507
TX: 51532
") 51532 error: invalid arithmetic operator (error token is "
Total:

>>33
Thanks for trying to help, but it's not exactly what I need :)