Realtime bandwidth usage from shell

Hi

I'm using Tomato Firmware v1.28.0502 MIPSR2Toastman-RT-N K26 USB VLAN-Ext On an Asus RT-N66u. Rock solid for 18 months.

Is it possible to get the realtime bandwidth stats from shell? I need to run a cron job, log the realtime stats every 30 seconds, then run a curl call based on the values of the traffic. If traffic exceeds x, then do y. That kind of logic.

Does anyone know if this is possible?

Thanks

Will