A simply script that calculates bandwidth usage for a given interface. It is written for using with GNU Screen.

.screenrc

backtick 1 1 1 /home/$USER/bin/bwstat.sh eth0

hardstatus alwayslastline '%{= G}[%= %{= w}%?%-Lw%?%{= R}%n*%f %t%?%{= R}(%u)%?%{= w}%+Lw%?%= %{= g}][ %{y}%1`%{g} ][ %{y}L: %l %{g}]'

bwstat.sh

#!/bin/bash

# bwstat.sh -- Calculate bandwidth usage for a given interface.
#
# Copyright (C) 2013 Artyom V. Poptsov <poptsov.artyom@gmail.com>
#
# Copying and distribution of this file, with or without modification,
# are permitted in any medium without royalty provided the copyright
# notice and this notice are preserved.  This file is offered as-is,
# without any warranty.

BWSTAT_FILE=/tmp/bwstat
IFACE=$1

RES=""

if [ -s $BWSTAT_FILE ]; then
    TIME_OLD=$(cat $BWSTAT_FILE | cut -d ' ' -f 1)
    RXB_OLD=$(cat $BWSTAT_FILE | cut -d ' ' -f 2)
    TXB_OLD=$(cat $BWSTAT_FILE | cut -d ' ' -f 3)

    RXB_NEW=$(</sys/class/net/"$IFACE"/statistics/rx_bytes)
    TXB_NEW=$(</sys/class/net/"$IFACE"/statistics/tx_bytes)

    TIME_NEW=`date +%s`
    PERIOD=$((TIME_NEW - TIME_OLD))

    RXDIF=$(echo $((RXB_NEW - RXB_OLD)))
    TXDIF=$(echo $((TXB_NEW - TXB_OLD)))

    RES="U$((TXDIF / 1024 / PERIOD)) D$((RXDIF / 1024 / PERIOD))"

    echo "$TIME_NEW $RXB_NEW $TXB_NEW" > $BWSTAT_FILE
else
    RXB=$(</sys/class/net/"$iface"/statistics/rx_bytes)
    TXB=$(</sys/class/net/"$iface"/statistics/tx_bytes)
    echo "$(date +%s) $RXB $TXB" > $BWSTAT_FILE
    chmod 600 $BWSTAT_FILE
    RES="U0 D0"
fi

echo -e "$RES"