Simple interface byte counter script for Linux

usage.sh

#!/bin/ash
#
# Track bytes in/out on selected interface
#
# Run this script once a month/week/day to calculate how much data usage occuring
# on given interface
#

STOR="bwmon.store"
STAT_DIR="/sys/class/net/$1/statistics/"

[ $# -ne 1 ] && echo "Usage: $0 <interface>" && exit 1

for X in tx rx; do
	B=`cat ${STAT_DIR}/${X}_bytes`

	OLD_B=`grep ${1}_${X} $STOR | awk '{print $2}'`
	[ -z "$OLD_B}" ] && OLD_B="0"

	DIFFB=$(( B - OLD_B ))

	echo "New ${X}:  $B"
	echo "Old ${X}:  $OLD_B"
	echo "Diff ${X}: $DIFFB"

	grep "${1}_${X}" $STOR > /dev/null
	if [ $? -ne 0 ]; then
		echo ${1}_${X} $B >> $STOR
	else
		sed -i "s/^${1}_${X}.*$/${1}_${X} $B/" $STOR
	fi
done