network monitor without any requirements

gistfile1.txt

#!/bin/bash
#improved from https://www.commandlinefu.com/commands/view/10095/network-traffic-on-nics-in-mbps-without-sar-iperf-etc...
while true; do
	[ -z ${TX2} ] && TX2=0
	[ -z ${RX2} ] && RX2=0
	[ -z ${INTERFACE} ] && INTERFACE=$(ip link | egrep -e '^[0-9]+:' | grep -v 'lo:' | grep -v 'state DOWN'| cut -d " " -f2 | cut -d ":" -f1 | head -n1)  && echo "INTERFACE=${INTERFACE}"
	RX1=`cat /sys/class/net/${INTERFACE}/statistics/rx_bytes`
	TX1=`cat /sys/class/net/${INTERFACE}/statistics/tx_bytes`
	DOWN=$(($RX1-$RX2))
	UP=$(($TX1-$TX2))
	DOWN_Bits=$(($DOWN * 8 ))
	UP_Bits=$(($UP * 8 ))
	DOWNmbps=$(( $DOWN_Bits >> 20 ))
	UPmbps=$(($UP_Bits >> 20 ))
	echo -e "RX:${DOWN}\tTX:${UP} B/s | RX:${DOWNmbps}\tTX:${UPmbps} Mb/s"
	RX2=$RX1; TX2=$TX1
sleep 1; done