Linux interface bytes per second using only /proc

pps.sh

#!/bin/bash

time="1"     # one second
int="eth1"   # network interface

while true
do
  txbytes_old="`cat /sys/class/net/$int/statistics/tx_bytes`" # sent bytes
  rxbytes_old="`cat /sys/class/net/$int/statistics/rx_bytes`" # recv bytes
  sleep $time
  txbytes_new="`cat /sys/class/net/$int/statistics/tx_bytes`" # sent bytes
  rxbytes_new="`cat /sys/class/net/$int/statistics/rx_bytes`" # recv bytes
  txbytes="`expr $txbytes_new - $txbytes_old`"		     # evaluate expressions for sent bytes
  rxbytes="`expr $rxbytes_new - $rxbytes_old`"		     # evaluate expressions for recv bytes
  echo "tx $txbytes bytes/s - rx $rxbytes bytes on interface $int"
done

Adapted from https://www.unixteacher.org/blog/display-packets-per-second-on-linux/