Monitor network interface usage rate with minimal external tooling on linux

netifbw.sh

#!/bin/sh
# Run like so:   NET_IF=ens5 UNIT=K sh netifbw.sh

NET_IF="${NET_IF:-eth0}"
SLEEP_INTERVAL="${SLEEP_INTERVAL:-1}"
UNIT="${UNIT:-B}"
MODIFIER=1
case "${UNIT}" in
    B|[Bb]ytes)
        MODIFIER=1
        ;;
    K|[Kk]ilo)
        MODIFIER=1024
        ;;
    M|[Mm]ega)
        MODIFIER=1048576
        ;;
    G|[Gg]iga)
        MODIFIER=1073741824
        ;;
    *)
        echo "unknown unit: ${UNIT}" && exit 1
        ;;
esac

old_in_bytes=0 && old_out_bytes=0
in_bytes=0 && out_bytes=0

while true; do
    proc_net_dev=$(grep "${NET_IF}:" /proc/net/dev) || ( echo "NET_IF ${NET_IF} not found" && exit 1 )
    old_in_bytes=${in_bytes}
    old_out_bytes=${out_bytes}
    in_bytes=$(echo ${proc_net_dev} | awk '{print $2}')
    out_bytes=$(echo ${proc_net_dev} | awk '{print $10}')
    in_rate=$(echo "(${in_bytes} - ${old_in_bytes}) / ${MODIFIER}" | bc)
    out_rate=$(echo "(${out_bytes} - ${old_out_bytes}) / ${MODIFIER}" | bc)

    echo "${NET_IF}: ${in_rate} in-${UNIT}bytes/${SLEEP_INTERVAL}sec ${out_rate} out-${UNIT}bytes/${SLEEP_INTERVAL}sec"
    sleep ${SLEEP_INTERVAL:-1}
done