Show Real-time Net Speed

#!/bin/zsh

get_ninfo() {
    # get network info: rx bytes & tx bytes at a timepoint
    # (rx, tx, tp) in $ninfo, as a list

    interface=`ip ro g 1.1.1.1 2>/dev/null | awk '{print $5}'` # active network interface
    ninfo=(`awk -v p=$interface '$0 ~ p {print $2, $10}' /proc/net/dev`)
    ninfo+=`date +%s%N` # nanosec
}

nspd() {
    # calc rx/tx (down/up) net speed
    deltat=$((($3-$6)/1e9)) # sec
    spd=(`numfmt --to=iec $((($1 - $4)/$deltat)) $((($2 - $5)/$deltat))`)
    echo $spd[1]B/s $spd[2]B/s
}

get_ninfo
old_ninfo=($ninfo)
while ((1)) {
    get_ninfo
    nspd $ninfo $old_ninfo
    old_ninfo=($ninfo)
    sleep 1
}