Admins eHow

The following shell script shows current download and upload speeds for the network interface you choose.

Copy the shell script in a file named, i.e: net_speed.sh

Then after setting execution permissions:

chmod a+x net_speed.sh

You can run the shell script passing as the first argument the network interface you want to monitor:

./net_speed.sh eth0

You will get a line like that:
eth0 DOWN:15 KB/s UP:880 B/s

This script works parsing /proc/net/dev file and calculating the difference between current transmitted or received bytes and their values one second ago.

#!/bin/bash

# This shell script shows the network speed, both received and transmitted.

# Usage: net_speed.sh interface
#   e.g: net_speed.sh eth0


# Global variables
interface=$1
received_bytes=""
old_received_bytes=""
transmitted_bytes=""
old_transmitted_bytes=""


# This function parses /proc/net/dev file searching for a line containing $interface data.
# Within that line, the first and ninth numbers after ':' are respectively the received and transmited bytes.
get_bytes()
{
    line=$(cat /proc/net/dev | grep $interface | cut -d ':' -f 2 | awk '{print "received_bytes="$1, "transmitted_bytes="$9}')
    eval $line
}


# Function which calculates the speed using actual and old byte number.
# Speed is shown in KByte per second when greater or equal than 1 KByte per second.
# This function should be called each second.
get_velocity()
{
    value=$1    
    old_value=$2

    let vel=$value-$old_value
    let velKB=$vel/1024
    if [ $velKB != 0 ];
    then
 echo -n "$velKB KB/s";
    else
 echo -n "$vel B/s";
    fi
}

# Gets initial values.
get_bytes
old_received_bytes=$received_bytes
old_transmitted_bytes=$transmitted_bytes

# Shows a message and waits for one second.
echo "Starting...";
sleep 1;
echo "";


# Main loop. It will repeat forever.
while true; 
do

    # Get new transmitted and received byte number values.
    get_bytes

    # Calculates speeds.
    vel_recv=$(get_velocity $received_bytes $old_received_bytes)
    vel_trans=$(get_velocity $transmitted_bytes $old_transmitted_bytes)

    # Shows results in the console.
    echo -en "$interface DOWN:$vel_recv\tUP:$vel_trans\r"

    # Update old values to perform new calculations.
    old_received_bytes=$received_bytes
    old_transmitted_bytes=$transmitted_bytes

    # Waits one second.
    sleep 1;

done

Source : Linux Clues