How to find network usage in linux programatically

i know that the question is some weeks old but maybe this answer can still be helpful :)

You could read the device statistics from /proc/net/dev. Read the transmitted / received bytes in a interval and calculate the difference. Here is some simple Python script i hacked together

import re
import time


# A regular expression which separates the interesting fields and saves them in named groups
regexp = r"""
  \s*                     # a interface line  starts with none, one or more whitespaces
  (?P<interface>\w+):\s+  # the name of the interface followed by a colon and spaces
  (?P<rx_bytes>\d+)\s+    # the number of received bytes and one or more whitespaces
  (?P<rx_packets>\d+)\s+  # the number of received packets and one or more whitespaces
  (?P<rx_errors>\d+)\s+   # the number of receive errors and one or more whitespaces
  (?P<rx_drop>\d+)\s+      # the number of dropped rx packets and ...
  (?P<rx_fifo>\d+)\s+      # rx fifo
  (?P<rx_frame>\d+)\s+     # rx frame
  (?P<rx_compr>\d+)\s+     # rx compressed
  (?P<rx_multicast>\d+)\s+ # rx multicast
  (?P<tx_bytes>\d+)\s+    # the number of transmitted bytes and one or more whitespaces
  (?P<tx_packets>\d+)\s+  # the number of transmitted packets and one or more whitespaces
  (?P<tx_errors>\d+)\s+   # the number of transmit errors and one or more whitespaces
  (?P<tx_drop>\d+)\s+      # the number of dropped tx packets and ...
  (?P<tx_fifo>\d+)\s+      # tx fifo
  (?P<tx_frame>\d+)\s+     # tx frame
  (?P<tx_compr>\d+)\s+     # tx compressed
  (?P<tx_multicast>\d+)\s* # tx multicast
"""


pattern = re.compile(regexp, re.VERBOSE)


def get_bytes(interface_name):
    '''returns tuple of (rx_bytes, tx_bytes) '''
    with open('/proc/net/dev', 'r') as f:
        a = f.readline()
        while(a):
            m = pattern.search(a)
            # the regexp matched
            # look for the needed interface and return the rx_bytes and tx_bytes
            if m:
                if m.group('interface') == interface_name:
                    return (m.group('rx_bytes'),m.group('tx_bytes'))
            a = f.readline()


while True:
    last_time  = time.time()
    last_bytes = get_bytes('wlan0')
    time.sleep(1)
    now_bytes = get_bytes('wlan0')
    print "rx: %s B/s, tx %s B/s" % (int(now_bytes[0]) - int(last_bytes[0]), int(now_bytes[1]) - int(last_bytes[1]))

There's probably a better way, but an ugly workaround I used to estimate network rates from the command line is:

ifconfig; sleep 10; ifconfig;

Then just subtract the "TX bytes" (in case of transmission) on the relevant interface and divide by 10 (the sleep time) for a rough estimate in Bytes per second.