Get the information about the internet usage from the commandline.

Use it like this below:

vnstat -u -i interface forces a database update for interface or creates the database if it doesn't exist. This is
       usually the first command used after a fresh install.

       vnstat  -u  -i interface --nick nick gives interface the nickname nick and that information will be later included
       with queries.

       vnstat -u -r --disable -i interface resets the internal counters of interface and disables it from  being  updated
       before  enabled again with the --enable parameter. This feature is especially useful for interfaces like ppp0 that
       aren't always active.

And one of out put will be below,

sudo /usr/bin/vnstat -l
Monitoring ppp0...    (press CTRL-C to stop)

   rx:       0.00 kB/s     0 p/s            tx:       0.00 kB/s     0 p/s^C


 ppp0  /  traffic statistics

                             rx       |       tx
--------------------------------------+----------------------------------------
  bytes                         0 kB  |          0 kB
--------------------------------------+----------------------------------------
          max              0.05 kB/s  |     0.03 kB/s
      average              0.01 kB/s  |     0.00 kB/s
          min              0.00 kB/s  |     0.00 kB/s
--------------------------------------+----------------------------------------
  packets                          1  |             1
--------------------------------------+----------------------------------------
          max                  0 p/s  |         0 p/s
      average                  0 p/s  |         0 p/s
          min                  0 p/s  |         0 p/s
--------------------------------------+----------------------------------------
  time                    17 seconds

It has got so many wonderful switch to get the exact data you need.