vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network traffic for the selected interface(s). It uses the network interface statistics provided by the kernel as information source. This means that vnStat won’t actually be sniffing any traffic and also ensures light use of system resources.

Step 1. Installation:

apt-get install vnstat

Step 2. Select the interface you wish to monitor:

for eth0 :

vnstat -u -i eth0

Repeat this changing eth0 for each interface you wish to monitor.

NOTE: Once the text database have been initialised you’re ready to monitor your bandwidth usage. The package installs a cronjob which will update the database every five minutes.

The most simple way of doing so is to just invoke the command with no arguments:

vnstat

The output will be something like this:

$ vnstat
Database updated: Sat Jun  19 23:50:01 2010

   eth1 since 1/17/10

          rx:  3.32 TiB      tx:  2.81 TiB     total:  6.13 TiB

   monthly
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
       Jul '09    609.40 GiB |  282.21 GiB |  891.61 GiB |    2.79 Mbit/s
       Aug '09     16.95 GiB |   10.46 GiB |   27.40 GiB |    2.80 Mbit/s
     ------------------------+-------------+-------------+---------------
     estimated    552.14 GiB |  340.70 GiB |  892.83 GiB |

   daily
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
     yesterday     19.19 GiB |    8.63 GiB |   27.82 GiB |    2.70 Mbit/s
         today     16.95 GiB |   10.46 GiB |   27.40 GiB |    2.80 Mbit/s
     ------------------------+-------------+-------------+---------------
     estimated     17.81 GiB |   10.99 GiB |   28.80 GiB |

U can use vnstat –help for more options and info:

$ vnstat --help
 vnStat 1.10 by Teemu Toivola 

         -q,  --query          query database
         -h,  --hours          show hours
         -d,  --days           show days
         -m,  --months         show months
         -w,  --weeks          show weeks
         -t,  --top10          show top10
         -s,  --short          use short output
         -u,  --update         update database
         -i,  --iface          select interface (default: eth0)
         -?,  --help           short help
         -v,  --version        show version
         -tr, --traffic        calculate traffic
         -ru, --rateunit       swap configured rate unit
         -l,  --live           show transfer rate in real time

See also "--longhelp" for complete options list and "man vnstat".

Done.