vnStat is a network tranffic monitor for specific interface like (eth0, eth1,..), we can monitoring bandwidth on console.

Features:

Its a light weight and minimal resource usage
Set the network traffic report at specific time in cronjob
Can monitoring multiple interfaces (ethX) at the same time
Its shows the reports possibilities are hourly, daily, monthly, weekly, average, top 10 days,
Can be used without root permission
 Easy way install and configure it.

1. How to Install a vnStat:

Centos/Fedora/RHEL
 # yum install vnstat

Debian/Ubuntu
 # apt-get install vnstat

Install Manually:

Download the package from the below url and install it

# wget http://humdi.net/vnstat/vnstat-1.11.tar.gz

# cd vnstat-1.11

# ./configure

# make && make install

Once you have installed it can find out installed location,

# whereis vnstat

vnstat: /usr/bin/vnstat /usr/sbin/vnstat.cron /usr/share/man/man1/vnstat.1.gz

# which vnstat

/usr/bin/vnstat

Update Bandwidth :

execute the below command to update,
 
# vnstat -u

Network Bandwidth Reports:
 
TX= transmitted traffic
RX= Received traffic 

# vnstat 

Database updated: Wed Aug 27 20:10:01 2014

   eth0 since 05/28/14

          rx:  173.25 GiB      tx:  592.98 GiB      total:  766.23 GiB

   monthly
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
       Jul '14     30.49 GiB |  218.83 GiB |  249.32 GiB |  780.85 kbit/s
       Aug '14     21.76 GiB |  192.70 GiB |  214.46 GiB |  775.79 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated     25.13 GiB |  222.57 GiB |  247.70 GiB |

   daily
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
     yesterday    886.22 MiB |    8.15 GiB |    9.01 GiB |  874.93 kbit/s
         today    812.13 MiB |    6.37 GiB |    7.16 GiB |  827.09 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated       966 MiB |    7.57 GiB |    8.52 GiB |
Query Database Report:

# vnstat -q    
Database updated: Wed Aug 27 20:10:01 2014

   eth0 since 05/28/14

          rx:  173.25 GiB      tx:  592.98 GiB      total:  766.23 GiB

   monthly
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
       Jul '14     30.49 GiB |  218.83 GiB |  249.32 GiB |  780.85 kbit/s
       Aug '14     21.76 GiB |  192.70 GiB |  214.46 GiB |  775.79 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated     25.13 GiB |  222.57 GiB |  247.70 GiB |

   daily
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
     yesterday    886.22 MiB |    8.15 GiB |    9.01 GiB |  874.93 kbit/s
         today    812.13 MiB |    6.37 GiB |    7.16 GiB |  827.09 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated       966 MiB |    7.57 GiB |    8.52 GiB |

Show Hourly Reports
 
# vnstat -h
 eth0                                                                     20:10 
  ^                                                t                            
  |                                                t           t                
  |                                                t           t  t             
  |   t                                      t  t  t  t  t  t  t  t             
  |   t     t                                t  t  t  t  t  t  t  t             
  |   t  t  t                             t  t  t  t  t  t  t  t  t     t       
  |   t  t  t  t           t           t  t  t  t  t  t  t  t  t  t  t  t       
  |   t  t  t  t  t        t        t  t  t  t  t  t  t  t  t  t  t  t  t       
  |   t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t  t rt  t  t  t  t  t       
  |   t  t  t  t  t  t  t  t  t  t  t  t  t  t  t rt  t rt  t  t  t  t  t       
 -+---------------------------------------------------------------------------> 
  |  21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20    
                                                                                
 h  rx (KiB)   tx (KiB)      h  rx (KiB)   tx (KiB)      h  rx (KiB)   tx (KiB) 
21      40781     421358    05      29270     129691    13      40147     451605
22      26993     351584    06      16354     167081    14     170773     442355
23      31056     399914    07      20764     212141    15      36508     439376
00      20413     270040    08      25438     249406    16      51216     587212
01      18229     211535    09      25359     330828    17      53637     504272
02      15236     135354    10      36803     451424    18      34071     280340
03      13034     148547    11      49231     437033    19      40051     352470
04      45167     242102    12      86296     588615    20       3622      42860

Show Daily Reports
 
# vnstat -d
 eth0  /  daily

         day         rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
      07/29/14    735.79 MiB |    8.06 GiB |    8.78 GiB |  852.01 kbit/s
      07/30/14      1.00 GiB |    7.79 GiB |    8.79 GiB |  853.74 kbit/s
      07/31/14    727.67 MiB |    6.81 GiB |    7.53 GiB |  730.63 kbit/s
      08/01/14    744.88 MiB |    6.37 GiB |    7.10 GiB |  689.40 kbit/s
      08/02/14    774.92 MiB |    5.89 GiB |    6.65 GiB |  645.55 kbit/s
      08/03/14    622.54 MiB |    5.97 GiB |    6.58 GiB |  638.88 kbit/s
      08/04/14      1.20 GiB |    7.13 GiB |    8.33 GiB |  808.94 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated       966 MiB |    7.57 GiB |    8.52 GiB |

Show Monthly Reports
 
# vnstat -m
 eth0  /  monthly

       month        rx      |     tx      |    total    |   avg. rate
    ------------------------+-------------+-------------+---------------
      May '14     20.12 GiB |    1.96 GiB |   22.08 GiB |   69.15 kbit/s
      Jun '14    100.88 GiB |  179.49 GiB |  280.37 GiB |  907.38 kbit/s
      Jul '14     30.49 GiB |  218.83 GiB |  249.32 GiB |  780.85 kbit/s
      Aug '14     21.76 GiB |  192.70 GiB |  214.46 GiB |  775.79 kbit/s
    ------------------------+-------------+-------------+---------------
    estimated     25.13 GiB |  222.57 GiB |  247.70 GiB |

Show Weekly Reports
 
# vnstat -w
 eth0  /  weekly

                      rx      |     tx      |    total    |   avg. rate
   ---------------------------+-------------+-------------+---------------
    last 7 days      5.22 GiB |   49.08 GiB |   54.30 GiB |  770.74 kbit/s
      last week      5.24 GiB |   48.98 GiB |   54.22 GiB |  752.05 kbit/s
   current week      2.50 GiB |   22.70 GiB |   25.20 GiB |  861.37 kbit/s
   ---------------------------+-------------+-------------+---------------
      estimated      6.17 GiB |   56.08 GiB |   62.25 GiB |

Show Top 10 Reports
 
# vnstat -t
 eth0  /  top 10

    #      day          rx      |     tx      |    total    |   avg. rate
   -----------------------------+-------------+-------------+---------------
    1   06/04/14      24.08 GiB |    5.15 GiB |   29.23 GiB |    2.84 Mbit/s
    2   06/05/14      22.75 GiB |    5.57 GiB |   28.32 GiB |    2.75 Mbit/s
    3   06/02/14      21.11 GiB |    3.68 GiB |   24.78 GiB |    2.41 Mbit/s
    4   06/03/14      11.68 GiB |    4.48 GiB |   16.16 GiB |    1.57 Mbit/s
    5   05/31/14      12.22 GiB |    1.66 GiB |   13.88 GiB |    1.35 Mbit/s
    6   07/10/14       3.80 GiB |    6.67 GiB |   10.47 GiB |    1.02 Mbit/s
    7   07/25/14     723.59 MiB |    9.60 GiB |   10.31 GiB |    1.00 Mbit/s
    8   07/24/14     943.91 MiB |    9.15 GiB |   10.08 GiB |  978.28 kbit/s
    9   08/14/14     935.94 MiB |    8.92 GiB |    9.83 GiB |  954.81 kbit/s
   10   08/13/14     817.88 MiB |    9.00 GiB |    9.80 GiB |  951.08 kbit/s
   -----------------------------+-------------+-------------+---------------

Short Output
 
# vnstat -s
                      rx      /      tx      /     total    /   estimated
 eth0:
       Jul '14     30.49 GiB  /  218.83 GiB  /  249.32 GiB
       Aug '14     21.76 GiB  /  192.70 GiB  /  214.46 GiB  /  247.70 GiB
     yesterday    886.22 MiB  /    8.15 GiB  /    9.01 GiB
         today    812.13 MiB  /    6.37 GiB  /    7.16 GiB  /    8.52 GiB

Calculate Tranffic
 
# vnstat -tr
282 packets sampled in 5 seconds    
Traffic average for eth0

      rx        60.80 kbit/s            37 packets/s
      tx        72.00 kbit/s            19 packets/s

Rate Unit Montly / Daily
 
# vnstat -ru
Database updated: Wed Aug 27 20:15:16 2014

   eth0 since 05/28/14

          rx:  173.25 GiB      tx:  593.01 GiB      total:  766.26 GiB

   monthly
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
       Jul '14     30.49 GiB |  218.83 GiB |  249.32 GiB |    97.61 KiB/s
       Aug '14     21.76 GiB |  192.73 GiB |  214.49 GiB |    96.97 KiB/s
     ------------------------+-------------+-------------+---------------
     estimated     25.13 GiB |  222.57 GiB |  247.70 GiB |

   daily
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
     yesterday    886.22 MiB |    8.15 GiB |    9.01 GiB |   109.37 KiB/s
         today    815.35 MiB |    6.39 GiB |    7.19 GiB |   103.38 KiB/s
     ------------------------+-------------+-------------+---------------
     estimated       965 MiB |    7.58 GiB |    8.52 GiB |

Show Live Transfer Rate:
 
# vnstat -l
Monitoring eth0...    (press CTRL-C to stop)

   rx:       20 kbit/s    44 p/s          tx:       16 kbit/s    18 p/s^C


 eth0  /  traffic statistics

                           rx         |       tx
--------------------------------------+------------------
  bytes                       52 KiB  |         751 KiB
--------------------------------------+------------------
          max              44 kbit/s  |      968 kbit/s
      average           32.00 kbit/s  |   462.15 kbit/s
          min              20 kbit/s  |       16 kbit/s
--------------------------------------+------------------
  packets                        842  |             442
--------------------------------------+------------------
          max                 98 p/s  |          55 p/s
      average                 64 p/s  |          34 p/s
          min                 44 p/s  |          18 p/s
--------------------------------------+------------------
  time                    13 seconds

vnStat Help:
 
# vnstat --help

or

# help -?
 vnStat 1.11 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".

Set cronjob execute every 30 minutes

Open cronjob file and add the below line 
# crontab -e
*/30 * * * * /usr/bin/vnstat -u
# /etc/init.d/crond restart
Error / Solution : You may get an below error when execute vnstat command, you need to set the network interface ( ethX )  
# vnstat

No database found, nothing to do. Use --help for help.

To check your network interface using ifconfig command,

# ifconfig

You can configure it

# vnstat -u -i eth0