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. However, in Linux at least a 2.2 series kernel is required.
It is very simple to install and use. It can monitor multiple network interfaces at the same time. It will display the output summary in hourly, daily, weekly and as well as monthly basis. And it can be used without root permissions.

Install vnstat
On Debian/Ubuntu:

On RHEL/CentOS:
vnstat will not be found in Official repositories, so add EPEL repository to install vnstat:

Install vnstat with following command:

Usage
To view all network interfaces statistics, enter the following command:

This shows network bandwidth usage of all network interfaces:

To view a particular interface bandwidth usage, enter the command:

The above command will display the bandwidth usage of eth1:

To view the hourly usage, use the parameter “-h” without quotes:

To view daily usage, use the parameter “-d”:

To view weekly usage, use the parameter “-w”:

And to view monthly usage, use “-m”:

To see top ten network bandwidth usage of eth1:

To view the bandwidth usage of 5 seconds, enter the following command:

To view the usage of 10 seconds, enter the following command:

Finally to see the real live bandwidth usage of eth1:

This command will collect the network bandwidth usage of eth1. You can stop the process by pressing CTRL+C at any time.
Sample Output:

Hope you have a basic idea about vnstat.
Happy Monitoring!

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