vnStat is a command line utility which helps users in monitoring, logging and viewing network statistics over various time periods for various network interfaces.
##How to install?
For Arch and its derivatives:

pacman -S vnstat

For Debian and its derivatives:

apt install vnstat

##How to setup?
vnStat monitors data usage on a network interface basis so first you'll need to know the name of the network interface you want to monitor.
For that, run below command in terminal:

ip link

To initialize monitoring on any network interface, run following command:

vnstat -u -i wlp0s20u2

Here wlp0s20u2 is name of my network interface. Replace it with your network interface's name.
Now start vnStat daemon using below command, so that it can start monitoring network traffic.

systemctl enable --now vnstat

##How to use?
Type "vnstat" in the terminal to get default data usage which is in the form monthly summary.

To see data usage of a particular network interface:

vnstat -i wlp0s20u2

To see daily data usage of a particular network interface:

vnstat -i wlp0s20u2 -d

To see hourly data usage of the past 24 hours of a particular network interface:

vnstat -i wlp0s20u2 -h

You can monitor weekly, monthly, yearly and live(at this moment) data usage by using various flags which I leave up to you to find out.