There are various way to display filesystem usage in ubuntu system tray. You can see it through terminal or through various tools.
1. Using SpaceView
Run following command to install spaceview in your ubuntu.
sudo add-apt-repository ppa:vlijm/spaceview sudo apt-get update sudo apt-get install spaceview
2. Using Udisks-Indicator
Type below command to use this tool.
sudo apt-add-repository ppa:udisks-indicator-team/ppa sudo apt-get update sudo apt-get install udisks-indicator
Now run the following command after installing
Now you can see the exact usage in real time without having hassle to type command.