Ubuntu : Adding a new repository (PPA)

To add a repository ppa:

sudo add-apt-repository ppa:user/ppa-name
sudo apt-get update

Sometimes adding a repository may result in error related to missing key. Then copy that key and run following command

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com KEY

Do this for each of the missing keys.
Now re-run the above two commands.

If you are under proxy network or seen a error like

Cannot add PPA: 'ppa:user/ppa-name'.
Please check that the PPA name or format is correct.

and if you are sure that the PPA is working, the try

sudo -E add-apt-repository ppa:user/ppa-name