There is a utility called y-ppa-manager https://launchpad.net/y-ppa-manager which helps to manage your ppa repositories in ubuntu. One of its best features is to backup your repository list. Is there any command line or graphical tool to backup your repositories without installing any ppa tool?
There is a folder called sources.list.d and a file called sources.list both need to be backed up, found in /etc/apt
I just navigated there using Nautilus and dragged them to my desktop.
To restore I opened Nautilus as admin: in terminal
GKSU Nautilus and drag back in to the same folder.
Then in terminal:
sudo apt-get update
There may be a better way, but I am not big on terminal commands.
1. Open Terminal
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup
sudo cp /etc/apt/sources.list.d /etc/apt/sources.list.d.backup