Back up sources.list
If you open a terminal window and type this command, it will make a copy of the sources.list file, adding the current date and time to the backed up file’s name, and place the copy into the same directory as the original:
sudo cp -p /etc/apt/sources.list /etc/apt/sources.list.`date +%b-%d-%Y~%T`
Obligatory Happy Ending
And they all lived happily ever after. The end.