saw this at askubuntu, and it looks quite complete:

A quick way of backing up a list of programs is to run this:

dpkg --get-selections > ~/Package.list
sudo cp /etc/apt/sources.list ~/sources.list
sudo apt-key exportall > ~/Repo.keys
It will back them up in a format that dpkg can read for after your
reinstall, like this:

sudo apt-key add ~/Repo.keys
sudo cp ~/sources.list /etc/apt/sources.list
sudo apt-get update
sudo apt-get install dselect
sudo dpkg --set-selections < ~/Package.list
sudo apt-get dselect-upgrade -y
Settings and Personal Data
Before you reinstall, you should probably back up the settings from some of
your programs, this can easily be done by grabbing folders from /etc and
all the content from your user directory (not just the stuff you can see in
nautilus!):

rsync --progress /home/`whoami` /path/to/user/profile/backup/here
After you reinstall, you can restore it with:

rsync --progress /path/to/user/profile/backup/here /home/`whoami`
So all together as a pseudo-bash script.
This assumes there is only one user on the machine (remove /'whoami'
otherwise) and that you used the same username on both installs (modify
dest. of rsync otherwise).

dpkg --get-selections > ~/Package.list
sudo cp /etc/apt/sources.list ~/sources.list
sudo apt-key exportall > ~/Repo.keys
rsync --progress /home/`whoami` /path/to/user/profile/backup/here

##  Reinstall now

rsync --progress /path/to/user/profile/backup/here /home/`whoami`
sudo apt-key add ~/Repo.keys
sudo cp ~/sources.list /etc/apt/sources.list
sudo apt-get update
sudo apt-get install dselect
sudo dpkg --set-selections < ~/Package.list
sudo dselect


On Fri, Dec 27, 2013 at 1:21 PM, Michael Havens <bmike1@gmail.com> wrote:

> well.... it looks as if I may have to reinstall my laptop (which sucks) if
> we can't get x working again. I  just realized something I didn't do: I did
> not write down all the ppa addresses. So what I would like to know is in
> which file are the ppa s kept? then I can scp that to the tv server and if
> we can't get x working again it won't be as painful a bite. Thanks:-)
> :-)~MIKE~(-:
>
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