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If you have a Ubuntu installation that you want to convert to a live/installer disk
image (able to be burned to DVD, "burned" to bootable USB, or used as an .iso to set up a virtual machine), this is one way to do it. I'm using Ubuntu 14.04.3 in this example, because it's the latest LTS (Long-Term Support release for Ubuntu). If you're using the newest version of Ubuntu (15.10, as of this writing), you may find How to create a custom live/installer disk image for Ubuntu 15.10 more helpful.
Make sure you have your Ubuntu installation exactly the way you want it before proceeding.
First, go to the Remastersys PPA.
Expand Technical details about this PPA.
Under Display sources.list entries for:, select Trusty (14.04).
You'll then see the sources in the input box change to something like this:
deb http://ppa.launchpad.net/mutse-young/remastersys/ubuntu trusty main
deb-src http://ppa.launchpad.net/mutse-young/remastersys/ubuntu trusty main
Highlight and copy them, and keep the webpage open.
Put this command into the terminal to edit the sources.list (while also making a backup):
sudo nano -B /etc/apt/sources.list
Then, at the very bottom of the file (use Control-V to scroll down), you can paste (Control-Shift-V) the text you'd copied earlier, and then save (Control-X) the file.
Back on the PPA webpage, check out the bold text under Adding this PPA to your system—in this case, ppa:mutse-young/remastersys.
Back in the terminal, paste in
sudo add-apt-repository ppa:mutse-young/remastersys
and then hit Enter when prompted to add the gpg keys to say you're trusting this repository.
Now that the repository is set up, let's pull from it to get Remastersys actually installed:
sudo apt-get update && sudo apt-get install remastersys
Optional: Remastersys, by default, will just back up your whole system but not any of the logged-in users. If you want to make one of your user's the default user profile, run this command:
sudo remastersys-skelcopy username
is the username of the user whose profile you want to make the default user profile in the new live/installer disk image.
Required: To make the actual disk image, make sure all other programs are closed (you don't want to make changes to the system while it's being backed up / converted), and then run this command:
sudo remastersys backup dist
Just let it run. It may take a while, depending on how large your installation is.
Once it's done, you'll see the location of the created file, which should be /home/remastersys/remastersys/dist (not sure why the .iso file extension isn't appended).
That's your disk image (I'd recommend renaming it from dist to dist.iso.
You can then burn it to a DVD (as a disk image—not data), use Startup Disk Creator to make a bootable USB, or use the .iso to install to a virtual disk using something like VMWare or VirtualBox.