So, here's my setup. I have one partition with the newest version installed, a second partition with an older version installed (as a backup just in case), a swap partition that both share, and a boot partition so the bootloader doesn't need to be setup after each upgrade.

Partitions:

• sda1 ext3 /boot
• sda2 ext4 / (current version)
• sda3 ext4 / (old version)
• sda4 swap /swap
• sda5 ntfs (contains folders symbolically linked to /home on /)

So far it has been a very good setup. I can create new boot loaders without screwing it up and adding my personal files into a new install is as simple as creating some symbolic links (the partition is NTFS in case I need to load windows on the system again).

Here's the issue. I'd like to be able to drop the install into /distro on the current version and install a new version on / on the old version effectively replacing/upgrading it. The goal is to be able to just swap out new versions as they are released while maintaining redundancy in case I don't like th update.

So far I have:

• downloaded the install.iso
• created a folder in /distro
• copied the install.iso into /distro
• extracted vmlinuz and initrd.lz into /distro

Then I modified /boot/grub/menu.lst with the following entry:

title Install Linux
root (hd0,1)
kernel /distro/vmlinuz
initrd /distro/initrd.lz

vmlinuz loads perfectly but it says it can't find initrd.lz on boot.

I have also tried to uncompress the image with:

unlzma < initrd.lz > initrd.img

And, updating the menu.lst file to match; but that doesn't work either.

I'm assuming that vmlinuz (linux kernel) loads, fires up the virtual filesystem by creating a ramdisk (initrd), mounts the iso, and launches the installer.

Am I missing something here?

Update: First, I wanted to say that the accepted answer would have been the best option if I was doing a normal Ubuntu install. Unfortunately, I was installing Linux Mint (which lacks the script needed to make debootstrap work.

So the problem I with the above approach was, I was missing the command that vmlinuz (linux kernel) needed to execute to start boot into LiveCD mode. By looking in the /boot/grub/grub.cfg file I found what I was missing. Although this method will work, it requires that the installation files reside on their own partition. I took the easy route and used unetbootin to drop the LiveCD on a usb drive and booted from that.

Like I said before. Debootstrap would have been the ideal solution here. Even though I couldn't use it I wrote down the steps it would've taken to use it.

Step One: Format sda3 (the partition with the old copy of linux that's being overwritten)

I used gparted to format it as ext4 from within the current linux install. How this is done varies based on what tools you prefer to use.

Step Two: Mount the newly formatted partition (we'll call the mount ubuntu for simplicity)

sudo mkdir /mnt/ubuntu
sudo mount -o -loop /dev/sda3 /mnt/ubuntu

Step Three: Get debootstrap

sudo apt-get install debootstrap

Step Four: Mount the install disk (replace ubuntu.iso with the name if your install disk)

sudo mkdir /media/cdrom
sudo mount -o loop ~/ubuntu.iso /media/cdrom

Step Five: Install the OS using debootstrap (replace fiesty with the version you're installing and amd64 with your processor's architecture)

sudo debootstrap --arch amd64 fiesty /mnt/ubuntu file:/media/cdrom

The settings here varies. While I loaded debootstrap using an install iso, you can also have debootstrap automatically download and install if with a repository link (While most of these repositories contain debian versions I'm still not clear as to whether Ubuntu has similar repositories). Here a list of the debian package repositories and their mirrors.

This is how you'd deploy debootstrap if you were doing it directly from a repository:

sudo debootstrap --arch amd64 squeeze /mnt/debian http://ftp.us.debian.org/debian

Here's the link that I primarily used to figure this out.