Install Ubuntu without a USB Stick

Disclaimer: I recommend that you experiment the method written in this post on a virtual machine first, because it can get quite tricky.

Let's say you want to wipe out your entire system and install Ubuntu. The easiest way is perhaps
1. download Ubuntu Live Image,
2. create a bootable USB stick, and
3. boot from the USB.

Well, if you were like me, who wipe out the entire system often, you will find it quite annoying to locate the USB, create the bootable stick, and so on. Furthermore, what if you don't have a USB stick in possession?

This post is to rescue you in such situations. You can simply download the image and boot from the image stored on your disk! Let's see how we can do this. Some of the references are here and here.

Here is the setup. First, you will need at least two partitions on your disk. One is Linux installation partition, and the other is to hold the iso image. Throughout the post, I am going to assume that your first partition is mounted as / and your second partition is mounted as /data.

You will need to download the Ubuntu Live image to the second partition, say
$ wget http://releases.ubuntu.com/18.04/ubuntu-18.04-desktop-amd64.iso -P /data

Now, you will have the iso image file saved as /data/ubuntu-18.04-desktop-amd64.iso. Make sure that you save the image in the partition other than where the Linux will be installed.

Next, you need to add a grub menu entry.
$ sudo vim /etc/grub.d/40_custom

Your file should look like below:
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

menuentry "Ubuntu 18.04 LTS" {
set isofile="/ubuntu-18.04-desktop-amd64.iso"
loopback loop (hd0,2)$isofile
echo "Starting $isofile..."
linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=${isofile} quiet splash
initrd (loop)/casper/initrd.lz
}

Let me go over the system partition scheme once more. The above file applies to the partition scheme where
partition 1: /dev/sda1 --> currently mounted as /; will install Linux on this partition
partition 2: /dev/sda2 --> currently mounted as /data; holds the iso image

Since you downloaded the iso image on the /data directory, this is the root of the second partition. Therefore, this is specified as (hd0,2) in the grub menu entry, corresponding to /dev/sda2; we must omit /data here because /data is just the mount-point in currently-running system and grub won't know anything about it. If you have different partition scheme from mine, you will need to edit the entry accordingly.

Finally, you will need to update grub
$ sudo update-grub

Let's reboot the system and see if we can indeed boot from the iso from the current disk.
$ sudo reboot

Make sure to press and hold <Shift> key while booting up, so that grub entry appears. Otherwise, it is likely that grub menu entry won't even appear.

If you have correctly followed till now, you should be able to boot from the iso image. You should even be able to install Ubuntu on partition 1 using the iso image saved in partition 2. However, you will notice that during the installation, it complains that /isodevice cannot be unmounted. You can resolve this issue by running the following in the terminal within the Live Image system (not your currently installed system):
$ sudo umount -l -r -f /isodevice

After running this command, you should be able to successfully wipe out partition 1 and install Ubuntu 18.04 fresh!