Install Linux from Linux without a USB or a CD-DVD

I switched from Windows to Linux (Ubuntu) a few months ago and decided that it'd be a good idea to delete my Windows partition, which I did using the Disks application on Ubuntu.

However during the transition I made the Linux partition only 80 GB big and now I am stuck with 399 GBs of space just laying around.

I don't have a USB or a CD/DVD and I was wondering if I can install Linux from Linux or if there is a way to make use of the free space and save my files there.

I noticed that there is a related post here but as far as I understand this is a Windows only tutorial.

Here is what my Disks application looks like and I have a UEFI system.

Thanks in advance.

Booting ISO from GRUB Menu

The method on this page should work for booting an Ubuntu ISO from the GRUB bootloader 20.04 booting .iso from GRUB menu No need for a USB here.

To reiterate:

• Add the following menuentry to /etc/grub.d/40_custom:

  menuentry "isoname ISO" {
    set root=(hdX,Y)
    set isofile="/[path]/[isoname].iso"
        loopback loop $isofile
        linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile toram --
        initrd (loop)/casper/initrd
  }
  

Where (hdX,Y) is the disk and partition the ISO is on, [path] is the path to the folder the ISO file is in, and [isoname] is the name of the ISO

• In Terminal run:

  sudo update-grub
  

• Reboot and select Ubuntu from the grub menu. Install as usual.

• Please let us know how this works for you.