I had intended to install Devuan in an empty machine, using an iso file on a USB stick, and of course it is still possible to do that; however, I am interested in finding our whether one can make the iso file accessible to the existing Ubuntu system, and install Devuan by overwriting from there. Has anyone any ideas on that?

It's extremely easy if you have a home filesystem on different partition. Just perform clean installation overwriting existing system and don't format partition with home directories. The best way is to boot iso from usb stick. Another way is to make the root filesystem via debootstrap. In this case you don't even need an iso file with installer but you'l need to reconfigure GRUB or you won't be able to boot Devuan. You can also use qemu.

I am interested in finding our whether one can make the iso file accessible to the existing Ubuntu system, and install Devuan by overwriting from there.

Not sure I understand what you want to do but it is possible to boot an ISO image saved on the Ubuntu system by making a custom GRUB menu entry for it pointing to the installer kernel & initrd images.

Something like this saved to /boot/grub/custom.cfg should do it:

menuentry 'Start Installer' {
   isofile=example.iso
   loopback loop (hd0,1)/$isofile
   linux (loop)/install/vmlinuz vga=normal quiet toram
   initrd (loop)/install/initrd.gz
}

^ That presumes you have saved the image to /example.iso on /dev/sda1, mutatis mutandis.

I'm not 100% sure if the toram option will work with the installer kernel but it's needed to stop it being over-written during the installation process if you're installing to /dev/sda1. If you're installing to another partition then remove the toram option.

Unlock the root account by assigning a password:

sudo passwd root

I use this to generate a menu entry for /usr/bin/x-terminal/emulator:

# ~/.local/share/applications/x-terminal-emulator.desktop
[Desktop Entry]
Type=Application
Encoding=UTF-8
Name=x-terminal-emulator
Comment=Terminal emulator
Exec=x-terminal-emulator
Icon=terminal
Categories=System;

Then set xterm to the x-terminal-emulator alternative:

# update-alternatives --set x-terminal-emulator /usr/bin/lxterm

Or you could just back up the hard disk from the live system, eg over the network to one of your existing systems.

Or use the live system to edit /etc/shadow on the new system to set the root password hash to the same as on a system where you know the root password.

Chris

But it is filled with Gnome bloatware and garbage, and I am hampered in investigating what is there by not knowing the password set up by the vendor, and Ubuntu's ridiculous attitude about root accounts. I had intended to install Devuan in an empty machine, using an iso file on a USB stick, and of course it is still possible to do that; however, I am interested in finding our whether one can make the iso file accessible to the existing Ubuntu system, and install Devuan by overwriting from there. Has anyone any ideas on that?

In passing, I must comment that if I wanted a desktop system filled with bloatware, and a Gnome menu devoid of xterms, I might as well stay with Windows. Linux does of course allow access to tty's using function keys, but that isn't any use without a password to log in. I can get the password, either from the vendor, or by diverting the boot process, and would like to investigate what Ubuntu has provided, before I erase it and set up my familiar Devuan.