Comment 39 for bug 1922342

Hi,

just in case somebody wants to make experiments with changed grub.cfg,
here are instructions how to create a modified ISO from an Ubuntu amd64
original (but not from a "powerpc" ISO):

Set paths to the original ISO image and for the emerging ISO image:

  ORIG=ubuntu-21.04-desktop-amd64.iso
  NEW=ubuntu-21.04-test.iso

Extract grub.cfg from the original ISO to the current working directory:

  xorriso -indev "$ORIG" -osirrox on -extract /boot/grub/grub.cfg ./grub.cfg

Make the extracted file writable:

  chmod u+w ./grub.cfg

Now edit the file.
My proposal is to remove the 5 lines about grub_platform.

Make sure that a file with path "$NEW" does not yet exist.
Then create the new modified ISO:

  xorriso -indev "$ORIG" \
          -outdev "$NEW" \
          -map ./grub.cfg /boot/grub/grub.cfg \
          -boot_image any replay

This run of xorriso will issue some righteous warnings
  xorriso : WARNING : -volid text problematic as automatic mount point name
  xorriso : WARNING : -volid text does not comply to ISO 9660 / ECMA 119 rules
because of the blanks and lower case characters in
  Volume id : 'Ubuntu 21.04 amd64'
It will analyze "$ORIG" for boot equipment and boot entry points in order to
create the same in "$NEW"
  xorriso : NOTE : Replayed 22 boot related commands
  xorriso : NOTE : Copying to System Area: 32768 bytes from file '--interval:imported_iso:0s-15s:zero_mbrpt,zero_gpt:ubuntu-21.04-desktop-amd64.iso'

Then it begins to write to "$NEW" an ISO image of roughly the same size as
the size of "$ORIG". Enjoy the progress messages. :))

When done, put "$NEW" onto the USB stick and try what happens.
(Maybe mount "$NEW" and check whether your changes really did get into
 effect in /boot/grub/grub.cfg, before the lengthy dd to USB stick.)

Have a nice day :)

Thomas