Make a USB drive that can boot Ubuntu ISOs

grub.cfg

# this goes at boot/grub/grub.cfg
# inspired by https://github.com/thias/glim

insmod all_video

set gfxpayload=keep

loadfont unicode

terminal_output gfxterm

# https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1851311
rmmod tpm

for isofile in /ubuntu-*.iso; do
  # check if the glob didn't expand
  if [ ! -e "${isofile}" ]; then break; fi

  menuentry "${isofile}" "${isofile}" {
    # title is $1
    set isofile=$2

    # loopback.cfg expects `iso_path` to be set
    set iso_path="${isofile}"
    export iso_path
  
    loopback loop "${isofile}"
    set root=(loop)
    configfile /boot/grub/loopback.cfg
    loopback --delete loop
  }
done

menuentry "Shut down" {
  halt
}

menuentry "Restart" {
  reboot
}

mkisoboot

#!/bin/bash

# Assume: a formatted volume with the label "IsoBoot"
# I used FAT32.

TARGET=/media/ubuntu/IsoBoot

sudo grub-install \
    --target=x86_64-efi \
    --uefi-secure-boot \
    --efi-directory="${TARGET}" \
    --boot-directory="${TARGET}/boot" \
    --removable

(cd "${TARGET}" && curl -L -O 'https://releases.ubuntu.com/20.04/ubuntu-20.04-desktop-amd64.iso')