Bootloader configuration for multiple .iso files on a USB drive.

grub_iso.sh

#!/bin/sh
# Use grub to boot .iso files directly from /boot/iso.
#
# This file lives at /etc/grub.d/25_iso and is executable.
#
# Open .iso files to inspect internal /boot files for grub config hints.
# /boot is on the third partition of a gpt-partitioned USB drive.

ISO_PART_UUID="0a5086cd-8fdf-4ae0-a248-67411ebbbb18" # UUID of /boot `blkid`.
ISO_PART_NUMBER=3

ISO_HINT_BAREMETAL="ahci0,gpt${ISO_PART_NUMBER}"
ISO_HINT_BIOS="hd0,gpt${ISO_PART_NUMBER}"
ISO_HINT_EFI="hd0,gpt${ISO_PART_NUMBER}"

iso() {
    filename="$1"
    class="$2"

    if [ $# -ge 3 ]; then
        platform="$3"
    else
        platform=""
    fi

    if [ -n "$platform" ]; then
        echo "if [ \"\${grub_platform}\" = \"${platform}\" ]; then"
    fi

    cat <<EOF
menuentry "$filename" --class "$class" {
	gfxmode \$linux_gfx_mode
	insmod part_gpt
	insmod ext2

	set iso="/iso/$filename"
	set root=$ISO_HINT_BIOS

	if [ x\$feature_platform_search_hint = xy ]; then
		search --no-floppy --fs-uuid --set=root --hint-bios=$ISO_HINT_BIOS --hint-efi=$ISO_HINT_EFI --hint-baremetal=$ISO_HINT_BAREMETAL $ISO_PART_UUID
	else
		search --no-floppy --fs-uuid --set=root $ISO_PART_UUID
	fi

	loopback loop (\${root})\${iso}
EOF

    cat -

    echo "}"

    if [ -n "$platform" ]; then
        echo 'fi'
    fi

    echo
}

freenas() {
    iso "$1" freebsd pc <<EOF
	insmod ufs2
	insmod iso9660

	kfreebsd          (loop)/boot/kernel/kernel
	kfreebsd_loadenv  (loop)/boot/device.hints
	kfreebsd_loadenv  (loop)/boot/loader.conf
	kfreebsd_module   (\${root})\${iso} type=mfs_root

	set FreeBSD.vfs.root.mountfrom=cd9660:/dev/md0
	set FreeBSD.vfs.root.mountfrom.options=ro
EOF
}
 
iso FreeDOS-1.3-LIVE.iso windows pc <<EOF
	insmod fat

	linux16  (loop)/isolinux/memdisk
	initrd16 (loop)/isolinux/fdlive.img
EOF

freenas FreeNAS-11.2-U8.iso
freenas FreeNAS-11.3-U5.iso

iso kali-linux-2021.2-live-amd64.iso kali <<EOF
	linux   (loop)/live/vmlinuz boot=live components quiet splash noeject findiso=\$iso
	initrd  (loop)/live/initrd.img
EOF

iso kali-linux-2021.2-live-i386.iso kali pc <<EOF
	linux   (loop)/live/vmlinuz boot=live components quiet splash noeject findiso=\$iso
	initrd  (loop)/live/initrd.img
EOF

iso KNOPPIX_V9.1DVD-2021-01-25-EN.iso knoppix pc <<EOF
	linux   (loop)/boot/isolinux/linux bootfrom=/dev/sda1\$iso acpi=off keyboard=us language-us
	initrd  (loop)/boot/isolinux/minirt.gz
EOF

freenas TrueNAS-12.0-U4.1.iso

iso ubuntu-20.04.2.0-desktop-amd64.iso ubuntu <<EOF
	linux   (loop)/casper/vmlinuz boot=casper iso-scan/filename=\$iso quiet noeject noprompt splash
	initrd  (loop)/casper/initrd
EOF

iso ubuntu-20.04.2-live-server-amd64.iso ubuntu <<EOF
	linux   (loop)/casper/vmlinuz boot=casper iso-scan/filename=\$iso quiet noeject noprompt splash
	initrd  (loop)/casper/initrd
EOF

This is how I manage my multi-boot USB drive; it's a personal configuration.

The KNOPPIX and i386 .iso menu entries require "legacy" (or "CSM") boot from BIOS on x86_64 systems (else GRUB complains about the 64-bit kernel and doesn't boot); to support this, I have a 2MB GRUB BIOS partition (GPT type ef02) alongside the EFI partition so that the resulting grub.cfg can be read in either "legacy" or UEFI modes.

Adjust ISO_PART_UUID and ISO_PART_NUMBER for the target USB drive. Download .iso files to the /iso directory inside the boot partition (typically mounted at /boot/iso).

Useful articles:

• https://blog.kylemanna.com/linux/boot-linux-isos-from-usb-sticks-using-grub/
• https://help.ubuntu.com/community/Grub2/ISOBoot/Examples

Protip: procure a USB 3.0 drive with (well) over 100MB/s read/write speeds; USB thumb drives have arrived.

GRUB snippets for reference:

• https://github.com/mbusb/multibootusb/tree/master/data/multibootusb/grub/menus

See also:

• https://github.com/ventoy/Ventoy