This is an example on how to use GRUB2 as a bootstrap loader. This allows you to burn ISO9660 files within a partition rather than to burn it on the entire disk, creating the possiblity to have a single flash drive with multiple ISOs to boot from. It's like Ventoy, but using a different approach, maybe more suitable for other use cases.
For now, this project covers only UEFI-compatible system compatibility. For legacy/BIOS systems, take a look here: stefan-golinschi/microserver-bootstrap.
Disk partitioning is done using gdisk, by creating an EFI system partition for the bootloader and some other partitions, depending on your specific requirements.
A sample partition layout is layout.sfdisk, which can be easily imported using sfdisk like this:
sfdisk /dev/sdg < layout.sfdisk
Remember, the partition code for EFI ESP is ef00.
sudo mkfs.vfat -F32 /dev/sdg1
If you use Fedora, you will need to install this package grub2-efi-x64.x86_64.
sudo mount /dev/sdg1 data
sudo grub2-install --target=x86_64-efi --efi-directory=./data/ --boot-directory=./data/boot --removable --force /dev/sdg
# TODO: For legacy/BIOS systems
# sudo grub2-install --target=i386-pc --boot-directory=./data/boot --removable /dev/sdg
# sudo grub2-install --target=i386-efi --efi-directory=./data/ --boot-directory=./data/boot --removable --force /dev/sdg
For example, we will use two alpine linux ISOs.
sudo dd if=alpine-standard-3.19.1-x86_64.iso of=/dev/sdg2 status=progress
sudo dd if=alpine-extended-3.18.3-x86_64.iso of=/dev/sdg3 status=progress
After this, make sure to update the grub.cfg file accordingly. For example, I used GRUB2's search --label function to set the root for each menuentry.
If you want to know the labels of the ISOs, you can use file alpine-standard-3.19.1-x86_64.iso, or you can do a lsblk -o +LABEL after you burn the ISOs to the flash drive.
Finally, copy the grub config file to the EFI ESP partition on the flash drive.
sudo cp grub.cfg data/boot/grub2/
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