How to create a bootable disk image that works with bios and uefi

make-bootable-image.sh

#!/bin/sh
# apt install gdisk syslinux syslinux-efi mtools
IMG=boot-image.img
SIZE=300
KERNEL=vmlinz
INITRAMFS=initrd.img
MD="mmd -i ${IMG}@@1M"
CP="mcopy -i ${IMG}@@1M"
RN="mren -i ${IMG}@@1M"
dd if=/dev/zero bs=1M count=$SIZE of=$IMG
sgdisk -n 1::+$(($SIZE-2))M -c 1:EFI-SYSTEM -t 1:ef00 --hybrid=1 --attributes=1:set:2 ${IMG}
mkfs.fat -S 512 --offset 2048 ${IMG} -n EFI-SYSTEM
$MD ::efi
$MD ::efi/boot
$CP /usr/lib/SYSLINUX.EFI/efi64/syslinux.efi ::efi/boot/bootx64.efi
$CP /usr/lib/syslinux/modules/efi64/ldlinux.e64 ::efi/boot/ldlinux.e64
$MD ::system
$CP $INITRAMFS $KERNEL ::system
dd bs=440 count=1 conv=notrunc if=/usr/lib/syslinux/mbr/gptmbr.bin of=${IMG}
syslinux --offset $((1024*1024)) --install ${IMG}
$CP syslinux.cfg ::syslinux.cfg

syslinux.cfg

DEFAULT linux
LABEL linux
 SAY Now booting the kernel from SYSLINUX...
 KERNEL /system/vmlinuz
 APPEND initrd=/system/initrd.img

After quite some tinkering I came up with this little script for creating bootable disk images, that work both in MBR BIOS and in UEFI setups. The cool thing about this script is that it does NOT require any privileged access. No loop back mounting or such.

If you don't know where to get the KERNEL and INITRAMFS files, look in your /boot directory