Create a bootable .iso from a folder in Linux

If you have ever tried to modify a Linux installer you will know that you must mount the .iso file:

sudo mkdir /mnt/ubuntu
sudo mount -o loop ubuntu-xxx.iso /mnt/ubuntu

then copy it to another directory:

sudo cp -ra /mnt/ubuntu ~/ubuntu

because you can only mount it read-only. Then you can make your modifications to the files in the copied folder. In my case I provided a default value for the keyboard (US) to see if the installer would skip asking me for the setting. Then the bootable .iso must be recreated from the modified folder. But how to do it? I used xorriso:

sudo xorriso -as mkisofs \
   -J -R -V 'UNUNTU-20.04 [CUSTOM]' \
   -o ubuntu-20.04.custom.iso \
   -J -joliet-long -cache-inodes \
   -isohybrid-mbr isohdpfx.bin \
   -b isolinux/isolinux.bin \
   -c isolinux/boot.cat \
   -boot-load-size 4 -boot-info-table -no-emul-boot \
   -eltorito-alt-boot \
   -e boot/grub/efi.img \
   -no-emul-boot -isohybrid-gpt-basdat -isohybrid-apm-hfsplus \
   "ubuntu"

Note the use of the file isohdpfx.bin. I couldn't locate this in the Ubuntu distribution so I just copied the first 432 bytes of the existing Ubuntu .iso installer and saved it using hexedit. Without this it will fail to mount and hence won't be bootable. Now make an installable USB out of the .iso using a tool like Startup Disk Creator or Balena Etcher and it will boot happily from it.