On Linux
Installing mkisofs
apt-get install mkisofs
Editing ISO image
mkdir /tmp/custom_iso
cd /tmp/custom_iso/
mount -t iso9660 -o loop ~/original.iso /mnt/
cd /mnt
mnt is read-only, thus we need to copy the files in it in order to modify the files.
tar cf - . | (cd /tmp/custom_iso; tar xfp -)
You can now modify the files for making a preseed for example!
Back in .iso
If not, go back to your /tmp/custom_iso folder
cd /tmp/custom_iso
Copy this line, modify it and paste it in order to create the .iso
mkisofs -o output.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -J -R -V "Ubuntu Custom ISO Preseed" .
Make it bootable with isohybrid
Install isohybrid:
sudo apt install syslinux-utils
Then simply type the following to make it bootable:
sudo isohybrid output.iso
Creating UEFI-Bootable ISO
use xorriso to create an ISO from directory:
xorriso -as mkisofs -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin -c isolinux/boot.cat -b isolinux/isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table -eltorito-alt-boot -e boot/grub/efi.img -no-emul-boot -isohybrid-gpt-basdat -o /dest/iso /tmp/ubnt-custom/
On macOS
Install brew
Follow https://brew.sh/
Install requirements
In order to install mkisofs, you need to get the cdrtools package for macOS:
brew install cdrtools
Prepare folders
Make two folders: iso and custom.
ro_iso is going to be where the files of the iso will be mounted, read only.
custom will be where you will be able to modify the files
mkdir ro_iso && mkdir custom
Attaching, mounting and editing
hdiutil attach -nomount ubuntu.iso
List the disks and get the disk image name usually /dev/disk2:
diskutil list
We will use /dev/disk2.
Now you can mount the disk into the read-only folder:
mount -t cd9660 /dev/disk2 ro_iso
We now need to copy the files in the custom folder in order to modify the files:
tar cf - . | (cd $HOME/yourfolder/custom; tar xfp -)
You can now begin editing the files.
Back in .iso
If not yet, go back to your custom folder.
Copy this line, modify it and paste it in order to create the .iso
sudo mkisofs -o output.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -J -R -V "Ubuntu Custom ISO Preseed" .
Make it bootable with isohybrid
Use the isohybrid.pl file of this gist like the following:
sudo perl isohybrid.pl output.iso
umount and detach
Don't forget to do it, or you will can't be able to delete the ro_iso folder
umount /dev/disk2
hdiutil detach /dev/disk2
You now have a bootable USB iso file :)