Accordind to
archwiki , "a boot loader is the first software program that runs when a computer starts. It is responsible for selecting, loading and transferring control to an operating system kernel."
Installing a bootloader is often considered a bit complicated and risky task. And strangely enough, it's a common practice to install it after the main system installed. The situation is even more complicated if a user has other system(s) on a hard disk, and installing a new system may be not so easy.
Let's consider grub bootloader. It may be installed from obarun.iso before the main system. Then a simple grub.config will be created manually for loading obarun (and any other system(s) if needed).
Caution!: Tested for a hard disk (and a usb flash drive) with MBR. For the UEFI, GPT it is recommended to read and understand their bootloading.
1. Boot obarun.iso and, as a root user, mount a hard disk partition for boot (it must be any primary partition - for example, sda1) to /mnt folder:
# mount /dev/sda1 /mnt
2. Create a boot directory:
# mkdir /mnt/boot_dir
3. Install grub to MBR of the hard disk (/dev/sda) and grub files to a boot directory:
# grub-install --target=i386-pc /dev/sda --boot-directory=/mnt/boot_dir
Note: For other --target=TARGET, see # grub-install --help
4. Make a simple grub config, for example:
# nano /mnt/boot_dir/grub/grub.cfg
# Default entry 0
set default=0
# Defaul entry loading in 5 sec
set timeout=5
menuentry "Obarun" {
UUID=4e2695d1-9838-423b-9063-8cg797b406c1 # Note: run # blkid in terminal to get UUID of Obarun partition
search --fs-uuid $UUID --set root
linux /boot/vmlinuz-linux root=UUID=$UUID ro
initrd /boot/initramfs-linux.img
}
# menuentry "Gentoo" {
# set root=(hd0,8)
# linux /boot/vmlinuz-4.12.12-gentoo root=/dev/sda8
# initrd /boot/initramfs-genkernel-x86_64-4.12.12-gentoo
# }
5. Unmount the boot partition:
# umount /mnt
P.S. For menu entries for other systems, search archwiki and the internet.