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How can we add a kernel to grub2? i knew the stuff we could do with the former versions, but it seems to have changed. What files should we edit?

Amir Zadeh
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7 Answers7

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To add a new kernel to grub2:

  1. Move your kernel to /boot/
  2. Run sudo update-grub

update-grub will scan your computer for kernels, and create a listing of available kernels at boot.

In order for you to select which kernel to boot at boot time, you may have to hold the SHIFT button down right after your BIOS does its posting.

You can edit /etc/default/grub to change default boot options and parameters that you may need.

Non-Debian Linux

See the grub notes for your distribution, such as these Fedora GRUB 2 docs.

  • grub2-mkconfig -o /boot/grub2/grub.cfg
earthmeLon
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  • What if you have 2 Linux distros without a separate /boot partition? Do you have to copy the kernel from the secondary rootfs to the primary one? Or does 'grub-update' also check other partitions for new kernels? – jiggunjer Jan 28 '17 at 09:22
  • This is a separate question, but either executable used should read `/boot/` and generate correctly. The grub configuration file (ex: `/etc/default/grub` ) might be different per OS, so make sure you sync them up. Best practice might be to have a single OS generate grub entries for you, however. – earthmeLon Jan 28 '17 at 18:51
  • Not really a separate question, OP didn't specify his configuration, and dual boots a are common one. Thought it might add value to your answer, which is already good. So in other words, the secondary OS doesn't even need it's own grub, the grub from the primary one will scan for /boot folders on all partitions and find the kernels. – jiggunjer Jan 28 '17 at 19:03
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    How do I move a kernel to /boot? `mv kernel /boot`? Where do I find a kernel that could be moved to /boot? – Thomas Weller Jan 10 '18 at 22:02
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I'm running Fedora 20, and the command to update grub2 is grub2-mkconfig. Also, the entire process can be accomplished with the kernel build make:

make xconfig (make config, etc)
make bzImage
make modules
sudo make modules_install
sudu make install

This (last step) will copy the kernel into /boot, and update the grub2 boot loader. Very easy, and it worked correctly in my case. My only issue is that you don't really learn any important details of the process, everything is automated. If you have problems and have to figure out what's wrong, you could get stuck.

Arjan
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philbrooksjazz
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Try update-grub or update-grub2 depending on you grub version. You will have to run these as root, sudo. This worked for me when I installed a second Linux distro without reinstalling grub.

beatgammit
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For Grub2, the file you should edit is called 40_custom and it's located at /etc/grub.d/. As this file states:

# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment.

Here is an example of what a menu entry looks like:

menuentry 'Debian with Linux 5.10.0-21-amd64 (in /dev/sda8)' --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-97a0ac1a-9298-4dfc-baf5-ae8cfb37e3f0' {
    insmod part_gpt
    insmod ext2
    set root='hd0,gpt8'
    if [ x$feature_platform_search_hint = xy ]; then
        search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt8 --hint-efi=hd0,gpt8 --hint-baremetal=ahci0,gpt8  97a0ac1a-9298-4dfc-baf5-ae8cfb37e3f0
    else
        search --no-floppy --fs-uuid --set=root 97a0ac1a-9298-4dfc-baf5-ae8cfb37e3f0
    fi
    linux /boot/vmlinuz-5.10.0-21-amd64 root=UUID=97a0ac1a-9298-4dfc-baf5-ae8cfb37e3f0 ro quiet
    initrd /boot/initrd.img-5.10.0-21-amd64
}

You add your own modified menu entry (without deleting 40_custom's initial content) and then you run sudo update-grub2 or grub2-mkconfig -o /boot/grub2/grub.cfg (they do the same thing). To confirm that your entry was added, you can check the /boot/grub/grub.cfg file:

...
### BEGIN /etc/grub.d/40_custom ###
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

>>Your entry should appear here<<

### END /etc/grub.d/40_custom ###
...
gene.bustam
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/boot/grub/grub.cfg is the file that replaced menu.lst from grub1

In Debian/Ubuntu systems this is generated by update-grub, which runs the scripts in /etc/grub.d using something similar to run-parts.

Flexo
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If the new kernel is installed with dpkg (as if it's compiled with make-kpkg), update-grub2 is enough (it removes no more existent kernels, too)

Daniele Santi
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You don't "edit" to add kernels anymore. It scans and adds them dynamically. If you MUST add one a scan doesn't find you should look in /etc/grub.d and modify or copy and custom40(it is added last so it's a great place to test your config before you put it at the top of your list).

RobotHumans
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