Set default kernel in GRUB

Question

How can I pick which kernel GRUB2 should load by default? I recently installed a the linux realtime kernel and now it loads by default. I'd like to load the regular one by default.

So far I only managed to pick the default OS.. and for some reason the /boot/grub.cfg already assumes that I want to load the rt-kernel and put it into the generic linux menu entry (in my case Arch Linux).

Answer 1

As mentioned in the comments, you can set the default kernel to boot into using the grub-set-default X command, where X is the number of the kernel you want to boot into. In some distributions you can also set this number by editing the /etc/default/grub file and setting GRUB_DEFAULT=X, and then running update-grub.

The number is the index to an array of kernels/kernel settings shown in the GRUB menu during boot, with 0 being the first (top-most) entry. You can usually find the right number by looking for menuentry lines in /boot/grub/grub.cfg, like so:

grep menuentry /boot/grub/grub.cfg

You'll see each kernel listed with the name that is shown in the GRUB boot menu. The first one is 0, the second is 1, and so on.

Answer 2

I think most distributions have moved additional kernels into the advanced options sub menu at this point, as TomTom found was the case with his Arch.

I didn't want to alter my top level menu structure in order to select a previous kernel as the default. I found the answer here:

http://www.humans-enabled.com/2014/08/how-to-set-default-grub-kernel-boot.html

To summarize:

1) Find the $menuentry_id_option for the submenu:

grep submenu /boot/grub/grub.cfg

submenu 'Advanced options for Debian GNU/Linux' $menuentry_id_option 'gnulinux-advanced-65c9af03-3d9b-411c-99b2-a9ada0961a40' {

2) Find the $menuentry_id_option for the menu entry for the kernel you want to use:

grep gnulinux /boot/grub/grub.cfg

excerpt: ... menuentry 'Debian GNU/Linux, with Linux 4.7.0-1-amd64' --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.7.0-1-amd64-advanced-65c9af03-3d9b-411c-99b2-a9ada0961a40' { ...

3) Comment out your current default grub in /etc/default/grub and replace it with the sub-menu's $menuentry_id_option from step one, and the selected kernel's $menuentry_id_option from step two separated by ">".

In my case the modified GRUB_DEFAULT is:

#GRUB_DEFAULT=0

GRUB_DEFAULT="gnulinux-advanced-65c9af03-3d9b-411c-99b2-a9ada0961a40>gnulinux-4.7.0-1-amd64-advanced-65c9af03-3d9b-411c-99b2-a9ada0961a40"

4) Update grub to make the changes. For Debian this is done like so:

sudo update-grub

Done. Now when you boot, the advanced menu should have an asterisk and you should boot into the selected kernel. You can confirm this with uname.

uname -a

Linux NAME 4.7.0-1-amd64 #1 SMP Debian 4.7.8-1 (2016-10-19) x86_64 GNU/Linux

Changing this to the most recent kernel is as simple as commenting out the new line and uncommenting #GRUB_DEFAULT=0, then rerunning update-grub.

Answer 3

Simply doing grep 'menuentry' /boot/grub/grub.cfg lists additional entries that are not related to the actual kernel or OS versions. For instance, it lists

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
  menuentry_id_option=""
export menuentry_id_option

I would like to propose a small improvement to the method of searching that file: use awk

$ awk '/menuentry/ && /class/ {count++; print count-1"****"$0 }' /boot/grub/grub.cfg                                            
0****menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-86df21bf-d95f-435c-9292-273bdbcba056' {
1****   menuentry 'Ubuntu, with Linux 3.19.0-26-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.19.0-26-generic-advanced-86df21bf-d95f-435c-9292-273bdbcba056' {
2****   menuentry 'Ubuntu, with Linux 3.19.0-26-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.19.0-26-generic-recovery-86df21bf-d95f-435c-9292-273bdbcba056' {
3****   menuentry 'Ubuntu, with Linux 3.13.0-62-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-62-generic-advanced-86df21bf-d95f-435c-9292-273bdbcba056' {
4****   menuentry 'Ubuntu, with Linux 3.13.0-62-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-62-generic-recovery-86df21bf-d95f-435c-9292-273bdbcba056' {

With the command bellow and its output, you can see the awk code match actual OS version, and give you the number which you then may use in /etc/default/grub file.

In addition to editing the /etc/default/grub file by hand, I suggest using sed. In the command bellow, replace X with appropriate number you got from the awk command above:

sudo sed -i 's/GRUB_DEFAULT=0/GRUB_DEFAULT=X/g' /etc/default/grub; sudo update-grub

Answer 4

jkt123's will work for most distributions I guess. However for Arch Linux it didn't work, at least not with the packages I have available.

The indices you can set with grub-set-default only correspond to the main menu entries. The kernel options are however in a submenu. So either you move the kernel entry out of the submenu into the main menu or you put the entry on top of the submenu list and select the submenu.

My Grub Menu

• Arch Linux
• Advanced options for Arch Linux
  • Kernel 1
  • Kernel 2
• Windows

To be able to boot Kernel 2 you have to either swap it with Kernel 1 or you put it outside the submenu on the same level as Arch Linux or Windows. And then set the default number to one of the main menu indices. For example in the menu above "0" boots "Arch Linux" and "1" boots Kernel 1.

To change the hierarchy and swap, open /boot/grub/grub.cfg and move the entry you wish to move. An entry could look like this

menuentry 'ENTRY NAME'
   ... some code ...
}

Then you need to apply your changes. In my case with grub-mkconfig. But this might vary from system to system.

Answer 5

As recently as CentOS-6.6 grub.conf looked like this can could be manually edited to change the default selection:

default=1
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz