Setting default kernel in grub2

With newer systems like CentOS 7 and Ubuntu 14.04 and 16.04 using Grub2, you can no longer simply update a single file to have your kernel boot off an older or newer kernel. There are a series of steps that must be followed. The examples below will show how to boot off an older kernel for their respective operating systems.

Please note, the instructions in this article will lock your kernel on whichever one you selected. Even if your system receives automatic kernel updates, those new kernels will have to be manually enabled within grub if you want to use them.

CentOS 7

First, check to see which kernel is currently running:

[[email protected] ~]# uname -r
3.10.0-514.16.1.el7.x86_64

That shows us we’re running 3.10.0-514.16.1, however I need to be running 3.10.0-327.36.3. So to use this specific named kernel, first changed the GRUB_DEFAULT to ‘saved’ in /etc/default/grub by:

[[email protected]~]# cp /etc/default/grub /etc/default/grub.bak
[r[email protected]~]# vim /etc/default/grub
...
GRUB_DEFAULT=saved
...

Now create a backup of the grub config for recovery purposes if needed, then rebuild grub:

[[email protected] ~]# cp /boot/grub2/grub.cfg /boot/grub2/grub.cfg.bak
[[email protected] ~]# grub2-mkconfig -o /boot/grub2/grub.cfg

Determine what the full kernel name is you want to use. Get the listing by running:

[[email protected]~]# grep "^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2
CentOS Linux (3.10.0-514.16.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-514.2.2.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-327.36.3.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-327.22.2.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-327.3.1.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-c11d017c89ca4e8685ae3d9791d472ca) 7 (Core)

I want to use the 3.10.0-327.36.3 kernel. Setting that to be the default kernel is simple. Set the desired kernel by running:

[[email protected] ~]# grub2-set-default "CentOS Linux (3.10.0-327.36.3.el7.x86_64) 7 (Core)"

Now verify that the change got applied in the configs by running:

[[email protected] ~]# grub2-editenv list
saved_entry=CentOS Linux (3.10.0-327.36.3.el7.x86_64) 7 (Core)

Reboot the system so it boots off the older kernel:

[email protected]:~# reboot

Finally, once the system comes back online, verify the desired kernel is running by:

[[email protected] ~]# uname -r
3.10.0-327.36.3.el7.x86_64

If the system rebooted, and dropped you into a grub shell with an error, you can boot up off of the backup grub.cfg file that was created by:

grub2> configfile (hd0,1)/boot/grub2/grub.cfg.bak

Ubuntu 14.04 and Ubuntu 16.04