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:
[root@web01 ~]# 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:
[root@web01~]# cp /etc/default/grub /etc/default/grub.bak [root@web01~]# vim /etc/default/grub ... GRUB_DEFAULT=saved ...
Now create a backup of the grub config for recovery purposes if needed, then rebuild grub:
[root@web01 ~]# cp /boot/grub2/grub.cfg /boot/grub2/grub.cfg.bak [root@web01 ~]# grub2-mkconfig -o /boot/grub2/grub.cfg
Determine what the full kernel name is you want to use. Get the listing by running:
[root@web01~]# 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:
[root@web01 ~]# 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:
[root@web01 ~]# 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:
root@web01:~# reboot
Finally, once the system comes back online, verify the desired kernel is running by:
[root@web01 ~]# 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
First, check to see which kernel is currently running:
root@web01:~# uname -r 4.4.0-48-generic
That shows us we’re running 4.4.0-48, however I need to be running 4.4.0-47. So to use this specific named kernel, first changed the GRUB_DEFAULT to ‘saved’ in /etc/default/grub by:
root@web01:~# cp /etc/default/grub /etc/default/grub.bak root@web01:~# vim /etc/default/grub ... GRUB_DEFAULT=saved ...
Now create a backup of the grub config for recovery purposes if needed, then rebuild grub:
root@web01:~# cp /boot/grub/grub.cfg /boot/grub/grub.cfg.bak root@web01:~# update-grub
Determine what the full kernel name is you want to use. Get the listing by running:
root@web01:~# egrep "^[[:space:]]?(submenu|menuentry)" /boot/grub/grub.cfg | cut -d "'" -f2 Ubuntu Advanced options for Ubuntu Ubuntu, with Linux 4.4.0-78-generic Ubuntu, with Linux 4.4.0-78-generic (recovery mode) Ubuntu, with Linux 4.4.0-47-generic Ubuntu, with Linux 4.4.0-47-generic (recovery mode)
I want to use the 4.4.0-47-generic kernel. Setting that to be the default kernel is simple. However, you MUST prepend ‘Advanced options for Ubuntu’ to the kernel name as shown below since Ubuntu makes use of sub menus in the kernel listing. So set the desired kernel by running:
root@web01:~# grub-set-default 'Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-47-generic'
Now verify that the change got applied in the configs by running:
root@web01:~# grub-editenv list saved_entry=Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-47-generic
Reboot the system so it boots off the older kernel:
root@web01:~# reboot
Finally, once the system comes back online, verify the desired kernel is running by:
root@web01:~# uname -r 4.4.0-47-generic
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