Grub2-reboot, which allows you to temporarily change the kernel, is convenient. After all, I researched GRUB2 in various ways.

1.First of all

Have you ever used Linux and experienced "I installed a new kernel module and it didn't boot" or "I changed the boot options I passed to the kernel and it didn't boot"?

The ** grub2-reboot ** command, which changes the kernel only at the next boot, is useful in this situation. It's especially useful in the cloud, where console access is restricted, as a subsequent reboot will revert to the original kernel. It is also useful when switching between multiple kernels for testing purposes.

It was a command that was unexpectedly unknown, so I was thinking of introducing it, but as I investigated it, I ended up writing the following.

1-1. Target environment

2. When is it convenient?

I explained it at the beginning, but I will explain it in a little more detail. When a server that has been normally started up to now suddenly stops starting, the following causes / triggers are possible.

When such a trouble occurs, it is necessary to troubleshoot the following procedure.

** How to troubleshoot when not starting **

  1. Reboot the server to display the console screen
  2. From the boot menu, select a kernel that has been booted
  3. After successful boot, change the default kernel

There is no problem when the server itself is at hand, but when it is in a remote location such as the cloud or data center, the console screen cannot be displayed and the old kernel may not be selected.

** Public Cloud: ** Some vendors do not offer console login functionality ** Data center: ** For servers that do not have a BMC / IPMI board or KVM device (PC switch) set, you need to go to the data center.

The grub2-reboot command only temporarily changes the boot kernel, so recovery is possible as long as you can reboot. However, be aware that the default kernel will also be updated when you install with Yum / DNF.

  1. GRUB2

The GRUB configuration file is basically /etc/grub.conf or /boot/grub/grub.conf for GRUB up to 6 series, and /etc/grub2.cfg for GRUB2 after 7 series. That's right. However, there is a difference between BIOS mode and UEFI mode. Especially in the case of physical servers, most of them are used in UEFI. I will explain about that.

3-1. GRUB2 configuration file

GRUB2 has many files, the following files are typical.

Of these, /etc/grub2.cfg is referenced when changing the default kernel. The general procedure for changing the default kernel in 7 series is as follows. 8 series uses grubby.

  1. Display the kernel list.

python


awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2.cfg
  1. Set the default kernel entry number.
grub2-set-default entry number
  1. /boot/grub2.cfg is a symbolic link, so to ensure reliability, specify the actual /boot/grub2/grub.cfg and rebuild.
grub2-mkconfig -o /boot/grub2/grub.cfg

** Tips </ font> ** 8 series requires grubby operation, but 7 series can also use grubby. However, older versions may not be available. </ font>

** Caution </ font> ** For GRUB up to 6 series, modified /boot/grub/grub.conf with an editor to change the default kernel. However, in GRUB2 you should not change /boot/grub2/grub.cfg by hand. Be sure to change it using commands such as grub2-mkconfig and grubby.

3-2. Watch out for UEFI mode

In BIOS mode, the explanation so far is fine, but in UEFI mode it is different. Generally, it is explained as follows, but in addition to the substance, the symbolic link is also different. explain in detail.

** BIOS boot **: /boot/grub2/grub.cfg ** UEFI boot **: /boot/efi/EFI/*/grub.cfg

3-2-1. Determine UEFI mode

You can determine if the server or virtual machine you are currently using is in UEFI mode with the following command: When I examined the cloud, AWS and Azure were in BIOS mode, and Oracle Cloud was in UEFI mode. I haven't checked all instance types, so it's just for reference.

In UEFI mode


$ ls -l /sys/firmware/efi
total 0
-r--r--r--.  1 root root 4096 Aug 13 10:00 config_table
drwxr-xr-x.  2 root root    0 Aug 13 09:06 efivars
-r--r--r--.  1 root root 4096 Aug 13 10:00 fw_platform_size
-r--r--r--.  1 root root 4096 Aug 13 10:00 fw_vendor
-r--r--r--.  1 root root 4096 Aug 13 10:00 runtime
drwxr-xr-x.  9 root root    0 Aug 13 10:00 runtime-map
-r--------.  1 root root 4096 Aug 13 10:00 systab
drwxr-xr-x. 29 root root    0 Aug 13 10:00 vars

In BIOS mode


$ ls -l /sys/firmware/efi
ls: cannot access /sys/firmware/efi: No such file or directory

3-2-2. UEFI mode entity file

The configuration file in UEFI mode was introduced as /boot/efi/EFI/*/grub.cfg. The asterisk part depends on the distribution.

Distribution file name
RHEL /boot/efi/EFI/redhat/grub.cfg
CentOS /boot/efi/EFI/centos/grub.cfg
Oracle Linux /boot/efi/EFI/redhat/grub.cfg
Amazon Linux /boot/efi/EFI/amzn/grub.cfg
Ubuntu /boot/grub/grub.cfg(UEFI/Common to BIOS)

3-2-3. Relationship between symbolic links and entities

Perhaps the confusing thing is symlinks. In UEFI mode, /etc/grub2-efi.cfg has been added.

BIOS mode


$ ls -l /etc/grub2*
lrwxrwxrwx. 1 root root 22 Sep  3 13:31 /etc/grub2.cfg -> ../boot/grub2/grub.cfg

UEFI mode


$ ls -l /etc/grub2*
lrwxrwxrwx. 1 root root 22 Sep  3 13:31 /etc/grub2.cfg -> ../boot/grub2/grub.cfg
lrwxrwxrwx. 1 root root 31 Sep  3 13:31 /etc/grub2-efi.cfg -> ../boot/efi/EFI/
centos/grub.cfg

If you're using symlinks, you're likely to do it. However, /etc/grub2.cfg refers to /boot/grub2/grub.cfg, but the entity does not exist.

$ ls -l /boot/efi/EFI/centos/grub.cfg
-rwx------. 1 root root 10223 Sep  3 13:35 /boot/efi/EFI/centos/grub.cfg
$ ls -l /boot/grub2/grub.cfg ★ does not exist
ls: cannot access /boot/grub2/grub.cfg: No such file or directory

So the next command will fail in UEFI mode.

python


awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2.cfg

It is necessary to recognize that it is UEFI and specify /etc/grub2-efi.cfg or /boot/efi/EFI/*/grub.cfg.

python


awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2-efi.cfg

3-3. Other files

The following files, which have not been explained so far, are explained. This time, the contents of CentOS 8 are displayed, but it is almost the same for 7 series.

  • /etc/default/grub
  • /boot/grub2/grubenv
  • /etc/sysconfig/kernel

3-3-1. /etc/default/grub This file contains the GRUB2 overall settings. Related to the default kernel is the ** GRUB_DEFAULT ** directive. The key is "** saved **".

GRUB_TIMEOUT=5 ★ Display time of kernel selection menu at startup
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved ★ The name of the directive used by the default kernel
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true

3-3-2. /boot/grub2/grubenv This file contains the default kernel settings. Since it was "** saved **" earlier, the kernel specified by the ** saved_entry ** directive becomes the default kernel.

In RHEL7 series, "menu number" or "menu name" is set, but in RHEL8 series, "ID + kernel version" is set.

7 series


# GRUB Environment Block
saved_entry=CentOS Linux (3.10.0-1062.el7.x86_64) 7 (Core)

8 series


# GRUB Environment Block
saved_entry=0aa24a7da6284581821b2215d0757580-4.18.0-147.el8.x86_64
kernelopts=root=/dev/mapper/cl-root ro resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet
boot_success=0
boot_indeterminate=0

In RHEL8 series, the matching configuration file exists here. This is a specification called BootLoaderSpecByDefault (BLS).

RHEL8 series only


# ls /boot/loader/entries/
0aa24a7da6284581821b2215d0757580-0-rescue.conf
0aa24a7da6284581821b2215d0757580-4.18.0-147.el8.x86_64.conf
0aa24a7da6284581821b2215d0757580-4.18.0-193.14.2.el8_2.x86_64.conf

3-3-3. /etc/sysconfig/kernel This file contains instructions on how to update the kernel.

# UPDATEDEFAULT specifies if new-kernel-pkg should make
# new kernels the default
UPDATEDEFAULT=yes

# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel-core

** UPDATEDEFAULT ** specifies whether to make the default kernel when the kernel is updated. If yes, it will be the default kernel, but if no, it will not be the default kernel.

For example, specify no when you do not want to use a new kernel even if you install an additional kernel with yum update.

In the first place, if you want to suppress the kernel installation itself, you should exclude it in the Yum / DNF configuration file as follows.

shell-session:/etc/yum.conf(/etc/dnf/dnf.conf)


exclude=kernel

Set the default kernel type in ** DEFAULTKERNEL **. The normal kernel is kernel and kernel-core, but the debug kernel is kernel-debug. Also, in UEK of Oracle Linux, it is kernel-uek.

4. Change the kernel to boot

I'm going to explain how to use grub2-reboot, but I've taken a detour. The method of changing the default kernel is almost the same, so I will explain it together. The main procedure is as follows.

  1. Examine the default kernel
  2. View the list of kernels
  3. Set the kernel to boot next (permanent or temporary)
  4. Reboot

The command to change the kernel differs depending on the version, so each will be explained. If you want to use grubby even in 7 series, please see the explanation of 8 series.

** 7 series: ** grub2-mkconfig / grub2-set-default or grubby ** 8 series: ** grubby

4-1. RHEL7 series (non-grubby)

4-1-1. Check the default kernel

The current default can be displayed with the grub2-editenv command. The value of the saved_entry directive is the default kernel.

# grub2-editenv list
saved_entry=CentOS Linux (3.10.0-1127.19.1.el7.x86_64) 7 (Core)
# uname -a
Linux hostname 3.10.0-1127.19.1.el7.x86_64 #1 SMP Tue Mar 17 23:49:17 UTC 2020
 x86_64 x86_64 x86_64 GNU/Linux

4-1-2. Display the list of kernels

Display a list of installed kernels. Combined with the previous results, index number 0 is the default kernel.

** BIOS mode **

# awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2.cfg

** UEFI mode **

# awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2-efi.cfg

Output example


0 : CentOS Linux (3.10.0-1127.19.1.el7.x86_64) 7 (Core)
1 : CentOS Linux (3.10.0-1062.18.1.el7.x86_64) 7 (Core)
2 : CentOS Linux (3.10.0-1062.el7.x86_64) 7 (Core)
3 : CentOS Linux (0-rescue-b5a029229fd6464ca40d2e93351d5c5f) 7 (Core)

4-1-3. Change the default kernel

In the following example, one version specifies an older entry number 1 kernel.

  1. Change the default kernel to entry number 1.
# grub2-set-default 1
  1. You can see that the default kernel has changed.
# grub2-editenv list
saved_entry=1

4-1-4. Reboot

Reboot for the modified kernel to take effect. That's it.

# systemctl reboot

4-2. RHEL7 / RHEL8 series (grubby)

4-2-1. Check the default kernel

grubby can display kernel filenames and indexes.

# grubby --default-kernel
/boot/vmlinuz-4.18.0-193.14.2.el8_2.x86_64
# grubby --default-index
0

4-2-2. Display the list of kernels

Display a list of installed kernels. This example also shows the next line that matches grep with the -A 2 option.

# grubby --info=ALL | grep -A 2 index
index=0
kernel="/boot/vmlinuz-4.18.0-193.14.2.el8_2.x86_64"
args="ro crashkernel=auto resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet $tuned_params"
--
index=1
kernel="/boot/vmlinuz-4.18.0-147.el8.x86_64"
args="ro crashkernel=auto resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet $tuned_params"
--
index=2
kernel="/boot/vmlinuz-0-rescue-0aa24a7da6284581821b2215d0757580"
args="ro crashkernel=auto resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"

4-2-3. Change the default kernel

To permanently change the default kernel, use the grubby command as follows: In this example, an equal (=) is specified between the --set-default option and the value, but a blank space may be used.

# grubby --set-default=/boot/vmlinuz-4.18.0-147.el8.x86_64

You can also specify the index number as follows.

# grubby --set-default-index=1

Confirm that it has been changed.

# grubby --default-kernel
/boot/vmlinuz-4.18.0-147.el8.x86_64

You can use --info to display the entire entry for the specified kernel. In this example, the kernel file name is specified, but the index can also be specified.

# grubby --info=/boot/vmlinuz-4.18.0-147.el8.x86_64
index=1
kernel="/boot/vmlinuz-4.18.0-147.el8.x86_64"
args="ro crashkernel=auto resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv
=cl/swap rhgb quiet $tuned_params"
root="/dev/mapper/cl-root"
initrd="/boot/initramfs-4.18.0-147.el8.x86_64.img $tuned_initrd"
title="CentOS Linux (4.18.0-147.el8.x86_64) 8 (Core)"
id="0aa24a7da6284581821b2215d0757580-4.18.0-147.el8.x86_64"

4-2-4. Reboot

Reboot for the modified kernel to take effect. That's it.

# systemctl reboot

4-3. Temporarily change the kernel

I changed the kernel permanently earlier, but you can change the kernel only at the next boot. Rebooting again will restore the original kernel, which is useful for the following purposes:

  • I want to change to another kernel for testing purposes
  • I want to use a debug kernel
  • I want to test the boot options passed to the kernel

Use ** grub2-reboot ** to make changes. Specify the menu entry number (index number) as follows.

grub2-reboot 1

In order to confirm the operation, a series of steps will be described.

  1. The current kernel version is "4.18.0-147".
# uname -r
4.18.0-147.el8.x86_64
  1. The default kernel is of course the same.
# grubby --default-kernel
/boot/vmlinuz-4.18.0-147.el8.x86_64

# grubby --default-index
1
  1. Use grub2-reboot to change the index to "0" only the next reboot.
# grub2-reboot 0
  1. The default kernel has not changed, so the index has not changed.
# grubby --default-index
1
  1. Reboot.
# systemctl reboot
  1. When I log in after rebooting, the kernel changes to index "0".
# uname -r
4.18.0-193.14.2.el8_2.x86_64

# grubby --info=/boot/vmlinuz-4.18.0-193.14.2.el8_2.x86_64
index=0
kernel="/boot/vmlinuz-4.18.0-193.14.2.el8_2.x86_64"
args="ro crashkernel=auto resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet $tuned_params"
root="/dev/mapper/cl-root"
initrd="/boot/initramfs-4.18.0-193.14.2.el8_2.x86_64.img $tuned_initrd"
title="CentOS Linux (4.18.0-193.14.2.el8_2.x86_64) 8 (Core)"
id="0aa24a7da6284581821b2215d0757580-4.18.0-193.14.2.el8_2.x86_64"
  1. Reboot again.
# systemctl reboot
  1. You are back to the default kernel. It was confirmed that grub2-reboot is effective only at the next reboot.
# uname -r
4.18.0-147.el8.x86_64

5. Summary

  • ** In GRUB2 after RHEL7, use a command to change the default kernel. Do not modify the file by hand. ** **
  • ** GRUB configuration file names are different between BIOS mode and UEFI mode. ** **
  • ** To change the default kernel, use grub2-mkconfig / grub2-set-default or grubby on RHEL7 series. Grubby is used for RHEL8 series. ** **
  • ** You can use the grub2-reboot command to temporarily change the kernel only at the next boot. These characteristics are useful for cloud and testing where console access is difficult. ** **

6. Reference