How do I change the default kernel in GRUB that is loaded at startup?

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux 4
  • Red Hat Enterprise Linux 5
  • Red Hat Enterprise Linux 6
  • Red Hat Enterprise Linux 7
  • Red Hat Enterprise Linux 8
  • Red Hat Enterprise Linux 9
  • Red Hat Enterprise Linux 10

Issue

  • The kernel booted by default (using GRUB) is different from the one you would like to be automatically selected on startup. How do I change the default kernel in GRUB that is loaded at startup?

Resolution

  • Use the command grubby --bootloader-probe to find out which bootloader you have installed.

For RHEL 4, 5, 6 Systems:

  • GRUB is the most common bootloader for RHEL 4, 5, 6 systems
    Below is an example GRUB configuration file:

        default=0                                                  <===
    timeout=10
    splashimage=(hd0,0)/grub/splash.xpm.gz

    title Red Hat Enterprise Linux ES (kernel number zero)
            root (hd0,0)
            kernel /vmlinuz-zero ro root=/dev/hard_drive
            initrd /initrd-zero.img
        
    title Red Hat Enterprise Linux ES (kernel number one)
            root (hd0,0)
            kernel /vmlinuz-one ro root=/dev/hard_drive
            initrd /initrd-one.img

  • In this example file, notice the line at the top that reads default=0. The number 0 (zero) indicates which stanza to select by default. A stanza is the indented portion after the line starting with title. GRUB will then use this default stanza to boot, after a number of seconds has passed (specified in the line timeout=10). The 0 (zero) in this case is referring to the first stanza that starts with "title Red Hat Enterprise Linux ES (kernel number zero)". It includes all of the indented lines up to but not including the next "title" line.

  • For example, to instead set the second stanza, "title Red Hat Enterprise Linux ES (kernel number one)", to be the default, change the default line to:

    default=1

  • The effect of changes will be seen on the next boot.

For RHEL 7 Systems:

  • GRUB2 is the supported bootloader for x86 RHEL 7 systems

  • Note that ordering of menu entries means very little in GRUB2. By default, each new kernel package install changes the default kernel regardless of its location in the ordering of kernel menu entries. To override this, use grub2-set-default 'ID', where ID is either a number or a name, e.g.:

    # grub2-set-default 'Red Hat Enterprise Linux Server, with Linux 3.10.0-123.el7.x86_64'

  • By default, the key for the GRUB_DEFAULT directive in the /etc/default/grub file is the word saved. This instructs GRUB 2 to load the kernel specified by the saved_entry directive in the GRUB 2 environment file, located at /boot/grub2/grubenv. One can set another GRUB record to be the default, using the grub2-set-default command, which will update the GRUB 2 environment file.

  • By default, the saved_entry value is set to the name of latest installed kernel of package type kernel. This is defined in /etc/sysconfig/kernel by the UPDATEDEFAULT and DEFAULTKERNEL directives. The file can be viewed by the root user as follows:

      $ cat /etc/sysconfig/kernel
  # UPDATEDEFAULT specifies if new-kernel-pkg should make
  # new kernels the default
  UPDATEDEFAULT=yes
  
  # DEFAULTKERNEL specifies the default kernel package type
  DEFAULTKERNEL=kernel

  • To force a system to always use a particular menu entry, use the menu entry name as the key to the GRUB_DEFAULT directive in the /etc/default/grub file. To list the available menu entries, run the following command as root:

    # awk -F\' /^menuentry/{print\$2} /etc/grub2.cfg
Red Hat Enterprise Linux Server, with Linux 3.10.0-123.el7.x86_64                        <=== Entry 0
Red Hat Enterprise Linux Server, with Linux 3.10.0-123.9.2.el7.x86_64                    <=== Entry 1
Red Hat Enterprise Linux Server, with Linux 3.10.0-123.6.3.el7.x86_64                    <=== Entry 2
Red Hat Enterprise Linux Server, with Linux 0-rescue-b03fd2e1e769493994c88 b8da4257178   <=== Entry 3

  • GRUB 2 supports using a numeric value as the key for the saved_entry directive to change the default order in which the kernel or operating systems are loaded. To specify which kernel or operating system should be loaded first, pass its number to the grub2-set-default command. For example:

# grub2-set-default 1
  • Check the below file to see the kernel which will be loaded at next boot, crosscheck the numeric value with the menuentry in the /etc/default/grub file.
# cat /boot/grub2/grubenv |grep saved

Eg:

# cat /boot/grub2/grubenv |grep saved
saved_entry=1

  • Changes to /etc/default/grub require rebuilding the grub.cfg file as follows:

  • Rebuild the /boot/grub2/grub.cfg file by running the grub2-mkconfig -o command as follows:

    • On BIOS-based machines: ~]# grub2-mkconfig -o /boot/grub2/grub.cfg

    • On UEFI-based machines: ~]# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

For RHEL 8, 9 & 10 Systems:

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