kernel

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Backup the GRUB configuration Files

Make copy of the GRUB2 configuration files as a precaution:

For an MBR (BIOS-based) system:

# cp /etc/default/grub /etc/default/grub-backup

# cp /boot/grub2/grub.cfg /boot/grub2/grub.cfg-backup

For a GPT (UEFI-based) system:

# cp /etc/default/grub /etc/default/grub-backup # cp /boot/efi/EFI/redhat/grub.cfg /boot/efi/EFI/redhat/grub.cfg-backup

Editing /etc/default/grub file

1. Open the /etc/default/grub file for editing:

# vi /etc/default/grub

2. This file contains multiple GRUB2 options. Kernel boot parameters are specified by the GRUB_CMDLINE_LINUX option. Below is a sample /etc/default/grub configuration file.

# cat /etc/default/grub  GRUB_TIMEOUT=1 GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)" GRUB_DEFAULT=saved GRUB_DISABLE_SUBMENU=true GRUB_TERMINAL="serial console" GRUB_SERIAL_COMMAND="serial --speed=115200" GRUB_CMDLINE_LINUX="console=ttyS0,115200 console=tty0 vconsole.font=latarcyrheb-sun16 crashkernel=auto  vconsole.keymap=us" GRUB_DISABLE_RECOVERY="true"

3. Once the file is edited, save it.

Generating new grub.cfg file

1. Once the file is edited, save it and run the command specified below to generate a new grub.cfg file:

For an MBR (BIOS-based) system:

# grub2-mkconfig -o /boot/grub2/grub.cfg

For a GPT (UEFI-based) system:

# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

2. Finally, reboot the system to apply the changes:

# systemctl reboot

Verify

Once the system is up, you can verify the kernel command line with the below command.

# cat /proc/cmdline BOOT_IMAGE=/boot/vmlinuz-3.10.0-693.21.1.el7.x86_64 root=UUID=0f790447-ebef-4ca0-b229-d0aa1985d57f ro console=ttyS0,115200 console=tty0 vconsole.font=latarcyrheb-sun16 crashkernel=auto vconsole.keymap=us

CentOS / RHEL 7 : GRUB2 configuration file /boot/grub2/grub.cfg explained

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