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Welcome to this comprehensive guide on installing, configuring, and troubleshooting the GRUB2 bootloader on CentOS Stream 8. You’ll learn how to recover a non-booting system using rescue media, install GRUB in BIOS or UEFI mode, customize its settings, and verify your changes.
The image shows a boot menu for CentOS Stream 8-stream, offering options to install, test media, or troubleshoot.

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1. Prerequisites

Before you begin, ensure you have:
  • A CentOS Stream 8 installation USB/DVD.
  • Physical or virtual access to the target machine.
  • A basic understanding of Linux shell commands.
You will need root (or sudo) privileges to run most commands in this tutorial.

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2. Boot into Rescue Environment

  1. Insert your CentOS Stream 8 installation media and power on the machine.
  2. At the main menu, select Troubleshooting → Rescue a CentOS Stream System.
  3. Wait for system messages, then choose 1) Continue at the rescue prompt: 
    The rescue environment will now attempt to find your Linux installation
and mount it under /mnt/sysroot. You can then make any changes required.

1) Continue
2) Read-only mount
3) Skip to shell
4) Quit (Reboot)
The installer will mount your root filesystem at /mnt/sysroot.

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3. Chroot into Your System

Change root into the mounted filesystem: 
chroot /mnt/sysroot
You’re now operating inside your installed system as if you had booted normally.

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4. Generate a New GRUB Configuration

GRUB’s configuration file must be regenerated after installation or any changes to /etc/default/grub.
Boot ModeConfiguration File Path
BIOS/boot/grub2/grub.cfg
UEFI/boot/efi/EFI/centos/grub.cfg
Use one of these commands: 
# For BIOS-based systems
grub2-mkconfig -o /boot/grub2/grub.cfg

# For UEFI-based systems
grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg

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5. Install GRUB to the Disk (BIOS Mode)

  1. Identify your disks: 
    lsblk
    Example output: 
    NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda      8:0    0   20G  0 disk
├─sda1   8:1    0    1G  0 part /boot
├─sda2   8:2    0    2G  0 part [SWAP]
└─sda3   8:3    0   17G  0 part /
    Here, /dev/sda is our target disk.
  2. Install GRUB to the MBR: 
    grub2-install /dev/sda
You should see: 
Installing for i386-pc platform.
Installation finished. No error reported.
Ensure you specify the correct disk (e.g., /dev/sda). Installing GRUB to the wrong device can overwrite another OS or data.

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6. Exit Rescue Mode and Reboot

Exit the chroot environment and reboot the system: 
exit     # leaves chroot
exit     # leaves rescue shell
Remove the installation media when prompted so the machine boots from the hard drive.

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7. Customize GRUB Settings

Once back in your CentOS Stream system, you can fine-tune GRUB by editing /etc/default/grub: 
sudo vi /etc/default/grub
A typical configuration: 
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto resume=UUID=… rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true

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Change the Boot Timeout

To reduce the wait time: 
-GRUB_TIMEOUT=5
+GRUB_TIMEOUT=1
Save and exit (:wq in vi).

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8. Regenerate Configuration After Edits

Apply your changes by regenerating grub.cfg: 
# BIOS
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

# UEFI
sudo grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg
Look for: 
Generating grub configuration file ...
done

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9. Verify and Test

Reboot one final time: 
sudo reboot
You should see the GRUB menu for the new timeout duration, and your kernel options will reflect any changes made in /etc/default/grub.

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References

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