Modern GRUBing

Rocky Linux 9 does not use the /etc/default/grub and /boot/grub2/grub.cfg Files any longer. Instead the BLS (Boot Loader Specification) 'method' is used. If one reads the documentation from RedHat, it seems like thing got really messy (and confusing).

Quick D for Cloning, err, "Recreating" a drive from a BackUp (IE, the closest thing to Acronis that Linux seems to have)

• Create a duplicate or functionally similar Partition layout to the original
  • CFDISK and "Logical Volume Commands"
  • Use Blivet from a Rocky 8 Live Boot Device
    

Rough D

• Start from a 'Clean Slate': wipefs -a /dev/sdX (make dang sure the proper 'sdX' is chosen!)
• Set the Partition Type (MBR or GPT): cfdisk /dev/sdX, (select DOS or GPT, save it (write) )
• Create Partitions: cfdisk
  • https://en.wikipedia.org/wiki/Partition_type
  • Linux EXT4: $83 (that's Commodore Speaky for 83h)
  • Linux LVM (and LVM2): $8E
• LVM Stuff using Commands or BLIVET (sadly only Rocky Linux 8)
  • pvcreate /dev/sdX
  • vgcreate VG.NVMe.ROOT /dev/sdX
    • vgrename WhatEverOldName WhatEverNewName
  • lvcreate -L 64G -n LV.ROOT VG.NVMe.ROOT
• Format Partitions
  • mkfs.vfat -F32 /dev/sdXy
  • mkfs.ext4 -v -L ROOT.EXT4 /dev/sdX ( -v = Verbose, -L = Label )
  • mkswap -L SWAP /dev/sdaX
• MBR Stuff if desired
• UEFI Stuff
  • Make sure UEFI Stuff is installed (if cloning from an MBR / BIOS based system): dnf install -y grub2-efi-x64 grub2-efi-x64-modules shim-x64 efibootmgr
  • mkdir /mnt/efi
  • mount /dev/nvme0n1p1 /mnt/efi
  • grub2-install --target=x86_64-efi --efi-directory=/mnt/efi --bootloader-id=rocky --recheck --force --no-nvram --removable ( --force = Do it on a supposedly 'non-secure' system, --no-nvram = Do it even if a target drive is being prepared as a UEFI Booted system even if the current OS is an MBR / BIOS system, --removable installs the 'generic' BOOTX64.EFI module so the system can boot before )
  • Command to show information on how a system was booted (and can boot): bootctl status
  • Command for EFIBootMgr: efibootmgr -v
• ...and the tribulations continue as the below command won't work from an MBR / BIOS system or theoretically from any system that isn't the one being booted, run from etc.;
  • Entry for Hardware Firmware (AKA BIOS Settings): efibootmgr --create --disk /dev/sdb --part 1 --loader /EFI/rocky/grubx64.efi --label "Rocky Linux" --verbose
  • Work Around:

BLS (Boot Loader Specification)

Nope, not anymore (sort of): /etc/fstab

Nope, not anymore (sort of): grub2-mkconfig -o /tmp/boot/grub2/grub.cfg --root-directory=/tmp/LV.ROOT

Look in /boot/loader/entries and / or use GRUBBY

...see below for key information on /etc/default/grub and BLS

GRUB "Installation" for MBR / BIOS

Installs the Stage 1 Boot Code (program) in the MBR and Stage 1.5 Boot Code (program) in the "MBR Gap": grub2-install --target=i386-pc /dev/sdd

...and don't forget to set the Boot Partition active using CFDISK or something else

OR

Use DD to clone the Boot Partition

Sources

https://www.baeldung.com/linux/grub-menu-management

https://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/ > https://uapi-group.org/specifications/specs/boot_loader_specification/

https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/9/html/managing_monitoring_and_updating_the_kernel/configuring-kernel-command-line-parameters_managing-monitoring-and-updating-the-kernel

ChatGPT's Opinion

GRUB "Installation" (What Does grub2-install Do?)

The grub2-install command is used to install the GRUB bootloader onto a disk or partition. It places the necessary GRUB boot code onto the target disk and copies GRUB modules required for booting.

How grub2-install Works

When executed, grub2-install performs the following actions:

1. Determines the Installation Target

• If given a disk (e.g., /dev/sda), it installs GRUB to the MBR or GPT boot partition.
• If given a partition (e.g., /dev/sda1), it may fail or install to the partition's boot sector (not recommended).
• If on a BIOS system, it installs the i386-pc GRUB bootloader.
• If on a UEFI system, it installs x86_64-efi and places EFI files in the EFI system partition.

2. Copies GRUB Core to the Correct Location

• On BIOS/MBR systems, GRUB places its boot code in the MBR and additional code in the post-MBR gap (sectors after the MBR).
• On GPT systems using BIOS (BIOS-GPT), it places GRUB in the BIOS Boot Partition.
• On UEFI systems, it installs the grubx64.efi file into the EFI System Partition (ESP).

3. Selects the Correct GRUB Modules

• grub2-install determines which GRUB modules are needed based on the filesystem, partition layout, and system firmware.
• It installs these modules into /boot/grub2/ (BIOS) or /boot/efi/EFI/ (UEFI).

4. Writes GRUB Boot Code

• BIOS (MBR) Installation:
  • The Stage 1 boot code is written into the MBR.
  • The Stage 1.5 boot code is stored in the post-MBR gap (if available).
  • The Stage 2 boot code (GRUB core modules) is stored in /boot/grub2/.
• UEFI Installation:
  • The grubx64.efi file is copied to the EFI System Partition.
  • GRUB is registered as a boot entry with the system's UEFI firmware.

5. Registers the Bootloader (If UEFI)

• On UEFI systems, grub2-install registers grubx64.efi with the UEFI boot manager using efibootmgr.

Comprehensive Guide to Installing GRUB on Rocky Linux (UEFI & BIOS)

This guide covers:

• UEFI bootloader installation on a target drive.
• Legacy BIOS (MBR) bootloader installation.
• BLS (Boot Loader Specification) and its effect on grub2-mkconfig.
• GRUB Boot Stages Explanation (Stage 1, Stage 1.5, Stage 2).
• How to properly configure bootloader entries and verify installation.

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1. Understanding GRUB Boot Stages

GRUB Stage	Description	Applies To
Stage 1	MBR code (first 512 bytes of disk) loads Stage 1.5 or Stage 2.	Legacy BIOS (MBR) Only
Stage 1.5	Located in the first 30KB after MBR, it helps find /boot/grub2.	Legacy BIOS (MBR) Only
Stage 2	Loads the GRUB menu, kernel, and initrd.	Both UEFI and BIOS
UEFI Firmware Boot	Uses the EFI System Partition (ESP) instead of MBR. Loads grubx64.efi.	UEFI Only

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2. Installing GRUB for UEFI Booting

A. Identify the Target Drive

List available drives and partitions:

Look for the EFI System Partition (ESP), usually formatted as FAT32 (vfat).

Example: If the target drive is /dev/sdb, and the EFI partition is /dev/sdb1, mount it:

B. Bind System Directories (for Chroot)

If configuring an external system:

D. Install GRUB to the Target Drive

• --target=x86_64-efi → Specifies UEFI mode.
• --efi-directory=/boot/efi → Ensures GRUB is installed in the EFI partition.
• --bootloader-id=rocky → Labels the boot entry as "Rocky".
• --recheck → Forces a re-scan of the drive layout.

E. Generate the GRUB Configuration

⚠ However, Rocky Linux follows BLS, which makes this step less critical (explained in Section 4).

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3. Installing GRUB for Legacy BIOS (MBR Boot)

A. Mount the Target Drive's Root Partition

If the system has a separate /boot partition:

D. Install GRUB to the MBR of the Target Drive

• --target=i386-pc → Required for Legacy BIOS (MBR) booting.
• /dev/sdb → Must be the whole disk (not a partition like /dev/sdb1).
• --recheck → Forces a disk layout rescan.

E. Generate the GRUB Configuration

⚠ But due to BLS, grub.cfg is no longer the main configuration source.

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4. Understanding BLS (Boot Loader Specification)

Rocky Linux follows BLS (Boot Loader Specification), meaning boot entries are controlled by systemd-boot entries, not grub2-mkconfig.

How to Manually Manage BLS Entries

1. List Existing Boot Entries:
2. Manually Create a BLS Entry:
3. Example BLS Entry (rocky.conf)
4. Enable BLS Support in GRUB (if needed)

Why BLS Makes grub2-mkconfig Less Relevant

• Traditional GRUB entries are no longer used in Rocky Linux.
• The bootloader reads BLS entries instead.
• grub.cfg is still used for bootloader settings, but kernel entries are controlled by BLS.

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6. Summary

Boot Mode	Target Option for GRUB	Install Command	Config Location
UEFI	x86_64-efi	grub2-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=rocky	/boot/efi/EFI/rocky/grub.cfg
BIOS (MBR)	i386-pc	grub2-install --target=i386-pc /dev/sdb	/boot/grub2/grub.cfg
BLS (Rocky Linux Default)	systemd-boot controlled	bootctl list	/boot/loader/entries/

1. The Presence (or Absence) of /boot/grub2/grub.cfg

• If grub.cfg exists, GRUB will parse it.
• If grub.cfg does NOT exist, GRUB will attempt to read the BLS entries directly instead.

This means:

✔ Deleting grub.cfg forces GRUB to rely entirely on BLS.

✔ Regenerating grub.cfg (grub2-mkconfig) puts GRUB back into a "hybrid mode" where it may still reference BLS but also maintain compatibility with legacy boot setups.

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2. GRUB’s Bootloader Installation (grub2-install)

The actual bootloader installed to the disk controls whether it defaults to BLS mode or grub.cfg mode.

When GRUB is installed with:

(where /dev/sdX is your boot disk, like /dev/sda or /dev/nvme0n1), it does the following:

• Sets up GRUB’s core image (/boot/grub2/i386-pc/core.img for BIOS systems or /boot/efi/EFI/rocky/grubx64.efi for UEFI systems).
• Embeds the logic for how GRUB will find and load boot entries.
• If BLS is active, it instructs GRUB to skip parsing grub.cfg and go directly to BLS entries.

This means: ✔ If GRUB was installed with BLS enabled, even if grub.cfg exists, it may not be used.

✔ If GRUB was installed without BLS support, it will fall back to grub.cfg.

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How to Explicitly Switch Between BLS and grub.cfg

To Force BLS Mode (and Ignore grub.cfg Completely)

1. Ensure BLS entries exist: If this folder is empty, something is broken.
2. Delete grub.cfg (this ensures GRUB won’t use it):
3. Ensure GRUB is installed with BLS support: (For UEFI systems; adjust for BIOS if needed.)
4. Reboot and verify that grub.cfg is no longer in use.

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To Force Legacy grub.cfg Mode (and Ignore BLS)

1. Ensure grub.cfg is present:
2. Reinstall GRUB to disable BLS mode: (The --no-bootloader-spec flag explicitly prevents GRUB from using BLS.)
3. Reboot and verify that grub.cfg is being used.

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Final Answer

✔ The real switch between BLS and grub.cfg is NOT in /etc/default/grub—it's controlled by how GRUB is installed and configured.

✔ Deleting grub.cfg forces BLS mode, provided GRUB was installed with BLS support.

✔ Reinstalling GRUB with --no-bootloader-spec forces it to use grub.cfg instead of BLS.

✔ To change modes, you must either delete grub.cfg (to force BLS) or reinstall GRUB (to force legacy mode).