Understanding grub2-editenv list Output: Saved Entry Hash

When sudo grub2-editenv list prints a saved_entry hash, this indicates GRUB is using an environment block file (grubenv) to store persistent variables, including the default boot entry.

1. What is the grubenv File?

  • Location: /boot/grub2/grubenv (or /boot/grub/grubenv).

  • Purpose: Stores GRUB environment variables that persist across reboots.

  • Key Variable: saved_entry specifies the default menu entry (by hash/index).

2. Decoding the saved_entry Hash

  • The value is a hash or numeric index referring to an entry in grub.cfg.

  • Example output: .. code-block:: bash

    saved_entry=1a2b3c4d

3. Why a Hash Instead of a Name?

  • GRUB uses hashes to: - Avoid dependency on menu entry titles (which may change). - Ensure consistency if grub.cfg is regenerated.

  • The hash is derived from the menu entry’s content (kernel paths, boot args, etc.).

4. Mapping the Hash to a Boot Entry

To find which menu entry the hash refers to: 1. List all menu entries with their hashes:

sudo grep -oP 'menuentry.* --id \K[^ ]+' /boot/grub2/grub.cfg

  1. Cross-reference the saved_entry hash with the output.

5. Example Workflow

  1. Check the current saved_entry: .. code-block:: bash

    sudo grub2-editenv list

  2. Find the matching menu entry: .. code-block:: bash

    sudo grep -B1 – ‘–id 3e4567f8’ /boot/grub2/grub.cfg

6. Implications for Your VM

  • If grub.cfg lacks linux/initrd lines, GRUB might: - Use the saved_entry hash to load a statically configured entry. - Rely on embedded paths in core.img.

7. Next Steps

  1. Verify the saved_entry mapping.

  2. Check for embedded paths in core.img: .. code-block:: bash

    strings /boot/grub2/i386-pc/core.img | grep -iE ‘linux|initrd’

  3. Review grubenv for other variables.