Regenerate GRUB Safely with update-grub

Change one line in /etc/default/grub, forget to regenerate, and your setting does nothing at the next boot: that is what update-grub is for. On this machine it comes from grub2-common version 2.12-1ubuntu7.3, and it is a thin wrapper around grub-mkconfig, not a menu editor.

You will regenerate /boot/grub/grub.cfg, confirm the new file actually landed, and know where to look when generation fails. Allow about ten minutes, plus time to review whatever boot setting you are changing.

• An administrator shell and a working GRUB installation.
• Recovery access (console or rescue boot) before you touch anything, since this changes what the next boot reads.

1. Check the setting before regenerating

GRUB reads its main config from /etc/default/grub, then fragments from /etc/default/grub.d/, then runs the executable scripts in /etc/grub.d/. Look at all three before changing anything:

$ sudo sed -n '1,220p' /etc/default/grub
$ sudo find /etc/default/grub.d -maxdepth 1 -type f -name '*.cfg' -print 2>/dev/null
$ sudo find /etc/grub.d -maxdepth 1 -type f -perm /111 -printf '%f\n' | sort

Use sudo if your account cannot read these files.

2. Make one controlled configuration change

For something like a menu timeout, edit the existing setting in /etc/default/grub with an editor you trust:

$ sudoedit /etc/default/grub

• Keep it valid POSIX shell. The file is sourced as shell code during generation, so a simple assignment like GRUB_TIMEOUT=5 is safe, but never paste untrusted text into it.
• Quote values with spaces or shell punctuation. An unquoted value can break parsing in ways that are hard to spot later.

Back the file up first so an undo is one command away:

$ sudo cp -p /etc/default/grub /etc/default/grub.backup-before-grub-update

To undo before regenerating, restore that copy and check it:

$ sudo cp -p /etc/default/grub.backup-before-grub-update /etc/default/grub
$ sudo sh -n /etc/default/grub

3. Regenerate the GRUB configuration

Run the wrapper as root:

$ sudo update-grub
Sourcing file `/etc/default/grub'`
Generating grub configuration file ...
done

The exact source-file lines vary with whatever fragments and scripts you have installed. What matters is a clean exit and the final done. Under the hood the wrapper runs grub-mkconfig -o /boot/grub/grub.cfg, so no output filename is needed, and update-grub2 is just an alias for the same command on this system.

4. Verify the generated file

In a script or automation, check the exit status straight away:

$ sudo update-grub
$ status=$?
$ printf 'update-grub exit status: %s\n' "$status"
update-grub exit status: 0

Then confirm the target exists, belongs to root, and holds generated content:

$ sudo stat -c '%U %G %a %s %n' /boot/grub/grub.cfg
root root 600 12345 /boot/grub/grub.cfg
$ sudo sed -n '1,16p' /boot/grub/grub.cfg

The size shown is only an example; yours will differ. The opening comments should say the file is automatically generated. A plausible file size is not proof every menu entry you expected is present, so search for a specific one when you have one in mind:

$ sudo grep -nF 'menuentry' /boot/grub/grub.cfg | head

5. Diagnose a failed update without guessing

If the command exits non-zero, read the error before retrying rather than running it again and hoping. A syntax error in /etc/default/grub or in an executable script under /etc/grub.d/ can stop generation cold:

$ sudo sh -n /etc/default/grub
$ sudo ls -l /etc/grub.d
$ sudo find /etc/default/grub.d -maxdepth 1 -type f -name '*.cfg' -exec sh -n '{}' \;

Generation writes to a temporary file beside the target and validates it before installing it. A failed run is never permission to hand-edit the existing grub.cfg: fix the reported source file, re-run the checks, then regenerate.

For a harmless capability check that exits before any generation happens:

$ grub-mkconfig --help
Usage: grub-mkconfig [OPTION]
Generate a grub config file

6. Recover from an unwanted change

If you have not rebooted yet, the previous boot configuration may still sit in your backups or a package-managed recovery process. Restore whatever caused the problem, then regenerate. Using the example backup from earlier:

$ sudo cp -p /etc/default/grub.backup-before-grub-update /etc/default/grub
$ sudo sh -n /etc/default/grub
$ sudo update-grub