GRUB & Boot Process

On a BIOS-based machine, you swap out a failed software-RAID disk, add its partitions back with mdadm, and the array resyncs cleanly. Why can that disk still fail to boot the machine?

You want to encrypt an entire Linux system including the root partition. Why can't you simply encrypt every partition and be done?

What role does GRUB play the instant a PC is switched on, and what three jobs does it handle?

On an EFI computer with three operating systems installed side by side, which component actually decides which OS boots — and where does that leave GRUB's menu?

On an EFI machine, where does GRUB's code physically live, and what must be true of that partition?

The kernel file is named /boot/vmlinuz. What does the trailing z signify, and what single parameter must the bootloader always pass to it?

The kernel needs modules to read the root file system, but those modules live on that same file system it can't yet read. How does the boot process escape this deadlock?

In this chapter, why does installing GRUB mean something different from installing a normal software package?

With UEFI Secure Boot active, GRUB is not the first thing EFI runs. What runs before it, and why was that extra step introduced?

A user with an NVIDIA GPU or a self-compiled kernel finds it won't load under Secure Boot. What's the underlying cause, and what mechanism lets them proceed without disabling Secure Boot?

On a BIOS boot from a GPT disk, the MBR is only 512 bytes — far too small for GRUB. How is GRUB split up, and what extra partition is recommended on GPT?

Why must a fresh initrd be built every time you install a new kernel — and why is the name initrd technically wrong on modern distributions?

Your distribution uses update-initramfs. What does that tell you about the distro family, and which tool would the other major family use instead?

After a hardware change on a Fedora/RHEL box, the normal boot fails because dracut's compact initrd lacks the needed driver. What's the recovery move?

At the GRUB menu you press E, append a kernel option to work around a hardware issue, and boot. What happens to that option on the next reboot?

You edit /boot/grub2/grub.cfg directly to change the boot menu, but the file is read-only and the change is discouraged. What's the intended way to change the menu?

In /etc/default/grub, what does GRUB_DEFAULT=saved do, and in what situation does it silently stop working?

In /etc/default/grub, what's the practical difference between GRUB_CMDLINE_LINUX and GRUB_CMDLINE_LINUX_DEFAULT?

On Fedora/RHEL you want to add a kernel option to every boot entry without opening a config file in an editor. Which tool does that?

When update-grub rebuilds grub.cfg, it runs the /etc/grub.d scripts in order. What do the two most important scripts, 10_linux and 30_os-prober, each contribute?

You dual-boot Windows and Linux, but after a Linux update the GRUB menu no longer lists Windows. On Ubuntu 22.04+ / Debian 12+, why — and how do you get it back?

In GRUB's own syntax, how do you assign and read a variable, and what does the disk notation hd0,1 versus hd1,gpt8 mean?

Numbered disk names like hd0,1 are fragile if disks get reordered. How does GRUB locate a partition robustly instead, and how does it pull in extra capabilities at runtime?

Inside a GRUB menuentry, what are the three essential keywords, and which two kernel parameters must always appear on the linux line?

Your kernel and initrd sit on an LVM logical volume, or a software-RAID array. Why does a plain menuentry fail to boot it, and what's missing?

To manually (re)install GRUB on an EFI PC you run grub-install / grub2-install. What does it actually create and register?

Booting a live USB to repair a broken GRUB, why isn't it enough to just mount the system partition and run grub-install — what environment must you build first?

Where does EFI store the list of installed operating systems, and what does Linux need in place to read or edit that list?

You want Fedora to start on the next reboot only, then revert to normal order. Which efibootmgr option does that, and how does it differ from changing the boot order?

systemd-boot skips a step that GRUB always performs. What step does it eliminate, and what does that let it avoid dealing with?

Where does systemd-boot keep its configuration, and which command reinstalls it?

A notebook with preinstalled Windows has a 250 MiB EFI partition. Why can that be too small under systemd-boot even for a single Linux install?

On a Raspberry Pi 4 running Raspberry Pi OS, which component drives the boot process — and what unusual role does the hardware play?

You install Ubuntu and Linux Mint side by side and one stops booting. What conflict causes this?

On Debian/Ubuntu, update-initramfs isn't the real generator. What tool does the work, and where do you configure which modules go into the initrd?

Your distro boots straight into Linux with no GRUB menu, and even when it shows, GRUB's extra functions seem locked behind graphics. How do you get the menu and the full interface?

At a locked-down GRUB menu, the edit and command-line keys do nothing until you do one thing. What protection is in place, and which key unlocks it?

In /etc/default/grub, which settings control how long the menu waits and whether it shows in graphics mode at all?

You want GRUB to pass the kernel a device name like /dev/sda1 instead of a UUID, and to stop generating recovery entries. Which two /etc/default/grub settings do that?

On Fedora/RHEL, grub.cfg is unusually short and the actual boot entries aren't in it. Which setting causes this, and where do the entries live?

After a crash, your headless server sits at the GRUB menu for ~30 seconds on the next boot and looks hung. Which variable controls that, and what's the fix?

Before grub-install can succeed on an EFI machine, several prerequisites must hold — including one kernel module that only loads if you booted a certain way. What are they?

After changing /etc/default/grub or installing a new kernel, which command regenerates grub.cfg — and how does it differ by distribution?

On Debian/Ubuntu vs Fedora, GRUB isn't one package. How are its files split, and which package keeps the config current after a kernel update?

Inside grub.cfg, what does insmod do, and where does GRUB find the module it loads?

Older Linux installs carved out a separate /boot partition. What did it hold, and why is it usually pointless today?

Repairing GRUB from a live system, the standard chroot recipe breaks when the system sits on LVM. What changes for the LVM case?

With efibootmgr, how do you add a brand-new EFI boot entry and later remove one — and what's the trap in the loader path?

On a Raspberry Pi 4, GRUB is nowhere in the boot chain. Where does the second-stage bootloader live, and how is the kernel image chosen?

A broken init configuration leaves the system unbootable. Which kernel boot options drop you into a shell or a minimal mode, and what caveats apply to the shell?

Among kernel boot options, which one hides startup messages, and how do you tell the kernel to load no initrd at all?

A BIOS server mirrors its boot partition across two disks with RAID-1, yet may not boot after swapping the first disk. What extra step makes it boot from either disk?

You can't put an EFI System Partition inside a RAID array, so how do you make an EFI server survive losing a disk — and which fstab option is critical?