Levels 50–54: Boot Process & Kernel

Level 50: Boot Process Overview

Command	What it does
systemctl get-default	Show the default boot target
systemctl set-default multi-user.target	Set a headless (no GUI) default
systemctl list-units --type=target	List every available target
systemd-analyze	Boot time breakdown: firmware, loader, kernel, userspace
systemd-analyze blame	Every unit ranked by startup time, slowest first

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Level 51: GRUB2

Command	What it does
update-grub (Debian) / grub2-mkconfig -o /boot/grub2/grub.cfg (RHEL)	Regenerate grub.cfg after an edit
grub-install /dev/sda	Reinstall GRUB’s boot code onto a disk
cat /etc/default/grub	The human-edited source, not the generated config
ls (at a grub rescue> prompt)	List partitions GRUB can see
e (at the GRUB menu)	Edit a boot entry’s kernel parameters for one boot

Caution

Always edit /etc/default/grub, never grub.cfg directly. update-grub regenerates the real config from it.

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Level 52: Alternative Boot Managers

GRUB isn’t the only option. systemd-boot is minimal and ships with systemd; rEFInd is a graphical multi-OS picker.

Command	What it does
bootctl status	Current systemd-boot status and entries
bootctl update	Update systemd-boot binaries on the ESP
bootctl list	List every boot entry
bootctl install	First-time install onto the EFI system partition

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Level 53: Kernel Panics & Recovery

The kernel dumps everything it knows right before it dies. Reading that output is how you fix it.

Command	What it does
systemd.unit=rescue.target (kernel param)	Boot straight into a rescue shell
journalctl -b -1	Read the PREVIOUS boot’s logs, where the panic actually happened
dmesg	Kernel ring buffer, direct from the kernel
update-initramfs -u (Debian) / dracut -f (RHEL)	Rebuild the initramfs after a driver fix
touch /forcefsck	Force a filesystem check on the next boot

Tip

The current boot’s logs won’t show a crash, it happened on the boot before. journalctl -b -1 is the single most useful command after an unexpected reboot.

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Level 54: Building a Kernel

make menuconfig              # interactive configuration menu
make -j$(nproc)              # compile, using every CPU core
sudo make modules_install    # install built modules
sudo make install            # install the kernel image
sudo update-grub             # make it a selectable boot entry

Almost nobody builds a kernel from source for daily use, distros handle that. But knowing how, adding a missing driver or enabling a feature flag, is the deepest level of Linux mastery, and it demystifies everything above it.