// learn · linux · rhcsa · 9 min

Linux boot process.

From the moment you press the power button to the login prompt — every stage explained, the files that drive it, and the rescue moves that save you when something breaks. Maps to the RH134 boot objectives on RHCSA.

// linux boot process

Linux booting is a chain. Each link hands control to the next, after doing its narrow job: power-on test, firmware, bootloader, kernel, root-finder, service manager, login. Knowing the chain is how you debug a system that won't come up.

// before you start

you should know

·What a 'kernel' is at a high level (the OS core)
·Basic file paths in /boot, /etc, /var
·Comfortable editing config files in vim or nano

by the end you'll

Name each stage in the boot chain, point to the file that drives it, and know which rescue lever to pull when a system won't boot.

pace: 9 minutes

step 1 / 7

⚡

Power on — firmware wakes up

You press the button. The CPU starts running tiny instructions that ship inside the motherboard itself — the firmware. On modern systems this firmware is called UEFI; on older ones it's called BIOS. Either way, its first job is to check that the hardware actually works.

// what happens

›Runs POST (Power-On Self-Test): the firmware inventories RAM, CPU, storage, and GPU to make sure they respond.
›If POST fails you get beep codes or a dead screen — boot stops here.
›After POST succeeds, the firmware looks at the configured boot order (set in firmware setup) and finds a bootable device.

firmware lives in→motherboard ROM

boot order config→UEFI / BIOS setup menu

// key insight

When boot fails, the failure happens AT a specific stage. The recovery move depends on which one: GRUB 'e' for kernel-param edits, rd.break for initramfs shell, init=/bin/bash for the ultimate root shell, rescue.target for a minimal system. Knowing the chain tells you which lever to pull.

// rescue moves · exam-ready

$press 'e' at GRUB menuedit a single boot entry

$append rd.break to kernel cmdlinedrop to dracut emergency shell (initramfs)

$append init=/bin/bash to kernel cmdlineroot shell, before init runs — reset root pw

$append systemd.unit=rescue.targetboot to rescue target (minimal)

$append systemd.unit=emergency.targetboot to emergency (bare bones)

$mount -o remount,rw /sysrootmake initramfs root writable (after rd.break)

$chroot /sysrootoperate as if booted into real root

$touch /.autorelabelforce SELinux relabel on next boot

$passwdset the root password (after chroot)

$grub2-mkconfig -o /boot/grub2/grub.cfgregenerate GRUB cfg after edits

// check yourself

4 quick questions

Which stage does POST run during?

What is the initramfs FOR?

You're at the GRUB menu and need to boot with init=/bin/bash to reset the root password. How?

What process ID does systemd run as?

These aren't graded — they're just for active recall, which is what actually makes the lesson stick.

back to RHCSA track all lessons