What Actually Happens When Linux Boots?
Most of us press the power button, wait a few seconds and arrive at the desktop, but there's quite a lot happening behind the scenes before Linux gets there.On a modern Linux PC the process is roughly:
1. UEFI/BIOS starts
The motherboard firmware performs its initial hardware checks and looks for a bootable device.
2. The bootloader takes over
On many systems this will be GRUB. It lets you select which operating system or kernel to boot.
This is particularly useful on a dual-boot machine where you might have Linux and Windows installed.
3. The Linux kernel loads
The kernel starts detecting and controlling the hardware — CPU, memory, storage, USB devices and so on.
4. The init system starts
On most modern distributions this is systemd.
It begins starting the services required by the system, including networking, audio, login services and the graphical environment.
You can see how long the complete boot took with:
systemd-analyze
You can also see which services took the longest:
systemd-analyze blame
Another useful command is:
systemctl --failed
This shows any systemd units that failed during startup.
I've found that understanding the boot process makes Linux troubleshooting a bit less mysterious. If a machine boots but something isn't working, you can start working out which stage actually had the problem rather than treating startup as one big process.
Has anyone here used systemd-analyze to track down a slow Linux boot?