What actually happens between pressing power and seeing a login prompt — knowing the chain makes boot failures debuggable instead of mysterious.
The chain
1. Firmware (UEFI/BIOS) — power-on self-test, find a boot device
2. Bootloader (GRUB) — load the kernel + initramfs, pass kernel params
3. Kernel — init hardware, mount initramfs, then the real root fs
4. init (systemd) — start services, reach the target (login)Where things break (and where to look)
| Symptom | Likely stage |
|---|---|
| No boot device / black screen | Firmware / disk |
| GRUB menu errors | Bootloader config |
| Kernel panic | Kernel / initramfs / bad fstab |
| Boots but no services | systemd / failed units |
Useful tools
journalctl -b # logs from this boot
systemd-analyze blame # what made boot slow
systemctl --failed # units that failed to startA failed
/etc/fstabentry is a top cause of "won't boot" — the system waits forever for a disk that isn't there. Boot to a recovery shell and fix the file.
Related: Understanding systemd · File Permissions Explained