How Linux Boots
Motivation
Nothing is more frustrating then not being able to boot into your system. If it hangs a boot with a cryptic message, then what do you do? People running other Operating Systems then tend to try to re-install their OS at this point. Once you know how Linux boots then it is almost always possible to boot into your system again as long as your hard-drive is not damaged.
Stages of Booting Linux
Simplified:
- your PC boots (BIOS from EEPROM chips on your motherboard. e.g when you press F12 during boot this is handled by the BIOS)
- loads the first ca 300 bytes from block 0 of your hard drive
- loads the rest of GRUB boot loader
- GRUB can readonly read most filesystems and load kernel (vmlinuz) and initial-ramdisk (initrd)
- kernel unpacks itself and intializes hardware
- kernel unpacks initial ramdisk and starts script there (e.g. /init )
- initial ramdisk scripts loads additional modules and mount real /
- starts init process. (today usually systemd)
- systemd runs boot scripts for all configured items. (e.g. ssh, X11, ..)
