Booting

How Linux boots up

Here is how linux starts:

bios - basic input output system

bios first perform some integrity check on sdd or hdd
bios then loads mbr master boot record boot loader
you can set up where the mbr is form the boot menu
- you can set it to pendrive or a cd rom
once the boot loader program is detected it's loaded into memory and the bios give control of the system to it

responsible for loading and executing the grub boot loader
the mbr is located ar the 1st sector of the bootable disk which is typically /dev/hsa /dev/sda depending on your hardware.

grub is bootloader and lets you to select the os you want to run
configuration file in, /etc/grub.conf or /boot/grub/grub.conf
on ubuntu it is in /etc/default/grub, you change this file and then you run the utility command provided by ubuntu update-grub which verifies if this configuration is valid or not, and then make necessary updates. This extra layer is provided so user won't damage the system.

now the kernel linux is loaded if you have selected some linux distribution
now the root file system is mounted, which was specified in the grub configuration
then /sbin/init is executed, it have pid one therefore
the kernel then establishes a temporary root file system using initial ram disk initrd until the real file system is mounted.

now run level programs are executed,
there is a directory for each level of programs
Run Level 0 - /etc/rc0.d/
...
Run Level 6 - /etc/rc6.d/
Inside each directory there are applications, the one starting with S (start) are executed on startup, and the one starting with K (kill) are executed on shutdown.
each of the runlevel defines specific tasks
- 0 - poweroff.target
- 1 - rescue.target
- 2 -
- 3 - multi-user.target
- 4 -
- 5 - graphical.target
- 6 - reboot.target
- emergency.target
https://www.tecmint.com/change-runlevels-targets-in-systemd/