Linux Boot process – IT – Engineering – Cloud

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Let’s describe Linux Boot Process:

BIOS	Basic INPUT/OUTPUT System. Executes MBR
MBR	Master Boot Record Executes GRUB
GRUB	Grand Unified Bootloader Executes kernel
KERNEL	Kernel Executes /sbin/init
INIT	Init Executes Run level programs
Run Level	Run Level Programs are executed from /etc/rc.d/rc*.d/

As power comes up the BIOS is given control
BIOS runs self tests, usually including cursory memory tests.
The BIOS then loads the first sector of the disk to be used for booting and transfers control to it.
The MBR code varies. One version will chain to the code in the first sector of the boot partition (Windows), another will load a bootloader. Windows boot proceeds from code and information in the boot partition.
The bootloader chooses kernel location and version
The bootloader prepares kernel and initrd image in memory, transfers control to kernel
Loading kernel modules
Discovering hardware and load additional kernel modules to support it
Looking for disks
R/O mount of / partition so that it can potentially be checked and repaired
init process spawn
/etc/inittab read and executing
Mounting all FSes from /etc/fstab
runlevels running (based on default runlevel in /etc/inittab) or another init method such as systemd or upstart
rc.local
login prompt