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**28/10/2021 - AnthonyF**
AnthonyF
<style> section { font-family: 'Century Gothic', serif !important; } </style>

Linux Startup Process

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Table of Contents

  • Importance of understanding Linux startup
  • Main process during Linux startup
  • Some commands to manipulate Linux runlevel

Importance of understanding Linux startup

  • Good for knowledge
  • Being able to configure and resolve startup issues

Main process during Linux startup

Two main sequences during a Linux distro startup

  • Boot
    • ==> When the computer is turned on, and completed when the kernel is initialized and systemd is launched.
  • Startup
    • ==> When the booting sequence is over and it launches all the process necessary of making the computer operational for the user.

boot and startup sequences are composed of 6 steps :

  1. BIOS (POST)
  2. MBR
  3. Bootloader (GRUB2)
  4. Kernel (Linux)
  5. Init (Systemd)
  6. Runlevel and scripts

BIOS (Basic Input Output System)

  • Stored in EEPROM (Electrically-Erasable Programmable Read-only Memory).
  • Written in Assembly Language.
  • First interaction with the physical material.
  • Loads and executes the 512 bytes of the disk (MBR).

Nowadays the BIOS is replaced by UEFI (Unified Extensible Firmware Interface)

MBR (Master Boot Record)

  • Contains Bootstrap code which contains information about the boot loader (446 bytes).
  • Partitions table to index all the partitions of the disk (64 bytes).
  • Boot signature to check if the disk is bootable or not (2 bytes).

GRUB (GRand Unified Bootloader)

  • Loads all the available operating system or other boot loaders.
  • Loads and executes automatically the default Linux kernel (vmlinuz) and initrd (inital ramdisk) images.
  • Contains all the additional modules and drivers for the kernel.

Kernel

  • The Linux kernel first mounts the root file system set in grub.conf in the line root=.
  • Then executes the /sbin/init program as the fisrt program with root privileges which executes some others scripts.

Init has the PID (Process IDentifier) of 1.

  • Establishes a temporary root file system with initrd until the real file system is mounted. It also contains necessary drivers compiled inside.

Init

  • init program reads its initialization files which are in /etc/init.d/ (/etc/inittab before with SysV).
  • It sets everything the system needs for its initialization. Then it set the default run level.

There are 7 run level from 0 to 1 :

Level Description
0 Halt
1 Single user mode
2 Multi-user
3 Full multi-user mode
4 Unused
5 X11 (Full multi-user graphical mode)
6 Reboot

Modern Linux systems use systemd which refers with this :

Level Target
0 poweroff.target
1 rescue.target
2,3,4 multi-user.target
5 graphical.target
6 reboot.target

Runlevels and scripts

  • The scripts in /etc/init.d are not directly executed by the init process.
  • Each of the directories /etc/rc0.d through /etc/rc6.d contain symbolic links to scripts in the /etc/init.d directory.

"S" stands for "start" and the "K" stands for "kill"

Some commands to manipulate Linux runlevel

Commands

Current runlevel of the system sudo runlevel

"N" means has not changed since the boot.

Default runlevel systemctl get-default

Current loaded targetssystemctl list-units --type target

Change runlevel sudo systemctl set-default runlevel.target

Thank you !