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‎README.md

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# Abstract-Linux-Startup-Process
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Abstract about Linux Startup Process
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Abstract about Linux Startup Process

‎img/logo.png

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‎presentation-linux-startup-process.md

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---
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marp: true
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paginate: true
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color: #ffff
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backgroundColor: #2A2A2A
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header: '![width:100px height:100px](./img/logo.png)'
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footer: "**28/10/2021 - AnthonyF**"
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author: AnthonyF
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---
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<style>
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section {
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font-family: 'Century Gothic', serif !important;
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}
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</style>
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<!-- _class: invert -->
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<!-- _header: Séance -->
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# Linux Startup Process <!-- fit -->
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![width:300px](./img/logo.png)
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---
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<!-- _class: invert -->
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# Table of Contents
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- Importance of understanding Linux startup
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- Main process during Linux startup
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- Some commands to manipulate Linux runlevel
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---
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<!-- _class: invert -->
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# Importance of understanding Linux startup
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---
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<!-- _class: invert -->
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- Good for knowledge
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- Being able to configure and resolve startup issues
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---
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<!-- _class: invert -->
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# Main process during Linux startup
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---
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<!-- _class: invert -->
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# Two main sequences during a Linux distro startup
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- Boot
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- ==> When the computer is turned on, and completed when the kernel is initialized and systemd is launched.
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- Startup
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- ==> When the booting sequence is over and it launches all the process necessary of making the computer operational for the user.
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---
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<!-- _class: invert -->
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**boot** and **startup** sequences are composed of 6 steps :
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1. BIOS (POST)
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2. MBR
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3. Bootloader (GRUB2)
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4. Kernel (Linux)
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5. Init (Systemd)
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6. Runlevel and scripts
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---
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<!-- _class: invert -->
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## BIOS (**B**asic **I**nput **O**utput **S**ystem)
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- Stored in EEPROM (**E**lectrically-**E**rasable **P**rogrammable **R**ead-**o**nly **M**emory).
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- Written in Assembly Language.
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- First interaction with the physical material.
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- Loads and executes the 512 bytes of the disk (MBR).
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>Nowadays the BIOS is replaced by UEFI (Unified Extensible Firmware Interface)
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---
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<!-- _class: invert -->
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# MBR (**M**aster **B**oot **R**ecord)
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- Contains Bootstrap code which contains information about the boot loader (446 bytes).
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- Partitions table to index all the partitions of the disk (64 bytes).
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- Boot signature to check if the disk is bootable or not (2 bytes).
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---
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<!-- _class: invert -->
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# GRUB (**GR**and **U**nified **B**ootloader)
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- Loads all the available operating system or other boot loaders.
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- Loads and executes automatically the default Linux kernel (vmlinuz) and initrd (inital ramdisk) images.
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- Contains all the additional modules and drivers for the kernel.
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---
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<!-- _class: invert -->
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# Kernel
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- The Linux kernel first mounts the root file system set in `grub.conf` in the line `root=`.
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- Then executes the `/sbin/init` program as the fisrt program with root privileges which executes some others scripts.
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>Init has the PID (Process IDentifier) of 1.
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- Establishes a temporary root file system with initrd until the real file system is mounted. It also contains necessary drivers compiled inside.
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---
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<!-- _class: invert -->
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# Init
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- init program reads its initialization files which are in /etc/init.d/ (/etc/inittab before with SysV).
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- It sets everything the system needs for its initialization.
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Then it set the default run level.
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---
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<!-- _class: invert -->
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There are 7 run level from 0 to 1 :
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| Level | Description |
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| ----- | ------------------------------------ |
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| 0 | Halt |
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| 1 | Single user mode |
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| 2 | Multi-user |
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| 3 | Full multi-user mode |
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| 4 | Unused |
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| 5 | X11 (Full multi-user graphical mode) |
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| 6 | Reboot |
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---
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<!-- _class: invert -->
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Modern Linux systems use systemd which refers with this :
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| Level | Target |
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| ----- | ----------------- |
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| 0 | poweroff.target |
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| 1 | rescue.target |
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| 2,3,4 | multi-user.target |
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| 5 | graphical.target |
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| 6 | reboot.target |
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---
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<!-- _class: invert -->
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## Runlevels and scripts
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- The scripts in /etc/init.d are not directly executed by the init process.
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- Each of the directories /etc/rc0.d through /etc/rc6.d contain symbolic links to scripts in the /etc/init.d directory.
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> "S" stands for "start" and the "K" stands for "kill"
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---
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<!-- _class: invert -->
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# Some commands to manipulate Linux runlevel
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---
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<!-- _class: invert -->
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# Commands
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## Current runlevel of the system `sudo runlevel`
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> "N" means has not changed since the boot.
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## Default runlevel `systemctl get-default`
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## Current loaded targets`systemctl list-units --type target`
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## Change runlevel `sudo systemctl set-default runlevel.target`
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---
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<!-- _class: invert -->
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# Thank you ! <!-- fit-->

‎presentation-linux-startup-process.pdf

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‎presentation-linux-startup-process.pptx

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