Merge pull request #4 from jpcairesf/linux-boot

Linux boot

content/posts/linux-boot/index.md

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+title = "Understanding the Linux Boot Process"
+date = 2024-09-22T20:35:51-03:00
+draft = false
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+
+When you turn on your computer, several steps happen before you can use your operating system. In this post, I’ll explain the Linux boot process in a simple way. Most of the content is based on the [Introduction to Linux course, from The Linux Foundation](https://training.linuxfoundation.org/training/introduction-to-linux/) which I did and highly recommend (free course, only pays for certificate).
+
+> **_TL;DR:_** When your computer starts, the BIOS/UEFI runs, GRUB loads the Linux kernel, and systemd takes over to start services and the user interface.
+
+---
+
+### Why understanding the boot process
+
+One reason Linux is considered more secure than other systems is because each program runs in isolation. This means that a virus or malicious program has a harder time spreading. In Linux, each process (or task) has its own set of resources and permissions. If something bad happens in one process, it won’t easily affect the others.
+
+Understanding how Linux boots helps you troubleshoot problems and customize your system, giving you more control over your computer.
+
+![Linux boot flow steps](linux-boot-flow-steps.png)
+
+### Step 1: BIOS, UEFI, and Boot Loader
+
+**BIOS** (Basic Input/Output System) is a small program stored on your computer's motherboard. It checks if your hardware (like the keyboard and monitor) is working when you turn the computer on. Newer computers use **UEFI** (Unified Extensible Firmware Interface), which is a more advanced version of BIOS.
+
+![BIOS flow to initialize hardware](bios-flow-to-initialize-hardware.png)
+
+After the hardware check, the BIOS/UEFI looks for a bootable partition on your hard drive or USB stick. This partition has a **/boot/efi** directory that contains the **boot loader**, a program that starts your operating system. 
+
+![BIOS UEFI boot flow comparation](bios-uefi-boot-flow-comparation.png)
+
+The second stage boot loader resides under **/boot**. For Linux, the boot loader is usually **GRUB** (GRand Unified Bootloader). GRUB has a configuration that tells it where the **kernel** and **initramfs** (initial RAM filesystem) are located. GRUB loads both the kernel and initramfs into memory and decompresses them.
+
+### Step 2: Kernel and Initramfs
+
+The **kernel** is the core part of an operating system. Once the kernel is loaded, it takes over control and uses initramfs as a temporary file system. It manages how software and hardware work together. In Linux, the kernel is usually compressed in a file called **vmlinuz**.
+
+When GRUB loads the kernel, it also mounts a temporary partition in RAM called **initramfs**. Initramfs contains important drivers and tools that help the system complete the boot process.
+
+After the kernel is loaded and decompressed, it mounts the real root file system (usually located in **/**), and the boot process can continue.
+
+![initramfs actions](initramfs-actions.png)
+
+The **/proc** directory holds virtual files that provide information about the system and kernel. These aren't actual files but represent real-time data, like system memory, mounted devices, and hardware details. It's a key place for checking how your system is running behind the scenes.
+
+![Linux directories cheatsheet](linux-directories-cheatsheet.png)
+
+### Step 3: Systemd and Runlevels
+
+Once the kernel is ready, it runs **/sbin/init** and starts a program called **systemd**. Systemd is a service manager that runs as process 1. It controls all the other programs and services on your computer.
+
+Before systemd, Linux used another program called **init**. This process controlled the **runlevels**, which defined which services or programs were running at different stages of the boot process. For example:
+
+- **Runlevel 0**: System is off
+- **Runlevel 1**: Single-user mode, used for system recovery
+- **Runlevel 2**: Multi-user mode without Network File System (NFS)
+- **Runlevel 3**: Multi-user mode with a command line
+- **Runlevel 4**: User-definable
+- **Runlevel 5**: Multi-user mode with a graphical interface (GUI)
+- **Runlevel 6**: Reboot the system
+
+Nowadays, systemd uses **targets** as an equivalent to runlevels. These targets decide which services to start based on the current system state. For example, `graphical.target` is similar to runlevel 5 (GUI mode), and `multi-user.target` is similar to runlevel 3 (command-line mode).
+
+You can manually change the runlevel by using:
+
+```bash
+sudo telinit {runlevel number}
+```