Linux Explained

By Sebastien Bronchard

Linux Explained is a series of articles I've published on my blog to help newcomers or beginners to Linux to discover and develop their knowledge about it.
With eight chapters, this small book will explain in simple words :

- What is Linux ?
- What is the bootloader, the init process, the shell
- What is a filesystem and how Linux work with them
- What are the file permissions
- How the users are managed
- What are the Standard Streams for input and output data
- How the memory is managed
- What is SSH and how remote access works

• Language: English
• Publisher: Sebastien Bronchard
• Release date: Sep 9, 2023
• ISBN: 9798223419624

Book preview

Linux Explained

Seb

2023-02-10

Creative Commons Attribution ShareAlike 4.0

Preface

About the author

I’m Seb and I’ve discovered Linux in the early 2000s, with a first attempt to install Debian Linux on my computer, but after printing kilometers of documentation and trying to have the display working, I’ve finally gave up. A few times later, I’ve found a Linux magazine with three Mandrake 9.1 CD-ROM. That was my actual first experience with Linux until I’ve started using Ubuntu Linux with its first release in 2004.

On a more professional side, I’ve started to see Linux for servers in my second job, mainly Red Hat Enterprise Linux. I had also some AIX system under my scope. Since I was using this system for my personal computer, I became more and more enthusiast with it and after 20 years using it, I think I can write something about it. Today, I’m a solution architect working on the integration of CICD tools and process for business context and promoting the DevOps philosophy. And Linux-based systems remain my daily drivers.

About this book

Linux Explained is addressed to beginners or newcomers to the Linux world. Additionally, if you also have some experience and want to develop your knowledge about what is Linux and its concepts, you’re welcome too ! This book will (try to) keep using simple words and explanation with the purpose of being the most accessible possible.

Linux Explained started as a series of articles I’ve wrote for my blog. Following this first publication, I had the idea to rewrite these articles to fit in a book. The original series is available at the following URL : https://blog.zedas.fr/tags/linux-explained/

The sources of this book (texts and diagrams) are available on GitHub and you are free to contribute to this work. This book is not set in stone and its content may evolve and improve with each contribution.

https://github.com/Wivik/linux-explained

Copyright

Unless other statement, this book content, including the images, is licensed under Creative Commons BY-SA 4.0.

CreativeCommons BY-SA CreativeCommons BY-SA

The cover page is illustrated with Tux, the Linux mascot, drawn by Larry Ewing and licensed under Creative Commons BY-SA.

Contributors

Chapter 1 : What is Linux ?

Usually, when we talk about Linux, we talk about a Linux Distribution. But actually, Linux itself is just one component of an operating system (ok, a very central one). Linux is a kernel¹, a core computer program that manages the entire operating system. It’s the interface between the hardware and the software : CPU access, Memory access, Filesystem access, devices access, all of this is made through the kernel. By misuse of langage, we assimilate Linux to an operating system but it’s not exactly accurate because the OS is the integration of the Linux Kernel and various other softwares (we will detail this in the What is a Linux Distribution part).

This distinction is not specific to Linux, even in Microsoft’s world you have it, but it’s less confusing as the software is managed by a unique company unlike a Linux Distribution which integrates various projects together. Actually, Microsoft Windows 11 or 2022 are both operating systems based on the Windows NT Kernel², the core program of Windows’ architecture. In Apple’s world, the kernel running behind every systems edited by the company is named XNU³. Since these two companies are providing a comprehensive product, these details are less advertised.

Linux is distributed under a free and open-source license, the General Public License 2.0, allowing anybody to use it, modify it, distribute it, and integrate it in other works. This is one of the many reasons that made the Linux kernel very popular for Servers systems, embedded systems, mobile devices (Android⁴) is based on Linux too), and even mainframes and supercomputers. If Linux was born for Intel x86 architecture at the beginning, the kernel now supports a multitude of platforms : ARM, PowerPC, etc.

Unix and Linux history

In August 2021, Linux celebrated its 30th birthday⁵. In this article, in French (I can translate it later if requested), I’ve made a little history about the origins of the Linux Kernel. Linux is what we commonly call a Unix-Like or a POSIX compatible kernel. But, what are Unix and POSIX ?

Back in the mid-1960, the MIT, Bell Labs and General Electric were developing an operating system for a mainframe computer. This OS, named Multics⁶, was innovative (with the idea of sharing the computer resources for multiple users, hierarchical filesystem, etc) but very difficult to develop and the project was eventually abandoned. The experimentation was reimplemented into a smaller scale project for a single-tasking system, Unics, later spelled Unix⁷ for reasons that seems to have been forgotten. The first version was released in 1970 after a development started in 1969, a date well known in the Information Technologies sector because it’s what we call the Unix epoch⁸, a date and time representation using the number or seconds elapsed since the 1st January 1970 00:00:00 UTC. The Unix epoch timestamp is widely used in programmation to manipulate date and time. Also another very noticeable fact related to Unix is the creation of the C programming language⁹ at the same moment. Unix itself has been rewritten in C with the release of its version 4 in 1973.

Unix has been later selected as the basis for a new operating system standard written in 1980 by the IEEE Computer society, the Portable Operating System Interface¹⁰, or POSIX. POSIX is a standard describing both the system and user-level application programming interfaces (APIs) with command line shells and various utilities. The reason of this standard creation is mainly because Unix has been progressively copied, adapted and fragmented into various Unix-like systems developed by several companies that were often mutually-incompatible (HP-UX from HP, SunOS/Solaris from Sun Microsystems, AIX from IBM, etc).

Today, the Single Unix Specification is still evolving and the operating system based on the POSIX standard can be certified for using the commercial brand Unix. This is why Linux is described as a Unix-like system, because it was developed as a compatible POSIX kernel, but is not certified.

Linux itself is inspired by Minix¹¹, another Unix-like system used by students. Created in August 1991 by Linus Torvalds, a 21 years old Finnish student, the source code of Linux has been submitted to the Minix mailing-list community. A couple of months later, the first official version of Linux able to run Bash, GCC, and some other GNU tools, was released. Despite its various limitations at this time, Linux has been adopted by numerous developers and later integrated by the GNU Project for its free and open source operating system, the GNU Operating System. The GNU Project was also developing its own kernel, GNU Hurd, since the mid-1980s.

In 1992, Linus adopts the GPLv2 license for the Linux source code and distributes it publicly, including the drivers code, in contrast of Unix which was mainly a proprietary system. Linux would eventually support the POSIX APIs and being able to run software and applications developed for Unix. The version 0.92 of