Add Linux subject

subjects/linux/README.md

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+## Linux
+
+### Introduction
+
+Linux is the most widely used kernel (the core of an operating system).
+
+Linux can be found everywhere there is a computer : data centers, phones, gaming consoles, cars, planes, submarines, space stations...
+
+Operating systems using the Linux kernel are called "Linux distributions".
+
+![Tux mascot](tux.png)
+
+> "But this wasn't to be just any penguin. Above all, Linus wanted one that looked happy, as if it had just polished off a pitcher of beer and then had the best sex of its life."
+>
+> _Just for Fun: The Story of an Accidental Revolutionary_ (Linus Torvalds, David Diamond)
+
+---
+
+This series of projects will introduce you to a Linux distribution called Debian and its use in servers (always-on computers providing the required services for Internet and applications).
+
+You'll become a better programmer by knowing what's underneath your programs and what's behind Cloud services.
+
+It will also allow you to learn some tricks to better control your machine and servers.
+
+### Virtualization
+
+To practice in a standardized way and without the risk of altering your own operating system, install the hypervisor [VirtualBox](https://www.virtualbox.org/wiki/Downloads).
+
+This will allow you to run virtual machines (virtual computers accessible through a window).
+
+This way, whatever your operating system, you can, for example, run Linux, Windows & macOS in 3 separate windows at the same time.
+
+### Installation
+
+Operating systems are usually installed with a "disk image" that can be written to a USB stick or CD/DVD for installation on a computer, or kept as is for installation in a virtual machine (which is the case here).
+
+Install Debian (ISO installer "netinst" is recommended) in a VM (Virtual Machine). The steps are :
+
+- Create the VM.
+- Start the VM and attach the ISO file as an optical drive (CD/DVD).
+- Follow the steps of the installer, it will reboot with a Debian system ready to use.
+
+### Shut down
+
+Gracefully close the VM using "ACPI Shutdown". It's like pressing the power button of a real computer.

subjects/linux/audit/README.md

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+#### Functional
+
+###### Is VirtualBox correctly installed ?
+
+###### Does the virtual machine boot Debian properly (in less than 2 minutes) ?
+
+###### After the boot is complete, does the system react to the ACPI Shutdown and does the virtual machine stop ?