This lesson will cover what Linux is and how it has evolved into different distributions.

What Is Linux?

The image shows a timeline with the following years:
1969 - Unix is developed at Bell labs by Dennis Ritchie and Ken Thompson
1985 - Richard Stallman begins GNU project
1989 - GNU project releases GPL
1991 - Linus Torvolds releases first version of Linux
1992 - Linux released under GPL
1993 - Debian distribution launched
1996 - tux mascot launched
1999 - red hat goes public on NASDAQ
2004 - Ubuntu Linux is founded
2008 - First Android OS is launched
2012 - First Raspberry Pi OS is launched making embedded Linux more available to public

To understand how Linux came to be, we must look back to the early days in computing, when Ken Thompson and Dennis Ritchie of Bell Laboratories began working on UNIX, the first ever portable operating system. The year was 1969. UNIX could run on a range of devices and was widely used as proprietary software. Then, Richard Stallman started the *GNU Project* in 1983 with goals to create a free operating system. At the time, the free software movement was advocating for the freedom for users to modify and share copies of software. The project created the [GNU General Public License](https://www.gnu.org/licenses/licenses.en.html) (GPL), a widely used free software license, which set the background for the Linux operating system. 

In 1991 there was not yet a free operating system, so a Finnish computer science student named Linus Torvalds began developing an operating system as a hobby. He released the code under GPL and slowly, the community around Linux grew as more and more developers contributed to the base code. 

Today, Linux refers to the many versions of operating systems built on top of the base code. This means anyone can create their own version of a Linux operating system!

Unix history

In this exercise, we'll break down the main components of the Linux operating system.  First, let's explain what an operating system is. It is a system program that provides an interface between the user and the hardware. When a computer boots up, the operating system is the first program that loads and manages all of the other application programs on a computer.

The *kernel* is the most important part of the operating system. It performs a variety of tasks:
* process management
* managing hardware devices
* task scheduling

The kernel controls all the major functions of the hardware, whether it’s a phone, laptop, or server. When we talked about what Linus Torvalds first published in 1991, it was the kernel.

The *shell* is the interface between the user and kernel. At its barest, it is a command-line interpreter where a user can enter commands which are interpreted by the computer to perform a certain task. In Linux, the shell language is called Bash.

Inside operating systems with desktop environments such as Windows and Mac OS, users may never interact with the shell, but in Linux, the shell plays a huge role. We can even install programs to use only within the shell. Some versions of Linux (like Ubuntu) have graphical desktop environments too. To navigate in a Linux environment and get the most out of it though, we have to be familiar with the *Bash command language*, which is what the Linux shell interprets.

In the next exercise, we'll learn more about Linux distributions that we can choose from for an operating system.

A circular diagram with the kernel in the center surrounded by another circle labeled "shell". Surrounding the shell is a circle labeled with "Graphical Desktop (some Linux versions)".  A drawing of a coder with a laptop can send user input to the Graphical Desktop circle through "click and point" and the shell circle through "Bash language".

OS vs Linux Kernel vs Shell 

Four hand-drawn coders with laptops in a circle surrounding the Open Source Initiative logo. Each coder has an arow pointing to the Open-Source logo with certificates saying "License to Code."

To better understand the popularity of the Linux operating system among developers, let's take a moment to talk more about open-source software and its ideals. Open-source software is software with source code that anybody can look at or contribute to and distribute based on the terms of the license. It grew out of the Free Software Movement which [guarantees certain freedoms](https://www.gnu.org/philosophy/free-sw.en.html#four-freedoms) to software users. The two movements overlap significantly and the terms are often used interchangeably, but they are respectively defined by the non-profit organizations, [Open Source Initiative](https://opensource.org/) and the [Free Software Foundation](https://www.fsf.org/).

The Free Software Foundation first published the GNU Public License (GPL) in 1989. The GPL is a copyleft (as opposed to copyright) license, which means that any derivative software must be distributed under the same or equivalent license terms. Linus Torvalds released the Linux operating system under GPL license in 1992. This non-proprietary way of sharing code has since been described as a "defining factor of Linux's success" as more and more developers began to contribute to the project. Over time, a rich Linux eco-system with different versions (called  *distributions* ) of the operating system and many open-source applications has developed.

In the next few exercises, we'll learn some of the key differences between Linux distributions.

If you'd like to learn more about open source, we have a [blog post dedicated to it](https://www.codecademy.com/resources/blog/what-is-open-source/).

Open Source Software

Tux, the Linux mascot, is sitting surrounded by many devices such as a phone, a router, car navigation system, a laptop, and a server tower.

Welcome to the world of Linux and open-source software! In this lesson, we will learn about the Linux Operating System, its history, and its applications. Linux has had an incredible impact on modern technology as a lightweight operating system for low-power devices to highly sophisticated supercomputers. Linux runs on over 95% of web servers. Today, Linux runs 85% of smartphones (Android is built on top of the Linux kernel). 

The reason that Linux is such a flexible operating system is that almost all of its features can be accessed using a text terminal, without a graphical desktop environment. This gives us faster access to a computer or server's resources. 

In the next few exercises, we'll talk about Linux's history and open-source ideas, the components of the Linux operating system, and the different "flavors" of Linux. Towards the end, we will tour a Linux desktop.

Let's get started!

Introduction to Linux

Linux refers to the family of operating systems based on the Linux kernel and each operating system is packaged as a *distribution*, or *distro*, of Linux. A Linux distribution is made from a software collection including the Linux kernel, GNU tools, and default software.

There are a large number of Linux distributions. Here is a short list of some of the most popular distributions:

* [*Debian*](https://www.debian.org/)  is one of the oldest distributions of Linux, first developed in 1993. It is a stable Linux operating system, and software updates are frequent but small.

* [*Ubuntu*](https://ubuntu.com/)  is the most popular distribution of Linux and is based on Debian Linux. It’s widely used and supported and looks most like other operating systems like OSX and Windows in terms of usability. 

* [*Linux Mint*](https://linuxmint.com/)  is a distribution that is based off of Ubuntu that comes with less pre-installed software than Ubuntu.

* [*Red Hat Enterprise Linux*](https://www.redhat.com/en/technologies/linux-platforms/enterprise-linux) , also known as RHEL, is a distribution of Linux developed by Red Hat. It has strict rules around its trademark which prevent free distribution, but it is mostly used in enterprise environments on servers.

* [*Fedora*](https://getfedora.org/)  is an open-source community-driven distribution of Linux that is backed by Red Hat. Think of it as the Ubuntu equivalent to Red Hat.

* [*Arch*](https://archlinux.org/)  is a rolling release distribution of Linux that is 100% developed by its community. It has a steeper learning curve than other distributions but it is a great lightweight distribution of Linux.

This is just a small selection of the distributions out there. There are forks and derivations of these distributions as well! We recommend starting with a free beginner-friendly distribution like Ubuntu, but feel free to do some research to see what works best for you. 

Timeline of Linux distribution forks.
1993 - Debian distribution launched
2004 - Ubuntu forked from Debian
2006 - Mint forked from Ubuntu
1993 - Red Hat Founded
2003 - Fedora from Red Hat
2002 - Arch Linux started

What's in a Linux distribution?

Throughout this lesson, we learned about the rich open-source history of Linux and how it has been developed by the community as one of the most successful open-source projects of all time.

We discussed the differences between the kernel and the shell and how they form the operating system. In the case of Linux, the shell is based on the command language Bash.

We also introduced an overview of popular distributions. Some are more beginner-friendly with a more conventional graphical user interface, while others require more technical know-how around the command line.

In the following video, we'll be showing a quick demo of a Linux Ubuntu desktop, what the default applications look like, and what we can do with powerful commands in the shell.

We hope this is the beginning of your exploration of Linux!
