Linux Evolution and Popular Operating Systems

The definition of the word Linux depends on the context in which it is used. Linux means the kernel of the system, which is the central controller of everything that happens on the computer (more on this later). People that say their computer “runs Linux” usually refer to the kernel and suite of tools that come with it (called the distribution). If you have “Linux experience”, you are most likely talking about the programs themselves, though depending on the context, you might be talking about knowing how to fine-tune the kernel. Each of these components will be investigated so that you understand exactly what roles each plays.

Further complicating things is the term UNIX. UNIX was originally an operating system developed at AT&T Bell Labs in the 1970’s. It was modified and forked (that is, people modified it and those modifications served as the basis for other systems) such that at the present time there are many different variants of UNIX. However, UNIX is now both a trademark and a specification, owned by an industry consortium called the Open Group. Only software that has been certified by the Open Group may call itself UNIX. Despite adopting all the requirements of the UNIX specification, Linux has not been certified, so Linux really isn’t UNIX! It’s just… UNIX-like.

Role of the Kernel

The kernel of the operating system is like an air traffic controller at an airport. The kernel dictates which program gets which pieces of memory, it starts and kills programs, and it handles displaying text on a monitor. When an application needs to write to disk, it must ask the operating system to do it. If two applications ask for the same resource, the kernel decides who gets it, and in some cases, kills off one of the applications in order to save the rest of the system.

The kernel also handles switching of applications. A computer will have a small number of CPUs and a finite amount of memory. The kernel takes care of unloading one task and loading a new task if there are more tasks than CPUs. When the current task has run a sufficient amount of time, the CPU pauses the task so that another may run. This is called pre-emptive multitasking. Multitasking means that the computer is doing several tasks at once, and pre-emptive means that the kernel is deciding when to switch focus between tasks. With the tasks rapidly switching, it appears that the computer is doing many things at once.

Each application may think it has a large block of memory on the system, but it is the kernel that maintains this illusion, remapping smaller blocks of memory, sharing blocks of memory with other applications, or even swapping out blocks that haven’t been touched to disk.

When the computer starts up it loads a small piece of code called a boot loader. The boot loader’s job is to load the kernel and get it started. If you are more familiar with operating systems such as Microsoft Windows or Apple’s OS X, you probably never see the boot loader, but in the UNIX world it’s usually visible so that you can tweak the way your computer boots.
The boot loader loads the Linux kernel, and then transfers control. Linux then continues with running the programs necessary to make the computer useful, such as connecting to the network or starting a web server.

Applications

Like an air traffic controller, the kernel is not useful without something to control. If the kernel is the tower, the applications are the airplanes. Applications make requests to the kernel and receive resources, such as memory, CPU, and disk, in return. The kernel also abstracts the complicated details away from the application. The application doesn’t know if a block of disk is on a solid-state drive from manufacturer A, a spinning metal hard drive from manufacturer B, or even a network file share. Applications just follow the kernel’s Application Programming Interface (API) and in return don’t have to worry about the implementation details.

When we, as users, think of applications, we tend to think of word processors, web browsers, and email clients. The kernel doesn’t care if it is running something that’s user facing, a network service that talks to a remote computer, or an internal task. So, from this we get an abstraction called a process. A process is just one task that is loaded and tracked by the kernel. An application may even need multiple processes to function, so the kernel takes care of running the processes, starting and stopping them as requested, and handing out system resources.

Role of Open Source

Linux started out in 1991 as a hobby project by Linus Torvalds. He made the source freely available and others joined in to shape this fledgling operating system. His was not the first system to be developed by a group, but since it was a built-from-scratch project, early adopters had the ability to influence the project’s direction and to make sure mistakes from other UNIXes were not repeated.

Software projects take the form of source code, which is a human readable set of computer instructions. The source code may be written in any of hundreds of different languages, Linux just happens to be written in C, which is a language that shares history with the original UNIX.
Source code is not understood directly by the computer, so it must be compiled into machine instructions by a compiler. The compiler gathers all of the source files and generates something that can be run on the computer, such as the Linux kernel.

Historically, most software has been issued under a closed-source license, meaning that you get the right to use the machine code, but cannot see the source code. Often the license specifically says that you will not attempt to reverse engineer the machine code back to source code to figure out what it does!

Open source takes a source-centric view of software. The open source philosophy is that you have a right to obtain the software, and to modify it for your own use. Linux adopted this philosophy to great success. People took the source, made changes, and shared them back with the rest of the group.
Alongside this, was the GNU project (GNU’s, not UNIX). While GNU was building their own operating system, they were far more effective at building the tools that go along with a UNIX operating system, such as the compilers and user interfaces. The source was all freely available, so Linux was able to target their tools and provide a complete system. As such, most of the tools that are part of the Linux system come from these GNU tools.

There are many different variants on open source, and those will be examined in a later chapter. All agree that you should have access to the source code, but they differ in how you can, or in some cases, must, redistribute changes.

Linux Distributions

 

Take Linux and the GNU tools, add some more user facing applications like an email client, and you have a full Linux system. People started bundling all this software into a distribution almost as soon as Linux became usable. The distribution takes care of setting up the storage, installing the kernel, and installing the rest of the software. The full featured distributions also include tools to manage the system and a package manager to help you add and remove software after the installation is complete.

Like UNIX, there are many different flavors of distributions. These days, there are distributions that focus on running servers, desktops, or even industry specific tools like electronics design or statistical computing. The major players in the market can be traced back to either Red Hat or Debian. The most visible difference is the package manager, though you will find other differences on everything from file locations to political philosophies.

Red Hat started out as a simple distribution that introduced the Red Hat Package Manager (RPM). The developer eventually formed a company around it, which tried to commercialize a Linux desktop for business. Over time, Red Hat started to focus more on the server applications such as web and file serving, and released Red Hat Enterprise Linux, which was a paid service on a long release cycle. The release cycle dictates how often software is upgraded. A business may value stability and want long release cycles, a hobbyist or a startup may want the latest software and opt for a shorter release cycle. To satisfy the latter group, Red Hat sponsors the Fedora Project which makes a personal desktop comprising the latest software, but still built on the same foundations as the enterprise version.

Because everything in Red Hat Enterprise Linux is open source, a project called CentOS came to be, that recompiled all the RHEL packages and gave them away for free. CentOS and others like it (such as Scientific Linux) are largely compatible with RHEL and integrate some newer software, but do not offer the paid support that Red Hat does.

Scientific Linux is an example of a specific use distribution based on Red Hat. The project is a Fermilab sponsored distribution designed to enable scientific computing. Among its many applications, Scientific Linux is used with particle accelerators including the Large Hadron Collider at CERN.

Open SUSE originally derived from Slackware, yet incorporates many aspects of Red Hat. The original company was purchased by Novell in 2003, which was then purchased by the Attachmate Group in 2011. The Attachmate group then merged with Micro Focus International. Through all of the mergers and acquisitions, SUSE has managed to continue and grow. While Open SUSE is desktop based and available to the general public, SUSE Linux Enterprise contains proprietary code and is sold as a server product.

Debian is more of a community effort, and as such, also promotes the use of open source software and adherence to standards. Debian came up with its own package management system based on the .deb file format. While Red Hat leaves non Intel and AMD platform support to derivative projects, Debian supports many of these platforms directly.

Ubuntu is the most popular Debian derived distribution. It is the creation of Canonical, a company that was made to further the growth of Ubuntu and make money by providing support.

Linux Mint was started as a fork of Ubuntu Linux, while still relying upon the Ubuntu repositories. There are various versions, all free of cost, but some include proprietary codecs, which can not be distributed without license restrictions in certain countries. Linux Mint is quickly supplanting Ubuntu as the world's most popular desktop Linux solution.

We have discussed the distributions specifically mentioned in the Linux Essentials objectives. You should be aware that there are hundreds, if not thousands more that are available. It is important to understand that while there are many different distributions of Linux, many of the programs and commands remain the same or are very similar.

What is a Command?

The simplest answer to the question, "What is a command?", is that a command is a software program that when executed on the command line, performs an action on the computer.

When you consider a command using this definition, you are really considering what happens when you execute a command. When you type in a command, a process is run by the operating system that can read input, manipulate data and produce output. From this perspective, a command runs a process on the operating system, which then causes the computer to perform a job.
However, there is another way of looking at what a command is: look at its source. The source is where the command "comes from" and there are several different sources of commands within the shell of your CLI:

• Commands built-in to the shell itself: A good example is the cd command as it is part of the bash shell. When a user types the cd command, the bash shell is already executing and knows how to interpret that command, requiring no additional programs to be started.
• Commands that are stored in files that are searched by the shell: If you type a ls command, then the shell searches through the directories that are listed in the PATH variable to try to find a file named ls that it can execute. These commands can also be executed by typing the complete path to the command.
• Aliases: An alias can override a built-in command, function, or a command that is found in a file. Aliases can be useful for creating new commands built from existing functions and commands.
• Functions: Functions can also be built using existing commands to either create new commands, override commands built-in to the shell or commands stored in files. Aliases and functions are normally loaded from the initialization files when the shell first starts, discussed later in this section.

Hardware Platforms

Linux started out as something that would only run on a computer like Linus’: a 386 with a specific hard drive controller. The range of support grew, as people built support for other hardware. Eventually, Linux started supporting other chips, including hardware that was made to run competitive operating systems!
The types of hardware grew from the humble Intel chip up to supercomputers. Later, smaller-size, Linux supported, chips were developed to fit in consumer devices, called embedded devices. The support for Linux became ubiquitous such that it is often easier to build hardware to support Linux and then use Linux as a springboard for your custom software, than it is to build the custom hardware and software from scratch.

Eventually, cellular phones and tablets started running Linux. A company, later bought by Google, came up with the Android platform which is a bundle of Linux and the software necessary to run a phone or tablet. This means that the effort to get a phone to market is significantly less, and companies can spend their time innovating on the user facing software rather than reinventing the wheel each time. Android is now one of the market leaders in the space.

Aside from phones and tablets, Linux can be found in many consumer devices. Wireless routers often run Linux because it has a rich set of network features. The TiVo is a consumer digital video recorder built on Linux. Even though these devices have Linux at the core, the end users don’t have to know. The custom software interacts with the user and Linux provides the stable platform.

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