Chapter 1

Introducing Linux

IN THIS CHAPTER

Getting acquainted with Linux

Discovering Linux distributions

The Linux operating system has become one of the most widely used operating systems, popular among researchers, application developers, and hobbyists alike. These days, the Linux operating system can be found in an amazing range of computer environments, from mobile phones to satellites.

This chapter examines just what the Linux operating system is and why there are so many different Linux distributions available to choose from. With this information, you can select the right Linux distribution for your environment.

What Is Linux?

If you've never worked with Linux before, you may be confused as to why there are so many different versions of it available. You've most likely come across terms such as distribution, LiveDVD, and GNU when looking at Linux packages, and you may have been confused. This section takes some of the mystery out of the Linux system for you.

Although people usually refer to the Linux operating system as just "Linux," in reality there are quite a few parts that make up a complete Linux system. The four main parts of a Linux system are

• The Linux kernel
• The GNU utilities
• The user interface
• Application software

Each of these four parts has a specific job in the Linux system. Although each of the parts by itself isn't very useful, put together, they create what people refer to as "Linux." Figure 1-1 shows the basic diagram of how these parts fit together to create the overall Linux system.

FIGURE 1-1: The Linux system.

The following sections describe these four parts in detail and give you an overview of how they work together to create a complete Linux system.

The Linux kernel

The core of the Linux system is the kernel. The kernel controls all the hardware and software on the computer system, allocating hardware when necessary and executing software when required.

If you've been following the Linux world at all, no doubt you've heard the name Linus Torvalds. Linus is the person responsible for creating the first Linux kernel software while he was a student at the University of Helsinki. He intended it to be a copy of the Unix system, at the time a popular operating system used at many universities.

After developing the Linux kernel, Linus released it to the Internet community and solicited suggestions for improving it. This simple process started a revolution in the world of computer operating systems. Soon Linus was receiving suggestions from students as well as professional programmers from around the world.

Allowing anyone to change programming code in the kernel would result in complete chaos. To simplify things, Linus acted as a central point for all improvement suggestions. It was ultimately Linus's decision whether to incorporate suggested code in the kernel. This same concept is still in place with the Linux kernel code, except that instead of just Linus controlling the kernel code, a team of developers has taken on the task.

The kernel is primarily responsible for four main functions:

• System memory management
• Software program management
• Hardware management
• File system management

The following sections explore the first three functions in more detail. File system management in Linux can be somewhat complicated; Book 2, Chapter 5 dives into that topic.

System memory management

One of the primary functions of the operating system kernel is memory management. Memory management is the ability to control how programs and utilities run within the memory restrictions of the system. Not only does the kernel manage the physical memory available on the system, but it can also create and manage virtual memory (memory that doesn't actually exist but is created on the hard drive and treated as real memory).

It does this by using space on the hard disk called the swap space. The kernel swaps the contents of virtual memory locations back and forth from the swap space to the actual physical memory. This allows the system to think there is more memory available than what physically exists, as shown in Figure 1-2.

The memory locations are grouped into blocks called pages. The kernel locates each page of memory either in the physical memory or the swap space. The kernel then maintains a table of the memory pages that indicates which pages are in physical memory and which pages are swapped out to disk.

FIGURE 1-2: The Linux system memory map.

The kernel keeps track of which memory pages are in use and automatically copies memory pages that have not been accessed for a period of time to the swap space area (called swapping out) - even if other memory is available. When a program wants to access a memory page that has been swapped out, the kernel must make room for it in physical memory by swapping out a different memory page and swapping in the required page from the swap space. Obviously, this process takes time and can slow down a running process. The process of swapping out memory pages for running applications continues for as long as the Linux system is running.

Software program management

With the Linux operating system, a running program is called a process. A process can run in the foreground, displaying output on a display, or it can run in the background, doing work behind the scenes. The kernel controls how the Linux system manages all the processes running on the system.

The kernel creates the first process, called the init process, to start all other processes on the system. When the kernel starts, it loads the init process into virtual memory. As the kernel starts each additional process, it gives it a unique area in virtual memory to store the data and code that the process uses.

A few different types of init process implementations are available in Linux, but these days, the two most popular are

• SysVinit: The SysVinit initialization method was the original method used by Linux and was based on the Unix System V initialization method. Though it is not used by many Linux distributions these days, you still may find it around in older Linux distributions.
• systemd: The systemd initialization method was created in 2010 and has become the most popular initialization and process management system used by Linux distributions.

The SysVinit initialization method primarily utilizes scripts to start and stop applications as needed, while the systemd initialization method uses configuration files. Book 4, Chapter 2 explores how each of these initialization methods works and how you can configure them to customize which applications your Linux system starts automatically.

Hardware management

Still another of the kernel's responsibilities is hardware management. Any device that the Linux system must communicate with needs driver code inserted inside the kernel code. The driver code in the kernel allows the kernel to pass data back and forth to the device, acting as a middleman between applications and the hardware. There are two methods used for inserting device driver code in the Linux kernel:

• Drivers compiled in the kernel
• Driver modules added to the kernel during runtime

Originally, the only way to insert device driver code was to recompile the kernel and restart the system. Each time you added a new device to the system, you had to recompile the kernel code and restart. This process became even more inefficient as Linux kernels supported more hardware and as removable storage devices (such as USB sticks) became more popular. Fortunately, Linux developers devised a better method to insert driver code into the running kernel.

Programmers developed the concept of kernel modules to allow the insertion of device driver code into a running kernel without having to recompile the kernel. Also, a kernel module could be removed from the kernel when the system had finished using the device. This greatly simplified and expanded using hardware with Linux.

Book 4, Chapter 2 also dives into driver modules and how to use them in your Linux system.

GNU utilities

Besides having a kernel to control memory, software, and hardware devices, a computer operating system needs utilities to perform standard functions, such as handling files and programs. Although Linus created the Linux system kernel, he had no system utilities to run on it. Fortunately for him, at the same time he was working, a group of people were working together on the Internet trying to develop a standard set of computer system utilities that mimicked the popular Unix operating system.

The GNU Project (GNU stands for GNU's Not Unix - a recursive acronym) developed a complete set of Unix utilities but had no kernel system to run them on. These utilities were developed under a software philosophy called open-source software (OSS).

The concept of OSS allows programmers to develop software and then release it to the world with no licensing fees attached. Anyone can use the software, modify it, or incorporate it into their own system without having to pay a license fee. Uniting Linus's...