What Is a Kernel?

An OS kernel is a software component responsible for managing various low-level features of the computer, including:

• Interfacing with hardware devices (network adapters, hard disks, and so on)
• Allocating memory to individual programs
• Allocating CPU time to individual programs
• Enabling programs to interact with one another

When you use a program (say, a web browser), it relies on the kernel for many of its basic functions. The web browser can communicate with the outside world only by using network functions provided by the kernel. The kernel allocates memory and CPU time to the web browser, without which it couldn’t run. The web browser may rely on plug-ins to display multimedia content; such programs are launched and interact with the web browser through kernel services. Any program you run on a computer relies on the kernel in a similar way, although the details vary from one OS to another and from one program to another.

The kernel is the software “glue” that holds the computer together. Without a kernel, a modern computer can do very little.

Kernels are not interchangeable; the Linux kernel is different from the macOS kernel used in Apple workstations and laptops, and from the Windows kernel used in Microsoft-compatible workstations and laptops. Each of these kernels uses a different internal design and provides different software interfaces for programs to use. Thus, each OS is built from the kernel up and uses its own set of programs that further define each OS’s features.

Linux uses a kernel called Linux—in fact, technically speaking, the word Linux refers only to the kernel. Other features that you might associate with Linux are provided by non-kernel programs, most of which are available on other platforms, as described shortly, in “What Else Identifies an OS?”

A student named Linus Torvalds created the Linux kernel in 1991. Linux has evolved considerably since that time. Today, it runs on a wide variety of CPUs and other hardware. The easiest way to learn about Linux is to use it on a desktop or laptop PC, so that’s the type of configuration emphasized in this book. The Linux kernel, however, runs on everything from tiny cell phones to powerful supercomputers.

What Else Identifies an OS?

The kernel is at the core of any OS, but it’s a component that most users don’t directly manipulate. Instead, most users interact with a number of other software components, many of which are closely associated with particular OSs. Such programs include the following:

 Command-Line Shells  Years ago, users interacted with computers exclusively by typing commands in a program (known as a shell) that accepted such commands. The commands would rename files, launch programs, and so on. Although many computer users today don’t use text-mode shells, they’re still important for intermediate and advanced Linux users, so we describe them in more detail in Chapter 5, “Getting to Know the Command Line,” and subsequent chapters rely heavily on your ability to use a text-mode shell. Many different shells are available, and which shells are available and popular differ from one OS to another. In Linux, a shell known as the Bourne Again Shell (Bash) is popular.

 Graphical User Interfaces  A graphical user interface (GUI) is an improvement on a text-mode shell, at least from the perspective of a beginning user. GUIs rely on icons, menus, and a mouse pointer rather than typed commands. Windows and macOS both have their own OS-specific GUIs. Linux relies on a GUI known as the X Window System, or X for short. X is a very basic GUI, so Linux also uses desktop environment program suites, such as the GNU Object Model Environment (GNOME) or the K Desktop Environment (KDE), to provide a more complete user experience. It’s the differences between a Linux desktop environment and the GUIs in Windows or macOS that will probably strike you most when you first begin using Linux.

Utility Programs  Modern OSs invariably ship with a wide variety of simple utility programs—calculators, calendars, text editors, disk maintenance tools, and so on. These programs differ from one OS to another. Even the names and methods of launching these programs can differ between OSs. Fortunately, you can usually find the programs you want by perusing menus in the main desktop environment.

Libraries  Unless you’re a programmer, you’re unlikely to have to work with libraries directly; nonetheless, we include them in this list because they provide critical services to programs. Libraries are collections of programming functions that can be used by a variety of programs. For instance, in Linux most programs rely on a library called libc. Other libraries provide features associated with the GUI or that help programs parse options passed to them on the command line. Many libraries exist for Linux, which helps enrich the Linux software landscape.

Productivity Programs  Major productivity programs—web browsers, word processors, graphics editors, and so on—are the usual reason for using a computer. Although such programs are often technically separate from the OS, they are sometimes associated with certain OSs. Even when a program is available on many OSs, it may have a different “feel” on each OS because of the different GUIs and other OS-specific features.

Using a Text-Mode User Interface

 In the past, and even sometimes today, Linux computers booted in text mode. After the system had completely booted, the screen would display a simple text-mode login prompt, which might resemble this:

Fedora 30 (Workstation Edition)
Kernel 5.0.9-301.fc30.x86_64 on an x86_64 (tty1)
 
essentials login:

The details of such a login prompt vary from one system to another. This example includes several pieces of information:

• The OS name and version: Fedora Linux version 30
• The computer’s name: essentials
• The name of the hardware device being used for the login: tty1
• The login prompt itself: login:

To log into such a system, you must type your username at the login: prompt. The system then prompts you for a password, which you must also type. If you entered a valid username and password, the computer is likely to display a login message, followed by a shell prompt:

[rich@essentials:~]$

In this book, we omit most of the prompt from example commands when they appear on their own lines. We keep the dollar sign ($) prompt, though, for ordinary user commands. Some commands must be entered as the root user account, which is the Linux administrative user. We change the prompt to a hash mark (#) for such commands, since most Linux distributions make a similar change to their prompts for the root user.

The details of the shell prompt vary from one installation to another, but you can type text-mode commands at the shell prompt. For instance, you could type ls (short for list) to see a list of files in the current directory. The most basic commands are shortened by removing vowels, and sometimes consonants, to minimize the amount of typing required to execute a command. This has the unfortunate effect of making many commands rather obscure.

Some commands display no information, but most produce some type of output. For instance, the ls command produces a list of files:

$ ls
chapter1.doc  figure01.png

This example shows two files in the current directory: chapter01.doc and figure01.png. You can use additional commands to manipulate these files, such as cp to copy them or rm to remove (delete) them. Chapter 5 (“Getting to Know the Command Line”) and Chapter 7 (“Managing Files”) describe some common file manipulation commands.

Some text-mode programs take over the display in order to provide constant updates or to enable you to interact with data in a flexible manner. Figure 1.1, for instance, shows the nano text editor, which is described in more detail in Chapter 10, “Editing Files.” When nano is working, you can use your keyboard’s arrow keys to move the cursor around, add text by typing, and so on.

Even if you use a graphical login, you can use a text-mode shell inside a window, known as a terminal. With common Linux GUIs, you can launch a terminal program, which provides a shell prompt and the means to run text-mode programs.

Using a Graphical User Interface

 Most users are more comfortable with GUIs than with text-mode commands. Thus, many modern Linux systems start up in GUI mode by default, presenting a login screen similar to the one shown in Figure 1.2. You can select your username from a list or type it, followed by typing your password, to log in.

Unlike Windows and macOS, Linux provides a number of different desktop environments for you to choose from. Which one you use depends on the specific variety of Linux you’re using, what software options you selected at installation time, and your own personal preferences. Common choices include GNOME, KDE Plasma, Cinnamon, and Xfce. Many other options are available as well. Many graphical desktops have assistive technology features built in. In Figure 1.2, the person icon at the top-right corner of the Fedora login window allows you to select an assistive technology such as a screen reader or onscreen keyboard to assist with the login entry.

Linux desktop environments can look quite different from one another, but they all provide similar functionality. Figure 1.3 shows the default Cinnamon desktop on a Mint 18.3 installation, with a couple of programs running. Chapter 4 describes common desktop environments and their features in more detail, but for now, you should know that they all provide features such as the following:

Program Launchers  You can launch programs by selecting them from menus or lists. Typically, one or more menus reside along the top, bottom, or side of the screen. In Figure 1.3, you can click the Mint leaf icon in the bottom-left corner of the screen to produce the menu that appears in that figure.

File Managers  Linux provides GUI file managers similar to those in Windows or macOS. A window for one of these is open in the center of Figure 1.3.

Window Controls  You can move windows by clicking and dragging their title bars, resize them by clicking and dragging their edges, and so on.

Multiple Desktops  Most Linux desktop environments enable you to keep multiple virtual desktops active, each with its own set of programs. This feature helps keep the screen uncluttered while you run several programs simultaneously. Typically, an icon in one of the menus enables you to switch between virtual desktops.

Logout Options  You can log out of your Linux session, which enables you to shut down the computer or let another user log in.

As you learn more about Linux, you’ll discover that its GUI environments are quite flexible. If you find you don’t like the environment that’s the default for your distribution, you can change it. Although they all provide similar features, some people have strong preferences about desktop environments. Linux gives you a choice in the matter that’s not available in Windows or macOS, so feel free to try multiple desktop environments.

Comparing Linux to Unix

 If you were to attempt to draw a “family tree” of OSs, you would end up scratching your head a lot. This is because OS designers often mimic one another’s features and sometimes even incorporate one another’s code into their OSs’ workings. The result can be a tangled mess of similarities between OSs, with causes ranging from coincidence to code “borrowing.” Attempting to map these influences can be difficult. In the case of Linux and Unix, though, a broad statement is possible: Linux is modeled after Unix.

Unix was created in 1969 at AT&T’s Bell Labs. Unix’s history is complex and involves multiple forks (that is, splitting of the code into two or more independent projects) and even entirely separate code rewrites. Modern Linux systems are, by and large, the product of open source projects that clone Unix programs, or of original open source code projects for Unix generally.

These projects include:

The Linux Kernel  Linus Torvalds created the Linux kernel as a hobby programming project in 1991, but it soon grew to be much more than that. The Linux kernel was designed to be compatible with other Unix kernels in the sense that it used the same software interfaces in source code. This made using open source programs for other Unix versions with the Linux kernel easy.

The GNU Project  The GNU’s Not Unix (GNU) project is an effort by the Free Software Foundation (FSF) to develop open source replacements for all the core elements of a Unix OS. In 1991, the FSF had already released the most important such tools, with the notable exception of the kernel. (The GNU HURD kernel is now available but is not as popular as the Linux kernel.) Alternatives to the GNU tools include proprietary commercial tools and open source tools developed for the BSD Unix variants. The tools used on a Unix-like OS can influence its overall “flavor,” but all of these tool sets are similar enough to give any Unix variety a similar feel compared to a non-Unix OS.

Xorg-X11  The X Window System is the GUI environment for most Unix OSs. Most Linux distributions today use the Xorg-X11 variety of X. As with the basic text-mode tools provided by the GNU project, choice of an X server can affect some features of a Unix-like OS, such as the types of fonts it supports. Wayland is a newer X Windows System software package used in Linux and is gaining in popularity.

Desktop Environments  GNOME, KDE Plasma, Cinnamon, Xfce, and other popular open source desktop environments have largely displaced commercial desktop environments even on commercial versions of Unix. Thus, you won’t find big differences between Linux and Unix in this area.

Server Programs  Historically, Unix and Linux have been popular as server OSs—organizations use them to run web servers, database servers, email servers, and so on. Linux runs the same popular server programs as do commercial Unix versions and the open source BSDs.

User Productivity Programs  In this realm, as in server programs, Linux runs the same software as do other Unix-like OSs. In a few cases, Linux runs more programs or runs them better. This is mostly because of Linux’s popularity and the vast array of hardware drivers that Linux offers. If a program needs advanced video card support, for example, it’s more likely to find that support on Linux than on a less popular Unix-like OS.

On the whole, Linux can be thought of as a member of the family of Unix-like OSs. Although Linux is technically not a Unix OS, it’s similar enough that the differences are unimportant compared to the differences between this family as a whole and other OSs, such as Windows. Because of its popularity, Linux offers better hardware support, at least on commodity PC hardware. Some Unix varieties offer specific features that Linux lacks, though. For instance, Oracle’s Solaris Unix uses built-in zones that handle virtual machines better than the tools currently available in Linux.

Comparing Linux to macOS

 Apple macOS is a commercial Unix-based OS that borrows heavily from the BSDs and discards the usual Unix GUI (namely X) in favor of its own user interface. This makes macOS both very similar to Linux and quite different from it.

You can open a macOS Terminal window and type many of the same commands described in this book to achieve similar ends. If a command described in this book isn’t present, you may be able to install it in one way or another. macOS ships with some popular Unix server programs, so you can configure it to work much like Linux or another Unix-like OS as a network server computer.

macOS differs from Linux in its user interface, though. The macOS user interface is known as Cocoa from a programming perspective, or Aqua from a user’s point of view. It includes elements that are roughly equivalent to both X and a desktop environment in Linux. Because Cocoa isn’t compatible with X from a programming perspective, applications developed for macOS can’t be run directly on Linux (or on other Unix-like OSs), and porting them (that is, modifying the source code and recompiling them) for Linux is a nontrivial undertaking. Thus, native macOS applications seldom make the transition to Linux.

macOS includes an implementation of X that runs under Aqua. This makes the transfer of GUI Linux and Unix programs to macOS relatively straightforward. The resulting programs don’t entirely conform to the Aqua user interface, though. They may have buttons, menus, and other features that look out of place compared to the usual appearance of macOS equivalents.

Apple makes macOS available only for its own computers. Its license terms forbid installation on non-Apple hardware, and even aside from licensing issues, installing macOS on non-Apple hardware is a nontrivial undertaking. A variant of macOS, known as iOS, runs on Apple’s iPad and iPhone devices, and is equally nonportable to other devices. Thus, macOS is largely limited to Apple hardware. Linux, by contrast, runs on a wide variety of hardware, including most PCs. You can even install Linux on Macintosh computers.

Comparing Linux to Windows

 Most desktop and laptop computers today run Microsoft Windows. Thus, if you’re considering running Linux, the most likely comparison is to Windows. Broadly speaking, Linux and Windows have similar capabilities; however, there are significant differences in details. These include the following:

Licensing  Linux is an open source OS whereas Windows is a proprietary commercial OS. Chapter 2, “Understanding Software Licensing,” covers open source issues in greater detail, but for now you should know that open source software gives you greater control over your computer than does proprietary software—at least in theory. In practice, you may need a great deal of expertise to take advantage of open source’s benefits. Proprietary software may be preferable if you work for an organization that’s only comfortable with the idea of software that’s sold in a more traditional way. (Some Linux variants, though, are sold in a similar way, along with service contracts.)

Costs  Many Linux varieties are available free of charge and so are appealing if you’re trying to cut costs. However, the expertise needed to install and maintain a Linux installation is likely to be greater, and therefore more expensive, than the expertise needed to install and maintain a Windows installation. Different studies on the issue of total cost of ownership of Linux versus Windows have gone both ways, but most tend to favor Linux.

Hardware Compatibility  Most hardware components require OS support, usually in the form of drivers. Most hardware manufacturers provide Windows drivers for their devices or work with Microsoft to ensure that Windows includes appropriate drivers. Although some manufacturers provide Linux drivers, too, for the most part the Linux community as a whole must supply the necessary drivers. This means that Linux drivers may take a few weeks or even months to appear after a device becomes available. At the same time, Linux developers tend to maintain drivers for old hardware for much longer than manufacturers continue to support their own old hardware. Thus, a modern Linux may run better than a recent version of Windows on old hardware. Linux also tends to be less resource intensive, so you can be productive on older hardware when using Linux.

Software Availability  Some popular desktop applications, such as Microsoft Office, are available on Windows but not on Linux. Although Linux alternatives such as LibreOffice are available, they haven’t caught on in the public’s mind. In other realms, the situation is reversed. Popular server programs, such as the Apache web server, were developed first for Linux or Unix. Although many such servers are available for Windows, they run more efficiently on Linux. If you have a specific program you must run, you may want to research its availability and practicality on any platforms you’re considering.

User Interfaces  Like macOS, Windows uses its own unique user interface. This fact contributes to poor inter-OS portability. (Tools exist to help bridge the gap, though; X Window System implementations for Windows are available, as are tools for running Windows programs in Linux.) Some users prefer the Windows user interface to any Linux desktop environment, but others prefer a Linux desktop environment.

Configurability  Linux is a much more configurable OS than is Windows. Although both OSs provide means to run specific programs at startup, change user interface themes, and so on, Linux’s open source nature means you can tweak any detail you want. Furthermore, you can pick any Linux variant you like to get a head start on setting up the system as you see fit.

Security  Advocates of each OS claim it’s more secure than the others. They can do this because they focus on different security issues. Many of the threats to Windows come from viruses, which by and large target Windows and its huge installed user base. Viruses are essentially a nonissue for Linux; in Linux, security threats come mostly from break-ins involving misconfigured servers or untrustworthy local users.

For over a decade, Windows has dominated the desktop arena. In both homes and offices, users have become familiar with Windows and are used to popular Windows applications, such as Microsoft Office. Although Linux can be used in such environments, it’s a less popular choice for a variety of reasons—its unfamiliarity, the fact that Windows comes preinstalled on most PCs, and the lack of any compelling Linux-only applications for most users.

Unix generally, and Linux in particular, on the other hand, have come to dominate the server market. Linux powers the web servers, database servers, email servers, and so on that make up the Internet and that many businesses rely on to provide local network services. Thus, most people use Linux daily even if they don’t realize it.

In most cases, it’s possible to use either Linux or Windows on a computer and have it do an acceptable job. Sometimes, though, specific needs dictate use of one OS or another. You might need to run a particular exotic program, for instance, or your hardware might be too old for a modern Windows or too new for Linux. In other cases, your own or your users’ familiarity with one OS or the other may favor its use.

Creating a Complete Linux-Based OS

 We’ve already described some of what makes up a Linux OS, but some details need reiteration or elaboration:

A Linux Kernel  A Linux kernel is at the core of any Linux OS, of course. We’ve written this item as a Linux kernel because the Linux kernel is constantly evolving. Two distributions are likely to use slightly different kernels. Distribution maintainers also often patch kernels—that is, they make small changes to fix bugs or add features.

Core Unix Tools  Tools such as the GNU tool set, the X Window System, and the utilities used to manage disks are critical to the normal functioning of a Linux system. Most Linux distributions include more or less the same set of such tools, but as with the kernel, they can vary in versions and patches.

Supplemental Software  Additional software, such as major server programs, desktop environments, and productivity tools, ships with most Linux distributions. As with core Unix software, most Linux distributions provide similar options for such software. Distributions sometimes provide their own “branding,” though, particularly in desktop environment graphics.

Startup Scripts  Much of a Linux distribution’s “personality” comes from the way it manages its startup process. Linux uses scripts and utilities to launch the dozens of programs that link the computer to a network, present a login prompt, and so on. These scripts and utilities vary between distributions, which means they have different features and may be configured in different ways.

An Installer  Software must be installed to be used, and most Linux distributions provide unique installation software to help you manage this important task. Thus, two distributions may install in very different ways, giving you options for key features such as disk layouts and initial user account creation.

Typically, Linux distributions are available for download from their websites. You can usually download a CD-R or DVD image file that you can then burn to an optical disc. When you boot the resulting disc, the installer runs and you can install the OS. You can also use the same image to create a bootable USB flash drive if your computer lacks an optical drive. There are also cloud versions of many Linux distributions, which allow you to download a complete Linux system to run in either a private virtual machine or a commercial cloud such as Amazon Web Services (AWS) or Google Cloud Platform.

Some Linux installers come complete with all the software you’re likely to install. Others come with only minimal software and expect you to have a working Internet connection so that the installer can download additional software. If your computer isn’t connected to the Internet, be sure to get the right type of installer.

A Summary of Common Linux Distributions

 Depending on how you count, there are about a dozen major Linux distributions for desktop, laptop, and small server computers, and hundreds more that serve specialized purposes. Table 1.1 summarizes the features of the most important distributions.

These features require explanation:

Availability  Most Linux distributions are entirely open source or free software; however, some include proprietary components and are sold for money, typically with a support contract. Red Hat Enterprise Linux (RHEL) and SUSE Enterprise Linux are the two most prominent examples of this type of distribution. Both have completely free cousins. For RHEL, CentOS is a near-clone that omits the proprietary components, and Fedora is an open version that serves as a testbed for technologies that may eventually be included in RHEL. For SUSE Enterprise, openSUSE is a free alternative.

Package Format  Most Linux distributions distribute software in packages, which are collections of many files in one. Package software maintains a local database of installed files, making upgrades and uninstallations easy. The RPM Package Manager (RPM) system is the most popular one in the Linux world, but Debian packages are very common, too. Other packaging systems work fine but are distribution-specific, such as the pacman package management system used in Arch Linux. Slackware is unusual in that it uses tarballs for its packages. These are package files created by the standard tar utility, which is used for backing up computers and for distributing source code, among other things. The tarballs that Slackware uses for its packages contain Slackware-specific information to help with package management. Gentoo is unusual because its package system is based on compiling most software from source code. This is time-consuming but enables experienced administrators to tweak compilation options to optimize the packages for their own hardware and software environments.

Release Cycle  We describe release cycles in more detail shortly, in “Understanding Release Cycles.” As a general rule, distributions with short release cycles aim to provide the latest software possible, whereas those with longer release cycles strive to provide the most stable environments possible. Some try to have it both ways; for instance, Ubuntu releases long-term support (LTS) versions in April of even-numbered years. Its other releases aim to provide the latest software.

Administrator Skill Requirements  The final column in Table 1.1 provides our personal estimation of the skill level required to administer a distribution. As you can see, we’ve described most Linux distributions as requiring “intermediate” skill to administer. Some, however, provide less in the way of user-friendly GUI administrative tools and so require more skill. Ubuntu aims to be particularly easy to use and administer.

Most Linux distributions are available for at least two platforms—that is, CPU types: x86 (also known as IA32, i386, and several variants) and x86-64 (also known as AMD64, EM64T, and x64). Until about 2007, x86 computers were the most common variety, but now x86-64 computers have become the standard. If you have an x86-64 computer, you can run either an x86 or an x86-64 distribution on it, although the latter provides a small speed improvement. More exotic platforms, such as ARM (for tablets), PowerPC, Alpha, and SPARC, are available. Such platforms are mostly restricted to servers and to specialized devices (described shortly).

Understanding Release Cycles

 Table 1.1 summarized the release cycles employed by a number of common Linux distributions. The values cited in that table are the time between releases. For instance, new versions of Ubuntu come out every six months, like clockwork. Most other distributions’ release schedules provide some “wiggle room”; if a release date slides a month, that may be acceptable.

After its release, a distribution is typically supported until sometime after the next version’s release—typically a few months to a year or more. During this support period, the distribution’s maintainers provide software updates to fix bugs and security problems. After the support period has passed, you can continue to use a distribution, but you’re on your own—if you need updated software, you’ll have to compile it from source code yourself or hope that you can find a compatible binary package from some other source. As a practical matter, therefore, it’s generally a good idea to upgrade to the latest version before the support period ends. This fact makes distributions with longer release cycles appealing to businesses, since a longer time between installations minimizes disruptions and costs associated with upgrades.

Two of the distributions in Table 1.1 (Arch and Gentoo) have rolling release cycles. Such distributions have no version numbers in the usual sense; instead, upgrades occur in an ongoing manner. Using such a distribution makes it unnecessary to ever do a full upgrade, with all the hassles that creates; however, you’ll occasionally have to do a disruptive upgrade of one particular subsystem, such as a major upgrade in your desktop environment.

 Before the release of a new version, most distributions make pre-release versions available. Alpha software is extremely new and very likely to contain serious bugs, whereas beta software is more stable but nonetheless more likely to contain bugs than is the final release software. As a general rule, you should avoid using such software unless you want to contribute to the development effort by reporting bugs or unless you’re desperate to have a new feature.

What Is Cloud Computing?

The first mention of the term cloud came in documentation for the original ARPANET network environment in 1977, the precursor to the modern-day Internet. In that documentation, the cloud symbol was commonly used to represent the large network of interconnected servers geographically dispersed. However, in this environment each server was self-contained and self-sufficient—there was no distributed computing.

The term cloud computing is related to distributed computing. In distributed computing, resources are shared among two or more servers to accomplish a single task, such as run an application. This environment became the precursor to what we know today as cloud computing, popularized by companies such as Amazon Web Services (AWS), Google Cloud Platform, and Microsoft Azure.

Cloud computing provides the ability to deliver computing resources across the Internet. Now customers can purchase both hardware and software resources as needed from cloud computing vendors. This includes servers, storage space, databases, networks, operating systems, and even individual applications. Figure 1.4 demonstrates the three methods for providing cloud computing services.

As you can see in Figure 1.4, there are three primary methods for providing cloud computing environments:

• Public: In the public cloud computing environments, a third party provides all of the computing resources outside of the organization. These resources are usually shared between multiple organizations.
• Private: In the private cloud computing environments, each individual organization builds its own cloud computing resources to provide resources internally.
• Hybrid: In hybrid cloud computing environments, computing resources are provided internally within the organization but also connected to an external public cloud to help supplement resources when needed.

What Are the Cloud Services?

Cloud computing environments can customize the level of resources provided to customers, depending on each customer’s needs. This section describes the three most popular models for providing resource levels that you’ll find from cloud computing vendors.

Infrastructure as a Service (IaaS)

In the infrastructure as a service (IaaS) model, the cloud computing vendor provides low-level server resources to host applications for organizations. These low-level resources include all of the physical components you’d need for a physical server, including CPU time, memory space, storage space, and network resources, as shown in Figure 1.5.

The server resources provided may be on a single server, or they may be distributed among several servers. In a distributed environment. the servers may be co-located in a single facility or they may be separated into multiple facilities located in separate cities. This helps provide for increased availability.

As shown in Figure 1.5, in an IaaS model the customer supplies the operating system and any applications that it needs to run. Most IaaS environments support a multitude of different operating systems, including Linux and Windows servers. The customer is responsible for any system administration work required for the operating system, as well as any application administration. The cloud computing vendor maintains the physical infrastructure environment.

Platform as a Service (PaaS)

In the platform as a service (PaaS) model, the cloud computing vendor provides both the physical server environment as well as the operating system environment to the customer, as shown in Figure 1.6.

With the PaaS model. the cloud computing vendor takes responsibility for both the physical components as well as the operating system administration. It provides system administration support to ensure the operating system is properly patched and updated to keep up with current releases and security features. This allows the customer to focus mainly on developing the applications running within the PaaS environment.

Software as a Service (SaaS)

In the software as a service (SaaS) model, the cloud computing vendor provides a complete application environment, such as a mail server, database server, or web server. The vendor provides the physical server environment, the operating system, and the application software necessary to perform the function. This is shown in Figure 1.7.