Linux Essentials Cap 1

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LINUX ESSENTIALS 1
1.1 Introduction
In this chapter we will explore the evolution of Linux® and popular operating systems. We
will also discuss the considerations for choosing an operating system.
1.2 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.
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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.
1.2.1 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.
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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.
1.2.2 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.
1.2.3 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.
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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.
1.2.4 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 specifictools 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.
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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.
1.2.4.1 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.
Consider This
While aliases will be covered in detail in a later section, this brief example may be helpful in
understanding the concept of commands.
An alias is essentially a nickname for another command or series of commands. For
example, the cal 2014 command will display the calendar for the year 2014. Suppose you
end up running this command often. Instead of executing the full command each time, you
can create an alias called mycal and run the alias, as demonstrated in the following graphic:
sysadmin@localhost:~$ alias mycal="cal 2014" 
sysadmin@localhost:~$ mycal
1.2.5 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.
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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.
1.3 Choosing an Operating System
You have learned that Linux is a UNIX-like operating system, which means that it has not
undergone formal certification and therefore can’t use the official UNIX trademark.There are
many other alternatives; some are UNIX-like and some are certified as UNIX. There are also
non-Unix operating systems such as Microsoft Windows.
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The most important question to ask when determining the configuration of a machine is
“what will this machine do?” If you need to run specialized software that only runs on Oracle
Solaris, then that’s what you’ll need. If you need to be able to read and write Microsoft Office
documents, then you’ll either need Windows or something capable of running LibreOffice or
OpenOffice.
1.3.1 Decision Points
The first thing you need to decide is the machine’s role. Will you be sitting at the console
running productivity applications or web browsing? If so, you have a desktop. Will the
machine be used as a Web server or otherwise sitting in a server rack somewhere? You’re
looking at a server.
Servers usually sit in a rack and share a keyboard and monitor with many other computers,
since console access is only used to set up and troubleshoot the server. The server will run
in non-graphical mode, which frees up resources for the real purpose of the computer. A
desktop will primarily run a GUI.
Next, determine the functions of the machine. Is there specific software it needs to run, or
specific functions it needs to do? Do you need to be able to manage hundreds or thousands
of these machines at the same time? What is the skill set of the team managing the
computer and software?
You must also determine the lifetime and risk tolerance of the server. Operating systems and
software upgrades come on a periodic basis, called the release cycle. Software vendors will
only support older versions of software for a certain period of time before not offering any
updates, which is called the maintenance cycle (or life cycle). For example, major Fedora
Linux releases come out approximately every 6 months. Versions are considered End of Life
(EOL) after 2 major versions plus one month, so you have between 7 and 13 months after
installing Fedora before you need to upgrade. Contrast this with the commercial server
variant, Red Hat Enterprise Linux, and you can go up to 13 years before needing to
upgrade.
The maintenance and release cycles are important because in an enterprise server
environment it is time consuming, and therefore rare, to do a major upgrade on a server.
Instead, the server itself is replaced when there are major upgrades or replacements to the
application that necessitate an operating system upgrade. Similarly, a slow release cycle is
important because applications often target the current version of the operating system and
you want to avoid the overhead of upgrading servers and operating systems constantly to
keep up. There is a fair amount of work involved in upgrading a server, and the server role
often has many customizations made that are difficult to port to a new server. This
necessitates much more testing than if only the application were upgraded.
If you are doing software development or traditional desktop work, you often want the latest
software. Newer software has improvements in both functionality and appearance, which
contributes to more enjoyment from the use of the computer. A desktop often stores its work
on a remote server, so the desktop can be wiped clean and the newer operating system put
on with little interruption.
Individual software releases can be characterized as beta or stable. One of the great things
about being an open source developer is that you can release your new software and quickly
get feedback from users. If a software release is in a state that it has many new features that
have not been rigorously tested, it is typically referred to as beta. After those features have
been tested in the field, the software moves to a stable point. If you need the latest features,
then you are looking for a distribution that has a quick release cycle and makes it easy to
use beta software. On the server side, you want stable software unless those new features
are necessary and you don’t mind running code that has not been thoroughly tested.
Another loosely related concept is backward compatibility. This refers to the ability for a later
operating system to be compatible with software made for earlier versions. This is usually a
concern if you need to upgrade your operating system, but aren’t in a position to upgrade
your application software.
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Of course, cost is always a factor. Linux itself might be free, but you may need to pay for
support, depending on which options you choose. Microsoft has server license costs and
may have additional support costs over the lifetime of the server. Your chosen operating
system might only run on a particular selection of hardware, which further affects the cost.
1.3.2 Microsoft Windows
The Microsoft world splits the operating systems according to the machine’s purpose:
desktop or server? The Windows desktop edition has undergone various naming schemes
with the current version (as of this writing) being simply Windows 8. New versions of the
desktop come out every 3-5 years and tend to be supported for many years. Backward
compatibility is also a priority for Microsoft, even going so far as to bundle virtual machine
technology so that users can run older software.
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In the server realm, there is Windows Server, currently (at this writing) at version 2012 to
denote the release date. The server runs a GUI, but largely as a competitive response to
Linux, has made amazing strides in command line scripting abilities through PowerShell. You
can also make the server look like a desktop with the optional Desktop Experience package.
1.3.3 Apple OS X
Apple makes the OS X operating system, which has undergone UNIX certification. OS X is
partially based on software from the FreeBSD project.
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At the moment, OS X is primarily a desktop operating system but there are optional
packages that help with management of network services that allow many OS X desktops to
collaborate, such as to share files or have a network login.
OS X on the desktop is usually a personal decision as many find the system easier to use.
The growing popularity of OS X has ensured healthy support from software vendors. OS X is
also quite popular in the creative industries such as video production. This is one area where
the applications drive the operating system decision, and therefore the hardware choice
since OS X runs on Apple hardware.
1.3.4 BSD
There are several open source BSD (Berkely Software Distribution) projects, such as
OpenBSD, FreeBSD, and NetBSD. These are alternatives to Linux in many respects as they
use a large amount of common software. BSDs are typically implemented in the server role,
though there are also variants such as GNOME and KDE that were developed for desktop
roles.
1.3.5 Other Commercial UNIXes
Some of the more popular commercial UNIXes are:
● Oracle Solaris
● IBM AIX
● HP-UX
Each of these runs on hardware from their respective creators. The hardware is usually large
and powerful, offering such features as hot-swap CPU and memory, or integration with
legacy mainframe systems also offered by the vendor.
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Unless the software requires the specific hardware or the needs of the application require
some of the redundancy built into the hardware, most people tend to choose these options
because they are already users of the company's products. For example, IBM AIX runs on a
wide variety of IBM hardware and can share hardware with mainframes. Thus, you find AIX
in companies that already have a large IBM footprint, or that make use of IBM software like
WebSphere.
1.3.6 Linux
One aspect where Linux is much different than the alternatives is that after an administrator
has chosen Linux theystill have to choose a distribution. Recall from Topic 1 that the
distribution packages the Linux kernel, utilities, and management tools into an installable
package and provides a way to install and update packages after the initial installation.
Some operating systems are available through only one vendor, such as OS X and
Windows, with system support provided through the vendor. With Linux, there are multiple
options, from commercial offerings for the server or desktop, to custom distributions made to
turn an old computer into a network firewall.
Often application vendors will choose a subset of distributions to support. Different
distributions have different versions of key libraries and it is difficult for a company to support
all these different versions.
Governments and large enterprises may also limit their choices to distributions that offer
commercial support. This is common in larger companies where paying for another tier of
support is better than risking extensive outages.
Various distributions also have release cycles, sometimes as often as every six months.
While upgrades are not required, each version can only be supported for a reasonable
length of time. Therefore, some Linux releases are considered to have long term support
(LTS) of 5 years or more while others will only be supported for two years or less.
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Some distributions differentiate between stable, testing, and unstable releases. The
difference being that unstable releases trade reliability for features. When features have
been integrated into the system for a long time, and many of the bugs and issues addressed,
the software moves through testing into the stable release. The Debian distribution warns
users about the pitfalls of using the “sid” release with the following warning:
1. ‘"sid" is subject to massive changes and in-place library updates. This can result in a
very "unstable" system which contains packages that cannot be installed due to
missing libraries, dependencies that cannot be fulfilled etc. Use it at your own risk!’
Other releases depend on Beta distributions. For instance, the Fedora distribution releases
Beta or pre-releases of its software ahead of the full release to minimize bugs. Fedora is
often considered the community oriented Beta release of RedHat. Features are added and
changed in the Fedora release before finding their way into the Enterprise ready RedHat
distribution.
1.3.7 Android
Android, sponsored by Google, is the world’s most popular Linux distribution. It is
fundamentally different from its counterparts. Linux is a kernel, and many of the commands
that will be covered in this course are actually part of the GNU (GNU's Not Unix) package.
That is why some people insist on using the term GNU/Linux instead of Linux alone.
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Android uses the Dalvik virtual machine with Linux, providing a robust platform for mobile
devices such as phones and tablets. However, lacking the traditional packages that are often
distributed with Linux (such as GNU and Xorg), Android is generally incompatible with
desktop Linux distributions.
This incompatibility means that a RedHat or Ubuntu user can not download software from
the Google Play store. Likewise, a terminal emulator in Android lacks many of the
commands of its Linux counterparts. It is possible, however, to use BusyBox with Android to
enable most commands to work.
Chapter 1 - Introduction to Linux
This chapter will cover the following exam objectives:
4.1: Choosing an Operating System
Weight: 1
Description: Knowledge of major operating systems and Linux distributions.
Key Knowledge Areas:
Windows, Mac, Linux differences
Section 1.3
Distribution Life Cycle Management
Section 1.3.6
1.1: Linux Evolution and Popular Operating Systems
Weight: 2
Description: Knowledge of Linux development and major distributions.
Key Knowledge Areas:
Open Source Philosophy
Section 1.2.3
Distributions
Section 1.2.4
Embedded Systems
Section 1.2.3
Android
Android, sponsored by Google, is the world’s most popular Linux distribution. Android
uses the Dalvik virtual machine with Linux, providing a robust platform for mobile devices
such as phones and tablets. Android is generally incompatible with desktop Linux
distributions.
Section 1.3.7
Beta and Stable
Some distributions differentiate between stable, testing, and unstable (beta) releases.
The difference being that unstable releases trade reliability for features. When features have
been integrated into the system for a long time, and many of the bugs and issues addressed,
the software moves through testing into the stable release.
Section 1.3.6
CentOS
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.
Section 1.2.4
Debian
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.
Section 1.2.4
Linux Mint
Linux Mint was started as a fork of Ubuntu Linux. There are various versions, all free
of cost, but some include proprietary codecs.
Section 1.2.4
Maintenance Cycles
Various distributions also have release cycles, sometimes as often as every six
months. While upgrades are not required, each version can only be supported for a
reasonable length of time.
Section 1.3.6
Open SUSE
Open SUSE originally derived from Slackware, yet incorporates many aspects of Red
Hat.
Section 1.2.4
Red Hat
Red Hat started out as a simple distribution that introduced the Red Hat Package
Manager (RPM). 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.
Section 1.2.4
Scientific Linux
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
Section 1.2.4
Ubuntu (LTS)
Ubuntu is the most popular Debian derived distribution.
Section 1.2.4
Prova:
WRONG - See Section 1.2
Unix is:
(choose two)
U An operating system✔
CORRECT - See Section 1.2.3
Most of the tools that are part of Linux systems come from:
U The GNU project✔
CORRECT - See Section 1.2.5
The Linux platform that runs on mobile phones is called:
U Android✔
CORRECT - See Section 1.2.1
The bootloader’s job is to:
U Load the kernel after the computer is powered on✔
CORRECT - See Section 1.2.4
Which distributions are made by, or clones of, Red Hat?
(choose two)
U Fedora✔
U CentOS✔
CORRECT - See Section 1.2.4
Ubuntu is derived from which distribution?
U Debian✔
CORRECT - See Section 1.2.3
Open source licenses differ, but generally agree that:
(choose two)
U You should have access to the source code of software✔
U You should be able modify the software as you wish✔
CORRECT - See Section 1.2.2
Applications make requests to the kernel and receive resources, such as memory, CPU, and
disk in return.
U True✔
CORRECT - See Section 1.3.3
Apple’s OS X is:
(choose three)
U Certified as UNIX compatible✔
U Derived from FreeBSD✔
U Only compatible with Apple hardware✔
CORRECT - See Section 1.3.5
Other commercial Unixes:
(choose two)
U Are tied to their vendor’s hardware✔
U Are UNIX certified✔