Also see Oracle Linux distributions.

Just like Microsoft Windows, Linux controls the computer hardware, does it's best to keep all your devices playing nicely together and gives you an interface to run things on the computer.  Unlike Microsoft Windows there are several groups (some commercial, some non-profit) who make Linux operating systems.

The Linux kernel

The heart of a Linux system is a program called the kernel.  The kernel controls security, processors, memory, disk and everything else about the system.

The kernel is what allows multiple users to be on a Linux system at the same time running dozens, possibly even thousands of things at once without interfering with each other.  While we won't talk too much more about the kernel, it's just important to remember that it's there keeping everything in check.

The Many Breeds of Linux

You may have already noticed that there are a ton of different Linux versions out there.  Many meet only a very specific need while others compete in the mass market for servers and workstations.  Here are a couple of the Linux distributions which are popular right now:

Red Hat Enterprise Linux

www.redhat.com/rhel/  - This is the commercial version of Linux used for the examples in this book.  It is a popular option for both servers and workstations because it is both feature-rich and well supported.

Fedora

www.redhat.com/fedora/  - Sponsored by Red Hat, this community supported version of Linux is available for free.  The free price tag makes this a great version to work with if you don't have (or want to pay for) a license for Red Hat Enterprise Linux, but be careful, some applications which are Red Hat compatible may not work on Fedora.

Ubuntu

www.ubuntu.com  - Ubuntu is a Linux-based operating system which has recently gained some popularity.  It is easy to install and the desktop version comes with many of the applications you would want on your workstation.

Mileage May Vary

Because no two distributions are quite the same you may find that some commands and options may not work, or may not work as described on your variety of Linux.  The examples in this book were done on RedHat Linux.  Almost all will work on most flavors of Linux but even a different version of RedHat will vary some.

Installation of Linux varies from distribution to distribution.  If you do not already have a Linux system to work on you may want to download one of the versions mentioned above and install it on a spare machine.  Unless you are experienced at installing Linux I suggest you choose a machine you will not use for anything else.  It is quite possible (even easy) to erase your entire hard drive while installing Linux, so take an old system (it doesn't have to be too fast) that you don't need any of the data on and follow the instructions that come with your distribution to install Linux.

What is a shell?

All the commands we run in this book are run in a Linux shell.  A shell in Linux is the software which allows you, a human, interact with the inner workings of the UNIX operating system.  Because of the shell we get to use commands that resemble our natural language (though sometimes only just barely) to control the behavior of the computer.  There are other types of shells in computing, but this is the only type we're concerned with for this book.

Beyond the ability to use somewhat intuitive commands individually, the shell also gives us the ability to combine these commands into shell scripts.  A shell script can contain one command or hundreds and the commands could be nearly anything you could do from within the shell.

The bash shell

There are, of course, many kinds of shells.  Each has its strengths and weaknesses but we're mostly concerned with the Bourne-Again Shell (bash) which is the default for users in Linux.

If you think your shell may not be bash don't worry.  We'll talk about how to change it later, but for now you can change it temporarily simply by typing bash.

Getting to the shell

You can access a Linux shell in several different ways.  You may access the shell by sitting at a Linux computer where the shell is displayed on screen or you may use a secure protocol to enter a shell on a system thousands of miles away.

In some environments you may use a telnet client or, more likely a secure ssh client to connect to a Linux machine.  This will depend largely on how you your machine and your environment is set up.  If you are connecting remotely through one of these clients you should talk with the person who set the machine up to find out how to connect.

Once you're at the Linux command line almost everything works the same whether you're at the keyboard of the system you work on or at a workstation half way around the world.

A few quick tips

Here are a few things that it might help you to know from the beginning.  They may seem out of place here, but will make more sense as we get into using Linux.

No news is good news

One thing you will have to get used to is that when you run many common Linux commands if things go right you will not get any feedback.  There are very few Linux commands which will tell you "Operation completed successfully."  More likely they will tell you nothing unless something goes wrong.  So basically don't worry if you don't get any feedback from Linux.  It'll give you an error if there's a problem.

Tab completion

One feature of the bash shell that we will be using is the ability to complete partially typed commands and file names with the tab key.  If you type in enough of the command or file to uniquely identify what you're looking for, then press tab Linux will complete the file or command.  Try it out once we get started.  It can save you a lot of typing.

Repeating recent commands with the up arrow

If you want to repeat a recently executed command, or even repeat it with a slight modification, the bash shell will allow you to recall it using the up arrow.  The up and down arrows will allow you to go back and forth through your recently typed commands to recall them for execution.  This can be very useful if you make a typo and don't want to retype the entire command again.

Man Pages

Whenever detailed help is needed with command syntax or with command options, the manual (man) command can be used to display all of the information about the specified command. The information will be displayed one screen at a time. Pressing the space bar will advance the next screen. At the end of the display, the Q command can be used to quit.

W(1)                          Linux User's Manual                         W(1)
NAME
       w - Show who is logged on and what they are doing. 

       w  - displays  information  about the users currently on the machine, and
       their processes.  The header shows, in this order,  the  current  time,
       how  long  the  system  has  been running, how many users are currently
       logged on, and the system load averages for the past 1, 5, and 15 minutes.

The  following entries are displayed for each user: login name, the tty name,
the remote host, login time, idle time, JCPU, PCPU, and the  command line
of their current process.

The  JCPU  time  is the time used by all processes attached to the tty.  It does
not include past background jobs, but  does  include  currently running background jobs.

FILES

              information about who is currently logged on

The man pages can be a bit confusing but they will often contain options which are not covered in this book.  Be careful when using a new option for any command.

Info

In situations where users are unsure which command should be used for a particular function, the info command can be used. The info subsystem contains information about all of the commands and utilities available within the system. The info system even gives tips for navigating within the info subsystem. The info command is entered to simply invoke the info subsystem.