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On The Nature of Linux

By Mark Finlay

If you come from a Microsoft background then your idea of an operating system is likely to largely involve a piece of software that provides a GUI. And as far as windows is concerned you would be right. But when you install Linux on your PC you install far more than a GUI.

If you installed a desktop orientated Linux distribution like Mandrake then you will likely see Linux the same way that you always saw Windows: Nothing more than a GUI. But if you have the courage to look under the hood you will realise that Linux is a hell of lot more than that.

NOTE: In this article I am not going to give you step-by-step instruction's on how to do anything. This article is meant as pure theory, it is designed to show you what Linux can do for you but not how to do it.

Where did Linux come from?

A Unix Ancestry

The two most important philosophies behind Unix are:

1. Everything is a file.

2. Each program should be for a single purpose and should perform that purpose well.

These idea's are the most important inherited by Linux. The fact that everything is a file has benefits that are beyond the reach of this article. The second philosophy is one of the reasons for Linux's infamous stability. Windows and other OSs have their GUI's integrated with their kernel which means that when the GUI fails then the whole system fails. Linux (as I will deal with later) is made up of layers of programs working with each other. This means that when the GUI, or any other part of the system, fails usually only that part needs to be restarted.

A mainframe in my Livingroom?

Linux as a multi-user System

So, you ask: How do I log in more than once? Linux uses virtual terminals, usually six of them are configured. They can be accessed by pressing CTRL + ALT + F[1-6] . To get back to the GUI use CTRL + ALT + F7. You can also login in other ways, like through telnet.

Layers of Linux

“Each program should perform one task and perform it well”

1. The boot manager

The boot manager is the first piece of software that the computer runs when it boots. It ask's you which OS you want to boot (eg. Windows or Linux). Then it hands over to the kernel of the chosen OS. When booting Linux it is used to pass settings to the kernel as well as information about which partition is root.

2. The Kernel

The kernel looks after the basic operations of the computer. It looks after, among other things, most of the the hardware. All input and output go through the kernel. The Linux kernel is superior to a lot of other kernels in many ways. One of these is that the BIOS is only used to load the kernel into memory – after that the kernel probes the hardware itself which circumvents some problems with out of date BIOS's.

3. INIT

The first thing the kernel does when it is finished loading is to look for the INIT scripts(it is told where by the boot manager). These scripts are what “boot up” your computer. They load network interfaces, kernel modules and any servers you run.

The last thing INIT will do is create the virtual consoles on F1 to F6. It will then display the contents of the file /etc/issue and run login on each.

4. Login

Login is a simple program that asks you for your user name and password, checks if they are correct and if they are it logs you in and runs the shell specified for your user account.

5. The shell

The shell is also know as the Command line interface (CLI). It is the MS-DOS like interface that allows you to enter commands to tell the computer what to do. Although to compare it to MS-DOS is like comparing a monster truck to a tricycle. The CLI is massively powerful, even more powerful that the GUI to those who know how to use it.

Like everything in Linux there are many different shells, to fit the needs of different people. The most common shell is Bash.

6. X

If your computer is set boot to X on startup; after it has run login, INIT will launch X. You will be asked to login via the GUI.

X is the piece of software that looks after you graphics card and mouse(and keyboard while you are using X). In the spirit of Unix is does this well and does no more. If you run X by itself all you will see is a background and a mouse pointer.

You might hear people online mention the X Server. It is natural to assume that the X server would only run on a Linux server and not on a Desktop machine. But in fact every X is a server. The applications connect to it the same way you connect to a website – through a network interface*. This means that applications are not limited to connect to the X server on their machine, they can connect to any X server that will let them.

Also multiple X servers can run on one machine. Different users can be logged into different X servers. Of course only one user can sit at the computer so the others would connect (usually from dumb terminals or under-powered machines) through a network.

* This does not mean that Linux needs to be on a network: a loopback interface (lo) creates a virtual network within the machine.

7. The Window Manager

X by can be used by itself but is much more useful if used with a window manager running on top of it. As the name suggests all a window manager does is looks after the windows. It provides multiple desktops, window decoration, and a menu of settings and programs when you click on the desktop(usually with the third or middle mouse button). An example of a window manager is sawfish.

8. The Desktop

The two major desktops are GNOME and KDE. They both run on top of window managers. They provide what most of us consider a GUI. They provide the equivalent of the windows “Start” menu and bar. They also provide the widget set, and common dialogues(open, save, print) used by programs.

The desktop is the topping on the cake and because it is the only thing most new users interact with they are rapidly getting easier to use and more intuitive.

9. The Applications

There are three types of applications on Linux: console applications, X applications and frontends.

9a. Console Applications

In the order of Layers of Linux these come just after the shell. They are command line programs and are very popular among Linux “Guru's” because they are more powerful than GUI apps, and give you more control, and if you know a command, it is quicker and easier just to type it in that to go through layers of GUI menu's.

9b. X applications

Applications that can only run from X. Most X applications use one of the standard widget sets. The most popular are GNOME's Gtk and KDE's Qt. When you hear an application described as a GNOME app or a KDE app that simply means that it uses either the GNOME or KDE widget sets. (GNOME apps can run in KDE and visa-versa)

Widget sets provide an easy way for programmers to give their programs a standard interface. The widget set provides all the things common to a GUI: menus, buttons, toolbars, etc ...

9c. Frontends.

Frontends are X applications that provide a GUI for a console command. As such the code to execute the actual task performed by the program isn't loaded until the console command is actually run. This makes frontends very fast to use and load. An example of a frontend is any of the Linux CD-Burning programs. All the frontend does is find out what files you want burned and at what speed , etc, and pass the information on to the console app. This is in the spirit of the “Each program should be for a single purpose and should perform that purpose well” philosophy.

Where is “C:” ?

The Linux Filesystem

The Linux equivalent of C: is the root filesystem. It is represented by / . The root filesystem is specified by the boot manager and is where the kernel finds the INIT scripts. So far this isn't that different to windows but it becomes very different when you start adding more partitions and removable media.

If I have a second partition in windows it will be known as D: , I have no choice about that. In Linux I mount the partition into the root filesystem. This means that I choose a directory* on the root partition to associate with this partition.

For example: on my computer I have a 30GB storage partition which is mounted under /mnt/storage. This means that when I browse to /mnt/storage I see what I would see in windows by clicking on D:

The same applies for cd-rom's and floppies. They are usually mounted to their own folders inside /mnt

An example of how this can be very useful: A while back I ran out of space on my root filesystem, and I don't like resizing partitions for fear of data loss. So I made a 1GB partition, mounted it to /mnt/usr. Then I moved all the files from /usr on the root partition to /mnt/usr. Then I remounted the new partition to /usr. This is the equivalent of being able to move the windows directory to a new partition when you run out of space. NOTE: moving /usr is more complicated than I described here – don't try it unless you know what you are doing.

* If you mount a partition to a directory with files in it – those files will be hidden until you unmount the partition.

When do I need to reboot?

Linux's modular nature

Because of Linux's modular nature, when you change a setting you only need to restart the part of the system that the setting applies to.

eg. If you change your IP address you then only need to restart networking. If you change your screen resolution then you only need to restart X. The only thing that I can think of that needs a reboot is a kernel upgrade.

Linux acts in the opposite way to windows: the longer you leave it on the faster it gets. There are people who have had a Linux desktop running for months without rebooting.

What about Linux NT?

A desktop built on top of a server.

When you install Linux onto your desktop you install the same OS that is installed on web servers all over the world. The only difference between a Linux server and a Linux desktop is that the server will have more servers apps installed and should have tighter security.

If you have a permanent connection to the internet you can run a hybrid desktop-server. You can have your own web server, ftp server, etc, at the same time as doing your word processing and playing games.

In terms of stability and performance it means that you are getting the best of the best.

Where do I get shareware for Linux?

Discovering the delights on your distribution CDs

I remember back in the day, having freshly installed Windows 95: It just sat there looking at me in 16 colours, at 640x480, offering me an arsenal of useful software like calculator and notepad. To make it come anywhere close to usable I had to add graphics and sound drivers, MS Office and IE4.

At that point I could get online so I went “shareware hunting”. After a few hours I was armed with PSP, Winzip, CuteFTP and a handful of other useful shareware apps. My PC was useful, well for 29 more days anyway.

When I did my first install of Linux I expected to encounter something similar. I downloaded at least 10 apps before I realised that they were all included as rpms on my Redhat CDs. The day I realised that, I took the phone off the hook and spent countless hours installing and playing with everything I could find on the Redhat CD's. The next day I woke up with a patchy memory of what I had installed, but when I booted up my computer I was reminded when I saw at least 10 servers starting during boot-up.

And now with such swish tools as Mandrake's RPMDrake you can browse through all the software available to you and even see descriptions of each one. The only things that I have downloaded for Linux that I actually needed were things like StarOffice and Java and NVidia Drivers that can't be included with the distro for legal reasons(although they are included in the box sets).

A favorite quote of mine sums it up nicely:

'Microsoft sells you Windows, Linux give you the whole house'

Conclusion

To link back to what I said at the beginning: Linux is far more that a GUI. It has a lot below the GUI and a lot above it. I hope that you have a better understanding of that now.

Enjoy...

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