"I don't understand how the kernel, OS and GUI are layered in Linux..."
This FAQ is a quick run-down that I have written to briefly introduce GNU/Linux (often referred to as just "Linux") to someone completely unfamiliar with how a typical Linux system is structured. The target audience for this is quite likely someone who is considering installing Linux for the first time, or who has installed it recently, but is having difficulty dealing with the variety of choices.
A Linux system usually consists of several layers, many of which have numerous alternatives available for selection. I will start at the base level, and then build up.
The Linux Kernel: The Engine
The heart of a Linux system is the Linux kernel. It is the very low-level software that deals directly with the computer's hardware. On its own, a kernel doesn't do
anything. It just sits there waiting to be told what is needed.
The Linux kernel has a production branch that uses an even
number as the first number after the decimal point (2.2, 2.4, 2.6, etc.). Any development work that is being worked on for the next major revision of the kernel will use an odd
number after the first decimal point (2.5, 2.7) Note, that at the time I am writing this, there is no 2.7 version, as all new features are being quickly incorporated into the production 2.6 series.
The GNU OS: What Really is Running
A kernel, by itself, doesn't get work done. This is where the GNU Project (part of the Free Software Foundation, or FSF) comes in. they provide the 'tools' needed to accomplish work. Think of these as the basic set of commands to perform operations.
GNU can be used by other kernels, as well. The FSF's own Hurd kernel (under development) uses it, and there are projects to get GNU on BSD (which typically is a self-contained kernel/OS). The nice thing about this is that the higher levels are independent of the kernel, meaning that you can run the same sets of apps (including Window Managers - more on that later) regardless what kernel you choose to run in the future. (and indeed, BSDs can even run the same apps in their BSD kernel)
The X Window System: Graphics!
The "X Window System", often called "X", is really just a system of handling graphics. There are two popular applications that handle the X protocol: XFree86 and X.org. X.org is a fork from the XFree86 code, just before XFree86 changed their licensing terms in 2004.
It is in X11 that your video driver is used. Also your other input and output devices (such as keyboard, mouse and screen) are declared here.
It is important to note that X does not provide for any sort of advanced windowing functions other than the display of them. User manipulation of the windows (and the decorations, such as titlebars, borders and widgets) is handled by the Window Manager, which we will cover next.
The Window Manager: So many choices...
The realm of managing windows in X is populated with a large variety of Window Managers. This is perhaps the most daunting and confusing thing for Microsoft Windows users to understand about the world of Linux.
In Linux, the user may choose a Window Manger that uses very little system resources, or they may choose ones with more features. This level will provide a fully functional GUI for your Linux box. Some examples include Fluxbox, Sawfish, FVWM, Enlightenment, Metacity and IceWM.
Compiz (or, more properly, Compiz Fusion) with its Emerald window decorator is another Window Manager with special appeal to many. It uses graphics card hardware to run visual effects of warping, zooming, shadows and more. Compiz is closely associated with Gnome, and replaces Metacity in a "3D" Gnome setup. If a user decides to switch back away from this Window Manager into a traditional (no GPU
) Window Manager, they must replace it, not just remove Compiz.
The Desktop Environment: The Big Two (plus one)
Where can you go beyond a Window Manager? Why, a full-blown Desktop Environment, of course! It continues where a Window Manager leaves off by incorporating things such as a "control panel" like item that lets you adjust your settings for a great deal of peripherals (scanners, printers, keyboards, mice, and anything else you can connect).
A Desktop Environment matches what most Microsoft Windows users expect in an operating system. All of these extra features come at the cost of extra resources being consumed. And, in fact, many people get a current Linux system running on an old box by not using the heavier KDE or Gnome, and getting more life out of the PC. The PC can still be updated with the latest kernel and apps (such as GIMP, OpenOffice.org and Firefox) and be a very capable desktop machine.
While KDE and Gnome have all the extra bells and whistles, if you want to slim down your resource usage, but don't want to sacrifice the convenience of having the added features of a Desktop Environment, XFCE may fit the bill. A self-described "lightweight desktop environment for various *NIX systems
", screenshots of these desktops are often featured in our "Desktop" threads.
How many layers, and which modular options you choose is up to you. The choices are daunting for someone first looking at them, but these choices make Open Source so exciting and you will have all the tools needed to make your PC work the way you want it to.
I hope this brief summary helps some of those who want to understand what makes up a Linux PC, and takes some of the confusion away. The forums here have many knowledgeable members, many of them are IT professionals (which I am not) and with the experience needed to help you out with guidance to get yourself out of a bind.
Those of you who are reading this and notice an error in what I have said, please let me know so that it can be corrected.