Linux Command Line and Shell Scripting Bible. Christine Bresnahan

I'm sure you have been confused by various terms such as distribution, LiveCD, and GNU when looking at Linux packages. Wading through the world of Linux for the first time can be a tricky experience. This chapter takes some of the mystery out of the Linux system before you start working on commands and scripts.

      First, four main parts make up a Linux system:

      ● The Linux kernel

      ● The GNU utilities

      ● A graphical desktop environment

      ● Application software

Each of these parts has a specific job in the Linux system. No part is very useful by itself. Figure 1.1 shows a basic diagram of how the parts fit together to create the overall Linux system.

Figure 1.1 The Linux system

      This section describes these four main parts in detail and gives you an overview of how they work together to create a complete Linux system.

Looking into the Linux Kernel

      The core of the Linux system is the kernel. The kernel controls all the hardware and software on the computer system, allocating hardware when necessary and executing software when required.

      If you've been following the Linux world at all, no doubt you've heard the name Linus Torvalds. Linus is the person responsible for creating the first Linux kernel software when he was a student at the University of Helsinki. He intended it to be a copy of the Unix system, at the time a popular operating system used at many universities.

      After developing the Linux kernel, Linus released it to the Internet community and solicited suggestions for improving it. This simple process started a revolution in the world of computer operating systems. Soon Linus was receiving suggestions from students as well as professional programmers from around the world.

      Allowing anyone to change programming code in the kernel would result in complete chaos. To simplify things, Linus acted as a central point for all improvement suggestions. It was ultimately Linus's decision whether or not to incorporate suggested code in the kernel. This same concept is still in place with the Linux kernel code, except that instead of just Linus controlling the kernel code, a team of developers has taken on the task.

      The kernel is primarily responsible for four main functions:

      ● System memory management

      ● Software program management

      ● Hardware management

      ● Filesystem management

      The following sections explore each of these functions in more detail.

System Memory Management

      One of the primary functions of the operating system kernel is memory management. Not only does the kernel manage the physical memory available on the server, but it can also create and manage virtual memory, or memory that does not actually exist.

It does this by using space on the hard disk, called the swap space. The kernel swaps the contents of virtual memory locations back and forth from the swap space to the actual physical memory. This allows the system to think there is more memory available than what physically exists, as shown in Figure 1.2.

Figure 1.2 The Linux system memory map

      The memory locations are grouped into blocks called pages. The kernel locates each page of memory either in the physical memory or the swap space. The kernel then maintains a table of the memory pages that indicates which pages are in physical memory and which pages are swapped out to disk.

      The kernel keeps track of which memory pages are in use and automatically copies memory pages that have not been accessed for a period of time to the swap space area (called swapping out), even if there's other memory available. When a program wants to access a memory page that has been swapped out, the kernel must make room for it in physical memory by swapping out a different memory page and swapping in the required page from the swap space. Obviously, this process takes time and can slow down a running process. The process of swapping out memory pages for running applications continues for as long as the Linux system is running.

Software Program Management

      The Linux operating system calls a running program a process. A process can run in the foreground, displaying output on a display, or it can run in the background, behind the scenes. The kernel controls how the Linux system manages all the processes running on the system.

      The kernel creates the first process, called the init process, to start all other processes on the system. When the kernel starts, it loads the init process into virtual memory. As the kernel starts each additional process, it gives it a unique area in virtual memory to store the data and code that the process uses.

      Some Linux implementations contain a table of processes to start automatically on bootup. On Linux systems, this table is usually located in the special file /etc/inittabs.

      Other systems (such as the popular Ubuntu Linux distribution) utilize the /etc/init.d folder, which contains scripts for starting and stopping individual applications at boot time. The scripts are started via entries under the /etc/rcX.d folders, where X is a run level.

      The Linux operating system uses an init system that utilizes run levels. A run level can be used to direct the init process to run only certain types of processes, as defined in the /etc/inittabs file or the /etc/rcX.d folders. There are five init run levels in the Linux operating system.

      At run level 1, only the basic system processes are started, along with one console terminal process. This is called single-user mode. Single-user mode is most often used for emergency filesystem maintenance when something is broken. Obviously, in this mode, only one person (usually the administrator) can log in to the system to manipulate data.

      The standard init run level is 3. At this run level, most application software, such as network support software, is started. Another popular run level in Linux is run level 5. This is the run level where the system starts the graphical X Window software and allows you to log in using a graphical desktop window.

      The Linux system can control the overall system functionality by controlling the init run level. By changing the run level from 3 to 5, the system can change from a console-based system to an advanced, graphical X Window system.

      In Chapter 4, you'll see how to use the ps command to view the processes currently running on the Linux system.

Hardware Management

      Still another responsibility for the kernel is hardware management. Any device that the Linux system must communicate with needs driver code inserted inside the kernel code. The driver code allows the kernel to pass data back and forth to the device, acting as a middle man between applications and the hardware. Two methods are used for inserting device driver code in the Linux kernel:

      ● Drivers compiled in the kernel

      ● Driver modules added to the kernel

      Previously, the only way to insert device driver code was to recompile the kernel. Each time you added a new device to the system, you had to recompile the kernel code. This process became even more inefficient as Linux kernels supported more hardware. Fortunately, Linux developers devised a better method to insert driver code into the running kernel.

      Programmers developed the concept of kernel modules to allow you to insert driver code into a running kernel without having to recompile the kernel. Also, a kernel module could be removed from the kernel when the device was finished being used. This greatly simplified and expanded using hardware with Linux.

      The Linux system identifies hardware devices as special files, called device files. There are three classifications of device files:

      ● Character

      ● Block

      ● Network

      Character