Linux Command Line and Shell Scripting Bible - Christine Bresnahan

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files are for devices that can only handle data one character at a time. Most types of modems and terminals are created as character files. Block files are for devices that can handle data in large blocks at a time, such as disk drives.

      The network file types are used for devices that use packets to send and receive data. This includes network cards and a special loopback device that allows the Linux system to communicate with itself using common network programming protocols.

      Linux creates special files, called nodes, for each device on the system. All communication with the device is performed through the device node. Each node has a unique number pair that identifies it to the Linux kernel. The number pair includes a major and a minor device number. Similar devices are grouped into the same major device number. The minor device number is used to identify a specific device within the major device group.

Filesystem Management

Unlike some other operating systems, the Linux kernel can support different types of filesystems to read and write data to and from hard drives. Besides having over a dozen filesystems of its own, Linux can read and write to and from filesystems used by other operating systems, such as Microsoft Windows. The kernel must be compiled with support for all types of filesystems that the system will use. Table 1.1 lists the standard filesystems that a Linux system can use to read and write data.

Table 1.1 Linux Filesystems

      Any hard drive that a Linux server accesses must be formatted using one of the filesystem types listed in Table 1.1.

      The Linux kernel interfaces with each filesystem using the Virtual File System (VFS). This provides a standard interface for the kernel to communicate with any type of filesystem. VFS caches information in memory as each filesystem is mounted and used.

The GNU Utilities

      Besides having a kernel to control hardware devices, a computer operating system needs utilities to perform standard functions, such as controlling files and programs. While Linus created the Linux system kernel, he had no system utilities to run on it. Fortunately for him, at the same time he was working, a group of people were working together on the Internet trying to develop a standard set of computer system utilities that mimicked the popular Unix operating system.

      The GNU organization (GNU stands for GNU's Not Unix) developed a complete set of Unix utilities, but had no kernel system to run them on. These utilities were developed under a software philosophy called open source software (OSS).

      The concept of OSS allows programmers to develop software and then release it to the world with no licensing fees attached. Anyone can use the software, modify it, or incorporate it into his or her own system without having to pay a license fee. Uniting Linus's Linux kernel with the GNU operating system utilities created a complete, functional, free operating system.

      While the bundling of the Linux kernel and GNU utilities is often just called Linux, you will see some Linux purists on the Internet refer to it as the GNU/Linux system to give credit to the GNU organization for its contributions to the cause.

The Core GNU Utilities

      The GNU project was mainly designed for Unix system administrators to have a Unix-like environment available. This focus resulted in the project porting many common Unix system command line utilities. The core bundle of utilities supplied for Linux systems is called the coreutils package.

      The GNU coreutils package consists of three parts:

      ● Utilities for handling files

      ● Utilities for manipulating text

      ● Utilities for managing processes

      Each of these three main groups of utilities contains several utility programs that are invaluable to the Linux system administrator and programmer. This book covers each of the utilities contained in the GNU coreutils package in detail.

The Shell

      The GNU/Linux shell is a special interactive utility. It provides a way for users to start programs, manage files on the filesystem, and manage processes running on the Linux system. The core of the shell is the command prompt. The command prompt is the interactive part of the shell. It allows you to enter text commands, and then it interprets the commands and executes them in the kernel.

      The shell contains a set of internal commands that you use to control things such as copying files, moving files, renaming files, displaying the programs currently running on the system, and stopping programs running on the system. Besides the internal commands, the shell also allows you to enter the name of a program at the command prompt. The shell passes the program name off to the kernel to start it.

      You can also group shell commands into files to execute as a program. Those files are called shell scripts. Any command that you can execute from the command line can be placed in a shell script and run as a group of commands. This provides great flexibility in creating utilities for commonly run commands, or processes that require several commands grouped together.

      There are quite a few Linux shells available to use on a Linux system. Different shells have different characteristics, some being more useful for creating scripts and some being more useful for managing processes. The default shell used in all Linux distributions is the bash shell. The bash shell was developed by the GNU project as a replacement for the standard Unix shell, called the Bourne shell (after its creator). The bash shell name is a play on this wording, referred to as the “Bourne again shell.”

In addition to the bash shell, we will cover several other popular shells in this book. Table 1.2 lists the different shells we will examine.

Table 1.2 Linux Shells

      Most Linux distributions include more than one shell, although usually they pick one of them to be the default. If your Linux distribution includes multiple shells, feel free to experiment with different shells and see which one fits your needs.

The Linux Desktop Environment

      In the early days of Linux (the early 1990s) all that was available was a simple text interface to the Linux operating system. This text interface allowed administrators to start programs, control program operations, and move files around on the system.

      With the popularity of Microsoft Windows, computer users expected more than the old text interface to work with. This spurred more development in the OSS community, and the Linux graphical desktops emerged.

      Linux is famous for being able to do things in more than one way, and no place is this more relevant than in graphical desktops. There are a plethora of graphical desktops you can choose from in Linux. The following sections describe a few of the more popular ones.

The X Window System

      Two basic elements control your video environment: the video card in your PC and your monitor. To display fancy graphics on your computer, the Linux software needs to know how to talk to both of them. The X Window software is the core element in presenting graphics.

      The X Window software is a low-level program that works directly with the video card and monitor in the PC, and it controls how Linux applications can present fancy windows and graphics on your computer.

      Linux isn't the only operating system that uses X Window; versions are written for many different operating systems. In the Linux world, several different software packages can implement it.

      The most popular package is X.org. It provides an open source software implementation of the X Window system and supports many of the newer video cards used today.

      Two other X Window packages are gaining in popularity. The Fedora Linux distribution is experimenting with the Wayland software, and the Ubuntu Linux distribution has developed the Mir display server for use with its desktop environment.

      When you first install a Linux distribution, it attempts to detect your video card and monitor, and then it creates an X Window configuration file that contains the required information. During installation, you may

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