Linux Command Line and Shell Scripting Bible. Christine Bresnahan

Чтение книги онлайн.

      Some Linux LiveCDs, such as Puppy Linux, are designed with a minimum number of Linux system files. The LiveCD boot scripts copy them directly into memory when the CD boots. This allows you to remove the CD from the computer as soon as Linux boots. Not only does this make your applications run much faster (because applications run faster from memory), but it also gives you a free CD tray to use for ripping audio CDs or playing video DVDs from the software included in Puppy Linux.

      Other Linux LiveCDs use an alternative method that allows you to remove the CD from the tray after booting. It involves copying the core Linux files onto the Windows hard drive as a single file. After the CD boots, it looks for that file and reads the system files from it. The dyne: bolic Linux LiveCD uses this technique, which is called docking. Of course, you must copy the system file to your hard drive before you can boot from the CD.

      A very popular technique for storing data from a live Linux CD session is to use a common USB memory stick (also called a flash drive or a thumb drive). Just about every Linux LiveCD can recognize a plugged-in USB memory stick (even if the stick is formatted for Windows) and read and write files to and from it. This allows you to boot a Linux LiveCD, use the Linux applications to create files, store those files on your memory stick, and then access them from your Windows applications later (or from a different computer). How cool is that?

      Summary

      This chapter discussed the Linux system and the basics of how it works. The Linux kernel is the core of the system, controlling how memory, programs, and hardware all interact with one another. The GNU utilities are also an important piece in the Linux system. The Linux shell, which is the main focus of this book, is part of the GNU core utilities. The chapter also discussed the final piece of a Linux system, the Linux desktop environment. Things have changed over the years, and Linux now supports several graphical desktop environments.

      The chapter also discussed the various Linux distributions. A Linux distribution bundles the various parts of a Linux system into a simple package that you can easily install on your PC. The Linux distribution world consists of full-blown Linux distributions that include just about every application imaginable, as well as specialized Linux distributions that include applications focused only on a special function. The Linux LiveCD craze has created another group of Linux distributions that allow you to easily test-drive Linux without even having to install it on your hard drive.

      In the next chapter, you look at what you need to start your command line and shell scripting experience. You'll see what you need to do to get to the Linux shell utility from your fancy graphical desktop environment. These days, that's not always an easy thing.

      Chapter 2

      Getting to the Shell

      Reaching the Command Line

      Before the days of graphical desktops, the only way to interact with a Unix system was through a text command line interface (CLI) provided by the shell. The CLI allowed text input only and could display only text and rudimentary graphics output.

      Because of these restrictions, output devices were not very fancy. Often, you needed only a simple dumb terminal to interact with the Unix system. A dumb terminal was usually nothing more than a monitor and keyboard connected to the Unix system via a communication cable (usually a multi-wire serial cable). This simple combination provided an easy way to enter text data into the Unix system and view text results.

      As you well know, things are significantly different in today's Linux environment. Just about every Linux distribution uses some type of graphical desktop environment. However, to enter shell commands, you still need a text display to access the shell's CLI. The problem now is getting to one. Sometimes finding a way to get a CLI in a Linux distribution is not an easy task.

      One way to get to a CLI is to take the Linux system out of graphical desktop mode and place it in text mode. This provides nothing more than a simple shell CLI on the monitor, just like the days before graphical desktops. This mode is called the Linux console because it emulates the old days of a hard-wired console terminal and is a direct interface to the Linux system.

      When the Linux system starts, it automatically creates several virtual consoles. A virtual console is a terminal session that runs in Linux system memory. Instead of having several dumb terminals connected to the computer, most Linux distributions start five or six (or sometimes even more) virtual consoles that you can access from a single computer keyboard and monitor.

The alternative to using a virtual console terminal is to use a terminal emulation package from within the Linux graphical desktop environment. A terminal emulation package simulates working on a console terminal, but within a desktop graphical window. Figure 2.1 shows an example of a terminal emulator running in a Linux graphical desktop environment.

Figure 2.1 A simple terminal emulator running on a Linux desktop

Graphical terminal emulation is responsible only for a portion of the Linux graphical experience. As a whole, the experience is accomplished via several components, including graphical terminal emulation software (called a client). Table 2.1 shows the different components in the Linux graphical desktop environment.

Table 2.1 Graphical Interface Elements

      For dealing with the command line from the desktop, the focus is on the graphical terminal emulator. You can think of graphical terminal emulators as CLI terminals “in the GUI” and virtual console terminals as CLI terminals “outside the GUI.” Understanding the various terminals and their features can enhance your command line experience.

      Accessing CLI via a Linux Console Terminal

      In the early days of Linux, when you booted up your system you would see a login prompt on your monitor, and that's all. As mentioned earlier, this is called the Linux console. It was the only place you could enter commands for the system.

      Even though several virtual consoles are created at boot time, many Linux distributions switch to a graphical environment after the boot sequence completes. This provides the user with a graphical login and desktop experience. Therefore, in this case, accessing a virtual console is done manually.

      In most Linux distributions, you can access one of the Linux virtual consoles using a simple keystroke combination. Usually, you must hold СКАЧАТЬ