Mastering Linux System Administration. Richard Blum

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      THE COMMAND‐LINE INTERFACE

      While having a fancy graphical desktop interface is nice, there are drawbacks. The extra processing power required to interact with the graphics card takes away crucial CPU time that can be used for other programs. Nowhere is this more important than in a server environment.

      Because of that, many Linux servers don't load a graphical desktop and instead rely on a text‐based interface, called the command‐line interface (CLI). The CLI provides a way for users to start programs, manage files on the filesystem, and manage processes running on the Linux system using simple text commands. The CLI is produced by a program called a shell. The shell allows you to enter text commands, and then it interprets the commands and then 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, there are several other popular shells you could run into in a Linux environment. Table 1.4 lists the more popular ones.

TABLE 1.4: Linux Shells

SHELL	DESCRIPTION
ash	A simple, lightweight shell that runs in low‐memory environments but has full compatibility with the Bash Shell
korn	A programming shell compatible with the Bourne shell but supporting advanced programming features like associative arrays and floating‐point arithmetic
tcsh	A shell that incorporates elements from the C programming language into shell scripts
zsh	An advanced shell that incorporates features from bash, tcsh, and korn, providing advanced programming features, shared history files, and themed prompts

      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.

Linux Distributions

      Now that you have seen the four main components required for a complete Linux system, you may be wondering how you are going to get them all put together to make a Linux system. Fortunately, there are people who have already done that for you.

      A complete Linux system package is called a distribution. There are lots of different Linux distributions available to meet just about any computing requirement you could have. Most distributions are customized for a specific user group, such as business users, multimedia enthusiasts, software developers, or average home users. Each customized distribution includes the software packages required to support specialized functions, such as audio‐ and video‐editing software for multimedia enthusiasts, or compilers and integrated development environments (IDEs) for software developers.

      The different Linux distributions are often divided into two categories.

       Core Linux distributions

       Specialized distributions

      The following sections describe these different types of Linux distributions and show some examples of Linux distributions in each category.

      Core Linux Distributions

A core Linux distribution contains a kernel, one or more graphical desktop environments, and just about every Linux application that is available, precompiled for the kernel. It provides one‐stop shopping for a complete Linux installation. Table 1.5 shows some of the more popular core Linux distributions.

TABLE 1.5: Core Linux Distributions

DISTRIBUTION	DESCRIPTION
Slackware	One of the original Linux distribution sets, popular with Linux geeks
Red Hat Enterprise	A commercial business distribution used mainly for Internet servers
Gentoo	A distribution designed for advanced Linux users, containing only Linux source code
openSUSE	Different distributions for business and home use
Debian	Popular with Linux experts and commercial Linux products

      In the early days of Linux, a distribution was released as a set of floppy disks. You had to download groups of files and then copy them onto disks. It would usually take 20 or more disks to make an entire distribution! Needless to say, this was a painful experience.

      Nowadays, Linux distributions are released as an ISO image file. The ISO image file is a complete disk image of a DVD as a single file. You use a software application to either burn the ISO image file onto a DVD or create a bootable USB stick. You then just boot your workstation from the DVD or USB stick to install Linux. This makes installing Linux much easier.

      However, beginners still often run into problems when they install one of the core Linux distributions. To cover just about any situation in which someone might want to use Linux, a single distribution has to include lots of application software. They include everything from high‐end Internet database servers to common games.

      While having lots of options available in a distribution is great for Linux geeks, it can become a nightmare for beginning Linux users. Most core distributions ask a series of questions during the installation process to determine which applications to load by default, what hardware is connected to the PC, and how to configure the hardware. Beginners often find these questions confusing. As a result, they often either load way too many programs on their computer or don't СКАЧАТЬ