In discussing computers and software, it's easy to become confused because it's not always clear when a word has its usual meaning and when it refers to a computer, file, program, command, or what have you. For this reason, this book uses certain typographic conventions to help clarify matters. Specifically:
The bulk of the text appears in a normal, proportionally- spaced font, like this.
Italicized text indicates an important term that's appearing for the first time in a chapter. It's also occasionally used for emphasis.
Monospaced text indicates a filename, computer name , the syntax used by a command, the contents of configuration files, or the output of commands typed at a command prompt. Sometimes program names appear in this way, when these names are really the software's filename.
Italicized monospaced text indicates a variable ”information that may differ on your system. For instance, instructions might say to create a file whose name is unimportant or system-specific. The instructions might then refer to this file as file.txt .
Bold monospaced text indicates information you should type exactly at a command prompt. When isolated on a line of its own, it's usually preceded by a monospaced but non-bold prompt, such as # , which the computer generates. This type of text may also be italicized, to indicate that what you type will depend upon your configuration or the results you intend to achieve.
When a command you type appears on a line of its own, the command is preceded by a command prompt. A pound sign ( # ) indicates a root command prompt. Such commands are usually entered by root, not by ordinary users (although there are exceptions to this rule). If the command prompt is a dollar sign ( $ ), ordinary users may, and often do, enter the command. Some unusually long commands use line continuation characters ”backslashes ( \ ) ”at the ends of all their lines but the first. You can type such commands exactly as they appear, including the backslashes, or you can omit the backslashes and type these commands entirely on one line. The backslashes exist just so that the command can be typeset in a reasonable font size .
This book also uses a number of special text elements that apply to entire paragraphs or larger segments of text. These elements are intended to highlight important or peripheral information. They are:
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A Note is not critical to the main discussion, but the information it contains is interesting or may be helpful in certain circumstances. For instance, a Note might point out how a feature differed in previous versions of a program.
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A Tip contains information that can help you achieve a goal in a non-obvious way, or that can point you to uses of a system or software that might not have occurred to you.
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A Warning describes a potential pitfall or danger. Warnings include software that could damage your system if used incorrectly, the potential to run afoul of ISP policies that forbid certain behaviors, and configurations that might leave your system vulnerable to outside intruders.
A Sidebar is like a Note, but it's usually longer ”typically at least two paragraphs. These components contain extended discussion of issues that don't fit neatly into the overall flow of the chapter, but that are nonetheless related , interesting, or even important.
In discussing networks, it's often necessary to give specific IP addresses as examples. In most cases, I've used IP addresses from the ranges reserved for private networks (192.168.0.0 “192.168.255.255, 172.16.0.0 “172.31.255.255, and 10.0.0.0 “10.255.255.255) even for systems that would normally be on the Internet at large. I've done this to avoid potential confusion or inadvertent offense that might occur if I were to pick random legitimate IP addresses.