  Getting StartedModern computing is highly interactive, and using the command line is just another form of interaction. Most people use the computer through its desktop or graphical interface, interacting at a rapid pace. They click on an object, drag and drop it, double-click another to open it, alter it, etc. Although interactions happen so fast you don't think about it, each click or keystroke is a command to the computer, which it reacts to. Using the command line is the same thing, but more deliberate. You type a command and press the Return or Enter key. For instance, in my terminal I type: date And the computer replies with: Thu Mar 12 17:15:09 EDT 2009 That's pretty computerish. In later chapters we'll explain how to request the date and time in a more congenial format. We'll also explain how working in different countries and with different languages changes the output. The idea is that you've just had an interaction. The Command Line Can Do Much BetterThe date command, as seen so far, compares poorly with the alternative of glancing at a calendar or clock. The main problem is not the unappetizing appearance of the output, mentioned already, but the inability to do anything of value with the output. For instance, if I'm looking at the date in order to insert it into a document I'm writing or update an event on my online calendar, I have to do some retyping. The command line can do much better. After you learn basic commands and some nifty ways to save yourself time, you'll find out more in this book about feeding the output of commands into other commands, automating activities, and saving commands for later use. What Do We Mean By a Command?At the beginning of this chapter we used the word "command" very generally to refer to any way of telling the computer what to do. But in the context of this book, a command has a very specific meaning. It's a file on your computer that can be executed. Except for a few commands that are executed directly (called built-in commands), each command is run by finding the file that bears its name and executing that file. We'll give you more details as they become useful. Ways to Enter CommandsTo follow along on this book, you need to open a command-line interpreter (called a shell or terminal in GNU/Linux) on your computer. Pre-graphical computer screens use to present people with this interpreter as soon as they logged in. Nowadays almost everybody except professional system administrators uses a graphical interface. So we'll show you how to pull up a shell.
Finding a TerminalVirtually every desktop interface provides a program that mimics the old text-only terminals that computers used to offer as interfaces. On your desktop, try looking through the menus of applications for a program called Terminal. Often it's on a menu named something such as "Accessories", which is not really fair because once you read this book you'll be spending a lot of time in the terminal every day. In GNOME you choose Applications -> Accessories -> Terminal
Wherever it's located, you can almost certainly find a terminal program. When you run the terminal program, it just shows a blank window; there's not much in the way of help. You're expected to know what to do--and we'll show you. The following figure shows the Terminal window opened on the desktop in GNOME.
Running an Individual CommandMany graphical interfaces also provide a small dialog box called something like "Run command". It presents a small text area where you can type in a command and press the Return or Enter key.
This box can be used as a shortcut to quickly start up a terminal program, as long as you know the name of a terminal program installed on your computer. If you are working on an unfamiliar computer and don't even know the name of the default terminal program, try typing "xterm" to start up a no-frills terminal program (no fancy menus allowing choice of color themes or fonts).
How We Show Commands and Output in This BookThere's a common convention in books about the command-line. When you start up a terminal, you see a little string of characters indicating that the terminal is ready to accept your command. This string is called a prompt, and it may be as simple as: $ After you type your command and press the Return or Enter key, the terminal displays the command's output (if there is any) followed by another prompt. So my earlier interaction would be shown in the book like this: $ date Thu Mar 12 17:15:09 EDT 2009 $ You have to know how to interpret examples like the preceding one. All you type here is date. Then press the return key. The word date in the example is printed in bold to indicate that it's something you type. The rest is output on the terminal.
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