Characters: letters (alpha) and numbers (numeric), including punctuation characters (such as _ and ?).

A suite of transport protocols first introduced in Mac OS 7 and included in all systems since that release. One advantage of AppleTalk is that it’s very easy to add and modify devices on an AppleTalk network.

The graphical appearance of Mac OS X.

The Berkeley Software Distribution version of Unix, BSD was the academic Unix, compared to System V, from AT&T Bell Telephone Labs, which had more of a commercial bent.

An instruction that you can give to a program running on the Unix system. For instance, you can type a program’s name and arguments on a command line, at a shell prompt; this command asks the shell to run that program. (The shell is a program itself; see shell.) Once a program starts running, it may accept commands of its own. For example, a text editor has commands for deleting and adding text to the file it’s editing.

The terms command and program are used almost interchangeably, probably because the program name is typed first on a command line (at a shell prompt). Shells have some built-in commands that don’t start a separate program running; one of these is cd, which changes the shell’s working directory.

A malicious person who tries to break into computer systems (usually via a network), disrupt computers and networks, steal secrets (such as passwords and credit card numbers), and exhibit other antisocial behavior.

Popular media often call these people hackers . But, to most computer people, a hacker is someone who enjoys computing and programming, and may be an expert at some area of it. (For instance, a Perl hacker is someone who’s good at programming in the Perl language.)

A hybrid of Mach (an operating system from Carnegie-Mellon University) and BSD Unix that serves as the underpinnings of Mac OS X (with the addition of many other Mac OS X technologies such as Aqua).

The part of a display that’s “behind” (not enclosed within) the windows, icons, and other items on the display.

Also known as a folder . A list of files and/or other directories. A directory is actually a special kind of file that has names and locations of other files and directories. See also working directory.

A graphical filesystem browser for your Macintosh. Prior to Mac OS X, the only way you could interact with your system was through the Finder, but now you can also opt to use the Terminal.

A group of volunteers who test, fix, and package open source software so it runs on Mac OS X. See http://fink.sourceforge.net/.

An open source operating system based on the original BSD Unix. Darwin derives some features from FreeBSD. See http://www.freebsd.org/.

An organization formed in 1985 that works for the rights of computer users to study, copy, modify, and redistribute computer programs. The FSF also distributes free software. See http://www.fsf.org/; see also GNU.

A project, started in 1984, to develop a completely free Unix-like operating system: the GNU system. GNU stands for “GNU’s Not Unix;” it is pronounced “guh-NEW.” See also Free Software Foundation.

An operating system that can run more than one program at a time is said to be a multitasking OS. On single-processor Macs, the programs don’t actually all run simultaneously; the OS can divide the computer’s time between the different programs very rapidly, so that they all appear to run at the same time. The system can still be overloaded and run slowly, if too many programs are trying to run at once.

Unix has always been multitasking, and Macintosh systems have been multitasking for many years too.

An open source operating system based on the original BSD Unix. Darwin derives some features from NetBSD. See http://www.netbsd.org/.

The location of a file or directory in a Unix filesystem: a series of names separated by slash (/) characters. Pathnames can be absolute (starting with a slash character, which means they begin at the filesystem’s root directory) or relative (not starting with a slash, which means the pathname starts from the current working directory). See also Section 2.1.

A program that’s been loaded into memory and has started executing. It has a state associated with it (such as running, suspended, or sleeping), and has a portion of memory allocated to it.

A set of instructions to the computer, written by a programmer, and stored in a file. The program is executed when you type its name as the first word on a command line, at a shell prompt (or when you choose the program from a menu or icon in a window system). Unix runs a program as a process, which you can suspend or terminate using job control, an interrupt key, or the kill command.

Unix systems have an account named root, also called the “superuser,” that has no protections or restrictions. System administrators and staff use this account to make changes to the system’s configuration and operation. Mac OS X users can access this account with sudo (see Section 2.3).

A Unix filesystem is like an upside-down tree with a branching structure of directories inside directories. The first directory, where the filesystem starts, is called the root directory.

The area of a terminal (usually glass or plastic) that shows computer output. .

When two programs, or two users running programs, communicate across a network, they typically start the communication by doing a certain thing — for instance, by logging in. The communication continues until it’s completed (or, possibly, aborted before it completes) — for instance, by logging out. The entire process, from start to completion, is called a session.

A program that runs other programs. There are several different kinds of shells, each with its own command-line syntax; Mac OS X uses tcsh as the default, but others are available including bash and zsh. All shells do the same basic job: read commands that you type interactively at a shell prompt or read commands noninteractively from a program file called a shell script.

When you start using Terminal, a shell program begins to run and prints a shell prompt. When you terminate that shell (by typing exit or Control-D at a prompt), you’re logged out from that Terminal.

The rules for, or the format of, the characters you use to make a command or other computer input. For example, the syntax of a Unix command line is explained in Section 1.2.

When you give Unix a relative pathname to a file or subdirectory, the working directory is the starting point — the directory where that relative pathname starts. For example, if your working directory is /Users/joe/food and you type the command less recipes/fish, Unix opens the file /Users/joe/food/recipes/fish. (Your working directory is still /Users/joe/food.)

If you type the command ls .. from any working directory, you get a listing of the files in your parent directory. That command uses the relative pathname to the parent directory (..). So if your working directory is /Users/joe/food, that command would list the parent directory /Users/joe. Or, if your working directory is /Users/joe, that same command would list the parent directory /Users.

Each process running on a Unix system has its own working directory, which the program can change at any time. For instance, you can give the shell the command cd to change its working directory.