The Terminal application (/Applications/Utilities) is your gateway between the candy-coated Aqua graphical interface and the no-nonsense command-line interface that Darwin uses. This book (as well as a lot of Apple documentation) tends to use the terms command line and Terminal interchangeably because, with Mac OS X, to get to the former you must go through the latter.
Each window in the Terminal represents a separate shell process—a command-line interpreter ready to accept your instructions, as described in "Introduction to the Shell" in Chapter 4.
The Terminal application's user settings control not just the application's look and feel, but the ways you interact with your shells. This section covers important application preferences to know about.
There are two ways to set a default shell when using your system, which are suggested by the "When creating a new Terminal window" radio buttons found in Terminal's Preferences window (Terminal → Preferences, or -,), seen in Figure 3-1.
The lazier way involves activating the "Execute this command" button and typing a shell's path into the neighboring text field. Henceforth, whenever you open a new Terminal window, that shell will launch in place of your default login shell. This is a nice solution if you use only Terminal as a command line and never ...