Mac OS X Leopard - ARCAism

Introducing the Shell
The Terminal application (go ahead, fire it up) is essentially an empty window that can display
and accept text input. In order for you to actually do anything, Terminal must run a shell program
inside of itself. A shell is a specialized application that helps you interact with Darwin; it
shares the same relationship to Darwin that the Finder shares with Quartz. By default, Leopard
includes five of the most common UNIX shells: Bash, Korn, Bourne, Tcsh, and Zsh (the C shell
is also listed as a shell—however, it is actually a link to Tcsh). Of the six, the shell Leopard uses
by default is Bash.
NOTE Originally, Mac OS X used Tcsh by default, but switched to Bash with the release of
OS X 10.3 Panther. If you are new to working with UNIX shells, I’d recommend sticking with
Bash, at least while you are learning. The lessons in this book, unless specifically noted, are all
given using the Bash shell.
The first time you launch Terminal, you’ll be greeted by some text similar to this:
Last login: Tue Sep 5 14:12:26 on ttyp1
Leopard:~ scott$ _
The first line tells you when you last logged in to Darwin (even if you’ve never logged in to
Darwin before intentionally, you’ll still get a message saying you did); followed by the default
prompt for Bash. The default prompt (which, like most everything else in Darwin, can be
changed) gives you some important information. First, it gives you the name of your computer,
which is either assigned by your network or taken from the Computer Name field in the system’s
Sharing preference pane. Next, following the :, the prompt gives you your location in the file system
(the ~ is a shortcut representing your home directory). Then the prompt gives you the
username you are logged in as, followed by the $ prompt (which will change to a # if the user is
logged in as the root user, otherwise known as the superuser). Finally, you get the cursor anxiously
waiting your command.
As you will soon see, shells possess some hidden powers that can make your interactions
with Darwin more pleasant and add a new level of power and flexibility to the command line.
Before you look at the shell in more depth, though, you should first learn a bit about the Darwin
file system as well as a few basic Darwin commands (walk before flying and all that).
Moving Around Darwin
One of the first things you need to learn about the shell is how to move around and view the file
system. To do this, there are three basic commands to start with: ls, cd, and pwd.
ls
ls is the “list” command, and by default lists all the visible files in a directory. If you’re familiar
with DOS, it would be the replacement for dir. By default, the ls command looks something like
this:
Leopard:~ scott$ ls
Desktop Library Music Public
Documents Movies Pictures Sites
CHAPTER 18 INTRODUCING DARWIN AND THE SHELL 293