Mac OS X Leopard - ARCAism

296
CHAPTER 18 INTRODUCING DARWIN AND THE SHELL
NOTE cd is kind of special in that, unlike ls, it is not an executable file—rather, it is a special
type of command referred to as a built-in command. This command, along with a few others
you will learn about, is a function of the shell, and not traditionally a separate executable file.
Although most common built-ins like cd exist in all common shells, it is possible that a built-in
command in one shell will behave slightly differently than one in another (cd isn’t one of
these—it behaves the same way in every shell I’ve used over the past 20 or so years).
To issue the cd command, simply type cd followed by your destination, like so:
Leopard:~ scott$ cd /
Leopard:/ scott$
If no argument is given, then cd will take you back to your home directory:
Leopard:/ scott$ cd
Leopard:~ scott$
Finally, if cd doesn’t recognize the argument as a file or directory, it will tell you so:
Leopard:~ scott$ cd /blah
-bash: cd: /blah: No such file or directory
Leopard:~ scott$
This is an error statement, and most well-written functions and executables will provide
some sort of error message if the information you provide doesn’t make sense to them.
pwd
CAUTION Just because well-written commands often give you an error message when you
do something wrong, it’s important to note that this only happens when the command has no
idea how to parse the information you’ve given it. However, this will not prevent all erroneous
commands from executing. If a command is recognized as valid, even if the information you
enter isn’t, the command will execute. While this is often harmless, it can have disastrous consequences
(the rm command, as you will soon learn, deletes files immediately and
permanently, and can cause all sorts of badness if used poorly).
The final command in this section is the pwd command. The pwd command returns your current
working directory, as this information is by default available in your prompt (after the :). You
may not need to use this command too often—however, it is useful in illustrating the file structure
of the system and can come in quite handy when you need to pass your current path into a
script. Also, it’s possible that you may find yourself stuck in a foreign shell on a foreign machine,
where you may actually need this. The basic pwd command looks like this:
Leopard:~ scott$ pwd
/Users/scott
In this case, pwd returns the absolute path of my home directory rather than the abbreviated
~ in the command prompt. This can come in handy on a foreign machine where you may not
know where the home directory is located, or one where the path info isn’t part of the prompt.
Being a rather simple command, pwd only offers two extra options:
• pwd -L will print the logical path to your working directory.
• pwd -D will print out the physical path to your working directory resolving any symbolic
links. This is the default behavior of pwd.