Although arguments to commands aren't always files, files are the most important types of "things" on any UNIX system. A file can contain any kind of information, and indeed there are different types of files. Three types are by far the most important:
Let's review the most important concepts about directories. The fact that directories can contain other directories leads to a hierarchical structure, more popularly known as a tree , for all files on a UNIX system. Figure 1.2 shows part of a typical directory tree; ovals are regular files and rectangles are directories.
The top of the tree is a directory called "root" that has no name on the system.  All files can be named by expressing their location on the system relative to root; such names are built by listing all of the directory names (in order from root), separated by slashes ( / ), followed by the file's name. This way of naming files is called a full (or absolute ) pathname .
For example, say there is a file called memo that is in the directory fred , which is in the directory users , which is in the root directory. This file's full pathname is /users/fred/memo .
Of course, it's annoying to have to use full pathnames whenever you need to specify a file. So there is also the concept of the working directory (sometimes called the current directory), which is the directory you are "in" at any given time. If you give a pathname with no leading slash, then the location of the file is worked out relative to the working directory. Such pathnames are called relative pathnames; you'll use them much more often than full pathnames.
When you log in to the system, your working directory is initially set to a special directory called your home (or login ) directory. System administrators often set up the system so that everyone's home directory name is the same as their login name, and all home directories are contained in a common directory under root.
For example, /users/billr is a typical home directory. If this is your working directory and you give the command lp memo , then the system looks for the file memo in /users/billr . If you have a directory called bob in your home directory, and it contains the file statreport , then you can print it with the command lp bob/statreport .
As you can well imagine, home directories occur often in pathnames. Although many systems are organized so that all home directories have a common parent (such as /home or /users ), you should not have to rely on that being the case, nor should you even have to know what the absolute pathname of someone's home directory is.
Therefore, the Korn shell has a way of abbreviating home directories: just precede the name of the user with a tilde ( ~ ). For example, you could refer to the file memo in user fred's home directory as ~fred/memo . This is an absolute pathname, so it doesn't matter what your working directory is when you use it. If fred's home directory has a subdirectory called bob and the file is in there instead, you can use ~fred/bob/memo as its name.
Even more convenient, a tilde by itself refers to your own home directory. You can refer to a file called notes in your home directory as ~/notes (note the difference between that and ~notes , which the shell would try to interpret as user notes home directory). If notes is in your bob subdirectory, then you can call it ~/bob/notes . This notation is handiest when your working directory is not in your home directory tree, e.g., when it's some "system" directory like /tmp .
If you want to change your working directory, use the command cd . If you don't remember your working directory, the command pwd tells the shell to print it.
cd takes as argument the name of the directory you want to become your working directory. It can be relative to your current directory, it can contain a tilde, or it can be absolute (starting with a slash). If you omit the argument, cd changes to your home directory (i.e., it's the same as cd ~ ).
Table 1.1 gives some sample cd commands. Each command assumes that your working directory is /users/billr just before the command is executed, and that your directory structure looks like Figure 1.2 .
The first four are straightforward. The next two use a special
The last two examples in the table use a new form of the cd command, which is not included in most Bourne shells. The form is cd old new . It takes the full pathname of the current working directory and tries to find the string old in it. If it finds the string, it substitutes new and changes to the resulting directory.
In the first of the two examples, the shell substitutes pete for billr in the current directory name and makes the result the new current directory. The last example shows that the substitution need not be a complete filename: substituting arry for illr in /users/billr yields /users/barry . (If the old string can't be found in the current directory name, the shell prints an error message.)
Another new feature of the Korn shell's cd command is the form cd - , which changes to whatever directory you were in before the current one. For example, if you start out in /usr/lib , type cd without an argument to go to your home directory, and then type cd - , you will be back in /usr/lib .
Sometimes you need to run a command on more than one file at a time. The most common example of such a command is ls , which lists information about files. In its simplest form, without options or arguments, it lists the names of all files in the working directory except special hidden files, whose names begin with a dot ( . ).
If you give ls filename arguments, it will list those files-which is sort of silly: if your current directory has the files bob and fred in it and you type ls bob fred , the system will simply parrot the filenames back at you.
Actually, ls is more often used with options that tell it to list information about the files, like the -l (long) option, which tells ls to list the file's owner, size, time of last modification, and other information, or -a (all), which also lists the hidden files described above. But sometimes you want to verify the existence of a certain group of files without having to know all of their names; for example, if you use the WordPerfect word processor, you might want to see which files in your current directory have names that end in .wp .
Filenames are so important in UNIX that the shell provides a built-in way to specify the pattern of a set of filenames without having to know all of the names themselves. You can use special characters, called wildcards , in filenames to turn them into patterns. We'll show the three basic types of wildcards that all major UNIX shells support, and we'll save the Korn shell's set of advanced wildcard operators for Chapter 4 . Table 1.2 lists the basic wildcards.
The asterisk (
Table 1.3 should give you a better idea of how the asterisk works. Assume that you have the files bob , darlene , dave , ed , frank , and fred in your working directory.
The remaining wildcard is the set construct. A set is a list of characters (e.g., abc), an inclusive range (e.g., a-z), or some combination of the two. If you want the dash character to be part of a list, just list it first or last. Table 1.4 should explain things more clearly.
In the original wildcard example, program.[co] and program.[a-z] both match program.c and program.o , but not program.log .
The range notation is handy, but you shouldn't make too many assumptions about what characters are included in a range. It's safe to use a range for uppercase letters, lowercase letters, digits, or any subranges thereof (e.g., [f-q] , [2-6] ). Don't use ranges on punctuation characters or mixed-case letters: e.g., [a-Z] and [A-z] should not be trusted to include all of the letters and nothing more. The problem is that such ranges are not entirely portable between different types of computers. 
The process of matching expressions containing wildcards to filenames is called wildcard expansion. This is just one of several steps the shell takes when reading and processing a command line; another that we have already seen is tilde expansion, where tildes are replaced with home directories where applicable. We'll see others in later chapters, and the full details of the process are enumerated in Chapter 7 .
However, it's important to be aware that the commands that you run only see the results of wildcard expansion. That is, they just see a list of arguments, and they have no knowledge of how those arguments came into being. For example, if you type ls fr* and your files are as on the previous page, then the shell expands the command line to ls fred frank and invokes the command ls with arguments fred and frank . If you type ls g* , then (because there is no match) ls will be given the literal string g* and will complain with the error message, g* not found. 
Here is another example that should help you understand why this is important. Suppose you are a C programmer. This just means that you deal with files whose names end in .c (programs, a.k.a. source files), .h (header files for programs), and .o (object code files that aren't human-readable) as well as other files.
Let's say you want to list all
source, object, and header files in your working directory.
In other words, ls will see the filenames just as if they were all typed in individually-but notice that we assumed no knowledge of the actual filenames whatsoever! We let the wildcards do the work.
As you gain experience with the shell, reflect on what life would be like without wildcards. Pretty miserable, we would say.