< Finding out where you are >

Like in a graphical file manager, you're always working in a single directory when you're at the CLI, and the directory can be anywhere in the filesystem tree. The directory contains files and a way to go to to its parent directory and its subdirectories. The directory where you're currently working in, is called (surprise surprise) the working directory. To find out where the working directory is, you use the pwd command that stands for print working directory. Now type it and see where you are.

me@puter: ~$ pwd
/home/me
me@puter: ~$

You're most likely in your home directory now. That's the directory where you put your personal files, and usually the working directory is set to your home directory when you log in to your Linux system. The home directories of users are located under /home, and usually the name of your home directory is /home/your_user_name.

< Listing the contents of a directory >

Now you know where you are. Next you might want to know what there is. For finding it out you use the ls command. Just type ls, and you'll get a list of all the files and subdirectories that your working directory contains.

You have a way of controlling what the ls command displays and how. Just like almost every Linux command, ls can have additional options that change the behavior of it. For example, try typing this:
me@puter: ~$ ls -l

You get again a list of the files in your working directory, but as you can see, the output of ls is now different with the -l option. You get additional info about the files your directory contains. What happened here? The shell, bash, took the -l you typed and passed it along to ls. This option told ls to display more info about the files in the current directory. These additions to commands are often called parameters or arguments or options. So, in this case, ls was the command and -l was the option.

You can separate the command from its options with a space, and you can also add more options and separate them from each other with a space as well. For example, the -r option tells ls to display the files in reverse order. If you want to display both the files in reverse order and get more info about the files, you could do it with:
me@puter: ~$ ls -l -r

In most cases you can use a shorter form when using multiple options. The following does the same thing:
me@puter: ~$ ls -lr

Many commands accept a lot of arguments, and some commands also accept file or directory names as arguments. For example, when you give the ls command without any arguments, it displays the contents of the current directory, as you already saw. If you want to display the contents of some other directory, you can give the pathname of the desired directory as an argument to ls. Like the name suggests, a pathname is the path you take along the directories to the destination directory. There are two kinds of pathnames, absolute pathnames and relative pathnames.

< Absolute pathnames >

An absolute pathname simply tells you what the complete path to a certain file or directory is. Because all the files on Linux are under the root directory, an absolute pathname must start from the root directory and then follow the filesystem tree directory by directory until you get to the desired file or directory.

All absolute file names start with a slash because the slash indicates the root directory. For example, there's a directory called /usr/X11R6/bin on your system. The path here means that you start from the root directory (/), go to its subdirectory "usr", which contains a directory called "X11R6", which in turn contains the "bin" directory. As you noticed, you use the slash not only for indicating the root directory, but also for separating the directories on the path. This is different from Windows where you use a backslash for separating the directories.

Now let's experiment a little. If you want to see what the /usr/X11R6/bin directory contains, but it isn't your working directory, you can give the path as an argument to ls so that it knows that it should work on /usr/X11R6/bin instead of the working directory:
$ ls /usr/X11R6/bin

You can also use the other options of ls, such as:
$ ls -l -r /usr/X11R6/bin

This would do the same:
$ ls -lr /usr/X11R6/bin

So this is the deal with absolute pathnames. Not hard. You know a pathname is an absolute one if it starts with the slash.

< Moving around in directories >

So far, you know how to find out your current working directory (pwd), list the contents of directories (ls), and you understand what are absolute pathnames. Soon you'll know what are relative pathnames, but before it I'll teach you how to change the working directory. You use the cd command, like in MS-DOS, and you give the desired directory as an argument to it. Now, change your working directory to /usr/X11R6:
me@puter: ~$ cd /usr/X11R6

If you want, you can check if your new working directory really is what you wanted:

me@puter: /usr/X11R6$ pwd
/usr/X11R6
me@puter: /usr/X11R6$

Maybe you noticed that the command prompt has changed. Usually it's configured to display the name of the working directory. This makes your life a little easier and you don't have to type pwd all the time!

< Relative pathnames >

Now it's finally time to discuss relative pathnames. As you've learned, an absolute pathname starts from the root directory. A relative pathname, however, starts from the working directory. This is why you need some special symbols for indicating the relative positions in the filesystem. These symbols are a dot (.) and two dots (..) and they mean the working directory and the parent directory, respectively.

Let's see how these things work. Now your current working directory should be /usr/X11R6. List the contents of it:

me@puter: /usr/X11R6$ ls
bin include lib man
me@puter: /usr/X11R6$

There should be a directory called "lib", and we want to find out what it contains but we don't want to change the working directory. There are two ways of doing this. We can use the absolute pathname:
me@puter: /usr/X11R6$ ls /usr/X11R6/lib

Or using the relative pathname:
me@puter: /usr/X11R6$ ls ./lib

Here, the dot in the path ./lib refers to the working directory, which is /usr/X11R6. This saved some typing! But it'll save even more typing if you omit the leading dot. In most cases you don't need it, so this would've been the same as the above:
me@puter: /usr/X11R6$ ls lib

Now let's change the working directory to /usr/X11R6/bin. There are again two ways of doing this. First, using absolute pathname:
me@puter: /usr/X11R6$ cd /usr/X11R6/bin

Or using relative pathname, remember that you don't have to type the dot:
me@puter: /usr/X11R6$ cd bin

Now your current working directory is /usr/X11R6/bin. Let's change it to /usr/X11R6 which is the parent directory. Again you could use the absolute pathname:
me@puter: /usr/X11R6/bin$ cd /usr/X11R6

But it's faster to use the two dots that mean "parent directory":
me@puter: /usr/X11R6/bin$ cd ..

Note the space between cd and the dots.

As you can see, in many cases using relative pathnames instead of absolute ones saves some typing.