Getting Down with ...

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Lesson n:


A file on a modern computer system is a named block of information accessible to a computer program and stored on some type of non-volatile memory.

Files can contain both programs and data, and data files can store text, images, sound, video, and many other kinds of data. The two most common types of data files for web designers are text files and image files. Web pages are written in text files.

File systems and directories

Operating systems organize files through the use of a file system. File systems provide tools for the management of files and directories. A directory (or folder) is a special type of file that can contain other files and directories within it. Directories are used to organize files to make them easier to find.

Since a directory can have directories inside it, and those directories can directories inside them, and so on, we end up with a tree structure for the file system.

Tree view of webappdev

The screenshot above shows the tree program display of a directory named webappdev that has four subdirectories named css, html, unix, and vim. Each of these subdirectories has two more subdirectories named examples and notes. The notes directory under unix contains two files, lesson1.txt and lesson2.txt. The .txt filename extention suggests these are text files.

A directory that contains a file or directory is called the parent directory of that file or subdirectory. In the example above, the notes directory is the parent directory of the lesson1.txt and lesson2.txt text files, and the webappdev directory is the parent of the css, html, unix, and vim subdirectories.


A modern computer system has hundreds of thousands of files and directories. By utilizing the tree structure of the file system, it becomes possible to organize and manage these hundreds of thousands of files. Each file is located in a nested series of directories. The sequence of these nested directories ending with the name of the file is called the path to that file.

Absolute Paths

At the top of the Unix file system is what is called the root directory, which is always just / in Unix. The absolute path to any file in the filesystem is the sequence of directories, starting from the root and ending with the file.

The easiest way to show how this works is by example. The following diagram shows part of a Unix filesystem tree.

Unix directory tree

The absolute path to the file lesson1.txt is:


Relative Paths and the Current Working Directory

The absolute path to a file in the file system can be long, and it is normally the case that the user is interested in files that are located in only one small part of the system, located near the user. By near the user we mean near in the tree to the user's current working directory. You can think of the current working directory as your present location as you move around the file system tree.

Again, it's much easier to show you than to tell you.

Inside webappdev

The screenshot above shows the pwd program displaying the current working directory as /home/jelkner/webappdev. Next it shows ls being run, with the css, html, unix, and vim directories as output. Lastly, tree is run again, this time with no argument.

The relative path from the current working directory to the file lesson1.txt is:


Notice that relative paths do not start with a slash (/).

Short Names for the Current and Parent Directories: . and ..

In expressing relative paths we need a way to indicate both our current working directory and its parent directory. Unix provides a very short way to do this. The current working directory is referred to with a single dot (.) and the parent directory is referred to with two dots (..). Note that since each file or directory can have only a single parent, using .. to refer to this parent directory is unambiguous.

Our First Commands: ls, pwd, cd, mkdir, and rmdir

The follow unix commands are used to list the contents of the current working directory, print the current working directory itself, move from one directory to another, create a new directory, and remove an existing directory.

List the contents of the current working directory. ls has several useful command line options which start with a - (dash) character followed by one or more letters.
ls -a
Show hidden files and directories (files and directories whose names begin with . (dot).
ls -l
Show file details using a long format.
ls -t
Sort listing by date, newest file first.
ls -S
Sort listing by size, largest file first.
These options can be combined, so ls -la, ls -lt, ls -lS, and ls -lat are all useful forms.
Print working (current) directory.
cd [directory_name]
Change working directory to directory_name.
mkdir [directory_name]
Create directory directory_name.
rmdir [directory_name]
Remove directory_name if it is empty. If not empty, a warning will be returned and the directory will not be removed.

In the webappdev directory tree displayed above with the webappdev directory located in the user's home directory (/home/[username]/), typing the command: cd webappdev/unix at the command prompt would change the current working directory to /home/[username]/webappdev/unix.

Typing the ls command would then reveal the two subdirectories, examples, and notes. Typing pwd will display the full path to the current working directory:


To move up one level in the directory tree, typing cd .. will change the current working directory to webappdev. Typing ls here will display the four subdirectories css, html, unix, and vim.


The following exercises will give you practice with each of these new commands. Note that the files that end in .txt in these exercises are text files, not directories. Use the following command to create these files:

    $ touch [file_name]

We will learn a better way to create and edit text files in the next lesson.

  1. What is information? Do you think this is an easy or difficult term to define? Why?
  2. Read over the wikipedia article on computer program. In your own words, summarize what a computer program is.
  3. According to Wikipedia, what is non-volatile memory? Give examples of non-volatile memory (or storage) on the computer systems you use. What do you think volatile memory is?
  4. Practice your new unix commands by following the step-by-step directions in this exercise sheet.
  5. Get more practice with unix commands by following the step-by-step directions in this exercise sheet.
  6. Use your new unix commands to create the directory pictured in this exercise sheet.
  7. Use your new unix commands to create the directory pictured in this exercise sheet.