Unit 4: Unix Directory Structure and Navigation

Unix Directory

Unix organizes files according to a directory structure. The following shows an example.

This structure is also known as a directory tree.

There are two important directories that you need to know.

Root Directory

The root directory is the directory at the top of the directory tree¹. It is simply referred to as /, without any name. Under the root directory are many other systems directory, which a casual user does not normally need to (and have no permission to) modify.

Home Directory

Each user has his/her own home directory. The above figure shows where the home directory of the user ooiwt resides in the directory tree. The user ooiwt may create files or directories in his/her home directory, but not elsewhere unless permission is given.

The home directory is referred to with the symbol ~ in bash. Sometimes we add the username behind ~ to indicate the home directory of the other user. E.g., ~bob means the home directory of a user named bob.

Current Working Directory

A user can navigate around the directory tree. The current working directory is the directory that the user is currently in. In contrast to the root and home directory, which are fixed², the current working directory changes as the user moves around. Knowing the current working directory is important since this is the default location in the directory tree a command executes. As such, many systems by default display the current working directory as part of the bash command prompt.

The current working directory is referred to with the symbol . in bash.

Parent Directory

The parent directory is directory one layer up from the current directory.

The parent directory is referred to with the symbol .. in bash.

To summarize, here are the short form representations:

Specifying a Path

To specify a directory or a file in the Unix directory tree, we can use either the absolute path or the relative path.

Absolute path

The absolute path is constructed as follows, starting from the root of the directory structure, find a path (a sequence of directories) to the location that you want to specify, then concatenate the names of the directories together, separated by the forward-slash /. This is a similar notation used for Web site URLs so you should already be familiar with it. For instance, the path /home/o/ooiwt is the absolute path of the directory named ooiwt in the figure above.

An absolute path is independent of the current working directory and always start with / or ~

Relative path

The relative path is dependent on the current working directory. To refer to another location, start from the current directory, and find a path (a sequence of directories) to the location that you refer to. When we go up a tree, we use .. to represent the directory.

For example, referring to the figure above, if we are in the directory /home/b, and we wish to refer to /home/o/ooiwt, we can use the relative path ../o/ooiwt. If we wish to refer to /home/b/bob, we can use the relative path bob.

A relative path never starts with /.

Directory-related Commands

Now, let's take a look at some basic commands available in bash that deals with navigation and directories.

pwd: Print Current Working directory

pwd shows you which directory you are currently in.
Type pwd into the command prompt, and it will print the absolute path to your current working directory. For instance, Suppose you are in /home/o/ooiwt, entering

pwd

/home/o/ooiwt

ls: LiSt content of a directory

The ls list the content in the current working directory.

mkdir: MaKe a subDIRectory

The mkdir command creates a subdirectory with the given name in the current directory.

In the example below, we assume that we start with an empty directory.

$ ls
$ mkdir workshop
$ ls
workshop
$ ls -F
workshop/

Here, we create a directory called workshop. Now, when we ls, you can see the directory listed.

You may also use ls -F for more information (-F is one of the many options/flags available for the ls command. To see a complete list of the options, refer to the man pages, i.e., man ls.)

The slash / beside the filename tells you that the file is a directory. A normal file does not have a slash beside its name when "ls -F" is used.

You may also use the ls -l command (hyphen el, not hyphen one) to display almost all the file information, include the size of the file and the date of modification.

go through it. Press the Ctrl+P until you find a previously executed command. You may then press Enter to execute it or edit the command before executing it. This is handy when you need to repeatedly execute a long bash command.

cd: Change Directory

To navigate in the directory tree, changing the current working directory from one to another, we use the cd command.

$ pwd
/home/o/ooiwt
$ cd workshop
$ pwd
/home/o/ooiwt/workshop

Suppose our starting working directory is /home/o/ooiwt, after we cd into workshop, the current working directory becomes /home/o/ooiwt/workshop. Note that cd can take in either an absolute path or a relative path. The example above takes in a relative path as the argument.

As mentioned in the Introduction to Shell unit, it is common to include the current working directory into the shell's prompt. So, you may see your command prompt updated to include the new working directory.

Entering cd alone (without argument) brings you back to your home directory.

rmdir: ReMove a subDIRectory

rmdir removes a subDIRectory in the current directory -- note that a directory must be empty before it can be removed.

The command

$ rmdir workshop