Unit 2: Unix Directory Structure and Navigation

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 user 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:

Symbol	Meaning
/	the root directory
~	the home directory
.	the current working directory
..	the parent directory

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/bob, entering

pwd

will give the output

/home/o/bob

ls: LiSt content of a directory

The ls list the content in the current working directory.

!!! note "Rule of Silence" Unix follows the economical rule of silence: programs should not print unnecessary output, to allow other programs and users to easily parse the output from one program. So, if ls has nothing to list, it will list nothing (as opposed to, say, printing "This is an empty 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.

!!! tip "Use Up Arrow for Command History" bash maintains a history of your previously executed commands, and you may use the Ctrl+P (previous) and Ctrl+N (next) to 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/bob
$ cd workshop
$ pwd
/home/o/bob/workshop

Suppose our starting working directory is /home/o/bob, after we cd into workshop, the current working directory becomes /home/o/bob/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

will remove the directory that you just created.