Which command would a user type on the command line to find out the current directory in the directory tree group of answer choices?

In this lesson, we will introduce our first three commands: pwd (print working directory), cd (change directory), and ls (list files and directories).

Those new to the command line will need to pay close attention to this lesson since the concepts will take some getting used to.

File System Organization

Like Windows, the files on a Linux system are arranged in what is called a hierarchical directory structure. This means that they are organized in a tree-like pattern of directories (called folders in other systems), which may contain files and subdirectories. The first directory in the file system is called the root directory. The root directory contains files and subdirectories, which contain more files and subdirectories and so on and so on.

Most graphical environments include a file manager program used to view and manipulate the contents of the file system. Often we will see the file system represented like this:

One important difference between Windows and Unix-like operating systems such as Linux is that Linux does not employ the concept of drive letters. While Windows drive letters split the file system into a series of different trees (one for each device), Linux always has a single tree. Different storage devices may be different branches of the tree, but there is always just a single tree.

pwd

Since the command line interface cannot provide graphic pictures of the file system structure, we must have a different way of representing it. To do this, think of the file system tree as a maze, and that we are standing in it. At any given moment, we are located in a single directory. Inside that directory, we can see its files and the pathway to its parent directory and the pathways to the subdirectories of the directory in which we are standing.

The directory we are standing in is called the working directory. To see the name of the working directory, we use the pwd command.

[me@linuxbox me]$ pwd /home/me

When we first log on to our Linux system, the working directory is set to our home directory. This is where we put our files. On most systems, the home directory will be called /home/user_name, but it can be anything according to the whims of the system administrator.

To list the files in the working directory, we use the ls command.

[me@linuxbox me]$ ls Desktop Downloads foo.txt Pictures Templates Documents examples.desktop Music Public Videos

We will come back to ls in the next lesson. There are a lot of fun things you can do with it, but we have to talk about pathnames and directories a bit first.

cd

To change the working directory (where we are standing in the maze) we use the cd command. To do this, we type cd followed by the pathname of the desired working directory. A pathname is the route we take along the branches of the tree to get to the directory we want. Pathnames can be specified two different ways; absolute pathnames or relative pathnames. Let's look with absolute pathnames first.

An absolute pathname begins with the root directory and follows the tree branch by branch until the path to the desired directory or file is completed. For example, there is a directory on your system in which most programs are installed. The pathname of the directory is /usr/bin. This means from the root directory (represented by the leading slash in the pathname) there is a directory called "usr" which contains a directory called "bin".

Let's try this out:

me@linuxbox me]$ cd /usr/bin me@linuxbox bin]$ pwd /usr/bin me@linuxbox bin]$ ls '[' mshortname 2to3-2.7 mshowfat 411toppm mtools a2ps mtoolstest a2ps-lpr-wrapper mtr aa-enabled mtrace aa-exec mtr-packet aclocal mtvtoppm aclocal-1.15 mtype aconnect mutter acpi_listen mxtar add-apt-repository mzip addpart namei and many more...

Now we can see that we have changed the current working directory to /usr/bin and that it is full of files. Notice how the shell prompt has changed? As a convenience, it is usually set up to display the name of the working directory.

Where an absolute pathname starts from the root directory and leads to its destination, a relative pathname starts from the working directory. To do this, it uses a couple of special notations to represent relative positions in the file system tree. These special notations are "." (dot) and ".." (dot dot).

The "." notation refers to the working directory itself and the ".." notation refers to the working directory's parent directory. Here is how it works. Let's change the working directory to /usr/bin again:

me@linuxbox me]$ cd /usr/bin me@linuxbox bin]$ pwd /usr/bin

O.K., now let's say that we wanted to change the working directory to the parent of /usr/bin which is /usr. We could do that two different ways. First, with an absolute pathname:

me@linuxbox bin]$ cd /usr me@linuxbox usr]$ pwd /usr

Or, with a relative pathname:

me@linuxbox bin]$ cd .. me@linuxbox usr]$ pwd /usr

Two different methods with identical results. Which one should we use? The one that requires the least typing!

Likewise, we can change the working directory from /usr to /usr/bin in two different ways. First using an absolute pathname:

me@linuxbox usr]$ cd /usr/bin me@linuxbox bin]$ pwd /usr/bin

Or, with a relative pathname:

me@linuxbox usr]$ cd ./bin me@linuxbox bin]$ pwd /usr/bin

Now, there is something important that we must point out here. In most cases, we can omit the "./". It is implied. Typing:

me@linuxbox usr]$ cd bin

would do the same thing. In general, if we do not specify a pathname to something, the working directory will be assumed. There is one important exception to this, but we won't get to that for a while.

A Few Shortcuts

If we type cd followed by nothing, cd will change the working directory to our home directory.

A related shortcut is to type cd ~user_name. In this case, cd will change the working directory to the home directory of the specified user.

Typing cd - changes the working directory to the previous one.

When you first login, your current working directory is your home directory. Your home directory has the same name as your user-name, for example, ee91ab, and it is where your personal files and subdirectories are saved.

To find out what is in your home directory, type

% ls

The ls command ( lowercase L and lowercase S ) lists the contents of your current working directory.

There may be no files visible in your home directory, in which case, the UNIX prompt will be returned. Alternatively, there may already be some files inserted by the System Administrator when your account was created.

ls does not, in fact, cause all the files in your home directory to be listed, but only those ones whose name does not begin with a dot (.) Files beginning with a dot (.) are known as hidden files and usually contain important program configuration information. They are hidden because you should not change them unless you are very familiar with UNIX!!!

To list all files in your home directory including those whose names begin with a dot, type

% ls -a

As you can see, ls -a lists files that are normally hidden.

ls is an example of a command which can take options: -a is an example of an option. The options change the behaviour of the command. There are online manual pages that tell you which options a particular command can take, and how each option modifies the behaviour of the command. (See later in this tutorial)

1.2 Making Directories

mkdir (make directory)

We will now make a subdirectory in your home directory to hold the files you will be creating and using in the course of this tutorial. To make a subdirectory called unixstuff in your current working directory type

% mkdir unixstuff

To see the directory you have just created, type

% ls

1.3 Changing to a different directory 

cd (change directory)

The command cd directory means change the current working directory to 'directory'. The current working directory may be thought of as the directory you are in, i.e. your current position in the file-system tree.

To change to the directory you have just made, type

% cd unixstuff

Type ls to see the contents (which should be empty)

Exercise 1a

Make another directory inside the unixstuff directory called backups

1.4 The directories . and ..

Still in the unixstuff directory, type

% ls -a

As you can see, in the unixstuff directory (and in all other directories), there are two special directories called (.) and (..)

The current directory (.)

In UNIX, (.) means the current directory, so typing

% cd .

NOTE: there is a space between cd and the dot

means stay where you are (the unixstuff directory).

This may not seem very useful at first, but using (.) as the name of the current directory will save a lot of typing, as we shall see later in the tutorial.

The parent directory (..)

(..) means the parent of the current directory, so typing

% cd ..

will take you one directory up the hierarchy (back to your home directory). Try it now.

Note: typing cd with no argument always returns you to your home directory. This is very useful if you are lost in the file system.

1.5 Pathnames

pwd (print working directory)

Pathnames enable you to work out where you are in relation to the whole file-system. For example, to find out the absolute pathname of your home-directory, type cd to get back to your home-directory and then type

% pwd

The full pathname will look something like this -

/home/its/ug1/ee51vn

which means that ee51vn (your home directory) is in the sub-directory ug1 (the group directory),which in turn is located in the its sub-directory, which is in the home sub-directory, which is in the top-level root directory called " / " .

Exercise 1b

Use the commands cd, ls and pwd to explore the file system.

(Remember, if you get lost, type cd by itself to return to your home-directory)

1.6 More about home directories and pathnames

Understanding pathnames

First type cd to get back to your home-directory, then type

% ls unixstuff

to list the conents of your unixstuff directory.

Now type

% ls backups

You will get a message like this -

backups: No such file or directory

The reason is, backups is not in your current working directory. To use a command on a file (or directory) not in the current working directory (the directory you are currently in), you must either cd to the correct directory, or specify its full pathname. To list the contents of your backups directory, you must type

% ls unixstuff/backups

~ (your home directory)

Home directories can also be referred to by the tilde ~ character. It can be used to specify paths starting at your home directory. So typing

% ls ~/unixstuff

will list the contents of your unixstuff directory, no matter where you currently are in the file system.

What do you think

% ls ~

would list?

What do you think

% ls ~/..

would list?

Summary

, © 9th October 2000