3.6 Linux File system parte 1: Directory e File

Files and Directories

The Linux filesystem is similar to other operating system’s filesystems in that it contains files and directories. Files contain data such as human-readable text, executable programs, or binary data that is used by the computer. Directories are used to create organization within the filesystem. Directories can contain files and other directories.

$ tree

Documents
├── Mission-Statement.txt
└── Reports
    └── report2018.txt

1 directory, 2 files

In this example, Documents is a directory that contains one file (Mission-Statement.txt) and one subdirectory (Reports). The Reports directory in turn contains one file called report2018.txt. The Documents directory is said to be the parent of the Reports directory.

Tip	If the command tree is not available on your system, install it using your Linux distribution’s package manager. Refer to the lesson on package management to learn how to do so.

File and Directory Names

File and directory names in Linux can contain lower case and upper case letters, numbers, spaces and special characters. However, since many special characters have a special meaning in the Linux shell, it is good practice to not use spaces or special characters when naming files or directories. Spaces, for example, need the escape character \ to be entered correctly:

$ cd Mission\ Statements

Also, refer to the filename report2018.txt. Filenames can contain a suffix which comes after the period (.). Unlike Windows, this suffix has no special meaning in Linux; it is there for human understanding. In our example .txt indicates to us that this is a plaintext file, although it could technically contain any kind of data.

Navigating the Filesystem

Getting Current Location

Since Linux shells such as Bash are text-based, it is important to remember your current location when navigating the filesystem. The command prompt provides this information:

user@hostname ~/Documents/Reports $

Note that information such as user and hostname will be covered in future sections. From the prompt, we now know that our current location is in the Reports directory. Similarly, the command pwd will print working directory:

user@hostname ~/Documents/Reports $ pwd
/home/user/Documents/Reports

The relationship of directories is represented with a forward slash (/). We know that Reports is a subdirectory of Documents, which is a subdirectory of user, which is located in a directory called home. home doesn’t seem to have a parent directory, but that is not true at all. The parent of home is called root, and is represented by the first slash (/). We will discuss the root directory in a later section.

Notice that the output of the pwd command differs slightly from the path given on the command prompt. Instead of /home/user, the command prompt contains a tilde (~). The tilde is a special character that represents the user’s home directory. This will be covered in more detail in the next lesson.

Listing Directory Contents

The contents of the current directory are listed with the ls command:

user@hostname ~/Documents/Reports $ ls
report2018.txt

Note that ls provides no information about the parent directory. Similarly, by default ls does not show any information about contents of subdirectories. ls can only “see” what is in the current directory.

Changing Current Directory

Navigation in Linux is primarily done with the cd command. This changes directory. Using the pwd command from before, we know our current directory is /home/user/Documents/Reports. We can change our current directory by entering a new path:

user@hostname ~ $ cd /home/user/Documents
user@hostname ~/Documents $ pwd
/home/user/Documents
user@hostname ~/Documents $ ls
Mission-Statement.txt Reports

From our new location, we can “see” Mission-Statement.txt and our subdirectory Reports, but not the contents of our subdirectory. We can navigate back into Reports like this:

user@hostname ~/Documents $ cd Reports
user@hostname ~/Documents/Reports $ pwd
/home/user/Documents/Reports
user@hostname ~/Documents/Reports $ ls
report2018.txt

We are now back where we started.

Absolute and Relative Paths

The pwd command always prints an absolute path. This means that the path contains every step of the path, from the top of the filesystem (/) to the bottom (Reports). Absolute paths always begin with a /.

/
└── home
    └── user
        └── Documents
            └── Reports

The absolute path contains all the information required to get to Reports from anywhere in the filesystem. The drawback is that it is tedious to type.

The second example (cd Reports) was much easier to type. This is an example of a relative path. Relative paths are shorter but only have meaning in relation to your current location. Consider this analogy: I am visiting you at your house. You tell me that your friend lives next door. I will understand that location because it is relative to my current location. But if you tell me this over the phone, I will not be able to find your friend’s house. You will need to give me the complete street address.

Special Relative Paths

The Linux shell gives us ways to shorten our paths when navigating. To reveal the first special paths, we enter the ls command with the flag -a. This flag modifies the ls command so that all files and directories are listed, including hidden files and directories:

user@hostname ~/Documents/Reports $ ls -a
.
..
report2018.txt

Note	You can refer to the man page for ls to understand what -a is doing here.

This command has revealed two additional results: These are special paths. They do not represent new files or directories, but rather they represent directories that you already know:.

Indicates the current location (in this case, Reports)...

Indicates the parent directory (in this case, Documents).

It is usually unnecessary to use the special relative path for the current location. It is easier and more understandable to type report2018.txt than it is to type ./report2018.txt. But the . has uses that you will learn in future sections. For now, we will focus on the relative path for the parent directory:

user@hostname ~/Documents/Reports $ cd ..
user@hostname ~/Documents $ pwd
/home/user/Documents

The example of cd is much easier when using .. instead of the absolute path. Additionally, we can combine this pattern to navigate up the file tree very quickly.

user@hostname ~/Documents $ cd ../..
$ pwd
/home

Guided Exercises

1. For each of the following paths, identify whether it is absolute or relative:/home/user/Downloads../Reports/vardocs/
2. Observe the following file structure. Note: Directories end with a slash (/):$ tree / / ├── etc/ │   ├── network/ │   │   └── interfaces │   ├── systemd/ │   │   ├── resolved.conf │   │   ├── system/ │   │   ├── system.conf │   │   ├── user/ │   │   └── user.conf │   └── udev/ │   ├── rules.d/ │   └── udev.conf └── home/ ├── lost+found/ └── user/ └── Documents/ 12 directories, 5 filesUse this structure to answer the following questions.A user enters the following commands:$ cd /etc/udev $ ls -aWhat will be the output of the ls -a command?
3. Enter the shortest possible command for each of the following:
   • Your current location is root (/). Enter the command to navigate to lost+found within the home directory (example):$ cd home/lost+found
   • Your current location is root (/). Enter the command to navigate to the directory named /etc/network/.
   • Your current location is /home/user/Documents/. Navigate to the directory named /etc/.
   • Your current location is /etc/systemd/system/. Navigate to the directory named /home/user/.
4. Consider the following commands:$ pwd /etc/udev/rules.d $ cd ../../systemd/user $ cd .. $ pwdWhat is the output of the final pwd command?

Explorational Exercises

1. Suppose a user has entered the following commands:$ mkdir “this is a test” $ ls this is a testWhat cd command would allow you to enter this directory?
2. Try this again, but after typing in cd this, press the TAB key. What is now displayed on the prompt?This is an example of autocompletion, which is an invaluable tool not only for saving time, but for preventing spelling errors.
3. Try to create a directory whose name contains a \ character. Display the directory’s name with ls and delete the directory.

Summary

In this lesson, you learned:

• The fundamentals of the Linux filesystem
• The difference between parent directories and subdirectories
• The difference between absolute file paths and relative file paths
• The special relative paths . and ..
• Navigate the filesystem using cd
• Show your current location using pwd
• List all files and directories using ls -a

The following commands were discussed in this lesson:cd

Change the current directory.pwd

Print the current working directory’s pathls

List the contents of a directory and display properties of filesmkdir

Create a new directorytree

Display a hierarchal listing of a directory tree

Answers to Guided Exercises

1. For each of the following paths, identify whether it is absolute or relative:/home/user/Downloadsabsolute../Reportsrelative/varabsolutedocsrelative/absolute
2. Observe the following file structure. Note: directories end with a slash (/). Use it to answer the following questions:$ tree / / ├── etc/ │   ├── network/ │   │   └── interfaces │   ├── systemd/ │   │   ├── resolved.conf │   │   ├── system/ │   │   ├── system.conf │   │   ├── user/ │   │   └── user.conf │   └── udev/ │   ├── rules.d/ │   └── udev.conf └── home/ ├── lost+found/ └── user/ └── Documents/ 12 directories, 4 filesA user enters the following commands:$ cd /etc/udev $ ls -aWhat will be the output of the ls -a command?. .. rules.d udev.conf
3. Enter the shortest possible command for each of the following:
   • Your current location is root (/). Enter the command to navigate to lost+found within the home directory (example):$ cd home/lost+found
   • Your current location is root (/). Enter the command to navigate to the directory named network:$ cd etc/network
   • Your current location is Documents. Navigate to the directory named etc:$ cd /etc
   • Your current location is system. Navigate to the directory named user:$ cd /home/user
4. Consider the following commands:$ pwd /etc/udev/rules.d $ cd ../../systemd/user $ cd .. $ pwdWhat is the output of the final pwd command?/etc/systemd

Answers to Explorational Exercises

1. Suppose a user has entered the following commands:$ mkdir “this is a test” $ ls this is a testWhat cd command would allow you to enter this directory?$ cd this\ is\ a\ test
2. Try this again, but after typing in cd this, press the TAB key. What is now displayed on the prompt?$ cd this\ is\ a\ testThis is an example of autocompletion, which is an invaluable tool not only for saving time, but for preventing spelling errors.
3. Try to create a directory whose name contains a \ character. Display the directory’s name with ls and delete the directory.You can either escape the backslash using another backslash (\\) or use hyphens around the whole directory name:$ mkdir my\\dir $ ls ‘my\dir’ $ rmdir ‘my\dir’