The Linux Filesystem

In this section, we will cover the:

  • the Linux filesystem and how it is structured and organized, and
  • the basic commands to navigate around and to work with directories and files

The terms directories and folders are synonymous, but as users of primarily graphical user interfaces, you are more likely familiar with the term folders. I will more often use the term directories since that is the command line (text user interface) convention. I will use the term folders when referring to a graphical environment.

Throughout this demonstration, I encourage you to connect to your remote server using the gcloud compute ssh command and follow along with the commands that I use. See Section 2.1 for details on connecting to the remote server.

Visualizing the Filesystem as a Tree

We will need to work within the filesystem quite a lot in this course, but the term filesystem may refer to different concepts, and it's important to clear that up before we start.

In come cases, a filesystem refers to how data (files) are stored and retrieved on a device like a hard drive, USB drive, etc. For example, macOS uses the Apple File System (APFS) by default, and Windows uses the New Technology File System (NTFS). Linux and other Unix-like operating systems use a variety of filesystems, but presently, the two major ones are ext4 and btrfs. The former is the default filesystem on distributions like Debian and Ubuntu; the latter is the default on the Fedora and openSUSE distributions. Opensource.com has a nice overview of filesystems under this concept.

A filesystem might also be used to refer to the directory structure or directory tree of a system. In graphical user interface parlance, this is simply how the folders are your disk are organized. This concept of a filesystem is related to the prior concept of a filesystem, but it's used here to refer to the location of files and directories on a system. For example, on Windows, the filesystem is identified by a letter, like the C: drive, regardless if the disk has a NTFS filesystem or a FAT filesystem. Additional drives (e.g., extra hard drives, USB drives, DVD drives, etc.), will be assigned their own letters (A:, B:, D:, etc.). macOS adheres to a tree like filesystem like Linux, UNIX, and other Unix-like operating systems, and this is because macOS is a registered UNIX® OS.

In Linux and Unix-like OSes, we have a top-level root directory identified by a single forward slash /, and then subdirectories under that root directory. Additional drives (e.g., extra hard drives, USB drives, DVD drives, etc.) are mounted under that root hierarchy and not separately like on Windows. Linux.com provides a nice overview of the most common directory structure that Linux distributions use along with an explanation for the major bottom level directories. In this section, we will learn about this type of filesystem.

On Linux, we can visualize the filesystem with the tree command. The tree command, like many Linux commands, can be run on its own or with options, like in the second example below:

  • tree : list contents of directories in a tree-like format
    • tree -dfL 1 : directories only, full path, one level
    • tree -dfL 1 / : list directories only at root / level

The root Directory and its Base Level Directories

As explained on the Linux.com page, here are the major sub directories under / (root) and a short description of their main purpose:

  • /bin : binary files needed to use the system
  • /boot : files needed to boot the system
  • /dev : device files -- all hardware has a file
  • /etc : system configuration files
  • /home : user directories
  • /lib : libraries/programs needed for other programs
  • /media : external storage is mounted
  • /mnt : other filesystems may be mounted
  • /opt : store software code to compile software
  • /proc : files containing info about your computer
  • /root : home directory of superuser
  • /run : used by system processes
  • /sbin : like /bin, binary files that require superuser privileges
  • /srv : contains data for servers
  • /sys : contains info about devices
  • /tmp : temp files used by applications
  • /usr : user binaries, etc that might be installed by users
  • /var : variable files, used often for system logs

Although there are 18 directories listed above that branch off from the root directory, we will use some more often than others. For example, the /etc directory contains system configuration files, and we will use the contents of this directory, along with the /var directory, quite a bit when we set up our web servers, relational database servers, and more later in the semester. The /home directory is where our default home directories are stored, and if you manage a multi-user system, then this will be an important directory to manage.

Source: Linux Filesystem Explained

Relative and Absolute Paths

macOS users have the Finder app to navigate their filesystem, to move files to different folders, to copy files, to trash them, etc. Window users have File Explorer for these functions. Linux users have similar graphical software options, but all of these functions can be completed on the Linux command line, and generally more efficiently. To get started, we need to learn two things first:

  1. how to specify the locations of files and directories in the filesystem
  2. the commands needed to work with the filesystem

To help specify the locations of files and directories, there are two key concepts to know:

  • absolute paths
  • relative paths

Above we learned about the / root directory and its subdirectories. All sorts of commands, especially those that deal with files and directories (like copying, moving, deleting), require us to specify on the command line the locations of the files and directories. It's common to specify the location in two different ways, by specifying their absolute path (or location) on the filesystem, or the relative path (or location).

To demonstrate, we might want to move around the filesystem. When we first log in to our remote system, our default location will be our home directory, sometimes referred to as $HOME. The path (location) to that directory will be.

/home/USER

Where USER is your username. Therefore, since my username is sean, my home directory is located at:

/home/sean

which we can see specified with the pwd (print working directory) command:

pwd
/home/sean

When I write $HOME, I am referring to a default, environmental variable that points to our home directory. It's variable because, depending on which account we're logged in as, $HOME will point to a different location. For me, then, that will be /home/sean, if I'm logged in as sean. For you it'll point to your home directory.

In my home directory, I have a subdirectory called public_html. The path to that is:

/home/sean/public_html

In a program like Finder (macOS) or File Explorer (Windows), if I want to change my location to that subdirectory (or folder), then I'd double click on its folder icon. On the command line, however, I have to write out the command and the path to the subdirectory. Therefore, starting in my home directory, I use the following command to switch to the public_html subdirectory:

cd public_html

Note that files and directories in Linux are case sensitive. This means that a directory named public_html can co-exist alongside a directory named Public_html. Or a file named paper.txt can co-exist alongside a file named Paper.txt. So be sure to use the proper case when spelling out files, directories, and even commands.

The above is an example of using a relative path, and that command would only be successful if I were first in my $HOME directory. That's because I specified the location of public_html relative to my default ($HOME) location.

I could have also specified the absolute location, but this would be the wordier way. Since the public_html directory is in my $HOME directory, and my $HOME directory is a subdirectory in the /home directory, then to specify the absolute path in the above command, I'd write:

cd /home/sean/public_html

Again, the relative path specified above would only work if I was in my home directory, because cd public_html is relative to the location of /home/sean. That is, the subdirectory public_html is in /home/sean. But specifying the absolute path would work no matter where I was located in the filesystem. For example, if I was working on a file in the /etc/apache2 directory, then using the absolute path (cd /home/sean/public_html) would work. But the relative path (cd public_html) command would not since there is no subdirectory called public_html in the /etc/apache2 directory.

Finally, you can use the ls command to list the contents of a directory, i.e., the files and subdirectories in a directory:

ls

We will cover this more next.

Conclusion

Understanding relative and absolute paths is one of the more difficult concepts for new commandline users to learn, but after time, it'll feel natural. So just keep practicing, and I'll go over this throughout the semester.

In this section, you learned the following commands:

  • tree to list directory contents in a tree-like format
  • cd to change directory
  • pwd to print working directory

You learned different ways to refer to the home directory:

  • /home/USER
  • $HOME
  • ~

You learned about relative and absolute paths. An absolute path starts with the root directory /. Here's an absolute path to a file named paper.txt in my home directory:

  • absolute path: /home/sean/paper.txt

If I were already in my home directory, then the relative path would simply be:

  • relative path: paper.txt

Additional Readings

Read through these two pages for more info on the file system and directories: