2.3 Lesson 2

This is an example of a normal file system in Linux:

Most of these directories are consistent across all Linux systems. From servers to supercomputers to tiny embedded systems, a seasoned Linux user can be confident that they can find the ls command inside /bin, can change the system configuration by modifying files in /etc, and read system logs in /var. The standard location of these files and directories is defined by the Filesystem Hierarchy Standard (FHS), which will be discussed in a later lesson. You will learn more about the contents of these directories as you continue learning about Linux, but for the time being, know that:

This means that normal users will be prohibited from modifying these files, and may also be prohibited from even reading these files. We will cover the topic of permissions in a later section.

Now, we will focus on the directory /home, which should be somewhat familiar at this point:

Our example system has three normal users, and each of our users has their own dedicated location, where they can create and modify files and directories without affecting their neighbor. For example, in the previous lesson we were working with the following file structure:

In actuality, the real filesystem may look like this:

…​and so on for lara.

In Linux, /home is similar to an apartment building. Many users may have their space here, separated into dedicated apartments. The utilities and maintenance of the building itself are the responsibility of the superintendent root user.

When you start a new terminal session in Linux, you see a command prompt similar to this:

The tilde (~) here represents our home directory. If you run the ls command, you will see some familiar output:

Compare that with the file system above to check your understanding.

Consider now what we know about Linux: it is similar to an apartment building, with many users residing in /home. So user 's home will be different from user michael 's home. To demonstrate this, we will use the su command to switch user.

The meaning of ~ changes depending of who the user is. For michael, the absolute path of ~ is /home/michael. For lara, the absolute path of ~ is /home/lara, and so on.

Using ~ for commands is very convenient, provided that you don’t switch users. We will consider the following example for user, who has begun a new session:

Note that users will always begin a new session in their home directory. In this example, user has traveled into their Documents/Reports subdirectory, and with the cd ~ command they have returned to where they started. You can perform the same action by using the cd command with no arguments:

One last thing to note: we can specify the home directories of other users by specifying the username after the tilde. For example:

Note that this will only work if michael has given us permission to view the contents of his home directory.

Let’s consider a situation where michael would like to view the file report2018.txt in user 's home directory. Assuming that michael has the permission to do so, he can use the less command.

Any file path that contains the ~ character is called a relative-to-home path.

In the previous lesson, we introduced the option -a for the ls command. We used ls -a to introduce the two special relative paths: . and ... The -a option will list all files and directories, including hidden files and directories.

Hidden files and directories will always begin with a period (.). By default, a user’s home directory will include many hidden files. These are often used to set user-specific configuration settings, and should only be modified by an experienced user.

The ls command has many options to change its behavior. Let’s look at one of the most common options:

-l creates a long list. Files and directories will each occupy one line, but additional information about each file and directory will be displayed.

Subjects such as ownership, permissions and links will be covered in future sections. As you can see, the long list version of the ls is oftentimes preferable to the default.

Below are some of the ways that we most commonly use the ls command. As you can see, the user can combine many options together to get the desired output.

Recursion refers to a situation when “something is defined in terms of itself”. Recursion is a very important concept in computer science, but here its meaning is far simpler. Let’s consider our example from before:

We know from before that user has a home directory, and in this directory there is one subdirectory. ls has up until now only shown us the files and subdirectories of a location, but cannot tell us the contents of these subdirectories. In these lessons, we have been using the tree command when we wanted to display the contents of many directories. Unfortunately, tree is not one of the core utilities of Linux and thus is not always available. Compare the output of tree with the output of ls -R in the following examples:

As you can see, with the recursive option, we get a far longer list of files. In fact, it is as if we ran the ls command in user 's home directory, and encountered one subdirectory. Then, we entered into that subdirectory and ran the ls command again. We encountered the file Mission-Statement and another subdirectory called Reports. And again, we entered into the subdirectory, and ran the ls command again. Essentially, running ls -R is like telling Bash: “Run ls here, and repeat the command in every subdirectory that you find.”

Recursion is particularly important in file modification commands such as copying or removing directories. For example, if you wanted to copy the Documents subdirectory, you would need to specify a recursive copy in order to extend this command to all subdirectories.