Let's start by logging in to our virtual machine via SSH:

ssh -p 2222 reader@localhost

Enter your password at the prompt and you should arrive at the default Ubuntu 18.04 login banner, which should look similar to the following:

reader@localhost's password: 
Welcome to Ubuntu 18.04.1 LTS (GNU/Linux 4.15.0-29-generic x86_64)
<SNIPPED>
  System information as of Sat Jul 28 14:15:19 UTC 2018

  System load:  0.09              Processes:             87
  Usage of /:   45.6% of 9.78GB   Users logged in:       0
  Memory usage: 15%               IP address for enp0s3: 10.0.2.15
  Swap usage:   0%
<SNIPPED>
Last login: Sat Jul 28 14:13:42 2018 from 10.0.2.2
reader@ubuntu:~$

When logging in (either via SSH or the Terminal console) you will end up at the home directory of the user. You can always find out where you are exactly by using the pwd command. pwd stands for print working directory:

reader@ubuntu:~$ pwd
/home/reader

So, we've ended up in the /home/reader/ directory. This is the default for most Linux distributions: /home/$USERNAME/. Since we created the primary user reader, this is where we expect to be. For those of you coming from Windows, this might look very foreign: where is the drive name (C:, D:, and so on) and why are we using (forward) slashes instead of backslashes?

Linux, as well as Unix and other Unix-like systems, uses a tree structure. It is referred to as a tree because it starts at a single origin point, the root (found at /). Directories are nested from there (like branches from a tree), not much differently from other operating systems. Finally, the tree structure ends in files that are considered the leaves of the tree. This might sound terribly complicated still, but it's actually relatively simple. Let's keep exploring to make sure we fully understand this structure! Under Linux, we use the cd command to change directories. It works by entering cd, followed by the location on the filesystem where we want to go as the argument to the command. Navigate to the filesystem root:

reader@ubuntu:~$ cd /    
reader@ubuntu:/$

As you can see, nothing much seems to have happened. However, there is one tiny difference in your Terminal prompt: the ~ character has been replaced by /. Under Ubuntu, the default configuration shows the location on the filesystem without needing to use the pwd command. The prompt is built as follows: <username>@<hostname>:<location>$. Why the ~ then? Simple: the tilde character is shorthand for the user's home directory! If the shorthand wasn't there, the prompt at login would be reader@ubuntu:/home/reader$.

Since we have navigated to the root of the filesystem, let's check out what we can find there. To list the contents of the current directory, we use the ls command:

reader@ubuntu:/$ ls
bin dev home initrd.img.old lib64 media opt root sbin srv sys usr vmlinuz
boot etc initrd.img lib lost+found mnt proc run snap swap.img tmp var vmlinuz.old

If you're using SSH, you'll most likely have some colors to differentiate between files and directories (and even permissions on directories, if you see tmp in a different manner; this will be discussed in the next chapter). However, even with color assistance, this still feels unclear. Let's clean it up a bit by using an option on the ls command:

reader@ubuntu:/$ ls -l
total 2017372
drwxr-xr-x  2 root root       4096 Jul 28 10:31 bin
drwxr-xr-x  3 root root       4096 Jul 28 10:32 boot
drwxr-xr-x 19 root root       3900 Jul 28 10:31 dev
drwxr-xr-x 90 root root       4096 Jul 28 10:32 etc
drwxr-xr-x  3 root root       4096 Jun 30 18:20 home
lrwxrwxrwx  1 root root         33 Jul 27 11:39 initrd.img -> boot/initrd.img-4.15.0-29-generic
lrwxrwxrwx  1 root root         33 Jul 27 11:39 initrd.img.old -> boot/initrd.img-4.15.0-23-generic
drwxr-xr-x 22 root root       4096 Apr 26 19:09 lib
drwxr-xr-x  2 root root       4096 Apr 26 19:07 lib64
drwx------  2 root root      16384 Jun 30 17:58 lost+found
drwxr-xr-x  2 root root       4096 Apr 26 19:07 media
drwxr-xr-x  2 root root       4096 Apr 26 19:07 mnt
drwxr-xr-x  2 root root       4096 Apr 26 19:07 opt
dr-xr-xr-x 97 root root          0 Jul 28 10:30 proc
drwx------  3 root root       4096 Jul  1 09:40 root
drwxr-xr-x 26 root root        920 Jul 28 14:15 run
drwxr-xr-x  2 root root      12288 Jul 28 10:31 sbin
drwxr-xr-x  4 root root       4096 Jun 30 18:20 snap
drwxr-xr-x  2 root root       4096 Apr 26 19:07 srv
-rw-------  1 root root 2065694720 Jun 30 18:00 swap.img
dr-xr-xr-x 13 root root          0 Jul 28 10:30 sys
drwxrwxrwt  9 root root       4096 Jul 28 14:32 tmp
drwxr-xr-x 10 root root       4096 Apr 26 19:07 usr
drwxr-xr-x 13 root root       4096 Apr 26 19:10 var
lrwxrwxrwx  1 root root         30 Jul 27 11:39 vmlinuz -> boot/vmlinuz-4.15.0-29-generic
lrwxrwxrwx  1 root root         30 Jul 27 11:39 vmlinuz.old -> boot/vmlinuz-4.15.0-23-generic

The option -l (hyphen lowercase l, as in long) to ls gives the long listing format. Among other things, this prints the permissions, the owner of the file/directory, the type of file, and its size. Remember, permissions and ownership are discussed in the next chapter, so no need to worry about this for now. The most important thing to take away from this is that each file/directory is printed on its own line, where the first character of that line denotes the type of file: d for directory, - for regular file, and l for symlinks (which are shortcuts under Linux).

Let's navigate deeper into the tree structure, back toward our home directory. At this point, you have two options. You can use a relative path (as in: relative to the current location) or a fully qualified path (which is not relative to the current directory). Let's try both:

reader@ubuntu:/$ cd home
reader@ubuntu:/home$

The preceding is an example of changing directories into a relative directory. We were positioned in the root directory, /, and we navigated to home from there, effectively ending up in /home. We could have navigated there from anywhere by using the fully qualified path:

reader@ubuntu:/$ cd /home
reader@ubuntu:/home$

Did you spot the difference? In the fully qualified example, the argument to cd started with a slash, but in the relative example it did not. Let's see what happens if you use both types incorrectly:

reader@ubuntu:/home$ ls
reader
reader@ubuntu:/home$ cd /reader
-bash: cd: /reader: No such file or directory

We listed the contents of the /home directory with ls. As expected, we saw (at least) the current user's home directory, reader. However, when we tried to navigate to it using cd /reader, we got the infamous error No such file or directory. This is not surprising though: there isn't actually a directory /reader. The directory we're looking for is /home/reader, which would be reached fully qualified with the command cd /home/reader:

reader@ubuntu:/home$ cd home
-bash: cd: home: No such file or directory
reader@ubuntu:/home$

The same error is presented if we try to use an incorrect relative path. In the preceding example, we are currently located in the /home directory and we use the cd home command. Effectively, this would put us in /home/home, which, as we saw when we used ls in the /home directory, does not exist!

Even though fully qualified is safer, it's much less efficient then relative. You saw how we can move deeper into the branches of the tree structure, but what if you had to go down a level, back toward the root? Luckily for us, that does not force us to use fully qualified paths. We can use the .. notation, which means as much as go up a level toward /:

reader@ubuntu:/home$ cd ..
reader@ubuntu:/$

Using cd .. to move up lands us back at the root of the filesystem. At this point, you might think If I do this again while I'm on the highest level of the filesystem, what would happen?. Give it a try:

reader@ubuntu:/$ cd ..
reader@ubuntu:/$

Fortunately for us, we do not get an error nor a crashing machine; instead, we just end up (or, depending on how you look at it, stay) on the root of the filesystem.