update README

README.md

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 # Introduction to the Linux Command Line 
 
+Material : https://kir-rescomp.github.io/training-intro-to-linux-cli/
+
 <p align="center">
-    <img src="./images/introduction_to_shell.png" alt="drawing" width="300">
+    <img src="./docs/images/introduction_to_shell.png" alt="drawing" width="300">
 </p>

docs/1_navigating_files_and_dirs.md → docs/2_navigating_files_and_dirs.md

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-# 2, Navigating Files and Directories
+# 2. Navigating Files and Directories
 
 !!! clipboard-list "Episode objectives"
 

docs/3_absolute_vs_relativepath.md

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+# 3. Absolute vs Relative Paths
+
+
+The `cd` command takes an argument which is a directory name. Directories can be specified using either a _relative_ path or a
+full _absolute_ path. The directories on the computer are arranged into a hierarchy. The full path tells you where a directory is in that
+hierarchy. Navigate to the home directory, then enter the `pwd` command.
+
+
+!!! terminal "code"
+
+    ```bash
+    $ cd
+    $ pwd
+    ```
+
+    You will see:
+
+    ```output
+    /users/group/user
+    ```
+
+This is the full name of your home directory. This tells you that you
+are in a directory called `training`, which sits inside a directory called
+`home` which sits inside the very top directory in the hierarchy. The
+very top of the hierarchy is a directory called `/` which is usually
+referred to as the _root directory_. So, to summarize: `training` is a
+directory in `home` which is a directory in `/`. More on `root` and
+`home` in the next section.
+
+!!! terminal-2 "Now enter the following command:"
+
+    ```bash
+    $ cd /users/group/user/shell_data/.hidden
+    ```
+
+    This jumps forward multiple levels to the `.hidden` directory.
+    Now go back to the home directory.
+
+    ```bash
+    $ cd
+    ```
+
+You can also navigate to the `.hidden` directory using:
+
+!!! terminal "code"
+
+    ```bash
+    $ cd ~/shell_data/.hidden
+    ```
+
+These two commands have the same effect, they both take us to the `.hidden` directory.
+The first uses the absolute path, giving the full address from the home directory. The
+second uses a relative path, giving only the address from the working directory. A full
+path always starts with a `/`. A relative path does not.
+
+A relative path is like getting directions from someone on the street. They tell you to
+"go right at the stop sign, and then turn left on Main Street". That works great if
+you're standing there together, but not so well if you're trying to tell someone how to
+get there from another country. A full path is like GPS coordinates. It tells you exactly
+where something is no matter where you are right now.
+
+You can usually use either a full path or a relative path depending on what is most convenient.
+If we are in the home directory, it is more convenient to enter the full path.
+If we are in the working directory, it is more convenient to enter the relative path
+since it involves less typing.
+
+Over time, it will become easier for you to keep a mental note of the
+structure of the directories that you are using and how to quickly
+navigate amongst them.
+
+# Relative path resolution
+
+!!! dumbbell "Using the filesystem diagram below, if `pwd` displays `/Users/thing`," what will `ls ../backup` display?"
+
+    1. `../backup: No such file or directory`
+    2. `2012-12-01 2013-01-08 2013-01-27`
+    3. `2012-12-01/ 2013-01-08/ 2013-01-27/`
+    4. `original pnas_final pnas_sub`
+
+    ![](images/filesystem-challenge.svg){alt='File System for Challenge Questions'}
+
+
+    ??? success "Solution"
+
+        1. No: there _is_ a directory `backup` in `/Users`.
+        2. No: this is the content of `Users/thing/backup`,
+           but with `..` we asked for one level further up.
+        3. No: see previous explanation.
+           Also, we did not specify `-F` to display `/` at the end of the directory names.
+        4. Yes: `../backup` refers to `/Users/backup`.
+
+
+### Navigational Shortcuts
+
+The root directory is the highest level directory in your file
+system and contains files that are important for your computer
+to perform its daily work. While you will be using the root (`/`)
+at the beginning of your absolute paths, it is important that you
+avoid working with data in these higher-level directories, as
+your commands can permanently alter files that the operating
+system needs to function. In many cases, trying to run commands
+in `root` directories will require special permissions which are
+not discussed here, so it's best to avoid them and work within your
+home directory. Dealing with the `home` directory is very common.
+The tilde character, `~`, is a shortcut for your home directory.
+In our case, the `root` directory is **two** levels above our
+`home` directory, so `cd` or `cd ~` will take you to
+`/home/<username>` and `cd /` will take you to `/`. Navigate to the
+`shell_data` directory:
+
+!!! terminal "code"
+
+    ```bash
+    $ cd
+    $ cd ~/shell_data
+    ```
+
+    Then enter the command:
+
+    ```bash
+    $ ls ~
+    ```
+
+    ```output
+    shell_data
+    ```
+
+This prints the contents of your home directory, without you needing to
+type the full path.
+
+!!! quote ""
+
+    The commands `cd`, and `cd ~` are very useful for quickly navigating back to your home directory. We will be using the `~` character in later lessons to specify our home directory.
+
+!!! graduation-cap "Summary"
+
+    - The `/`, `~`, and `..` characters represent important navigational shortcuts.
+    - Hidden files and directories start with `.` and can be viewed using `ls -a`.
+    - Relative paths specify a location starting from the current location, while absolute paths specify a location from the root of the file system.