Explain redirection to stderr and shebang

src/day_1/README.md

@@ -1,5 +1,5 @@
 # Day 1
 
-In this chapter, you will learn the basics of Linux in the terminal.
+In this day, you will learn the basics of Linux in the terminal.
 
 But before we start with the basics, the first section discusses why you should learn Linux in the first place.

src/day_2/README.md

@@ -1,3 +1,3 @@
 # Day 2
 
-In this chapter, we will learn how to glue commands together to benefit from their modularity. Later, we will write our own shell scripts to achieve some automation.
+In this day, we will learn how to glue commands together to benefit from their modularity. Later, we will write our own shell scripts to achieve some automation.

src/day_2/shell_scripting.md

@@ -4,9 +4,9 @@ Task automation requires multiple instructions that have to run on demand. To co
 
 Since `bash` is the default shell on most Linux distributions, we learn bash scripting.
 
-## Bash scripts
+## First script
 
-Lets write our first Bash script (also for some more demonstration of the two output forms):
+Lets write our first Bash script (also for some more demonstration of the two output forms from the last chapter):
 
 ```bash
 #!/usr/bin/bash
@@ -23,17 +23,63 @@ if [ "$ANSWER" == "1" ]
 then
     echo "Good choice!"
 else
-    echo "Nah, that can't be right! It must be an error!" > /dev/stderr
+    echo "Nah, that can't be right! It must be an error!" 1>&2
 fi
 ```
 
 Copy this code into a file called `which-os.sh`.
 
-Now run `chmod u+x which-os.sh`. Then run `./which-os.sh`.
+Now run `chmod u+x which-os.sh`. Then run `./which-os.sh`. Don't worry, everything will be explained afterwards.
 
-<!-- TODO: Shebang -->
+After running the script, you will see a prompt asking you to enter a number corresponding to an operating system. If you choose Linux, you get the output "Good choice". This output is in the standard output.
 
-<!-- TODO: /usr/bin/python3 -->
+If you don't choose Linux, you get the output "Nah, that can't be right! (...)". This output is redirected to the standard error.
+
+We did learn that we can redirect to a file using `>`. The syntax `1>&2` redirects the standard output to the standard error. On the other hand. `2>&1` redirects the standard error to the standard output.
+
+If you want to redirect both the standard output and error to a file, you can use this syntax: `COMMAND > FILE 2>&1`.
+
+The redirection **order** is important in this case! `COMMAND 2>&1 > FILE` will redirect the standard output **only** to the file `FILE`!
+
+`COMMAND > FILE 2>&1` has a useful shortcut: `COMMAND &> FILE`. You can also use `&>>` to append.
+
+All other aspects of the script above will be explained in the next sections.
+
+## Shebang
+
+The first line of our first script starts with `#!` which is called the _shebang_. The shebang is followed by the program that runs the script. Since the script is a bash script, we use the program `bash` to run it. But writing `bash` after the shebang is not enough. We have to specify the path to the program. We can find out the path of the program by using the command `which`:
+
+```console
+$ which bash
+/usr/bin/bash
+```
+
+You can also write a Python script and add a shebang at its beginning. We can find out the path to the Python program:
+
+```console
+$ which python3
+/usr/bin/python3
+```
+
+This means that we can now write this script:
+
+```python
+#!/usr/bin/python3
+
+print("Hello world!")
+```
+
+Lets save this tiny Python script as `hello_world.py`, make it executable (will be explained later) and then run it:
+
+```console
+$ chmod u+x hello_world.py
+$ ./hello_world.py
+Hello world!
+```
+
+We could have written the Python script without the shebang, but then we would have to run with `python3 hello_world.py`. Adding the shebang lets you see a script as a program and ignore what language it is written in when running it.
+
+You can use the shebang with any program that runs a script (interpreter).
 
 <!-- TODO: Variables -->