Explain variables

src/day_2/shell_scripting.md

@@ -16,13 +16,17 @@ echo "1. Linux"
 echo "2. Windows"
 echo "3. Mac"
 
+RIGHT_ANSWER=1
+
+# The option -n does not print a new line at the end
 echo -n "Enter a number: "
 read ANSWER
 
-if [ "$ANSWER" == "1" ]
+if [ $ANSWER == $RIGHT_ANSWER ]
 then
     echo "Good choice!"
 else
+    # Any answer other than 1
     echo "Nah, that can't be right! It must be an error!" 1>&2
 fi
 ```
@@ -81,15 +85,114 @@ We could have written the Python script without the shebang, but then we would h
 
 You can use the shebang with any program that runs a script (interpreter).
 
-<!-- TODO: Variables -->
+## Variables
 
-<!-- TODO: $ -->
+In our first bash script, we have the line `RIGHT_ANSWER=1`. This line defines a variable with the name `RIGHT_ANSWER` and the value `1`.
 
-<!-- TODO: Escaping '$' -->
+To define a variable in bash, you have to write the name of the variable _directly_ followed by an equal sign `=`. The equal sign has to be _directly_ followed by the value of the variable.
 
-<!-- TODO: ${} -->
+_directly followed_ means that spaces between the variable name, the equal sign `=` and the value are not allowed!
 
-<!-- TODO: $() -->
+But what if you want to set a variable equal to a string that contains spaces?
+
+In this case, you have to use quotation marks `"`. For example:
+
+```bash
+HELLO="Hello world!"
+```
+
+To use a defined variable, we use a dollar sign `$` before the variable name. For example:
+
+```bash
+echo $HELLO
+```
+
+The line above would output `Hello world!`.
+
+You can use defined variable inside a variable definition:
+
+```bash
+MESSAGE="Tux says: $HELLo"
+echo $MESSAGE
+```
+
+The two lines above would lead to the output `Tux says: Hello world!`.
+
+## Capturing command's output
+
+You can capture the standard output of a command using the the syntax `$(COMMAND)`. Example:
+
+```bash
+BASH_VERSION=$(bash --version)
+```
+
+The line above saves the output of the command `bash --version` in the variable `BASH_VERSION`.
+
+Lets run the command in the terminal first to see its output:
+
+```console
+$ bash --version
+GNU bash, version 5.1.16(1)-release (x86_64-redhat-linux-gnu)
+Copyright (C) 2020 Free Software Foundation, Inc.
+License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
+
+This is free software; you are free to change and redistribute it.
+There is NO WARRANTY, to the extent permitted by law.
+```
+
+Now lets output the variable that we did define above:
+
+```console
+$ echo $BASH_VERSION
+GNU bash, version 5.1.16(1)-release (x86_64-redhat-linux-gnu) Copyright (C) 2020 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software; you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
+```
+
+You can see that the lines are squashed into one line! If you want to output the lines without them beeing squashed, you have to use quotation marks `"`:
+
+```console
+$ echo "$BASH_VERSION"
+GNU bash, version 5.1.16(1)-release (x86_64-redhat-linux-gnu)
+Copyright (C) 2020 Free Software Foundation, Inc.
+License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
+
+This is free software; you are free to change and redistribute it.
+There is NO WARRANTY, to the extent permitted by law.
+```
+
+This is the output that we did expect 😃
+
+## Temporary variables
+
+Lets write the following tiny script `hello.sh`:
+
+```bash
+#!/usr/bin/bash
+
+echo "Hello $USER!"
+```
+
+Now we run the script. The output on my machine is:
+
+```console
+$ chmod u+x hello.sh
+$ ./hello.sh
+Hello mo!
+```
+
+We see `Hello mo!` as output. This is because my user name on my machine is `mo`. `USER` is a so called environment variable that is defined for all programs. If you run the script on your machine, you will get your user name instead of `mo`. There are more environment variables like `PATH` which we will learn about later.
+
+Now lets run the following:
+
+```console
+$ USER=Tux ./hello.sh
+Hello Tux!
+```
+
+We did define a temporary variable `USER` with the value `Tux`. This temporary variable overwrites the environment variable `USER` in our script. Therefore we get the output `Hello Tux!` and not our user name.
+
+You can use temporary variables not only to overwrite environment variables. These are basically variables that can be used by the program that you specify after their definition.
+
+<!-- Comments -->
 
 <!-- TODO: read -->
 
@@ -115,6 +218,8 @@ You can use the shebang with any program that runs a script (interpreter).
 
 <!-- TODO: Long command on multiple lines \ -->
 
+<!-- Why ./SCRIPT_NANEM -->
+
 <!-- TODO: Permissions -->
 
 <!-- TODO: r,w,x -->