update variable expansion

docs/04-redirection.md

@@ -593,16 +593,16 @@ Inside our for loop, we create a new name variable. We call the basename functio
     > done
     ```
 
-    Parentheses `()`
+    Why Parentheses `()` ?
 
-    - Command Substitution: The `$(...)` syntax is used for command substitution. It allows you to execute a command and use its output as a value in your script. In this case, $(basename ${filename} .fastq) executes the basename command and captures its output, which is then assigned to the variable name.
-    - Clarity: Using `$(...)` is preferred over backticks (`...`) for command substitution because it is more readable and can be nested more easily.
+    - **Command Substitution:** The `$(...)` syntax is used for command substitution. It allows you to execute a command and use its output as a value in your script. In this case, $(basename ${filename} .fastq) executes the basename command and captures its output, which is then assigned to the variable name.
+    - **Clarity:** Using `$(...)` is preferred over backticks (`...`) for command substitution because it is more readable and can be nested more easily.
 
-    Curly Braces {}
+    Why Curly Braces `{}` ?
 
-    - Variable Expansion: The `${filename}` syntax is used to clearly indicate that you are referencing the variable `filename`. This is particularly useful in cases where the variable name might be adjacent to other characters that could be interpreted as part of the variable name.
-    - Disambiguation: For example, if you had a variable named `filename_suffix`, writing `$filename_suffi`x could lead to confusion if you were trying to concatenate or manipulate it with other strings. Using `${filename}` makes it clear where the variable name ends.
-    - Consistency: While not always required, using curly braces for variable expansion is a common practice that enhances readability and reduces the risk of errors.
+    - **Variable Expansion:** The `${filename}` syntax is used to clearly indicate that you are referencing the variable `filename`. This is particularly useful in cases where the variable name might be adjacent to other characters that could be interpreted as part of the variable name.
+    - **Disambiguation:** For example, if you had a variable named `filename_suffix`, writing `$filename_suffi`x could lead to confusion if you were trying to concatenate or manipulate it with other strings. Using `${filename}` makes it clear where the variable name ends.
+    - **Consistency:** While not always required, using curly braces for variable expansion is a common practice that enhances readability and reduces the risk of errors.
 
 !!! dumbbell "Exercise"