Update question

_episodes/05-directory-structure.md

@@ -3,11 +3,15 @@ title: Directory structure
 teaching: 5
 exercises: 0
 questions:
-- "FIXME"
+- "Understanding the concept of Unix directory structure"
 objectives:
 - "FIXME"
 keypoints:
 - "FIXME"
 ---
 
+All Unix files are integrated in a single directory structure. The file-system is arranged in a structure like an inverted tree. The top of this tree is the root and is written as a slash ‘/’.
+
+## The `tree` command
+
 FIXME