# Module 02: Viewing History

## Learning Objectives

In this module, you will:
- Understand commit history and how to navigate it
- Use `git log` to view commit history with various formats
- Use `git show` to view specific commit details
- Use `git diff` to compare changes between commits
- Understand commit hashes and references

## Challenge

### Setup

Run the setup script to create your challenge environment:

```powershell
.\setup.ps1
```

This will create a `challenge/` directory with a Git repository that already has some commit history.

### Your Task

You'll explore an existing Git repository that contains multiple commits. Your goal is to use Git commands to discover information about the repository's history.

The setup script will create an `answers.md` file in the challenge directory with questions for you to answer. Fill in your answers directly in that file.

**Suggested Approach:**

1. Navigate to the challenge directory: `cd challenge`
2. Open `answers.md` to see the questions
3. View the commit history: `git log`
4. Try different log formats: `git log --oneline`, `git log --stat`
5. View specific commits: `git show <commit-hash>`
6. Compare commits: `git diff <commit1> <commit2>`
7. Fill in your answers in `answers.md`

> **Important Notes:**
> - You can use any Git commands you like to explore the repository
> - Fill in your answers directly in the `answers.md` file (there are placeholder sections for each answer)
> - Try different `git log` options to see which format you prefer
> - Commit hashes can be referenced by their full hash or just the first 7 characters

## Key Concepts

- **Commit Hash**: A unique identifier (SHA-1 hash) for each commit. You can use the full hash or just the first few characters.
- **Commit Message**: A description of what changed in that commit, written by the author.
- **Commit History**: The chronological record of all changes made to a repository.
- **HEAD**: A pointer to the current commit you're working from.
- **Diff**: A view showing the differences between two versions of files.

## Understanding Diff Output

When you run `git diff` between commits, the output can look confusing at first. Here's how to read it:

### Example Diff Output

```diff
diff --git a/app.py b/app.py
index 1a2b3c4..5d6e7f8 100644
--- a/app.py
+++ b/app.py
@@ -1,5 +1,7 @@
 # app.py - Main application file
+from auth import login, logout

 def main():
     print("Welcome to My App!")
-    # Application initialization code here
+    login("user", "password")
     pass
```

### Breaking It Down

**1. File Header**
```diff
diff --git a/app.py b/app.py
```
- Shows which file is being compared
- `a/app.py` = old version (before)
- `b/app.py` = new version (after)

**2. Metadata**
```diff
index 1a2b3c4..5d6e7f8 100644
--- a/app.py
+++ b/app.py
```
- `---` indicates the old version
- `+++` indicates the new version
- The hashes (1a2b3c4, 5d6e7f8) are internal Git identifiers

**3. Change Location (Hunk Header)**
```diff
@@ -1,5 +1,7 @@
```
- `@@ -1,5 +1,7 @@` tells you where changes occurred
- `-1,5` = in the old file, starting at line 1, showing 5 lines
- `+1,7` = in the new file, starting at line 1, showing 7 lines
- The file grew by 2 lines (from 5 to 7)

**4. The Actual Changes**

Lines are prefixed with symbols:
- ` ` (space) = unchanged line (context)
- `-` (minus) = line removed from old version (shown in red in terminal)
- `+` (plus) = line added in new version (shown in green in terminal)

In our example:
```diff
 # app.py - Main application file          ← unchanged
+from auth import login, logout             ← added (new)

 def main():                                ← unchanged
     print("Welcome to My App!")            ← unchanged
-    # Application initialization code here ← removed (old)
+    login("user", "password")              ← added (new)
     pass                                   ← unchanged
```

### Reading Multiple Files

If multiple files changed, you'll see multiple diff sections:

```diff
diff --git a/app.py b/app.py
[changes to app.py]

diff --git a/auth.py b/auth.py
[changes to auth.py]
```

### Pro Tips

- **Context lines**: Unchanged lines around changes help you understand where the change happened
- **Color coding**: In your terminal, deletions are usually red, additions are green
- **No newline warning**: If you see `\ No newline at end of file`, it means the file doesn't end with a newline character (usually not important for beginners)
- **Binary files**: For images or other binary files, Git just says "Binary files differ"

### Try It Yourself

In this module's challenge, you'll use:
```bash
git diff <commit1> <commit2> app.py
```

Pay attention to:
- Which lines were added (green, with `+`)
- Which lines were removed (red, with `-`)
- The surrounding context (white, with space)

## Useful Commands

```bash
git log                      # View commit history
git log --oneline            # Compact one-line format
git log --stat               # Show files changed in each commit
git log --graph              # Show branch graph (more useful with branches)
git show <commit>            # View specific commit details
git show <commit>:<file>     # View a file from a specific commit
git diff <commit1> <commit2> # Compare two commits
git diff <commit>            # Compare commit with current working directory
```

## Verification

Once you've filled in your answers in `answers.md`, verify your solution:

```powershell
.\verify.ps1
```

The verification script will check that your answers contain the expected information.

## Need to Start Over?

If you want to reset the challenge and start fresh:

```powershell
.\reset.ps1
```

This will remove the challenge directory and run the setup script again, giving you a clean slate.