Scanning for Changes

Detecting System Modifications

The git diff command is your scanner—it shows exactly what changed between different states of your files. Essential for reviewing work before committing.

Basic Diff Usage

See unstaged changes:

git diff

This compares your working directory to the staging area.

See Staged Changes

git diff --staged

Shows what's in the staging area vs. last commit.

Reading Diff Output

diff --git a/navigation.js b/navigation.js
index 3a8f2c1..7b9d4e2 100644
--- a/navigation.js
+++ b/navigation.js
@@ -10,7 +10,7 @@ function calculateCourse() {
     const distance = Math.sqrt(dx*dx + dy*dy);
-    const speed = 1000; // meters per second
+    const speed = 1500; // increased for faster travel
     return distance / speed;

• --- and +++: Old and new file versions
• - lines: Removed (shown in red)
• + lines: Added (shown in green)
• @@ -10,7 +10,7 @@: Line numbers context

Compare Specific Files

git diff navigation.js

Compare Commits

git diff commit1 commit2
git diff HEAD~2 HEAD    # Compare 2 commits ago to now

Compare Branches

git diff main feature-branch

Useful Diff Options

• --stat: Show summary statistics
• --color-words: Highlight changed words, not lines
• --name-only: Just list changed files

Why Use Diff?

• Review your work before committing
• Understand what teammates changed
• Verify fixes worked correctly
• Catch accidental changes

Next: Reviewing History

You can now see what changed. Next, we'll explore git log to review your complete mission history.