Git Diff: Compare Changes Between Commits and Branches

What Does git diff Do?

git diff shows the differences between two states in your repository — whether that is the working directory vs. staging area, two commits, two branches, or specific files.

Common Patterns

# Unstaged changes (working directory vs. staging area)
git diff

# Staged changes (staging area vs. last commit)
git diff --staged

# Compare two branches
git diff main..feature-branch

# Compare a specific file between branches
git diff main..feature-branch -- src/app.tsx

# Compare two commits
git diff abc1234..def5678

Understanding the Output

--- a/src/app.tsx
+++ b/src/app.tsx
@@ -10,7 +10,7 @@
 function App() {
-  const count = 0;
+  const count = useState(0);
   return <div>{count}</div>;
 }

Lines prefixed with - were removed; lines with + were added. The @@ header shows the line numbers in the old and new versions.

Useful Flags

# Show only file names that changed
git diff --name-only main..feature

# Show file names with change type (Added, Modified, Deleted)
git diff --name-status main..feature

# Show word-level diffs instead of line-level
git diff --word-diff

# Ignore whitespace changes
git diff -w

# Show stats summary
git diff --stat main..feature

Diffing Against the Merge Base

To see what a branch introduced since it diverged from main:

git diff main...feature-branch  # three dots

The triple-dot syntax compares feature-branch against the merge base (the common ancestor), which is usually what you want during code review. Mastering git diff is essential for understanding exactly what changed and why.