Git Diff — The Silent Merge That Deleted 200 Lines of Error Handling

Git diff is the single most important command for code review and debugging. Every PR diff is powered by git diff. Every merge conflict shows you a three-way diff. Every bisect session uses diff to verify the fix. Yet most developers only use the default git diff output, missing powerful flags that catch silent regressions. This guide covers the essential diff modes, the incident where a PR diff hid 200 deleted lines, and how to use diff to prevent invisible changes from reaching production.

Diff Modes and When to Use Each

git diff (no arguments) shows unstaged changes — working tree vs index. git diff --staged (or --cached) shows staged changes (index vs HEAD). git diff <commit> shows changes since a specific commit. git diff <commit1>..<commit2> compares any two arbitrary commits.

For branches: git diff main..feature shows what's in feature that isn't in main. git diff main...feature (three dots) shows changes since the common ancestor — this is what GitHub PRs use.

Use --stat for a summary of changed files and line counts. Use --name-only to list only file paths. Use --name-status to include what type of change (A=added, D=deleted, M=modified, R=renamed).

# Basic diffs
git diff                          # working tree vs index
git diff --staged                # index vs HEAD (staged changes)
git diff HEAD                    # working tree vs last commit

# Compare branches
git diff main..feature           # what feature has that main doesn't
git diff main...feature          # since common ancestor (like GitHub PR)

# Summary views
git diff --stat main...feature   # changed files with line counts
git diff --name-only main..HEAD  # just file paths
git diff --name-status main..HEAD  # A/D/M per file

# Compare specific file
git diff main...feature -- src/app.py

Word Diff, Ignore Whitespace, and Catching Invisible Changes

git diff --word-diff highlights changes at the word level instead of line level. This is ideal for documentation, prose, or when a single line has both additions and deletions. --word-diff-regex= allows custom word boundaries.

git diff -w ignores whitespace changes (spaces, tabs, indentation). --ignore-space-change ignores whitespace-only changes but still shows whitespace that affects meaning. --ignore-blank-lines ignores empty line changes.

git diff --check checks for whitespace errors (trailing spaces, spaces before tabs) and lists them. git diff --color-moved highlights moved blocks of code with different colors — crucial for refactors where code is reorganized.

# Word-level diff (not line-level)
git diff --word-diff

# Ignore whitespace changes
git diff -w                       # ignore all whitespace
git diff --ignore-space-change    # ignore whitespace-only changes

# Check for whitespace errors
git diff --check

# Show moved code blocks in different color
git diff --color-moved

# Show function context around changes
git diff --function-context

# Show staged changes with 10 lines of context
git diff --staged -U10