Q: What's the difference between `git diff`, `git diff --staged`, and `git diff HEAD`? When would you use each?

Git has three important snapshots: the last commit (HEAD), the staging area (index), and the working directory. Each diff variant compares two of these:

• git diff — working directory vs staging area. Shows changes you've made but haven't staged yet.
• git diff --staged (or --cached) — staging area vs HEAD. Shows what will go into the next commit if you run git commit right now.
• git diff HEAD — working directory vs HEAD. Shows *all* changes since the last commit, whether staged or not.

Other useful variations: For reviewing what you're about to commit, the workflow is: git diff to review unstaged changes → git add -p to selectively stage → git diff --staged to verify what's staged → git commit.

git diff main..feature     # diff between two branches
git diff HEAD~3..HEAD      # last 3 commits' changes
git diff --stat            # summary (files changed, insertions, deletions)
git diff --name-only       # just filenames
git diff -- path/to/file   # limit to specific file