git diff

Working tree, index, commits, branches — all comparable.

🎯 What & why

Show changes between two trees. With no arguments: working tree vs index. With one ref: working tree vs that ref. With two refs: between refs. With --cached: index vs HEAD. The same primitive backs every other 'what changed?' answer.

🧠 Mental model

Diff is a tree-to-tree operation. The 'tree' on either side may be HEAD's tree, the index, or your working tree. The output is a unified diff — a textual delta — but it's computed from object-database trees, not from text files. That's why git diff is fast on huge repos: it's comparing tree objects, not running diff(1) on every file.

🛠️ Synopsis

git diff [<options>] [<commit>] [--] [<path>...]
git diff [<options>] --cached [<commit>] [--] [<path>...]
git diff [<options>] <commit> <commit> [--] [<path>...]
git diff [<options>] <blob> <blob>

🎚️ Switches & options

Flag	What it does
`--cached, --staged`	Diff index vs HEAD — what would `git commit` write?
`--stat`	Show file-level summary instead of full hunks.
`--name-only`	Only file names that changed.
`--name-status`	File names plus an A/M/D letter.
`-w, --ignore-all-space`	Ignore whitespace differences entirely.
`-U<n>`	Lines of context around each hunk. Default 3.
`--word-diff`	Show changes at the word level instead of line level.
`--color-moved`	Highlight moved blocks in a different color. Surprisingly useful.
`--minimal`	Spend more CPU to produce a smaller diff. For review.

💡 Use cases

What did I just change? git diff.
What am I about to commit? git diff --cached.
What changed since main diverged? git diff main...feature.
Just the file names? git diff --name-only main..feature.

🧪 Examples

Working tree vs index.

$ git diff

Index vs HEAD.

$ git diff --cached

Between two branches.

$ git diff main..feature

Between two branches via merge-base (= 'what's new on feature?').

$ git diff main...feature

Just the changed files, with status letters.

$ git diff --name-status HEAD~5

🎓 Recommendations

Memorize the four common forms: diff, diff --cached, diff <ref>, diff <a>..<b>.
Use ... (three dots) when comparing branches: it diffs from the merge-base, which is what you usually mean.
Set diff.algorithm = histogram in config — better hunk grouping than default.
Try --color-moved=zebra for code-review diffs. Once you see it you can't go back.

🪤 Common pitfalls

A..B and A...B are different. Two dots: between commits. Three dots: from their merge-base.
git diff on a binary file says 'binary files differ' and stops. Use --text to force a textual diff (it'll be ugly).
Whitespace-only changes can hide real changes — always check with --no-color | wc or -w toggled.

🔗 Related modules

📝 Quiz

Hit each option, then Check answers. Score is recorded; Next is always open.