git diff

Compare any two versions of your content.

git diff

What it does

git diff highlights every difference between two states: working tree vs index, index vs HEAD, or any two arbitrary commits. It's the same engine that colors pull requests red and green. Read-only — it never changes a thing.

@@ -1,3 +1,3 @@ src/App.tsxMyers diff → edit script

const header = 'OLD'

const header = 'NEW'

const nav = [ 'home', 'about' ]

const nav = [ 'home', 'about', 'blog' ]

return <div>

Two blob versions, reduced to an edit script

Watch it — press play

A narrated, step-by-step walkthrough that shows how to do it — not just theory. It adapts to the learner mode you picked in the top bar.

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Step 1

What did I touch?

git diff compares your working files against the index — every edit you haven't staged yet.

git diff

@@ -1,3 +1,3 @@ src/App.tsxMyers diff → edit script

const header = 'OLD'

const header = 'NEW'

const nav = [ 'home', 'about' ]

const nav = [ 'home', 'about', 'blog' ]

return <div>

Worktree vs index = the red and green you just made

Under the hood

Blob-to-blob, using their SHA-1 hashes

Git compares content by hashing. If both blobs are identical, the hash matches and the file is skipped instantly. Only differing pairs enter the diff algorithm.

The Myers diff algorithm

Diffing is finding the shortest edit script between two sequences. Git uses the Myers diff (in the xdiff library): it hunts the minimal set of insertions/deletions to turn one blob into the other, capably finding moved-but-identical lines.

Hunks, not files

The output is grouped into hunks — contiguous changed regions — prefixed with @@ -old +new @@ line numbers. This is the exact format git apply consumes, which is how patches become commits.

The three comparison modes

Plain `git diff` compares the working tree against the index. `git diff --cached` compares the index against HEAD (what you've staged). `git diff A B` compares any two revisions.

Command anatomy

git diff [<revA>] [<revB>]

--cached

Diff the index against HEAD — what your next commit will do.

--stat

Just which files change, as a compact bar chart.

--name-only

Only the changed file names, nothing else.

--word-diff

Highlight changes word-by-word instead of line-by-line.

When to reach for it

Reviewing every edit you've made before staging: git diff.

Double-checking exactly what a commit changed: git diff HEAD~1 HEAD.

A quick sanity diff between two branches: git diff main..feature.

Practice

A real repo, pre-seeded with history, runs right here — stage, commit, branch, stash, diff, even hit and resolve a real merge conflict. Nothing to install, nothing leaves your machine.

Open practice playground

Go deeper with

git status

Compare the working tree, the index, and the last commit.

git add

Copy your edits into the staging area (the index).

git blame

Attribute every line of a file to the commit and author that made it.

git log

Walk the chain of parent links and read your history.