What git diff actually does
git diff is a command that compares two states of a Git repository — typically your working tree against the last commit, or two branches, or two commits. It outputs a unified diff: a standardised text format that shows removed lines prefixed with a minus sign and added lines prefixed with a plus sign, along with a few lines of context around each change.
git diff is deeply integrated into Git. It understands your repository structure, can diff individual files or entire directories, and pipes into other tools like git log, git show, and git stash. It is the canonical way to see what changed in a codebase under version control.
What an online diff tool does differently
An online diff tool like FreeDiffChecker takes two raw inputs — paste, upload, or type — and computes a diff without any knowledge of version history, branches, or repositories. It is stateless: you give it two texts, it gives you the differences.
This sounds less powerful, but it covers a large category of situations that git diff simply cannot handle: files not in a Git repository, content copied from emails or PDFs, configurations pasted from a server, two JSON API responses you captured manually, spreadsheets, images, or Word documents. Git is a version control system; it only knows about files that have been committed to a repository.
Use git diff when
You are working within a Git repository. git diff is always the right tool when both versions of the file are already tracked by Git. It is faster, terminal-native, and integrates with your existing workflow.
You want to see uncommitted changes. Running git diff with no arguments shows what has changed in your working tree since the last commit — the canonical "what did I change just now?" check.
You need to compare branches or commits. git diff main..feature shows every change between two branches. git diff abc123..def456 compares two specific commits. These are operations that require repository context that an online tool cannot replicate.
You are in a CI/CD pipeline. Automated scripts that need to detect changed files should use git diff or git diff --name-only — it integrates cleanly with shell scripting and produces machine-readable output.
Use an online diff tool when
The files are not in a Git repository. Configuration files on a production server, documents received from a client, exported reports, content pasted from an email — none of these are in Git. An online diff tool accepts any two pieces of text.
You want to share the diff with someone who is not technical. A colour-coded side-by-side diff in a browser is much easier for a non-developer to read than a unified diff in a terminal. FreeDiffChecker's Share feature generates a link anyone can open without any tools installed.
You are comparing non-text content. git diff cannot compare images, PDFs, or Excel files. Online diff tools purpose-built for those formats can.
You want to compare content across systems. Copying the old version from one system and the new version from another and pasting both into a browser diff tool is faster than setting up a temporary Git repo.
You are on a machine without Git installed. This is increasingly common on shared servers, client machines, or in browser-based development environments.
The hybrid approach: git diff output into an online tool
One underused workflow is to use git diff to extract the raw content of two file versions and then paste them into a visual diff tool for easier reading.
Run: git show HEAD:path/to/file.txt > old.txt and git show HEAD~1:path/to/file.txt > new.txt, then upload both to FreeDiffChecker. This gives you the version-controlled content with a much richer visual display than the terminal — useful when reviewing a complex change with a colleague who prefers a browser view.
Summary
Use git diff when you are in a repository and want to see changes in tracked files — it is faster and integrates with your workflow. Use an online diff tool when the content is outside Git, when you want to share results, when you need to compare non-text formats, or when you want a visual layout that is easier to read than terminal output. The two tools are complementary, not competing.