How to Compare Two Text Files or Code Snippets Online

Whether you are reviewing code changes, comparing two versions of a document, or debugging a configuration file, a diff checker is one of the most useful tools in a developer's toolkit. This guide explains how diff works and how to get the most from it.

What is a Diff?

A diff (from "difference") is a comparison between two text inputs that highlights what has been added, removed, or changed. The concept comes from the Unix diff command, used since the 1970s for comparing files.

Line Diff vs Word Diff

Line diff

Compares entire lines. A line is marked as removed and replaced with the new version — useful for code and configuration files where line structure matters.

Word diff

Highlights individual word or character changes within lines — useful for documents and prose where you care about specific word changes.

Practical Use Cases

Code review

Quickly see what changed between two versions of a function without reading every line.

Configuration comparison

# Original nginx.conf
worker_processes 1;
keepalive_timeout 65;

# Modified nginx.conf
worker_processes 4;     ← changed
keepalive_timeout 120;  ← changed
gzip on;               ← added

Document versioning

Compare two drafts of a contract or policy document to see exactly what was changed between versions.