How to Compare Text Files: A Guide to Diff Tools
Comparing two versions of a document is one of the most fundamental operations in writing and programming. Whether you are reviewing code changes, tracking document revisions, or checking what someone edited in your draft, a diff tool shows you exactly what changed.
- Learn how text diff tools work, when to use them, and best practices for comparing documents, code, configurations, and content revisions.
- Covers 1. how diff works.
- Covers 2. when to use text diff.
- Covers 3. how to read a diff.
- Covers 4. comparison options.
1. How Diff Works
A diff algorithm compares two texts line by line (or character by character) and identifies three types of changes: additions (lines in the new text that are not in the old), deletions (lines in the old text that are not in the new), and unchanged lines (present in both).
The algorithm finds the longest common subsequence — the maximum set of lines that appear in both texts in the same order. Everything else is classified as added or removed.
2. When to Use Text Diff
- Code review: See exactly what changed in a pull request or commit.
- Document revision: Track edits between drafts of articles, contracts, or proposals.
- Configuration changes: Spot differences between server configs, environment files, or settings.
- Content auditing: Verify that automated tools or AI edits made only the changes you expected.
3. How to Read a Diff
Green lines (prefixed with +) are additions — text that exists in the new version but not the old. Red lines (prefixed with -) are deletions — text that was in the old version but removed. Gray or unmarked lines are unchanged context.
-webkit-backdrop-filter alongside backdrop-filter for Safari support. Without the prefix, the effect is invisible to roughly 25% of mobile users.Character-level highlighting within changed lines shows exactly which words or characters were modified, making it easy to spot small edits in long lines.
4. Comparison Options
Ignore whitespace
When enabled, differences in spaces, tabs, and indentation are ignored. This is useful for comparing prose where formatting may have changed but content has not. Disable it when whitespace is meaningful (Python code, YAML files).
Ignore case
Case-insensitive comparison treats uppercase and lowercase as identical. Useful for comparing SQL queries, Windows file paths, or content where capitalization is inconsistent but irrelevant.
5. Best Practices
- Compare meaningful units: Compare full documents, not fragments. Context lines help you understand why changes were made.
- Use side-by-side for short texts: It is easier to see corresponding lines. Use inline view for long documents.
- Save diffs for audit trails: When reviewing legal or compliance documents, save the diff output as a record of what changed.