Have you ever needed to find what changed between two versions of a document? Or compare two similar files to spot differences? That's what diff tools do—they highlight additions, deletions, and modifications between two texts.
What Is a Diff?
A "diff" (short for difference) is a comparison between two pieces of text that shows what changed. The concept originated with the Unix diff command in 1974 and remains fundamental to version control systems like Git.
How Diff Works
Diff algorithms compare texts line by line (or sometimes character by character) and identify:
- Additions: Lines present in the new version but not the old
- Deletions: Lines present in the old version but not the new
- Modifications: Lines that changed (often shown as deletion + addition)
- Unchanged: Lines identical in both versions
Reading Diff Output
Traditional unified diff format looks like this:
--- old_file.txt +++ new_file.txt @@ -1,3 +1,4 @@ Unchanged line -Deleted line +Added line +Another addition Unchanged line
-prefix: Line was removed+prefix: Line was added- No prefix: Line unchanged (context)
@@: Shows line numbers affected
Visual Diff Tools
Modern diff tools use side-by-side or inline views with color coding:
- Red/strikethrough: Deletions
- Green/highlighted: Additions
- Yellow: Modifications
Visual diffs are easier to read than raw unified diff format.
Common Use Cases
Code Review
Reviewing pull requests and commits. See exactly what changed in the code before merging.
Document Comparison
Comparing contract versions, policy updates, or any revised documents to spot changes.
Configuration Files
Identifying differences between development and production configs.
Data Validation
Comparing expected output vs actual output in testing.
Merge Conflict Resolution
Understanding conflicting changes when multiple people edit the same file.
Git Tip: Use git diff to see uncommitted changes, git diff --staged for staged changes, or git diff branch1..branch2 to compare branches.
Character-Level vs Line-Level Diff
Line-level diffs mark entire lines as changed even if only one character differs. Character-level diffs highlight the specific changed characters within lines—more precise but sometimes harder to read.
Most modern tools offer both or a hybrid approach.
Limitations of Diff
- Moved text: If you move a paragraph, it shows as deletion + addition, not a move
- Formatting changes: Whitespace differences can create noise
- Binary files: Can't meaningfully diff images or compiled code
- Large differences: If files are very different, diff becomes less useful
Tips for Effective Comparison
- Compare clean, consistently formatted text when possible
- Use ignore whitespace options if formatting varies
- For large files, look at summary statistics first
- When diffing code, ensure both versions compile/run
Compare Text Instantly
Side-by-side diff tool with character-level highlighting.
Open Diff Checker →Conclusion
Diff is one of computing's most useful inventions. It makes version control possible, simplifies code review, and helps anyone track changes in text. Master diff tools and you'll save countless hours hunting for changes manually.