What Is a Diff?
A diff (short for difference) is a comparison between two pieces of text that shows exactly what changed. Diffs are a foundational concept in software development — they power version control systems like Git, code review tools, and deployment pipelines. But diffs are useful far beyond code. Anyone who needs to compare two versions of a document, configuration file, database query, or even plain text notes can benefit from a diff tool.
The concept was popularized by the Unix diff command, created in the early 1970s. It compares two files line by line and outputs the differences in a standardized format. Modern diff tools build on this foundation with color-coded highlighting, side-by-side views, and word-level granularity that makes changes easy to spot at a glance.
Understanding Unified Diff Format
The most common diff format you will encounter is the unified diff format, used by Git and most modern tools. It shows changes with context lines and uses + and - prefixes to indicate additions and deletions:
--- original.txt
+++ modified.txt
@@ -1,5 +1,5 @@
const config = {
- port: 3000,
+ port: 8080,
host: "localhost",
- debug: false,
+ debug: true,
};Lines starting with - (shown in red) were removed from the original text. Lines starting with + (shown in green) were added in the modified text. Lines without a prefix are unchanged context lines that help you locate the changes within the file. The @@ line (called a hunk header) tells you the line numbers where the changes occur.
Use Cases for Text Comparison
Code Review
Code review is the most common use case for diffs. When a developer submits a pull request, reviewers look at the diff to understand what changed. A good diff tool highlights not just which lines changed, but which specific words or characters within those lines were modified. This word-level highlighting is invaluable when reviewing changes to long lines of code or configuration values.
Configuration Comparison
System administrators and DevOps engineers frequently need to compare configuration files across different environments. For example, comparing the production Nginx config with the staging config to identify differences in upstream servers, timeouts, or SSL settings. A text diff tool makes these differences immediately visible.
Document Versioning
Writers, editors, and legal professionals use diffs to compare document versions. When a contract is revised, a diff shows exactly which clauses were added, removed, or modified. This is especially important for legal documents where even a single word change can have significant implications.
Database Query Comparison
When optimizing SQL queries, comparing the before and after versions helps you verify that only the intended changes were made. A diff can reveal accidental changes to WHERE clauses, JOIN conditions, or column selections that could produce incorrect results.
How to Use the PulpMiner Text Diff Tool
Using the tool is straightforward. Paste your original text in the left editor and your modified text in the right editor. Click "Compare" and the tool will instantly highlight the differences between the two texts. Additions are highlighted in green, deletions in red, and modifications show the old text in red and the new text in green.
The tool supports both side-by-side and inline diff views. The side-by-side view shows both versions next to each other with aligned line numbers, making it easy to scan through changes. The inline view interleaves deletions and additions in a single column, which is more compact and matches the traditional unified diff format.
Tips for Effective Comparisons
- Normalize whitespace first: If you only care about content changes, normalize indentation and trailing spaces before comparing. This prevents whitespace-only changes from cluttering the diff.
- Use consistent line endings: Mixing Windows (CRLF) and Unix (LF) line endings will produce false positives. Convert both texts to the same line ending format first.
- Sort unordered content: If you are comparing lists, environment variables, or configuration entries where order does not matter, sort both texts alphabetically before comparing.
- Break long lines: Very long lines make diffs hard to read. If possible, break long lines into shorter ones before comparing.
- Focus on meaningful changes: Ignore auto-generated content like timestamps, build hashes, or line numbers that change on every version.
Diff Algorithms
Under the hood, diff tools use algorithms to find the longest common subsequence (LCS) between two texts and then compute the minimal set of changes needed to transform one into the other. The classic Myers diff algorithm, used by Git, produces optimal diffs with O(ND) time complexity where N is the total length and D is the number of differences. For most practical use cases, this algorithm is fast enough to run in real time on texts with thousands of lines.