Unified Diff Format for Patch Files and Version Control

Unified Diff Format

The unified diff format is the standard output format for git diff, diff -u, and patch files. It combines both versions into a single stream with line prefixes indicating additions (+), deletions (-), and unchanged context lines (space).

Anatomy of a Unified Diff

--- a/config.json
+++ b/config.json
@@ -5,7 +5,8 @@
   "debug": false,
   "port": 3000,
-  "host": "localhost",
+  "host": "0.0.0.0",
+  "cors": true,
   "timeout": 30
 }

Header Lines

• --- a/file — the original file path
• +++ b/file — the modified file path
• @@ -5,7 +5,8 @@ — the hunk header indicating line ranges: starting at line 5 with 7 lines in the original, and starting at line 5 with 8 lines in the modified

Line Prefixes

Each line in a hunk is prefixed with exactly one character:

Prefix	Meaning
(space)	Context — unchanged line shown for reference
-	Deletion — line removed from original
+	Addition — line added in modified version

Context Lines

By default, git diff shows 3 context lines around each change. Context lines help readers understand where the change occurs within the file. You can adjust this with git diff -U<n> where n is the number of context lines.

Multiple Hunks

A single file diff can contain multiple hunks when changes occur in different parts of the file. Each hunk has its own @@ header and is separated by context lines.

Reading Hunk Headers

The hunk header @@ -10,4 +10,6 @@ means:

• Original: starts at line 10, spans 4 lines
• Modified: starts at line 10, spans 6 lines (2 lines were added)