Understanding Hunk Headers in Unified Diffs

Anatomy of a Hunk Header

Every change section in a unified diff starts with a hunk header: @@ -oldStart,oldCount +newStart,newCount @@. This header tells the patch engine exactly where to apply changes.

Header Format

@@ -10,8 +10,12 @@ optional function context

Component	Meaning
-10,8	In the original file, this hunk starts at line 10 and spans 8 lines
+10,12	In the new file, this hunk starts at line 10 and spans 12 lines
optional text	Some tools append the nearest function/class name for readability

Counting Rules

The counts include all lines in the hunk body for their respective side:

• Context lines ( ) count toward both old and new counts
• Removed lines (-) count toward the old count only
• Added lines (+) count toward the new count only

Example Verification

@@ -5,6 +5,8 @@
 context line          (old: 1, new: 1)
 context line          (old: 2, new: 2)
-removed line          (old: 3)
-removed line          (old: 4)
+added line            (new: 3)
+added line            (new: 4)
+added line            (new: 5)
+added line            (new: 6)
 context line          (old: 5, new: 7)
 context line          (old: 6, new: 8)

Old count: 2 context + 2 removed + 2 context = 6 . New count: 2 context + 4 added + 2 context = 8 .

Single-Line Shorthand

When a count is 1, it can be omitted: @@ -5 +5,2 @@ means the old side has 1 line starting at line 5, and the new side has 2 lines starting at line 5.