Applying a Patch That Removes Lines

Removing Lines with a Unified Diff Patch

When a patch removes lines, those lines appear prefixed with - in the diff. The patch engine matches the removed lines against the original text and deletes them from the output.

Example Original

import React from 'react';
import PropTypes from 'prop-types';
import lodash from 'lodash';

function App() {
  return <div>Hello</div>;
}

Example Patch

--- a/App.js
+++ b/App.js
@@ -1,7 +1,5 @@
 import React from 'react';
-import PropTypes from 'prop-types';
-import lodash from 'lodash';

 function App() {
   return <div>Hello</div>;
 }

Hunk Header Breakdown

@@ -1,7 +1,5 @@ means:

• The original has 7 lines starting at line 1
• The result will have 5 lines starting at line 1
• Net change: 2 lines removed

How Removal Works Internally

The patch engine walks through each line in the hunk:

1. Context lines ( prefix) — kept in the output, must match the original
2. Removed lines (- prefix) — must match the original but are not written to the output
3. Added lines (+ prefix) — written to the output (none in this example)

If a - line does not match the corresponding original line, the hunk fails to apply and the tool reports a context mismatch error.