How to apply a unified diff with git apply or patch

The example changes two settings in a TypeScript config file. The file sits at src/config.ts in the project, so that's the name to put in File name in the patch. The tool then writes this:

--- a/src/config.ts
+++ b/src/config.ts
@@ -1,5 +1,5 @@
 export const config = {
-  apiUrl: "https://api.example.com",
-  retries: 3,
+  apiUrl: "https://api.example.com/v2",
+  retries: 5,
   timeoutMs: 5000,
 };

Save it in the root folder of the repository, the folder that holds src, and run these commands there:

git apply --check change.patch
git apply change.patch

The first command only tests the patch. It prints nothing when the patch fits and an error when it doesn't, without touching your files. The second one applies it. git apply changes the files in your working folder and doesn't stage or commit anything, so review the result with git diff and commit it as usual.

Without git, the classic patch tool does the same job:

patch -p1 < change.patch

The -p1 option strips the first folder from each path, which turns a/src/config.ts into src/config.ts. Add --dry-run to test it first.

When a patch doesn't apply, it's usually one of these:

• The path is wrong. The name after a/ and b/ has to be the file's path from the folder you run the command in, which for git is the root of the repository.
• The file changed. Git checks that the context lines and the removed lines match your copy exactly. If someone edited those lines since the diff was made, make the diff again from the current file.
• The spacing is different. A patch copied with Ignore whitespace on keeps the original spacing in its context lines, so it only fits the original text.