How to create and apply a patch with Git

If you are an upstream developer or software engineer who works with Open Source Projects, definitely you have to use Git for collaborating and contributing your codes to upstream source codes. This article will show you the way to create and apply a patch with Git.

1. Create a branch for development

If you want to commit your codes to fix a bug or update some features, creating a new branch is the best solution for that. For example, let clone my sandbox repository:

$ git clone https://github.com/truongnh1992/sandbox.git
$ cd sandbox

Create a new branch: mybranch

$ git checkout -b mybranch
Switched to a new branch 'mybranch'

$ git branch -a
  master
* mybranch
  remotes/origin/HEAD -> origin/master
  remotes/origin/master
  remotes/origin/test

Update source codes from master branch

$ git checkout master
$ git pull origin master
$ git checkout mybranch
$ git rebase -i master

2. Create the patch

In the new branch mybranch, let update source codes, readme,… and whatever you want to fix. I’ve just created a commit.

$ git log
commit d0e4fcb56e2eeca01296cb9f9b78b7548280d517
Author: Nguyen Hai Truong <truongnh@vn.fujitsu.com>
Date:   Tue Oct 9 20:01:52 2018 -0700

    Update readme

    Signed-off-by: Nguyen Hai Truong <truongnh@vn.fujitsu.com>

Now, it’s time to create a patch for above commit.

$ git format-patch -1 --stdout > my_patch.patch

It will create a patch file has name my_patch.patch in the current directory.

$ vim my_patch.patch

From d0e4fcb56e2eeca01296cb9f9b78b7548280d517 Mon Sep 17 00:00:00 2001
From: Nguyen Hai Truong <truongnh@vn.fujitsu.com>
Date: Tue, 9 Oct 2018 20:01:52 -0700
Subject: [PATCH] Update readme

Signed-off-by: Nguyen Hai Truong <truongnh@vn.fujitsu.com>
---
 readme.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 readme.md

diff --git a/readme.md b/readme.md
new file mode 100644
index 0000000..5c2920f
--- /dev/null
+++ b/readme.md
@@ -0,0 +1 @@
+Test repository
--
2.7.4

By default, Git create a separate patch file for each commit.

3. Apply the patch

Verify that the patch will change the source codes?

$ git apply --stat my_patch.patch

 readme.md |    1 +
 1 file changed, 1 insertion(+)

 $ git apply --check my_patch.patch

The above command will not apply the patch, it only shows you the stats about what it will do? In order to apply the patch, you can use the command: git apply

$ git apply my_patch.patch