Apply a patch from a GitHub PR

Ever found yourself needing that one fix from a PR on GitHub that hasn't been released yet? How can you get the code, for example a Magento fix, into your code base? Copy-paste it? That might work for small changes, but for large and complex changes copy-pasting is no solution. Luckily, we can use GitHub to generate a patch file, which can then be applied with either the git or patch command.

patch-file what?

A patch file is a text file containing instructions on how to apply a change to files. If you run git diff and see the changed files with +/- in front of lines that were added (+) or removed (-) you are looking at a patch file.

Example code

For illustration purpose I created a example repo: https://github.com/KevinGimbel/blog-patch-example.

You can clone the main branch and then apply the patch from the Pull Request to it, if you want to follow along.

Getting the patch file

First we will need to get the patch file. GitHub makes this easy, but for some reason hides it from us. You can append .patch to the URL of any pull request to get the patch file. So for the example above, open the URL https://github.com/KevinGimbel/blog-patch-example/pull/1.patch to see the plain text patch file.

From 6da12536ff4da0efdabdf1a4dd55ded127fa247c Mon Sep 17 00:00:00 2001
From: Kevin Gimbel <hallo@kevingimbel.com>
Date: Fri, 4 Dec 2020 16:31:30 +0100
Subject: [PATCH] docs: add actual URL

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 0fe5e72..7f6398a 100644
--- a/README.md
+++ b/README.md
@@ -2,4 +2,4 @@

 This repo is part of a Blog Post on how to get and apply patch files from GitHub.com

-You can read the full post here: ENTER_URL_HERE
\ No newline at end of file
+You can read the full post here: [https://kevingimbel.de/blog/2020/12/apply-patch-from-github-pr/](https://kevingimbel.de/blog/2020/12/apply-patch-from-github-pr/)
\ No newline at end of file

The patch file is both human and machine readable. Now to continue, get the patchfile!

1. Open a terminal and navigate to the repo you cloned earlier (github.com/KevinGimbel/blog-patch-example)
2. Download the patch file with wget:

$ wget "https://github.com/KevinGimbel/blog-patch-example/pull/1.patch"

This downloads a file named 1.patch into the current directory, which we can verify by running ls -l

ls -l
total 16
-rw-r--r--@ 1 kevingimbel  staff  733 Dec  4 16:33 1.patch  # 👈 there it is
-rw-r--r--  1 kevingimbel  staff  155 Dec  4 16:33 README.md

You can look at the file if you want to, it contains the same text as the example in this article.

Applying a patch with git

We can use git to apply the patch. For manual patching git has the apply command:
If we want to test the changes to see if they can be applied but don't want to change any files yet, we can use the --check and -v (verbose) flag:

$ git apply -v --check 1.patch
Checking patch README.md...

To really apply the patch, we remove the --check flag:

$ git apply -v 1.patch
Checking patch README.md...
Applied patch README.md cleanly.

Now check the README.md file and you'll see the new content!

$ cat README.md 
# blog-patch-example

This repo is part of a Blog Post on how to get and apply patch files from GitHub.com

You can read the full post here: [https://kevingimbel.de/blog/2020/12/apply-patch-from-github-pr/](https://kevingimbel.de/blog/2020/12/apply-patch-from-github-pr/)

Patching without git

Sometimes you may not have git available, especially when patching software running on some server that - for whatever reasons - has no deployment process (no judging here!) or version control.

Even without the git command the patch can still be applied by using the patch tool installed on most (all?) Linux systems.
To do this, follow these steps:

1. Get the patch from the Pull Request by appending .patch to the URL
2. Apply the patch with the patch command

$ patch -p 1 < filename.patch
# To try out changes first, use `--dry-run`
$ patch -p 1 --dry-run < filename.patch

And that's it for today! Feel free to create PRs in the repo https://github.com/KevinGimbel/blog-patch-example/ if you want to play around with patching.

Additional resources

• patch man page
• git apply documentation

This post was cross-posted from kevingimbel.de

Feel free to follow me on Twitter @KevinGimbel for cloud stuff and DevOps/SysOps content. I swear I won't rant every day!