How to Use the Command `git format-patch` (with examples)

Git is a powerful version control system that allows developers to manage changes to source code over time. One of its features, git format-patch, brings forth the capability to generate patch files from commits. These patch files are text representations of changes made in commits, which can be easily emailed, shared, or applied to other repositories. This command is particularly useful in scenarios where direct access to a repository is unavailable, or when one needs to collaborate with others without pushing changes to a central Git repository.

Use Case 1: Create an auto-named .patch file for all the unpushed commits

Code:

git format-patch origin

Motivation:

Imagine you have been working on a feature and have made several commits on your local branch. Your feature is now ready, but you can’t push the changes directly to the remote repository due to lack of permission. Instead, you can generate a patch file for those commits and send it to someone with the necessary access.

Explanation:

• git format-patch: This initiates the process of converting one or more commits into patch files.
• origin: This specifies a comparison point. Here, origin refers to the remote repository. The command will create patch files for all the commits made locally that haven’t been pushed to the origin.

Example Output:

Running this command can produce several patch files, each named with the first line of the commit message, followed by the commit hash. For instance:

0001-Add-new-feature-X.patch
0002-Fix-bug-in-feature-X.patch

Use Case 2: Write a .patch file for all the commits between two revisions to stdout

Code:

git format-patch revision_1..revision_2

Motivation:

This use case is helpful when you’re interested in viewing the changes between two specific points in the repository’s history, perhaps for code review purposes or when preparing changes for a submission to an upstream repository.

Explanation:

• revision_1..revision_2: This syntax specifies the range of commits you want to generate patch files for. Commits ranging from revision_1 (exclusive) to revision_2 (inclusive) will be turned into patch files.
• Writing to stdout: By default, git format-patch writes patch files to the current directory. You can redirect the output to stdout if you want to inspect the patch in the terminal directly.

Example Output:

The changes displayed might look similar to a typical diff output in the terminal, showcasing what code was added or removed between revision_1 and revision_2.

diff --git a/file.txt b/file.txt
index 93c12ad..f93c1ef 100644
--- a/file.txt
+++ b/file.txt
@@ -1,3 +1,7 @@
+New changes

Use Case 3: Write a .patch file for the 3 latest commits

Code:

git format-patch -3

Motivation:

Sometimes, you need to generate patch files for a specific number of recent commits. This could be the case when you’re developing a small bug fix or feature that spans over a few commits, and you need to send it for review or apply it to a different branch or repository.

Explanation:

• -3: The hyphen followed by a number specifies how many of the most recent commits you want to include in the generated patches. In this case, the command will generate patch files for the latest three commits.

Example Output:

Once again, files will be named based on the first line of each commit:

0001-Refactor-codebase.patch
0002-Update-documentation.patch
0003-Improve-testing-suite.patch

Conclusion:

The git format-patch command is an essential tool for developers working in environments where collaboration via patch files is preferred or necessary. Whether you’re generating patches for unpushed local commits, specific commit ranges, or a defined number of recent changes, git format-patch offers versatile options to suit your development workflow. Through clear understanding and application of its arguments, developers can effectively manage and share their contributions across distributed and collaborative environments.