How to Use the Command 'git am' (with examples)

The git am command is an essential tool for developers and teams who frequently work with patch files. It is particularly useful when applying patches that come from external sources, such as patches distributed via email or remote files. This command takes a series of patches and applies them as a series of commits, which allows for a smooth integration of changes into the local repository. By using git am, you can automate the process of incorporating patches, making it easier to manage patches and maintain a coherent commit history.

Use case 1: Apply and commit changes following a local patch file

Code:

git am path/to/file.patch

Motivation:

This use case is relevant when you have received a patch file from another developer or generated one using git format-patch. By applying these patches, you ensure that the changes encapsulated in the patch are seamlessly integrated into your codebase without manually copying and pasting code. It is especially advantageous in collaborative environments where patches represent a concise and focused set of changes.

Explanation:

• git: This is the command-line interface for Git, the popular version control system.
• am: am stands for “apply mailbox,” and it is used to apply patches from a mailbox file.
• path/to/file.patch: This specifies the location of the patch file you wish to apply. The path can be absolute or relative, depending on where the file is located in your filesystem.

Example output:

Applying: Update README with installation instructions

This indicates that the patch was successfully applied, updating the README file with new content.

Use case 2: Apply and commit changes following a remote patch file

Code:

curl -L https://example.com/file.patch | git apply

Motivation:

You might encounter scenarios where a patch file is hosted on a remote server or shared via a URL. By using curl alongside git apply, you can fetch the patch directly from the network and apply it instantly. This method is particularly useful when patches are shared publicly or need to be integrated quickly during open-source contributions.

Explanation:

• curl: A command-line tool used for transferring data with URLs. Here, it’s used to download the patch file.
• -L: Instructs curl to follow any redirects that may occur before downloading the final file.
• https://example.com/file.patch: The URL where the patch file is hosted. It should point to a downloadable patch file.
• |: This is a pipe operator, directing the output from one command into another. Here, it takes the output from curl and provides it as input to git apply.
• git apply: This command reads a patch from the standard input, which is provided by curl, and applies it to the working tree.

Example output:

Updating file1.txt
Updating file2.txt

The output demonstrates that the patch has been downloaded and successfully applied to the specified files.

Use case 3: Abort the process of applying a patch file

Code:

git am --abort

Motivation:

During the process of applying patches, you may encounter conflicts or realize that you applied the wrong patch. In such cases, the ability to abort the current application process is crucial to maintaining repository stability. This command allows you to effectively roll back any partial changes and return to the pre-patch state.

Explanation:

• git: Refers to the Git command-line interface.
• am: The command used for dealing with patch files.
• --abort: This flag instructs git am to abort the current patch application process. It cleans up any changes that were staged, leaving the repository in the same state as before the process began.

Example output:

Patch application process is aborted and the index is reset to the pre-application state.

This indicates that any in-progress changes have been discarded, and the repository is stabilized.

Use case 4: Apply as much of a patch file as possible, saving failed hunks to reject files

Code:

git am --reject path/to/file.patch

Motivation:

When working with patches, certain parts of the code may not apply cleanly due to conflicts in the changes being made. By using --reject, one can apply as much of the patch as possible while saving conflicting portions in separate “.rej” files. This way, the collaborator can manually resolve these conflicts and ensure no changes are lost.

Explanation:

• git: The interface for the Git version control system.
• am: Used for applying patches stored in mbox formatted files.
• --reject: This option means to apply as much of the patch as possible and, for portions that cannot be applied, create “reject” files that list conflicts and allow for manual resolution later.
• path/to/file.patch: Specifies the location of the patch file to be used, relative or absolute.

Example output:

Applied some parts of file.patch with rejects in file.patch.rej.

This output signifies that while some parts of the patch were successfully applied, others were stored in a reject file for further manual inspection.

Conclusion:

The git am command is an indispensable feature in the Git toolkit for managing patches, whether they come from local files or remote URLs. These examples illustrate how to make efficient use of this command, applying patches in various scenarios while maintaining flexibility in managing conflicts and errors. Mastering git am empowers developers to smoothly integrate changes into their repositories, enhancing collaboration and ensuring a coherent commit history.