Git format-patch is a command-line tool used to prepare patch files in the Git version control system. This command is useful when you want to email commits to someone else for review or apply generated patch files using the git am command.

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

Code:

git format-patch origin

Motivation: When you have multiple commits that haven’t been pushed to a remote repository yet, it can be helpful to create a patch file for each of those commits. These patch files can be sent via email or transferred in other ways for code review or sharing purposes.

Explanation: In this use case, the git format-patch command is used with the argument origin. This specifies that you want to create patch files for all the commits that are not yet pushed to the origin remote repository. The command will generate a patch file for each commit, with an auto-named filename.

Example Output: The command will create multiple patch files, each representing an unpushed commit, in the current working directory. The patch filenames will follow the format 0001-commit-message.patch, 0002-commit-message.patch, and so on, where the number indicates the order of the commits.

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

Code:

git format-patch revision_1..revision_2

Motivation: When you need to get the changes between two specific revisions, creating a patch file can be a convenient way to capture those changes. Writing the patch file to the standard output (stdout) allows you to view the changes directly in the terminal or redirect them to a file of your choice.

Explanation: In this use case, the git format-patch command is used with the range of revisions revision_1..revision_2. This range specifies the starting commit and ending commit for which you want to generate the patch. By default, the patch file is written to the standard output.

Example Output: The command will output the generated patch file to the terminal. The content in the patch file will represent the changes introduced between revision_1 and revision_2. You can redirect the output to a file using the shell’s redirection operator (>).

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

Code:

git format-patch -3

Motivation: When you want to create patch files for a specific number of the latest commits, this use case comes in handy. It allows you to generate patch files for only the most recent commits, which can be useful for sharing or reviewing purposes.

Explanation: In this use case, the -3 option is provided to the git format-patch command. The -3 flag specifies that you want to create patch files for the three most recent commits in the repository.

Example Output: The command will create three patch files, each representing one of the three latest commits, in the current working directory. The patch filenames will follow the format 0001-commit-message.patch, 0002-commit-message.patch, and 0003-commit-message.patch.

Conclusion

The git format-patch command is a powerful tool to generate patch files from commits in the Git repository. These patch files can be used for code review, sharing, or applying changes to other repositories. By specifying different arguments and options, you can tailor its functionality to suit your specific needs.