Git patch workflow

 In the Git, you can apply patches using the git apply command for changes from a git diff or git am for patches generated by git format-patch. The method you choose depends on the patch's format and whether you want to create a new commit. 

Before you apply a patch

1. Back up your work. Before applying a patch, you should have a clean working directory. Either commit or stash your uncommitted changes to avoid complications.
2. Use a temporary branch. It's a best practice to create a new, temporary branch to test the patch. If something goes wrong, you can simply delete the branch without affecting your main codebase.
   sh

   git checkout -b temp-patch-branch
   

   Use code with caution.
3. Check the patch. To see if the patch will apply cleanly, run a check with the --check flag.
   sh

   git apply --check my_patch.patch
   

   Use code with caution.
    

Method 1: Using git apply

Use this command for patches created with git diff. It will apply the changes to your working directory and staging area but will not create a new commit. The commit message, author, and date from the original commit are not preserved.

How to use git apply

sh

# Apply the patch to your working directory
git apply my_patch.patch

# If the patch was generated from a different subdirectory, you may need to strip leading path components.
# The number 'n' represents the number of leading directories to ignore.
git apply -p<n> my_patch.patch

# To add the changes directly to the staging index
git apply --index my_patch.patch

Use code with caution.

Method 2: Using git am

Use this command for patches generated by git format-patch, which are in a more robust "mailbox" format that includes commit metadata. This command automatically applies the patch and creates a new commit, preserving the original author and commit message. 

How to use git am

sh

# Apply the patch and create a commit
git am my_patch.patch

# If you are applying multiple patches
git am *.patch

# For complex patches or if a patch fails to apply cleanly,
# use a three-way merge to help resolve conflicts
git am -3 my_patch.patch

Use code with caution.

Handling patch conflicts

If Git cannot apply the patch cleanly, you will encounter conflicts.

If using git apply:
The process will stop and leave .rej (rejected) files in your working directory. You will have to manually resolve the conflicts in your files using the .rej file as a guide, and then use git add and git commit to finalize the changes. 

If using git am:
Git will stop and inform you of the conflict.

1. Open the conflicted files and manually resolve the changes.
2. After resolving, stage the changes with git add ..
3. Continue the process with git am --continue.
4. To abort the process, use git am --abort. 

Reversing a patch

If you need to revert a patch, you can use the --reverse or -R flag with git apply. 

sh

git apply -R my_patch.patch

Use code with caution.

Git Diff, Git Format-Patch, and Git Apply: What’s the difference?