> For the complete documentation index, see [llms.txt](https://cover-docs.diffblue.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cover-docs.diffblue.com/features/cover-cli/writing-tests/patch-files.md).

# Patch files

Use the `--patch-only=<patch-file>` argument to define a patch file - Diffblue Cover will only write tests for the code changes defined in the patch file (any class in the patch and any related/dependent classes).

## Command line argument

**Usage:** `--patch-only=<patch-file>`

**Alternative:** `-p=<patch-file>`

**Example:** `--patch-only=path/to/file.patch`

* Use diff or a similar tool to create a patch file for your changes in your project - for example:\
  `git diff origin/develop > file.patch`.
* For a multi-module project, generate the patch at the root of the project and provide the absolute path to the patch file, using `--working-directory` with the relative path to the module. The same patch file can be used for each module.
* For a project without modules, or a project where tests will only ever be created for a single module, where `--working-directory` is not used, the relative path to the patch file for the project or module only may be used - for example:

```
    cd Module
    git diff origin/develop --relative > file.patch
```

* The `--patch-only` argument only accepts files in UTF-8 encoding. If using PowerShell on Windows, use the following command to get the patch in UTF-8 instead of the default UTF-16:

```
    git diff origin/develop | out-file file.patch -encoding utf8
```

## Create a patch file

The exact patch file you need depends on your setup, but here are some examples using `git diff`. The variable `DIFFBLUE_PATCH` represents the name of your patch file.

***

For a patch containing commits newer than the project's origin (i.e. the corresponding branch on the remote repository your local branch was created from):

```groovy
git diff $(git rev-parse --abbrev-ref HEAD)...HEAD > $DIFFBLUE_PATCH
```

***

For a patch containing commits on your branch newer than those on the local copy of `TARGET_BRANCH` (e.g. where `TARGET_BRANCH` is `develop`):

```groovy
git diff TARGET_BRANCH...HEAD > $DIFFBLUE_PATCH
```

***

For a patch containing commits on your branch newer than those on the remote host `TARGET_BRANCH` (e.g. where `REMOTE` is `origin` and `REMOTE/TARGET_BRANCH` is `origin/develop`):

```groovy
git fetch REMOTE
git diff REMOTE/TARGET_BRANCH...HEAD > $DIFFBLUE_PATCH
```

***

For a patch containing only currently uncommitted changes:

```groovy
git diff > $DIFFBLUE_PATCH
```