How can I remove an applied git patch?

On Windows

I'm using git on Windows and it works a bit differently than on Linux. Specifically, I found that when I ran:

git apply -R C:\downloads\mypatch.patch

This perhaps was running in the wrong directory. I had to copy it to the local directory where I was applying the patch. I also got errors like:

error: product/build.gradle: No such file or directory
error: main/generator/generator.js: No such file or directory

Which are super weird because those files are in directories specified inside of my patch. The only fix I could find for these was to modify the patch by hand. I had to change the filenames to prepend ./ to the directories and filenames. So I turned this:

Index: build_scripts/product/build.gradle
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- build_scripts/product/build.gradle  (revision 537fbcc1ebdf65652896eaaf2a315cc44a24ba6c)
+++ build_scripts/product/build.gradle  (date 1594123740523)
@@ -304,7 +304,7 @@
       from("$repoRootDir/src/main/nodejs/utils/") {
         include "common_*.js"
       }

Into this:

Index: ./build_scripts/product/build.gradle
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- ./build_scripts/product/build.gradle    (revision 537fbcc1ebdf65652896eaaf2a315cc44a24ba6c)
+++ ./build_scripts/product/build.gradle    (date 1594123740523)
@@ -304,7 +304,7 @@
       from("$repoRootDir/src/main/nodejs/utils/") {
         include "common_*.js"
       }

The only change is the ./ before the 3 different build_scripts/product/build.gradle strings.

Also, another nice hint is that git apply takes a -v parameter that gives a little more verbosity so you can see that it's first checking to see if the patch is safe to apply before applying it.

TL;DR

You can revert a patch with:

$ git apply -R <patch>

---

You can generate a patch either by one of the following:

This will generate a patch from a diff

$ git diff --patch > 0001-some-modifications.patch

If you want to generate a patch for just the HEAD commit:

$ git show --patch HEAD^ > 0001-some-modifications.patch

You can generate a patch for the previous 3 commits from HEAD:

$ git show --patch HEAD~3 > 0001-some-modifications.patch

You can apply the patch by:

$ git apply -- 0001-some-modifications.patch

You can revert a patch with:

$ git apply -R <patch>

When you generate a patch it is just a diff with metadata; files, line numbers adds/removes; something along the following:

commit 9dad147cbf16befecdef2e812c1249499bdef5ac
Author: My Name <[email protected]>
Date:   Mon Dec 21 20:46:01 2015 +0000

    Example commit message.

diff --git a/src/example.md b/src/example.md
new file mode 100644
index 0000000..ab73512
--- /dev/null
+++ b/src/example.md
@@ -0,0 +1,3 @@
+# Example document
+
+ Hello World

So when you use git apply you're essentially applying the edits as per to the tree.

When you then run git apply -R git will simply do the opposite to the patch.