scmx/split-git-commit-with-orig-head.md

## split-git-commit-with-orig-head.md

      
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    How to split a Git commit and preserve message using ORIG_HEAD

A colleague told me he'd accidentally committed something. Now he wanted to remove it from the previous
commit and then put in a new commit. This is called splitting a commit, and here's the best way I know of how to do it.
Solution


git reset --soft HEAD~ Undo the last commit, but preserve "Changes to be committed"
git reset <some/path> Exclude what you want
git commit -C ORIG_HEAD Commit with the same message as before, which was stored in ORIG_HEAD

Example

❯❯❯ echo hej > hej.txt

❯❯❯ echo hej2 > hej2.txt

❯❯❯ git add hej.txt hej2.txt

❯❯❯ git commit -m 'Add hej.txt'
[master f919e9d] Add hej.txt
 2 files changed, 2 insertions(+)
 create mode 100644 hej.txt
 create mode 100644 hej2.txt

❯❯❯ echo "Oops, I accidentally added hej2.txt as well"
Oops, I accidentally added hej2.txt as well

❯❯❯ git reset --soft HEAD~

❯❯❯ git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	new file:   hej.txt
	new file:   hej2.txt

❯❯❯ git reset hej2.txt

❯❯❯ git commit -C ORIG_HEAD
[master b880e4d] Add hej.txt
 Date: Fri Jan 19 11:22:31 2018 +0100
 1 file changed, 1 insertion(+)
 create mode 100644 hej.txt

❯❯❯ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	hej2.txt

nothing added to commit but untracked files present (use "git add" to track)
Background

I've done this many times before, but with different approaches that all were non optimal.
One of my previous crazy solutions was to undo it manually and then make a fixup-commit,
after that revert the fixup commit without committing and then use autosquash.
git commit --fixup HEAD, git revert HEAD, git rebase -i --autosquash HEAD~3
...not as good as the solution above with ORIG_HEAD! :)
Related reading:

man git-reset The git reset manual
https://stackoverflow.com/a/967611/2037928 HEAD and ORIG_HEAD in Git
https://emmanuelbernard.com/blog/2014/04/14/split-a-commit-in-two-with-git/#comment-3274061909

Crazy aliases

If you like git aliases, here's one I just (re)invented
~/.gitconfig
[alias]
split-head = "!f() { git reset --soft @~ && git reset $@ && git commit -C ORIG_HEAD; }; f"
❯❯❯ git split-head hej2.txt
[master b9ccd93] Add hej.txt
 Date: Fri Jan 19 12:45:40 2018 +0100
 1 file changed, 1 insertion(+)
 create mode 100644 hej.txt

❯❯❯ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	hej2.txt
❯❯❯ git split-head -p
diff --git a/hej.txt b/hej.txt
new file mode 100644
index 0000000..1b5b67f
--- /dev/null
+++ b/hej.txt
@@ -0,0 +1 @@
+hej
Unstage this hunk [y,n,q,a,d,/,e,?]? n

diff --git a/hej2.txt b/hej2.txt
new file mode 100644
index 0000000..fb793ab
--- /dev/null
+++ b/hej2.txt
@@ -0,0 +1 @@
+hej2
Unstage this hunk [y,n,q,a,d,/,e,?]? y

[master 2486910] Add hej.txt
 Date: Fri Jan 19 12:45:40 2018 +0100
 1 file changed, 1 insertion(+)
 create mode 100644 hej.txt