How to split last commit into two in Git

Solution 1:

You should use the index. After doing a mixed reset ("git reset HEAD^"), add the first set of changes into the index, then commit them. Then commit the rest.

You can use "git add" to put all changes made in a file to the index. If you don't want to stage every modification made in a file, only some of them, you can use "git add -p".

Let's see an example. Let's suppose I had a file called myfile, which contains the following text:

something
something else
something again

I modified it in my last commit so that now it looks like this:

1
something
something else
something again
2

Now I decide that I want to split it into two, and I want the insertion of the first line to be in the first commit, and the insertion of the last line to be in the second commit.

First I go back to HEAD's parent, but I want to keep the modifications in file system, so I use "git reset" without argument (which will do a so-called "mixed" reset):

$ git reset HEAD^
myfile: locally modified
$ cat myfile
1
something
something else
something again
2

Now I use "git add -p" to add the changes I want to commit to the index (=I stage them). "git add -p" is an interactive tool that asks you about what changes to the file should it add to the index.

$ git add -p myfile
diff --git a/myfile b/myfile
index 93db4cb..2f113ce 100644
--- a/myfile
+++ b/myfile
@@ -1,3 +1,5 @@
+1
 something
 something else
 something again
+2
Stage this hunk [y,n,a,d,/,s,e,?]? s    # split this section into two!
Split into 2 hunks.
@@ -1,3 +1,4 @@
+1
 something
 something else
 something again
Stage this hunk [y,n,a,d,/,j,J,g,e,?]? y  # yes, I want to stage this
@@ -1,3 +2,4 @@
 something
 something else
 something again
+2
Stage this hunk [y,n,a,d,/,K,g,e,?]? n   # no, I don't want to stage this

Then I commit this first change:

$ git commit -m "Added first line"
[master cef3d4e] Added first line
 1 files changed, 1 insertions(+), 0 deletions(-)

Now I can commit all the other changes (namely the numeral "2" put in the last line):

$ git commit -am "Added last line"
[master 5e284e6] Added last line
 1 files changed, 1 insertions(+), 0 deletions(-)

Let's check the log to see what commits we have:

$ git log -p -n2 | cat
Commit 5e284e652f5e05a47ad8883d9f59ed9817be59d8
Author: ...
Date: ...

    Added last line

Diff --git a/myfile b/myfile
Index f9e1a67..2f113ce 100644
--- a/myfile
+++ b/myfile
@@ -2,3 +2,4 @@
 something
 something else
 something again
+2

Commit cef3d4e0298dd5d279a911440bb72d39410e7898
Author: ...
Date: ...

    Added first line

Diff --git a/myfile b/myfile
Index 93db4cb..f9e1a67 100644
--- a/myfile
+++ b/myfile
@@ -1,3 +1,4 @@
+1
 something
 something else
 something again

Solution 2:

Goals:

• I want to split a past commit (splitme) into two.
• I want to maintain the commit message.

Plan:

1. rebase interactive from one before splitme.
2. edit splitme.
3. Reset the files to split into a second commit.
4. Amend commit, maintaining message, modify as necessary.
5. Add back the files split out from the first commit.
6. Commit with a new message.
7. Continue rebase.

The rebase steps (1 & 7) can be skipped if the splitme is the most recent commit.

git rebase -i splitme^
# mark splitme commit with 'e'
git reset HEAD^ -- $files
git commit --amend
git add $files
git commit -m "commit with just some files"
git rebase --continue

If I wanted the split files to be committed first, I'd then rebase -i again and switch the order

git rebase -i splitme^
# swap order of splitme and 'just some files'

Solution 3:

To change the current commit into two commits, you can do something like the following.

Either:

git reset --soft HEAD^

This undoes the last commit but leaves everything staged. You can then unstage certain files:

git reset -- file.file

Optionally restage parts of those files:

git add -p file.file

Make a new first commit:

git commit

The stage and commit the rest of the changes in a second commit:

git commit -a

Or:

Undo and unstage all of the changes from the last commit:

git reset HEAD^

Selectively stage the first round of changes:

git add -p

Commit:

git commit

Commit the rest of the changes:

git commit -a

(In either step, if you undid a commit that added a brand new file and want to add this to the second commit you'll have to manually add it as commit -a only stages changes to already tracked files.)