version control - How to split last commit into two in Git

Question

I have two working branches, master and forum and I've just made some modifications in forum branch, that I'd like to cherry-pick into master. But unfortunately, the commit I want to cherry-pick also contains some modifications that I don't want.

The solution would probably be to somehow delete the wrong commit and replace it with two separate commits, one with changes I want to pick in master, and others that doesn't belong there.

I've tried doing

git reset --hard HEAD^

which deleted all changes, so I had to go back with

git reset ORIG_HEAD

So my question is, what is the best way to split last commit into two separate commits?

Answer 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