git reset --soft seems to change index

Question

Having read through Mark Dominus's article and Scott Chanon's article I assumed that git reset abcd --soft would not affect the index, however, the following shows that it does.

Before Reset

c1.txt, c2.txt, c3.txt in the repo, c4.txt in the index. All in the working directory:

History:

$ git log --oneline --decorate
b91d91b (HEAD, master) C3
231a5df C2
7e7b2d7 C1

The index:

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

        new file:   c4.txt

The working directory:

$ ls
c1.txt  c2.txt  c3.txt  c4.txt

Reset softly to the C2 commit

$ git reset 231a --soft

$ git log --oneline --decorate
231a5df (HEAD, master) C2
7e7b2d7 C1

Index has changed

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

        new file:   c3.txt
        new file:   c4.txt


$ ls
c1.txt  c2.txt  c3.txt  c4.txt

What I thought would happen

From Scott's article, he says:

The first thing reset will do is move what HEAD points to.... If you add the flag --soft, this is the only thing it will do. With --soft, reset will simply stop there.

So, I thought that the index (which I hope I've understood to be what's shown in the git status outputs above) would not change, that being, it would still contain c4.txt. The output above shows that it actually contains c3.txt and c4.txt.

It appears that c3.txt has been moved into the index alongside c4.txt.

Answer 1

The fundamental misunderstanding seems to be about what the index is. The index isn't a set of differences, it's a set of files. What git status shows is not the index, but the difference between the last commit and the index.

Before your git reset --soft, the index contained files c1.txt, c2.txt, c3.txt and c4.txt. So after your git reset --soft, the index still contains those same files. git status will report this differently, but that isn't because the index changed, that's because the commit changed.