Re: git index: how does it work?

Shaun Cutts [off-list ref] writes:

I am wondering if someone could explain and/or point me to an explanation of how
the git index works.

For instance, suppose I have a tracked file: "foo.c"

1) [I modify "foo.c"]
2) git add foo.c
3) [modify again]
4) git commit -m "blah blah"

Since I don't include the "-a" switch, the version I added on step 2 is
committed. But how does the index keep track of these changes? Does the index
file actually contain the hunks of "foo.c" that have been modified? Or is there
a "temporary" blob created, which the index points to? 

Step 2 hashes foo.c and creates a blob object and registers it to the
index.  Step 4 writes out the index as a tree and makes a commit out of
it.

Running this sequence might be instructive.

	1$ edit foo.c
        2$ git add foo.c
        2a$ git ls-files -s foo.c
	2b$ git diff foo.c
        2c$ git diff --cached foo.c
        3$ edit foo.c
        3a$ git ls-files -s foo.c
	3b$ git diff foo.c
        3c$ git diff --cached foo.c
        4$ git commit -m 'half-edit of foo.c'
        4a$ git ls-files -s foo.c
	4b$ git ls-tree HEAD foo.c
        4c$ git diff foo.c
        4d$ git diff --cached foo.c

 - 2a shows the actual blob object that was created out of foo.c in step 2.

 - 2b shows the difference between that blob (now in the index) and foo.c,
   which should be empty.

 - 2c shows the difference between the HEAD commit and the index, which
   should show your edit in step 1.

 - 3a shows the blob in the index; you haven't added, so it should show
   the same as 2a.

 - 3b shows the difference between the index and foo.c, which should show
   the edit in step 3.

 - 3c shows the difference between the HEAD commit and the index, which
   should show your edit in step 1.

 - 4a shows the blob in the index; you haven't added, so it should show
   the same as 2a.

 - 4b shows the blob in the committed tree and the blob object should be
   identical to 2a.

 - 4c shows the difference between the index and foo.c, which should show
   the edit in step 3.

 - 4d shows the difference between the HEAD commit and the index, which
   should now be empty.