ask:

	show me the differences between HEAD and the current index
	contents (the ones I'd write using 'git write-tree')

For example, let's say that you have worked on your working directory, updated
some files in the index and are ready to commit. You want to see exactly
*what* you are going to commit, without having to write a new tree
object and compare it that way, and to do that, you just do

	git diff-index --cached HEAD

Example: let's say I had renamed `commit.c` to `git-commit.c`, and I had
done an `update-index` to make that effective in the index file.
`git diff-files` wouldn't show anything at all, since the index file
matches my working directory. But doing a 'git diff-index' does:

  torvalds@ppc970:~/git> git diff-index --cached HEAD
  :100644 000000 4161aecc6700a2eb579e842af0b7f22b98443f74 0000000000000000000000000000000000000000 D	commit.c
  :000000 100644 0000000000000000000000000000000000000000 4161aecc6700a2eb579e842af0b7f22b98443f74 A	git-commit.c

You can see easily that the above is a rename.

In fact, `git diff-index --cached` *should* always be entirely equivalent to
actually doing a 'git write-tree' and comparing that. Except this one is much
nicer for the case where you just want to check where you are.

So doing a `git diff-index --cached` is basically very useful when you are
asking yourself "what have I already marked for being committed, and
what's the difference to a previous tree".

NON-CACHED MODE
---------------
The "non-cached" mode takes a different approach, and is potentially
the more useful of the two in that what it does can't be emulated with
a 'git write-tree' + 'git diff-tree'. Thus that's the default mode.
The non-cached version asks the question:

  show me the differences between HEAD and the currently checked out
  tree - index contents _and_ files that aren't up to date

which is obviously a very useful question too, since that tells you what
you *could* commit. Again, the output matches the 'git diff-tree -r'
output to a tee, but with a twist.

The twist is that if some file doesn't match the index, we don't have
a backing store thing for it, and we use the magic "all-zero" sha1 to
show that. So let's say that you have edited `kernel/sched.c`, but
have not actually done a 'git update-index' on it yet - there is no
"object" associated with the new state, and you get:

  torvalds@ppc970:~/v2.6/linux> git diff-index --abbrev HEAD
  :100644 100644 7476bb5ba 000000000 M	kernel/sched.c

i.e., it shows that the tree has changed, and that `kernel/sched.c` is
not up to date and may contain new stuff. The all-zero sha1 means that to
get the real diff, you need to look at the object in the working directory
directly rather than do an object-to-object diff.

NOTE: As with other commands of this type, 'git diff-index' does not
actually look at the contents of the file at all. So maybe
`kernel/sched.c` hasn't actually changed, and it's just that you
touched it. In either case, it's a note that you need to
'git update-index' it to make the index be in sync.

NOTE: You can have a mixture of files show up as "has been updated"
and "is still dirty in the working directory" together. You can always
tell which file is in which state, since the "has been updated" ones
show a valid sha1, and the "not in sync with the index" ones will
always have the special all-zero sha1.

GIT
---
Part of the linkgit:git[1] suite