How do I see changes in the git index?

Say I do

git add foo.txt

Now, foo's changes are in the index (I'm assuming git was 开发者_Python百科already tracking that file). Now, when I do git diff, I can't see the changes in foo by doing

git diff

Are there some extra things that git diff wants before it shows me those changes?

• To show unstaged changes only:

git diff

• To show the staged/cached changes only:

git diff --cached

• To show both cached and uncached changes, compare the whole working tree to the named commit (HEAD):

git diff HEAD