[git] Show git diff on file in staging area

Is there a way I can see the changes that were made to a file after I have done git add file?

That is, when I do:

git add file
git diff file

no diff is shown. I guess there's a way to see the differences since the last commit but I don't know what that is.

In order to see the changes that have been staged already, you can pass the -–staged option to git diff (in pre-1.6 versions of Git, use –-cached).

git diff --staged
git diff --cached

You can also use git diff HEAD file to show the diff for a specific file.

See the EXAMPLE section under git-diff(1)