Question

Follow-up from Git: ability to stage a certain file content without touching the working tree:

I want to modify the index of one (text) file by staging a patch (against HEAD) without having to change the working tree file state. Is this possible?

Answer 1

You want git apply --cached

From the man page:

   --cached
       Apply a patch without touching the working tree. Instead take the cached data, apply the patch, and store
       the result in the index without using the working tree. This implies --index.