How to git-diff a single, staged file with same file in other branch

Question

If I understand correctly, if I do

git diff master devel ./my_file

I get the diff between the two HEADs of the branches. However, if I have made changes in, say, master and staged them, I thought I could get the diff between the unstaged file and the corresponding file in the other branch with

$ git diff --cached master devel ./my_file

but I get the following error:

usage: git diff [<options>] [<commit> [<commit>]] [--] [<path>...]

So what is the correct way to get the diff between staged changes in a file and the corresponding file in another branch?

Answer 1

Try the following syntax:

git diff --cached devel -- my_file.ext

This should compare my_file.ext as it is at the HEAD of the devel branch against the current stage in master (assuming you run this command on the master branch).

There is a lot going on in this command, but note that --cached does not refer to devel, which is a commit where the concept of stage does not apply. Instead, --cached refers to the file my_file.ext which follows the -- separator.

This command corresponds to the following pattern from the manual:

git diff [--options] --cached [<commit>] [--] [<path>…​]

Here devel is the commit and my_file.ext is the file path.