Git: ability to stage a certain file content without touching the working tree

Question

I want to modify the index of one (text) file without having to change the working tree file state. Is this possible?

Answer 1

Another take on "changing file in index without altering working dir" is to apply a patch to index only. This is often the way GUI git clients stage only selected lines from a given file.

You start out by (if you want) clearing out the changes from index for that file:

git reset path/to/file

Then extracting the full patch for it

git diff path/to/file > /path/to/tmpfile

Edit the patch file to include only the changes you want to apply, and apply just the edited patch:

git apply --cached /path/to/tmpfile

See:

git help apply