Selectively revert or checkout changes to a file in Git?

Question

Is there a command that allows you to partially undo the changes to a file (or files) in the working directory?

Suppose you have edited a file a lot but you realize that

Answer 1

git checkout $file reverts the state of the file $file to the last committed state. I think you can use git checkout SHA-1 -- $file to revert the file to the commit identified by SHA-1.