Commit only part of a file in Git

Question

When I make changes to a file in Git, how can I commit only some of the changes?

For example, how could I commit only 15 lines out of 30 lines that have been changed

Answer 1

Worth noting that to use git add --patch for a new file you need to first add the file to index with git add --intent-to-add:

git add -N file
git add -p file