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

Much like jdsumsion's answer you can also stash your current work but then use a difftool like meld to pull selected changes from the stash. That way you can even edit the hunks manually very easy, which is a bit of a pain when in git add -p:

$ git stash -u
$ git difftool -d -t meld stash
$ git commit -a -m "some message"
$ git stash pop

Using the stash method gives you the opportunity to test, if your code still works, before you commit it.