Stash only one file out of multiple files that have changed with Git?

How can I stash only one of multiple changed files on my branch?

git stash push -p -m "my commit message"

-p let's you select the hunks that should be stashed; whole files can be selected as well.

You'll be prompted with a few actions for each hunk:

   y - stash this hunk
   n - do not stash this hunk
   q - quit; do not stash this hunk or any of the remaining ones
   a - stash this hunk and all later hunks in the file
   d - do not stash this hunk or any of the later hunks in the file
   g - select a hunk to go to
   / - search for a hunk matching the given regex
   j - leave this hunk undecided, see next undecided hunk
   J - leave this hunk undecided, see next hunk
   k - leave this hunk undecided, see previous undecided hunk
   K - leave this hunk undecided, see previous hunk
   s - split the current hunk into smaller hunks
   e - manually edit the current hunk
   ? - print help

Disclaimer: the following answer is for git before git 2.13. For git 2.13 and over, check out another answer further down.

Warning

As noted in the comments, this puts everything into the stash, both staged and unstaged. The --keep-index just leaves the index alone after the stash is done. This can cause merge conflicts when you later pop the stash.

This will stash everything that you haven't previously added. Just git add the things you want to keep, then run it.

git stash --keep-index

For example, if you want to split an old commit into more than one changeset, you can use this procedure:

1. git rebase -i <last good commit>
2. Mark some changes as edit.
3. git reset HEAD^
4. git add <files you want to keep in this change>
5. git stash --keep-index
6. Fix things up as necessary. Don't forget to git add any changes.
7. git commit
8. git stash pop
9. Repeat, from #5, as necessary.
10. git rebase --continue