I have changes to a file, plus a new file, and would like to use git stash to put them away while I switch to another task. But git stash by itself stashes only the changes to the existing file; the new file remains in my working tree, cluttering up my future work. How do I stash this untracked file?

To stash your working directory including untracked files (especially those that are in the .gitignore) then you probably want to use this cmd:

git stash --include-untracked

Alternatively, you can use the shorthand -u instead of --include-untracked, or simply git stash --all (see warning below for this one) which stashes all files, including untracked and ignored files. This behaviour changed in 2018, so make sure your git is up to date.

Warning: there seems to be (or have been) situations in which contents of ignored directories could be deleted permanently. See this archived website for more information.