Stashing only staged changes in git - is it possible?

Yes, It's possible with DOUBLE STASH

1. Stage all your files that you need to stash.
2. Run git stash --keep-index. This command will create a stash with ALL of your changes (staged and unstaged), but will leave the staged changes in your working directory (still in state staged).
3. Run git stash push -m "good stash"
4. Now your "good stash" has ONLY staged files.

Now if you need unstaged files before stash, simply apply first stash (the one created with --keep-index) and now you can remove files you stashed to "good stash".

Enjoy

With latest git you may use --patch option

git stash push --patch   # since 2.14.6

git stash save --patch   # for older git versions

And git will ask you for each change in your files to add or not into stash.
You just answer y or n

UPD
Alias for DOUBLE STASH:

git config --global alias.stash-staged '!bash -c "git stash --keep-index; git stash push -m "staged" --keep-index; git stash pop stash@{1}"'

Now you can stage your files and then run git stash-staged.
As result your staged files will be saved into stash.

If you do not want to keep staged files and want move them into stash. Then you can add another alias and run git move-staged:

git config --global alias.move-staged '!bash -c "git stash-staged;git commit -m "temp"; git stash; git reset --hard HEAD^; git stash pop"'

TL;DR Just add -- $(git diff --staged --name-only) for your git <pathspec> parameter

Here is a simple one-liner:

git stash -- $(git diff --staged --name-only)

And to add a message simply:

git stash push -m "My work in progress" -- $(git diff --staged --name-only)

Tested on v2.17.1 and v2.21.0.windows.1

Limitations:

• Please be aware that this will stash every single thing, if you have no files staged.
• Also if you have a file that is only partially staged ( i.e. only some changed lines, are staged while some other changed lines are not), then the whole file will get stashed (including unstaged lines).