Really? `nix develop` still drops me into a bash shell, even though I pretty much always want to use zsh. Is there an option I have to enable somewhere?
`nix develop` is for entering an environment to build the packages, and packages are built with bash.
For "I use fish shell, and I want to load the shell.nix", it's more convenient to use direnv (which uses `nix print-dev-env`). https://github.com/direnv/direnv/wiki/nix
Another option is to use `nix develop --command fish`.
It used to be that running `nix-shell --package` would drop you into bash. With the newer nix command equivalent, `nix shell` retains the same SHELL.
When writing Nix packages, practically the only supported shell is bash. But, that's less of an issue, since it doesn't affect what interactive shell you use.