[duijf]

All the Nix commands that take an 'installable' can take GitHub URLs. For instance:

    $ nix run github:NixOS/nixpkgs#hello
    Hello world!

That command will download nixpkgs from GitHub, evaluate the `hello` flake attribute, build it (or download it from a cache), and run it.

> But to find out what the flake exposes at all, reading the flake (or its documentation) is pretty much necessary.

If the flake exposes default packages or apps, then you do not need to provide a flake attribute:

    $ nix run github:NixOS/nixpkgs
    error: flake 'github:NixOS/nixpkgs' does not provide attribute 'apps.aarch64-darwin.default', 'defaultApp.aarch64-darwin', 'packages.aarch64-darwin.default' or 'defaultPackage.aarch64-darwin'

So you can run e.g. Alejandra [1], a Nix formatter, like so:

    $ nix run github:kamadorueda/alejandra

[1]: https://github.com/kamadorueda/alejandra

EDIT: For what it's worth, I think this feature can be useful sometimes, but it does also suffer from the same typosquatting problems as we see in other ecosystems.