Refactor script to allow for environment variables

[eureka-cpu]

It would be cool to allow environment variables for variables in the script that rely on the command line flags so that the watcher could be declarative. Basically I'd like to be able to do:

{
  devShells.default = pkgs.mkShell {
    buildInputs = [ pkgs.nil ] ++ nix-watch.nix-watch.${system}.devTools;
    NIX_WATCH_COMMAND="nix build --keep-going";
    NIX_WATCH_DEBUG=true;
    # ... etc
  };
}

How the default variables are constructed would need to change. The following is the behaviour I'd expect:

• If a user passes a flag/option that conflicts with the environment variable, the flag/option should override it, i.e NIX_WATCH_COMMAND="nix build --keep-going" but the user specifies nix-watch -x "nix flake check"

There may be other quirks that will present themselves as I haven't entirely thought this out, but that's the gist.

[dzmitry-lahoda]

would you mind also move script into separate file out of nix and make builtins.readFile in nix? It will make script reusable out of nix.

also you may consider using https://github.com/ko1nksm/getoptions by refencing it as non flake input? it parses into ENV vars kind of as it seems.

[eureka-cpu]

would you mind also move script into separate file out of nix and make builtins.readFile in nix? It will make script reusable out of nix.

also you may consider using https://github.com/ko1nksm/getoptions by refencing it as non flake input? it parses into ENV vars kind of as it seems.

Do you have an example of how someone would use this in a non-nix setting? It's a nix developer tool, so I only really see anyone using it with nix.