dotnet run OR dotnet watch with development environment from command line?

You don't have to use environment variables if you adjust how the WebHostBuilder processes its configuration. This is merely the default for dotnet new -t web. For example, if you wanted to be able to set the default environment to "development" instead of production and facilitate overriding the environment in the command line, you could do that by modifying the normal Program.cs code from this ...

    public static void Main(string[] args) {
        var host = new WebHostBuilder()
            .UseKestrel()
            .UseUrls("http://0.0.0.0:5000")
            .UseContentRoot(Directory.GetCurrentDirectory())
            .UseIISIntegration()
            .UseStartup<Startup>()
            .Build();

        host.Run();
    }

... into something like this ...

    private static readonly Dictionary<string, string> defaults =
        new Dictionary<string, string> {
            { WebHostDefaults.EnvironmentKey, "development" }
        };

    public static void Main(string[] args) {
        var configuration =
            new ConfigurationBuilder()
                .AddInMemoryCollection(defaults)
                .AddEnvironmentVariables("ASPNETCORE_")
                .AddCommandLine(args)
                .Build();

        var host =
            new WebHostBuilder()
                .UseConfiguration(configuration)
                .UseKestrel()
                .UseUrls("http://0.0.0.0:5000")
                .UseContentRoot(Directory.GetCurrentDirectory())
                .UseIISIntegration()
                .UseStartup<Startup>()
                .Build();

        host.Run();
    }

Doing this, the environment variables would still work, but you can override it on the command line without any third-party dependencies, like so:

dotnet run environment=development
dotnet run environment=staging

This is actually what the yeoman generators do.

You can also set the variable inline when calling dotnet:

ASPNETCORE_ENVIRONMENT=Development dotnet run

I have found this is great for NPM scripts, but must always be called right before dotnet, e.g.:

{
  ...
  "scripts": {
    "start": "cd MyApp && ASPNETCORE_ENVIRONMENT=Development dotnet run",
    "watch": "cd MyApp && ASPNETCORE_ENVIRONMENT=Development dotnet watch"
  },
}

Note: This only works in OS X or Linux; for a cross-platform solution, you can use cross-env:

npm install cross-env -D

Then change the scripts to:

{
  ...
  "scripts": {
    "start": "cd MyApp && cross-env ASPNETCORE_ENVIRONMENT=Development dotnet run",
    "watch": "cd MyApp && cross-env ASPNETCORE_ENVIRONMENT=Development dotnet watch"
  },
}

Check documentation

https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-run?tabs=netcore21

https://learn.microsoft.com/en-us/aspnet/core/fundamentals/environments?view=aspnetcore-2.1

dotnet run --launch-profile EnvironmentsSample

launchSettings.json

{ 
  "profiles": {    
    "EnvironmentsSample": {
      "commandName": "Project",
      "launchBrowser": true,
      "environmentVariables": {
        "ASPNETCORE_ENVIRONMENT": "Staging"
      },
      "applicationUrl": "http://localhost:54340/"
    },   
  }
}

From Building Single Page Applications on ASP.NET Core with JavaScriptServices (styling added):

If you’re using PowerShell in Windows, execute $Env:ASPNETCORE_ENVIRONMENT = "Development"

If you’re using cmd.exe in Windows, execute setx ASPNETCORE_ENVIRONMENT "Development", and then restart your command prompt to make the change take effect

If you’re using Mac/Linux, execute export ASPNETCORE_ENVIRONMENT=Development

To set from CLI using dotnet run or dotnet watch:

dotnet run --environment Development

dotnet watch run --environment Development

With credit to @Technetium 's answer, here's an update to dotnetcore 2.0 that works really well for me:

public class Program
    {
        private static readonly Dictionary<string, string> defaults = new Dictionary<string, string> {{ WebHostDefaults.EnvironmentKey, "Development" }};
        private static IConfiguration config;
        public static void Main(string[] args)
        {
            config = new ConfigurationBuilder()
                .AddInMemoryCollection(defaults)
                .AddEnvironmentVariables("ASPNETCORE_")
                .AddCommandLine(args)
                .Build();
            BuildWebHost(args).Run();
        }
public static IWebHost BuildWebHost(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .UseConfiguration(config)
                /* Following setting already accounted for in CreateDefaultBuilder() : https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.webhost.createdefaultbuilder?view=aspnetcore-2.0 */
                // .UseContentRoot(Directory.GetCurrentDirectory())
                .UseStartup<Startup>()
                .Build();
    }

Thanks again, @Technetium