How to set environment via `ng serve` in Angular 6

You can try: ng serve --configuration=dev/prod

To build use: ng build --prod --configuration=dev

Hope you are using a different kind of environment.

This answer seems good.
however, it lead me towards an error as it resulted with
Configuration 'xyz' could not be found in project ...
error in build.
It is requierd not only to updated build configurations, but also serve ones.

So just to leave no confusions:

1. --env is not supported in angular 6
2. --env got changed into --configuration || -c (and is now more powerful)
3. to manage various envs, in addition to adding new environment file, it is now required to do some changes in angular.json file:
   • add new configuration in the build { ... "build": "configurations": ... property
   • new build configuration may contain only fileReplacements part, (but more options are available)
   • add new configuration in the serve { ... "serve": "configurations": ... property
   • new serve configuration shall contain of browserTarget="your-project-name:build:staging"

You need to use the new configuration option (this works for ng build and ng serve as well)

ng serve --configuration=local

or

ng serve -c local

If you look at your angular.json file, you'll see that you have finer control over settings for each configuration (aot, optimizer, environment files,...)

"configurations": {
  "production": {
    "optimization": true,
    "outputHashing": "all",
    "sourceMap": false,
    "extractCss": true,
    "namedChunks": false,
    "aot": true,
    "extractLicenses": true,
    "vendorChunk": false,
    "buildOptimizer": true,
    "fileReplacements": [
      {
        "replace": "src/environments/environment.ts",
        "with": "src/environments/environment.prod.ts"
      }
    ]
  }
}

You can get more info here for managing environment specific configurations.

As pointed in the other response below, if you need to add a new 'environment', you need to add a new configuration to the build task and, depending on your needs, to the serve and test tasks as well.

Adding a new environment

Edit: To make it clear, file replacements must be specified in the build section. So if you want to use ng serve with a specific environment file (say dev2), you first need to modify the build section to add a new dev2 configuration

"build": {
   "configurations": {
        "dev2": {

          "fileReplacements": [
            {
              "replace": "src/environments/environment.ts",
              "with": "src/environments/environment.dev2.ts"
            }
            /* You can add all other options here, such as aot, optimization, ... */
          ],
          "serviceWorker": true
        },

Then modify your serve section to add a new configuration as well, pointing to the dev2 build configuration you just declared

"serve":
      "configurations": {
        "dev2": {
          "browserTarget": "projectName:build:dev2"
        }

Then you can use ng serve -c dev2, which will use the dev2 config file