Add documentation on internal environment variables (#811)

Signed-off-by: Alex Vorona <alex@vorona.com.ua>

cmd/root.go

@@ -116,7 +116,7 @@ func NewRootCmd(globalConfig *config.GlobalOptions) (*cobra.Command, error) {
 func setGlobalOptionsForRootCmd(fs *pflag.FlagSet, globalOptions *config.GlobalOptions) {
 	fs.StringVarP(&globalOptions.HelmBinary, "helm-binary", "b", app.DefaultHelmBinary, "Path to the helm binary")
 	fs.StringVarP(&globalOptions.File, "file", "f", "", "load config from file or directory. defaults to `helmfile.yaml` or `helmfile.yaml.gotmpl` or `helmfile.d`(means `helmfile.d/*.yaml` or `helmfile.d/*.yaml.gotmpl`) in this preference. Specify - to load the config from the standard input.")
-	fs.StringVarP(&globalOptions.Environment, "environment", "e", "", `specify the environment name. defaults to "default"`)
+	fs.StringVarP(&globalOptions.Environment, "environment", "e", "", `specify the environment name. Overrides "HELMFILE_ENVIRONMENT" OS environment variable when specified. defaults to "default"`)
 	fs.StringArrayVar(&globalOptions.StateValuesSet, "state-values-set", nil, "set state values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2). Used to override .Values within the helmfile template (not values template).")
 	fs.StringArrayVar(&globalOptions.StateValuesFile, "state-values-file", nil, "specify state values in a YAML file. Used to override .Values within the helmfile template (not values template).")
 	fs.BoolVar(&globalOptions.SkipDeps, "skip-deps", false, `skip running "helm repo update" and "helm dependency build"`)

docs/index.md

@@ -499,6 +499,21 @@ releases:
 
 If you wish to treat your enviroment variables as strings always, even if they are boolean or numeric values you can use `{{ env "ENV_NAME" | quote }}` or `"{{ env "ENV_NAME" }}"`. These approaches also work with `requiredEnv`.
 
+### Useful internal Helmfile environment variables
+
+Helmfile uses some OS environment variables to override default behaviour:
+
+* `HELMFILE_DISABLE_INSECURE_FEATURES` - disable insecure features, expecting `true` lower case
+* `HELMFILE_DISABLE_RUNNER_UNIQUE_ID` - disable unique logging ID, expecting any non-empty value
+* `HELMFILE_SKIP_INSECURE_TEMPLATE_FUNCTIONS` - disable insecure template functions, expecting `true` lower case
+* `HELMFILE_EXPERIMENTAL` - enable experimental features, expecting `true` lower case
+* `HELMFILE_ENVIRONMENT` - specify [Helmfile environment](https://helmfile.readthedocs.io/en/latest/#environment), it has lower priority than CLI argument `--environment`
+* `HELMFILE_TEMPDIR` - specify directory to store temporary files
+* `HELMFILE_UPGRADE_NOTICE_DISABLED` - expecting any non-empty value to skip the check for the latest version of Helmfile in [helmfile version](https://helmfile.readthedocs.io/en/latest/#version)
+* `HELMFILE_V1MODE` - Helmfile v0.x behaves like v1.x with `true`, Helmfile v1.x behaves like v0.x with `false` as value
+* `HELMFILE_GOCCY_GOYAML` - use *goccy/go-yaml* instead of *gopkg.in/yaml.v2*.  It's `false` by default in Helmfile v0.x and `true` by default for Helmfile v1.x.
+* `HELMFILE_CACHE_HOME` - specify directory to store cached files for remote operations
+
 ## CLI Reference
 
 ```
@@ -540,7 +555,7 @@ Flags:
       --disable-force-update            do not force helm repos to update when executing "helm repo add"
       --enable-live-output              Show live output from the Helm binary Stdout/Stderr into Helmfile own Stdout/Stderr.
                                         It only applies for the Helm CLI commands, Stdout/Stderr for Hooks are still displayed only when it's execution finishes.
-  -e, --environment string              specify the environment name. defaults to "default"
+  -e, --environment string              specify the environment name. Overrides "HELMFILE_ENVIRONMENT" OS environment variable when specified. defaults to "default"
   -f, --file helmfile.yaml              load config from file or directory. defaults to helmfile.yaml or `helmfile.yaml.gotmpl` or `helmfile.d`(means `helmfile.d/*.yaml` or `helmfile.d/*.yaml.gotmpl`) in this preference. Specify - to load the config from the standard input.
   -b, --helm-binary string              Path to the helm binary (default "helm")
   -h, --help                            help for helmfile