Merge pull request #586 from sosedoff/cli-usage-env-vars

Print available environment variables in help message

pkg/cli/cli.go

@@ -131,8 +131,12 @@ func initClient() {
 func initOptions() {
 	opts, err := command.ParseOptions(os.Args)
 	if err != nil {
-		switch err.(type) {
+		switch errVal := err.(type) {
 		case *flags.Error:
+			if errVal.Type == flags.ErrHelp {
+				fmt.Println("Available environment variables:")
+				fmt.Println(command.AvailableEnvVars())
+			}
 			// no need to print error, flags package already does that
 		default:
 			fmt.Println(err.Error())

pkg/command/options.go

@@ -157,3 +157,18 @@ func getPrefixedEnvVar(name string) string {
 	}
 	return val
 }
+
+// AvailableEnvVars returns list of supported env vars.
+//
+// TODO: These should probably be embedded into flag parsing logic so we dont have
+// to maintain the list manually.
+func AvailableEnvVars() string {
+	return strings.Join([]string{
+		"  " + envVarPrefix + "DATABASE_URL  Database connection string",
+		"  " + envVarPrefix + "URL_PREFIX    HTTP server path prefix",
+		"  " + envVarPrefix + "SESSIONS:     Enable multiple database sessions",
+		"  " + envVarPrefix + "LOCK_SESSION  Lock session to a single database connection",
+		"  " + envVarPrefix + "AUTH_USER     HTTP basic auth username",
+		"  " + envVarPrefix + "AUTH_PASS     HTTP basic auth password",
+	}, "\n")
+}