feat(cmd/cupd): support env vars

cmd/cupd/main.go

@@ -2,9 +2,13 @@ package main
 
 import (
 	"context"
+	"errors"
+	"flag"
 	"os"
 
+	"github.com/peterbourgon/ff/v3"
 	"github.com/peterbourgon/ff/v3/ffcli"
+	"github.com/peterbourgon/ff/v3/ffyaml"
 	"go.flipt.io/cup/pkg/config"
 	"golang.org/x/exp/slog"
 )
@@ -17,9 +21,20 @@ func main() {
 		},
 	)))
 
+	set := flag.NewFlagSet("cupd", flag.ContinueOnError)
+	_ = set.String("config", "", "server config file")
+
 	cfg := &config.Config{}
 	root := &ffcli.Command{
-		Name: "cupd",
+		Name:    "cupd",
+		FlagSet: set,
+		Options: []ff.Option{
+			ff.WithEnvVarPrefix("CUPD"),
+			ff.WithConfigFileParser((&ffyaml.ParseConfig{
+				Delimiter: "-",
+			}).Parse),
+			ff.WithConfigFileFlag("config"),
+		},
 		Subcommands: []*ffcli.Command{
 			{
 				Name:       "serve",
@@ -34,8 +49,10 @@ func main() {
 	}
 
 	if err := root.ParseAndRun(context.Background(), os.Args[1:]); err != nil {
-		slog.Error("Exiting...", "error", err)
-		os.Exit(1)
+		if !errors.Is(err, flag.ErrHelp) {
+			slog.Error("Exiting...", "error", err)
+			os.Exit(1)
+		}
 	}
 }