Example of loading flags from a file

cmd/createtree/main.go

@@ -41,12 +41,14 @@ import (
 	"github.com/golang/protobuf/ptypes"
 	"github.com/golang/protobuf/ptypes/any"
 	"github.com/google/trillian"
+	"github.com/google/trillian/cmd"
 	"github.com/google/trillian/crypto/keyspb"
 	"github.com/google/trillian/crypto/sigpb"
 	"google.golang.org/grpc"
 )
 
 var (
+	configFile      = flag.String("config", "", "Config file containing flags")
 	adminServerAddr = flag.String("admin_server", "", "Address of the gRPC Trillian Admin Server (host:port)")
 
 	treeState          = flag.String("tree_state", trillian.TreeState_ACTIVE.String(), "State of the new tree")
@@ -175,6 +177,22 @@ func newOptsFromFlags() *createOpts {
 func main() {
 	flag.Parse()
 
+	if *configFile != "" {
+		if flag.NFlag() != 1 {
+			fmt.Printf("No other flags can be provided when --config is set")
+			os.Exit(1)
+		}
+
+		if err := cmd.ParseFlagFile(*configFile); err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to parse %v: %v\n", *configFile, err)
+			os.Exit(1)
+		}
+
+		// Alternative to printing error if more than just "--config" flag is provided:
+		// let command-line flags take precedent by re-parsing from the command-line.
+		// flag.Parse()
+	}
+
 	ctx := context.Background()
 	tree, err := createTree(ctx, newOptsFromFlags())
 	if err != nil {

cmd/flags.go

@@ -0,0 +1,24 @@
+package cmd
+
+import (
+	"flag"
+	"io/ioutil"
+
+	"github.com/mattn/go-shellwords"
+)
+
+func ParseFlagFile(path string) error {
+	file, err := ioutil.ReadFile(path)
+	if err != nil {
+		return err
+	}
+
+	p := shellwords.NewParser()
+	p.ParseEnv = true
+	args, err := p.Parse(string(file))
+	if err != nil {
+		return err
+	}
+
+	return flag.CommandLine.Parse(args)
+}