cmd/geth: add 'dumpgenesis' command (#20191)

Adds the 'geth dumpgenesis' command, which writes the configured
genesis in JSON format to stdout. This provides a way to generate the
data (structure and content) that can then be used with the 'geth init'
command.

cmd/geth/chaincmd.go

@@ -56,6 +56,18 @@ This is a destructive action and changes the network in which you will be
 participating.
 
 It expects the genesis file as argument.`,
+	}
+	dumpGenesisCommand = cli.Command{
+		Action:    utils.MigrateFlags(dumpGenesis),
+		Name:      "dumpgenesis",
+		Usage:     "Dumps genesis block JSON configuration to stdout",
+		ArgsUsage: "",
+		Flags: []cli.Flag{
+			utils.DataDirFlag,
+		},
+		Category: "BLOCKCHAIN COMMANDS",
+		Description: `
+The dumpgenesis command dumps the genesis block configuration in JSON format to stdout.`,
 	}
 	importCommand = cli.Command{
 		Action:    utils.MigrateFlags(importChain),
@@ -227,6 +239,17 @@ func initGenesis(ctx *cli.Context) error {
 	return nil
 }
 
+func dumpGenesis(ctx *cli.Context) error {
+	genesis := utils.MakeGenesis(ctx)
+	if genesis == nil {
+		genesis = core.DefaultGenesisBlock()
+	}
+	if err := json.NewEncoder(os.Stdout).Encode(genesis); err != nil {
+		utils.Fatalf("could not encode genesis")
+	}
+	return nil
+}
+
 func importChain(ctx *cli.Context) error {
 	if len(ctx.Args()) < 1 {
 		utils.Fatalf("This command requires an argument.")

cmd/geth/main.go

@@ -205,6 +205,7 @@ func init() {
 		copydbCommand,
 		removedbCommand,
 		dumpCommand,
+		dumpGenesisCommand,
 		inspectCommand,
 		// See accountcmd.go:
 		accountCommand,