Make `--file` optional for `generate-node-key` (#7043)

This pr makes the `--file` argument optional to `generate-node-key`.
If the argument is not given, the secret node key will be printed to
`stdout`. The public node key will always be printed to `stderr`.

substrate/bin/utils/subkey/src/lib.rs

@@ -31,7 +31,8 @@ use sp_core::crypto::Ss58Codec;
 	about = "Utility for generating and restoring with Substrate keys",
 )]
 pub enum Subkey {
-	/// Generate a random node libp2p key, save it to file and print its peer ID
+	/// Generate a random node libp2p key, save it to file or print it to stdout
+	/// and print its peer ID to stderr.
 	GenerateNodeKey(GenerateNodeKeyCmd),
 
 	/// Generate a random account

substrate/client/cli/src/commands/generate_node_key.rs

@@ -26,26 +26,31 @@ use libp2p::identity::{ed25519 as libp2p_ed25519, PublicKey};
 #[derive(Debug, StructOpt)]
 #[structopt(
 	name = "generate-node-key",
-	about = "Generate a random node libp2p key, save it to file and print its peer ID"
+	about = "Generate a random node libp2p key, save it to \
+			 file or print it to stdout and print its peer ID to stderr"
 )]
 pub struct GenerateNodeKeyCmd {
 	/// Name of file to save secret key to.
+	///
+	/// If not given, the secret key is printed to stdout.
 	#[structopt(long)]
-	file: PathBuf,
+	file: Option<PathBuf>,
 }
 
 impl GenerateNodeKeyCmd {
 	/// Run the command
 	pub fn run(&self) -> Result<(), Error> {
-		let file = &self.file;
-
 		let keypair = libp2p_ed25519::Keypair::generate();
 		let secret = keypair.secret();
 		let peer_id = PublicKey::Ed25519(keypair.public()).into_peer_id();
+		let secret_hex = hex::encode(secret.as_ref());
 
-		fs::write(file, hex::encode(secret.as_ref()))?;
+		match &self.file {
+			Some(file) => fs::write(file, secret_hex)?,
+			None => print!("{}", secret_hex),
+		}
 
-		println!("{}", peer_id);
+		eprintln!("{}", peer_id);
 
 		Ok(())
 	}

substrate/client/cli/src/commands/key.rs

@@ -28,10 +28,11 @@ use super::{
 	generate_node_key::GenerateNodeKeyCmd,
 };
 
-/// key utilities for the cli.
+/// Key utilities for the cli.
 #[derive(Debug, StructOpt)]
 pub enum KeySubcommand {
-	/// Generate a random node libp2p key, save it to file and print its peer ID
+	/// Generate a random node libp2p key, save it to file or print it to stdout
+	/// and print its peer ID to stderr.
 	GenerateNodeKey(GenerateNodeKeyCmd),
 
 	/// Generate a random account