Display custom command usage when it has no step (#264)

Use a Cobra PreRun function to end or abort a custom command when its
invocation is incorrect or incomplete.

Co-authored-by: Andriy Knysh <aknysh@users.noreply.github.com>

cmd/cmd_utils.go

@@ -51,6 +51,9 @@ func processCustomCommands(commands []cfg.Command, parentCommand *cobra.Command,
 				Use:   commandConfig.Name,
 				Short: commandConfig.Description,
 				Long:  commandConfig.Description,
+				PreRun: func(cmd *cobra.Command, args []string) {
+					preCustomCommand(cmd, args, parentCommand, commandConfig)
+				},
 				Run: func(cmd *cobra.Command, args []string) {
 					executeCustomCommand(cmd, args, parentCommand, commandConfig)
 				},
@@ -94,15 +97,26 @@ func processCustomCommands(commands []cfg.Command, parentCommand *cobra.Command,
 	return nil
 }
 
-// executeCustomCommand executes a custom command
-func executeCustomCommand(cmd *cobra.Command, args []string, parentCommand *cobra.Command, commandConfig *cfg.Command) {
+// preCustomCommand is run before a custom command is executed
+func preCustomCommand(cmd *cobra.Command, args []string, parentCommand *cobra.Command, commandConfig *cfg.Command) {
 	var err error
 
 	if len(args) != len(commandConfig.Arguments) {
 		err = fmt.Errorf("invalid number of arguments, %d argument(s) required", len(commandConfig.Arguments))
 		u.PrintErrorToStdErrorAndExit(err)
 	}
 
+	// no steps means a sub command should be specified
+	if len(commandConfig.Steps) == 0 {
+		cmd.Help()
+		os.Exit(0)
+	}
+}
+
+// executeCustomCommand executes a custom command
+func executeCustomCommand(cmd *cobra.Command, args []string, parentCommand *cobra.Command, commandConfig *cfg.Command) {
+	var err error
+
 	// Execute custom command's steps
 	for i, step := range commandConfig.Steps {
 		// Prepare template data for arguments