private void printHelpMessage() {
    for (CommandHandler commandHandler : commandHandlers) {
      commandHandler.printHelpMessage();
    }
    writer.println("HELP <COMMAND> to get help for a specific command.");
    writer.flush();
  }
Actually run the shell. Does not return until user choose to exit.
samza-sql-shell/src/main/java/org/apache/samza/sql/client/cli/CliShell.java