How to use

executeCommand

method

in

net.minecraft.command.ICommandManager

private Object[] doCommand(String command) {
  if (server != null && server.isCommandBlockEnabled()) {
    ICommandManager commandManager = server.getCommandManager();
    try {
      output.clear();
      int result = commandManager.executeCommand(this, command);
      return new Object[]{result > 0, Maps.newHashMap(output)};
    } catch (Throwable t) {
      return new Object[]{false, Collections.singletonMap(1, "Java Exception Thrown: " + t)};
    }
  }
  return new Object[]{false, Collections.singletonMap(1, ("Command blocks disabled by server"))};
}

public void execute(EntityPlayer player) {
  player.getServer().getCommandManager().executeCommand(player.getServer(), commandString.replaceAll("@p", player.getDisplayNameString()));
}

private static void summonFireworks(EntityLiving npc, Town town, BlockPos pos, int lifetime, int type, int flicker, int trail, String colors, String fadeColor) {
  pos = town.getCoordinatesFromOffset(HFBuildings.FESTIVAL_GROUNDS, pos);
  FMLCommonHandler.instance().getMinecraftServerInstance().getCommandManager().executeCommand(npc, "/summon FireworksRocketEntity "  + pos.getX() + " " + pos.getY() + " " + pos.getZ() + " {LifeTime:" + lifetime + ",FireworksItem:{id:fireworks,Count:1,tag:{Fireworks:{Explosions:[{Type:" + type + ",Flicker:" + flicker + ",Trail:" + trail + ",Colors:[" + colors + "],FadeColors:[" + fadeColor + "]}]}}}}");
}