U use scripts when you want to avoid repetition of tasks over and over.
In your case you can use batch, and that is the easiest way. Scripts inside TC don't exist, you can eventualy use plugin for that, which is not very well documented. Batch is documented on other hand and people know how to do it and it is beyond TC. U can alternatively use VBscript or JavaScript but, as I said before, for your primer, batch is no more then several lines.
or I have to use external programs running in background (and using resources...) ?
everything is using resources. Plugins are like 3thd party programs just with less overhead but if they are programmed bad, they can eat more resources then starting another application. So, you can't do anything here until TC have its own scripts. Batch use console applications, which have no GUI (usualy) and are usualy not made by lamers, so, again, batch is the best for you.
If you use batch u have some tricks to hide its black winodow from appearing.