Use rules on dicrionary

Hello,
really stupid and (probably) really simple question:

How to generate own dictionary based on rules?

I mean I have some directory (let's say rockyou.txt).
I and want to apply one rules file (like best64.rule).
And I want to make (big) directory rockyou+best64. How do I make so? (linux).

I found tons of tools, utils (https://hashcat.net/wiki/doku.php?id=hashcat_utils)
but non to like "play" directory+rules.

Please note that I dont want to use entire hashCat program (like with -1 option), but just small util that will apply "rules" to "txt file" .. and to pipe output somewhere else.

But I google for quite some time without success, thanks.