Changing the number in bars by macro?

Hi everyone, I have a game that i run with character sheet so the attack rolls are automated. i have made the Bar1 the health, and bar2 and 3 the defense and special defense. Can a macro apply an addition to the number in the bar1? The macro would be something like : bar1 = bar1 - [[?{What's the damage?|0} - @{selected|defense}]] As my understanding of macros, they are only graphicals and can't really interact with attributes but i'm asking to make sure. Also (if a macro can't) is there an API that do that?

Standard roll20 macro. Nope. With an API script, yes.

AlterBar Script - <a href="https://app.roll20.net/forum/permalink/1216260/" rel="nofollow">https://app.roll20.net/forum/permalink/1216260/</a> Alterbar is a simple script to alter the value of the bars on a token. TheAaron's TokenMod script is a much more powerful and complex script that can be used to alter all kinds of attributes and not just the token bars.

Thanks! I will check the alterbar and the tokenMod, the alterbar is maybe better because all my ennemy monsters are from rollable tables so their HP bar is not tied to a character sheet attribute.