You're welcome!
Since you're only taking action if 'arh' is greater than zero in string format*, the above could be written as:
(if (< "0" arh) (command "_.extrude" "_all" "" arh))
*Consider either converting the value to an integer or using the not equal operator '/=' since, whilst it is possible to compare strings in this manner, the strings will be compared on a character-by-character basis comparing the ASCII code for each character, which may not yield an expected result - e.g. (< "10" "2") would return True.