How to get control parameter in .ctl file

[Boyin Huang]

Assuming I have a gs control file for 5-day average data file, which
contains 2001 points data in time (But the number, 2001 in example, is
changing from time to time):

tdef 2001  linear 3jan1979 5dy

I only want to process last 10 average data points, nothing else.

This means I need to know the number of valid tdef.

What should I do? Is there any system variable could be used directly
since it should be available after open statement?

Thanks