Log shipping and shrinking transaction logs

I just solved a problem that had me worried for a bit. I'm log shipping from three primary servers to a single secondary server, and the transaction log disk on the secondary server was getting very full. I established that several primary databases had unused space that resulted from big, one-off updates so I could shrink their logs. But would this action be log shipped and applied to the secondary database too? I thought probably not. And, more importantly, would it break log shipping? My secondary databases are in a Standby / Read Only state so I didn't think I could shrink their logs. I RTFMd, Googled, and asked on a Q&A site (not the evil one) but was none the wiser.

So I was facing a monumental round of shrink, full backup, full secondary restore and re-start log shipping (which would leave us without a disaster recovery facility for the duration). Then I thought it might be worthwhile to take a non-essential database and just make absolutely sure a log shrink on the primary wouldn't ship over and occur on the secondary as well. So I did a DBCC SHRINKFILE and kept an eye on the secondary. Bingo! Log shipping didn't blink and the log on the secondary shrank too.

I just love it when something turns out even better than I dared to hope. (And I guess this highlights something I need to learn about what activities are logged.)