> 1- (priority) Emptying my trash seemed to delete over 30,000 items. But it seems to have stopped with message "The operation can’t be completed because the item “Cache_Data” is in use." Should I skip, stop, or continue?
Ultimately, some files might be in use - for example, if you're running an application that's using one of the cache files, it will have a 'handle' to that file which will stick even if the file is moved (say from ~/Library/Caches/ to ~/Desktop). You cannot delete 'open' files that applications are actively using.
In this case, I'd probably opt to skip these files. The Finder should continue to delete the other files. You can then quit the application(s) in question to delete the offending files (or reboot, which forces a release of the lock).
> 2- I'm trying to learn to take better care of my computer. I read that cache naturally clears on sleep mode.
You heard wrong. Sleep does nothing to cache files. You could have a thousand cache files, put your system to sleep for a week, and you'd still have a thousand cache files when you wake it back up. Nothing will affect these files during sleep.
> Is it the same for shutting it off? Clearly I built up a bunch over the years, should I make sure to close programs when I'm done with them?
Closing applications when you're not using them is always a good idea, IMHO.
Ordinarily applications are responsible for managing their cache files. Some applications will clean up their caches when you quit, and if you never quit, these files aren't going to get addressed.
Different applications also manage caches in different ways. For example, an application may limit itself to, say, 1GB of cache data, so as soon as something new is added to the cache that exceeds this limit, it throws away something it no longer needs, typically based on age (oldest item gets purged) or least used (content that hasn't been used/read the longest).
Only the application knows the 'relevance' of any data in the cache, so the OS isn't going to delete them arbitrarily.
> 3- Just to confirm: these caches are temporary to help apps function and go quickly, but don't hold my actual documents. for ex: if i downloaded a picture from the internet, the cache would remember to keep that handy, but if it got cleared i could just redownload it?
Absolutely - a good example of cache files. Nothing critical should be in the cache. If an application expects something to be there ("hmm... I'm pretty sure I downloaded this image before, so it should be in the cache...") but its not there because you manually deleted it, then it would just automatically recreate/redownload it.
