AFSC (Apple File System Compression) tool is an utility that can be used to apply HFS+ compression to file(s), decompress HFS+ compressed file(s), or get information about existing HFS+ compressed file(s). Mac OS 10.6 or later is required.
Latest version is 1.6.4 (build 34)
brkirch's Blog 
Great little tool! If you’re still developing this then it’d be great to have a flag for receiving more structured output for processing. For example, afsctool -p foo.bar could return 1 or 0 indicating whether a file is compressed (or not), with the -v flag it could also return extra tab-delimitted fields for the size before and size after etc., just to make it easier for a shell script to process the results?
Sure, if I decide to make any updates to afsctool in the future (that aren’t just bug fixes) then I’ll try to create options for easier to parse output.
I’m trying to use afsctool and naturally I’m starting with big files, like my World of Warcraft install, and I’m getting things like:
$ sudo afsctool -cv expansion4.MPQ
/Applications/World of Warcraft/Data/expansion4.MPQ:
Unable to compress file.
File content type: dyn.ah62d4rv4ge8046dv
File data fork size (reported size by Mac OS X Finder): 1561768336 bytes / 1.56 GB (gigabytes) / 1.45 GiB (gibibytes)
Number of extended attributes: 0
Total size of extended attribute data: 0 bytes
Approximate overhead of extended attributes: 0 bytes
Approximate total file size (data fork + resource fork + EA + EA overhead + file overhead): 1561772280 bytes / 1.56 GB (gigabytes) / 1.45 GiB (gibibytes)
Any idea why it says “unable to compress file”?
Neat! Feature request from me would be a daemon that automatically compresses any new files added to a compressed folder. Keep up the good work!
This is an awesome tool. Thank you.
It now lives as a cron job that gets run over my MobileSync and Mail directories in ~/Library every night. This is doing wonders to save my Air’s limited SSD space.
Thank you!