A caution for macOS users

["Brian L. Matthews" <blmatthews@gmail.com>]

macOS has things known as packages and bundles. At the filesystem level, 
they're just directories, but at the Finder level they look like files. 
Applications are both packages and bundles, and some applications write 
data files that are one of the two (for example, Quicken's data files 
and sparse bundle disk images are packages, etc.)

Packages and bundles should be considered atomic. Applications consist 
of numerous files--the actual executable code, localized strings, page 
layouts, etc.--and reasonably expect all that stuff to be there. 
Similarly, if an application stores its data files as a package, it 
would expect the various files in the package to be present, and if 
they're not some data may be lost or the file might not even be openable.

How this applies to tarsnap is that tarsnap is looking at the file 
system, so packages and bundles are just directories, and it would be 
easy to accidentally exclude some file or directory in a package that 
renders the package useless upon restore. If you're not specifying any 
excludes, it won't be a problem, but if you are excluding some files or 
directories, it wouldn't hurt to turn on -v and make sure everything in 
any packages you're backing up is being backed up.

While it's arguably a bug in tarsnap that this can happen--tarsnap's job 
is to back things up that can then be fully restored--it's also not 
trivial to fix in a reasonable way, for example as far as I know the 
only way to determine if a directory is a package is to query the 
metadata database, and while there's a command line tool to do this 
(mdls), running a command on (most) every directory would be expensive.

Brian