Fixing Archive Annoyances with Ruby
Both your pack and unpack scripts will fail if any of the targets contain spaces (or other shell meta-characters) in their name. Generally it's almost always a bad idea to insert arguments into commands using string interpolation.

I assume you're using backticks rather than system because you don't want the output of the invoked commands to go to the screen. In that case you can use IO.popen, which since ruby 1.9 can take an array as its first argument.

As a trivial sidenote: There doesn't seem to be any point in passing the -v flag to tar if you're not going to print its output, so I'd remove it.