I don't think that `/tmp` is always the OS's temp dir.  Consider that ruby provides an API to get this:

http://www.ruby-doc.org/stdlib-1.9.3/libdoc/tmpdir/rdoc/Dir.html#tmpdir-method

Thinking about it some more, is there any reason we should use the OS tmp dir?  Would it be better to use a local `./tmp` dir?  Then you don't have to worry about what the global temp dir is.