Increasingly, file-formats are not defined on single files, but on set of files usually bound together using an XML digest. One typical example of such file format is the OpenDocument format supported by OpenOffice. While Open-Office uses a Zip archive to bundle together the different files, many programs on Mac OS X use the notion of Bundle. A bundle is technically a directory with a special bit set that tells the file explorer to treat the directory as a single file.

While such files are less compact that a zip archive, the have the advantage of having all the logical entities (files) living directly on the file-system. This means in particular that you can use file-system tools like hard-links. One typical use of this trick I use to fill in forms. Many forms are distributed as PDF files, but without using PDF’s form functionality. They are just a page description of the form. To fill them in clearly, I typically import the PDF as a background image into Omnigraffle, and then add the text for the different fields. I like to keep a copy of the filled-in form, but also the clean form. What I do is I keep the Omnigraffle document and create a hard-link to the PDF file embedded inside the bundle outside it. This way I have two files, a pdf document (the form), and an omnigraffle document (the filled-in form), but the disk space is only used once.

The resulting files look something like this. Notice that the PDF file has a link count of two. In this particular the saving is negligible, but some PDF are quite large.

This trick can be used for all file-formats that rely on bundles, in particular Keynote files.