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Securely delete files in linux

I often use shred in linux, when deleting anything sensitive.  Any way I've used it has always been pretty simple and straight forward (ie shred -uvz filename).  Today I sold my old cell phone with microSD card on eBay and I wanted to get all of my data off securely.  Unfortunately, it seems shred does not know how to handle directories.  A little googling found me this:

find -type f -execdir shred -uvz '{}' \;

The find command lists every file in your present working directory and subdirectories and executes the shred command on it.  I finished up with an rm -rf * to delete any remaining directories and I was then comfortable sending the card out with the phone.