Shred tries to really remove any trace of a file from a disk. It seems like such an easy thing to do: even if you already understand that "rm" merely disassociates data blocks from an inode and returns them to the free list, why wouldn't just overwriting the file with a bigger file destroy the data? Well, because disks are mechanical devices and storage is quite fuzzy, so weak images of previous data are still present after a simple overwrite. Also, disk heads may move slightly out of track over time, leaving a magnetic trail "off to the side". Those problems and more are discussed in depth at Secure Deletion of Data from Magnetic and Solid-State Memory by Peter Gutmann.
Most of us have no need for such forensic obstructing scrubbing. Good thing that we don't, because the "info shred" page tells us that "shred --verbose /dev/fd0" could take 20 minutes. It's a bit quicker at running through a hard drive file but it still takes noticeable time.
Should you really need to do such a thing, be aware that there are lots of things that can trip you up - ordinary disk caching prevents "shred" from really doing what it intends. You can use "chattr +S" on the file before running shred to get the writes forced through the cache. Using "shred" is probably fine if you are giving away a computer and don't want its next owner to see your data (in which case you shred filesystems rather than files), but if you have serious reasons to need data securely destroyed, physical destruction of the media is the only real answer.
With Linux ext3, "shred" is a bit less necessary (from http://batleth.sapienti-sat.org/projects/FAQs/ext3-faq.html):
Q: How can I recover (undelete) deleted files from my ext3 partition? Actually, you can't! This is what one of the developers, Andreas Dilger, said about it: In order to ensure that ext3 can safely resume an unlink after a crash, it actually zeros out the block pointers in the inode, whereas ext2 just marks these blocks as unused in the block bitmaps and marks the inode as "deleted" and leaves the block pointers alone. Your only hope is to "grep" for parts of your files that have been deleted and hope for the best.
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