I am looking for the Linux command (or commands) that zeros any sectors on the hard drive that contain deleted data. Pretty much the same as what "secure delete" apps do for Windows, but without all of the hashing and rewriting. I am trying to make compression on my backup images of Vmware virtual machines more efficient and I read that an easy way to do that is to zero deleted sectors. Does anyone know how to do this?

I think you meant this http://www.howtoforge.com/how-to-shr...isk-files-vmdk

I don't think it makes that much difference, correct me if I'm wrong.

I'm a little fuzzy on what you want to do.

First, "deleted data" and "deleted sectors" are not the same thing.

"Secure delete" I assume refers to erasing a file in such a way that it could never be recovered. To do this on an individual file presumably involves replacing the file data with random data and then all zeroes---just as is done when "wiping" a whole drive or partition.

If you just want to zero things out in order to aid compression, it is easy.

dd if=/dev/zero of=/my/output/file/on/the/partition/to/be/zeroed

This command will run until the hard drive is full, then it will bomb. When it bombs all you do is delete the file you just wrote, and you have accomplished what you set out to accomplish.

That looks like a REALLY dangerous command. If it works the way you say, then the user might expect to zero out a file, and wind up zeroing much more.

It is no more dangerous than any other command that creates a file on a filesystem.

I disagree.... Again, if it works as you describe, it does a lot more than just create a file.

The original question (I think) is how to do secure erase of one or more specific files. Your command goes way beyond that.

Have you actually tried it? Cna you post an example?

I do it periodically. Especially when I want to image and compress a partition.

It works exactly as I describe. It writes a zero filled file. Lacking any instructions on the size of the file, it writes until the partition is full. This has the effect of zero-filling all previously unused space on the partition. Then you delete the file, thus freeing all the space and leaving it zero filled.

And why, exactly, do you think it does anything other than that? Because it uses dd? dd writes with user permissions, just like every other command out there. If you try to write as root to /dev/sd-something, I am sure it would be totally destructive. Short of that, it works perfectly.

Sorry---I was being dense.
Probably...

You could do cat /dev/zero > /filename till the hard drive fills also.

That's exactly what jiml was talking about.....