Some people make a living recovering data from old harddisks. If that knowledge makes you nervous, here is a command you will want to remember: "shred"
Shred is a real powerful command . . . . much better then just deleting-- it shreds a file, or wipes a partition, so that absolutely no recovery is possible ( even those businesses specialized in data-recovery will not be able to get it back . . . so be careful )
Here is the basics of the command:
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$ shred -n 50 -z -u secret.txt
The -n 50 means that the file "secret.txt" will be overwritten 50 times. The -z means add a final overwrite with zeros to hide the shredding. The -u makes sure the file is removed after shred is done with it.
You can shred a complete drive or partition too:
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# shred -n 50 -z /dev/hda
This will make sure that no data will be found when you send this drive to the local dump <g>
Bruno
NOTE: Shredding a single file on a partition with a journaled filesystem can be less permanent as one might think ( because of the journal ), shredding the complete partition is needed to be absolutely sure.