How to delete file permanently in Linux?

Most of Linux distributions have shred already installed. So it’s really easy to delete a file in a secure manner:

shred -u -z -n 26 topsecret.txt

Meaning of used switches:

-n [N]	Overwrite a file N times. For example, -n 20 will perform twenty passes over the file’s contents.
-u	Remove the file after you’ve shredded it. You’ll probably want to use this option in most cases.
-z	After shredding a file with random bits (ones and zeros), overwrite the file with only zeros. This is used to try and hide the fact that the file was shredded.

There is a Windows gui alternative available at http://www.fileshredder.org/.