
Terminal - securely overwrite a file with random junk, rename it to clear the directory entry and finally delete it
shred -vzu /tmp/junk-file-to-be-shredded
This is sample output - yours may be different.
$ shred -vzu /tmp/junk-file-to-be-shredded
shred: /tmp/junk-file-to-be-shredded: pass 1/26 (random)...
shred: /tmp/junk-file-to-be-shredded: pass 2/26 (dddddd)...
shred: /tmp/junk-file-to-be-shredded: pass 3/26 (bbbbbb)...
shred: /tmp/junk-file-to-be-shredded: pass 4/26 (333333)...
shred: /tmp/junk-file-to-be-shredded: pass 5/26 (249249)...
shred: /tmp/junk-file-to-be-shredded: pass 6/26 (eeeeee)...
shred: /tmp/junk-file-to-be-shredded: pass 7/26 (cccccc)...
shred: /tmp/junk-file-to-be-shredded: pass 8/26 (888888)...
shred: /tmp/junk-file-to-be-shredded: pass 9/26 (ffffff)...
shred: /tmp/junk-file-to-be-shredded: pass 10/26 (666666)...
shred: /tmp/junk-file-to-be-shredded: pass 11/26 (555555)...
shred: /tmp/junk-file-to-be-shredded: pass 12/26 (777777)...
shred: /tmp/junk-file-to-be-shredded: pass 13/26 (random)...
shred: /tmp/junk-file-to-be-shredded: pass 14/26 (444444)...
shred: /tmp/junk-file-to-be-shredded: pass 15/26 (db6db6)...
shred: /tmp/junk-file-to-be-shredded: pass 16/26 (b6db6d)...
shred: /tmp/junk-file-to-be-shredded: pass 17/26 (000000)...
shred: /tmp/junk-file-to-be-shredded: pass 18/26 (999999)...
shred: /tmp/junk-file-to-be-shredded: pass 19/26 (492492)...
shred: /tmp/junk-file-to-be-shredded: pass 20/26 (aaaaaa)...
shred: /tmp/junk-file-to-be-shredded: pass 21/26 (924924)...
shred: /tmp/junk-file-to-be-shredded: pass 22/26 (6db6db)...
shred: /tmp/junk-file-to-be-shredded: pass 23/26 (222222)...
shred: /tmp/junk-file-to-be-shredded: pass 24/26 (111111)...
shred: /tmp/junk-file-to-be-shredded: pass 25/26 (random)...
shred: /tmp/junk-file-to-be-shredded: pass 26/26 (000000)...
shred: /tmp/junk-file-to-be-shredded: removing
shred: /tmp/junk-file-to-be-shredded: renamed to /tmp/000000000000000000000000
shred: /tmp/000000000000000000000000: renamed to /tmp/00000000000000000000000
shred: /tmp/00000000000000000000000: renamed to /tmp/0000000000000000000000
shred: /tmp/0000000000000000000000: renamed to /tmp/000000000000000000000
shred: /tmp/000000000000000000000: renamed to /tmp/00000000000000000000
shred: /tmp/00000000000000000000: renamed to /tmp/0000000000000000000
shred: /tmp/0000000000000000000: renamed to /tmp/000000000000000000
shred: /tmp/000000000000000000: renamed to /tmp/00000000000000000
shred: /tmp/00000000000000000: renamed to /tmp/0000000000000000
shred: /tmp/0000000000000000: renamed to /tmp/000000000000000
shred: /tmp/000000000000000: renamed to /tmp/00000000000000
shred: /tmp/00000000000000: renamed to /tmp/0000000000000
shred: /tmp/0000000000000: renamed to /tmp/000000000000
shred: /tmp/000000000000: renamed to /tmp/00000000000
shred: /tmp/00000000000: renamed to /tmp/0000000000
shred: /tmp/0000000000: renamed to /tmp/000000000
shred: /tmp/000000000: renamed to /tmp/00000000
shred: /tmp/00000000: renamed to /tmp/0000000
shred: /tmp/0000000: renamed to /tmp/000000
shred: /tmp/000000: renamed to /tmp/00000
shred: /tmp/00000: renamed to /tmp/0000
shred: /tmp/0000: renamed to /tmp/000
shred: /tmp/000: renamed to /tmp/00
shred: /tmp/00: renamed to /tmp/0
shred: /tmp/junk-file-to-be-shredded: removed
securely overwrite a file with random junk, rename it to clear the directory entry and finally delete it
It's worth pointing out that this isn't reliable on most modern (journalling) filesystems, including many installed by default (particularly on Linux: ext3, ext4).
-n33 #when you overwrite more than the number of bits of your CPU you ensure that it is reliable
for 64-bit folks use -n65 ... have fun waiting ... or use screen ...