Secure way of deleting files in Unix

As I mentioned in the previous post there are simple techniques that can recover your deleted file (by using simple 'grep' or 'strings' utilities).
Even some data recovery tools does the same thing. So if you want to delete some data on the disk without being worried about the retrieval, then
you should probably over write the disk which has your file content.

shred utility available in Linux does the same thing.
shred actually overwrite the specified file repeatedly, in order to make it harder for even very expensive hardware probing to recover the data.
By default shred overwrites the file 25 times with the junk data.

you can chose to remove the file after over writing using -u (unlink)
There are multiple options with shred to explore.

$ shred -u filetobedeleted.txt

Just to see how it works, let say my script is writing some data to the file 'testlog.log' repeatedly after every 1 min.
I am tailing the file in one terminal. And in other terminal I did execute shred.
 

$ sh writetodisk.sh &

$ tail -f testlog.log
aaa
bbb
ccc
ddd

$ shred testlog.log

Now observe the terminal one

$ tail -f testlog.log
aaa
bbb
ccc
ddd
\{XÁÀà_ç æIƒòDÊ5žq­Æ 8<TÝõ ¬ S õŸt\Ê1’ïNÐ , éM‚?$Väé@. l"®ÎþÌÕæ ‡Ù+Ž’ bªO"× #f©ÎçN‰/h÷¡çÊhÇöŸz!*ÀA?RAo%æ} ÛZ½PSàpû7Íû3U_ ’e^u÷züê¾Ú6󚶄Ë[Fœ;½êê±î÷
                                                           Cý ]¤¥ˆi                                                                                  ÕÎ8ƒ:SÎq3®B h€'Q“ãªF¹X‘Q'†GÁ–oõ»hï eþ:½U4Úy_£È‘”f}"J_ŠÒ‡±Ê0íÕwº }rºŸoÇpÜ Wá‚À°xfeÒ?ÕC·         ‰JðhJë ™ÀQêM]ÞÑÅ,A {9b ÑùÇ@©}ÅŠ½°Ò¡øÜK-òõ ªLoLƒü
GýÑeÈ#WsG`Þ¼µÅ"–> T/~ [ºÝ ¸ýŒ<C8îzD±š¨š J
#Lwk{lû´köAٍ^0ê(9¿Ó Xnš¼¼ýc+7×Ãó ‡@ ;¥
                                        ŽBýˆÔ
                                              ÀF ⍠?’‰´q’+iQ‰ Y¸¯`± {·;²&%6ÈÄLYdù½­ š¼ÑÖi…ö±É* ÝÜ(Y2Ðc FÔ]þŠ ˜° ˜ƒTãðõ,l‚šl„bÜ8Å òU='µ YR™&iõqmôT ¤¿)“G[¡9îÎD $ˆÉšDÒ–„xFÀjKNs„)½3̆^¹°w

you can see that the shred filled the contents of the file with garbage data.