How to securely erase data from HDD in Linux

[ZF]

dd if=/dev/urandom of=/dev/sdb bs=8b conv=notrunc

Istead of using dd, you could use "dcfldd", which is dd with more features, including a "statusinterval" so you can see the progress:
Code:
sudo dcfldd if=/dev/zero of=/dev/sdb statusinterval=10 bs=10M conv=notrunc
//notrunc is required so that slack space in each blocks are not skipped.

Use the built in SHRED command :
shred -n 1 --random-source=/dev/random /dev/sda

[ZF]

badblocks -w -p 0 -s /dev/sda


Destructive BadBlock test (1gig ram in machine)
badblocks -b 4096 -c 98304 -p 0 -w -s /dev/hda
//-b block size
//-c is to use ram to speed things up
//-p is number of pass, if 0 = run once and stop
//-w is to write

For Non destructive, remove the -w
badblocks -b 4096 -c 98304 -p 0 -s /dev/hda

[ZF]

Indicator for dd:

>watch kill -USR1 <dd pid>

[ZF]

Securely erase files from a directory
find <foldername> -type f | while read line; do dd if=/dev/zero of=$line bs=512 conv=notrunc,noerror; done