Advanced Bash−Scripting Guide

# blotout.sh: Erase all traces of a file.
# This script overwrites a target file alternately
#+ with random bytes, then zeros before finally deleting it.
# After that, even examining the raw disk sectors
#+ will not reveal the original file data.
PASSES=7
BLOCKSIZE=1
E_BADARGS=70
E_NOT_FOUND=71
E_CHANGED_MIND=72
# Number of file−shredding passes.
# I/O with /dev/urandom requires unit block size,
#+ otherwise you get weird results.
if [ −z "$1" ] # No filename specified.
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
file=$1
if [ ! −e "$file" ]
then
echo "File \"$file\" not found."
exit $E_NOT_FOUND
fi
Advanced Bash−Scripting Guide
echo; echo −n "Are you absolutely sure you want to blot out \"$file\" (y/n)? "
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
*) echo "Blotting out file \"$file\".";;
esac
flength=$(ls −l "$file" | awk '{print $5}') # Field 5 is file length.
pass_count=1
echo
while [ "$pass_count" −le "$PASSES" ]
do
echo "Pass #$pass_count"
sync
# Flush buffers.
dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength
# Fill with random bytes.
sync
# Flush buffers again.
dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength
# Fill with zeros.
sync
# Flush buffers yet again.
let "pass_count += 1"
echo
done
rm −f $file
sync
# Finally, delete scrambled and shredded file.
# Flush buffers a final time.
Chapter 12. External Filters, Programs and Commands 210