Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo "File \"$file\" blotted out and deleted."; echo
# This is a fairly secure, if inefficient and slow method
#+ of thoroughly "shredding" a file. The "shred" command,
#+ part of the GNU "fileutils" package, does the same thing,
#+ but more efficiently.
# The file cannot not be "undeleted" or retrieved by normal methods.
# However...
#+ this simple method will likely *not* withstand forensic analysis.
# Tom Vier's "wipe" file−deletion package does a much more thorough job
#+ of file shredding than this simple script.
# http://www.ibiblio.org/pub/Linux/utils/file/wipe−2.0.0.tar.bz2
# For an in−depth analysis on the topic of file deletion and security,
#+ see Peter Gutmann's paper,
#+ "Secure Deletion of Data From Magnetic and Solid−State Memory".
# http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html
exit 0
od
The od, or octal dump filter converts input (or files) to octal (base−8) or other bases. This is useful for
viewing or processing binary data files or otherwise unreadable system device files, such as
/dev/urandom, and as a filter for binary data. See Example 9−26 and Example 12−10.
hexdump
Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough
equivalent of od, above, but not nearly as useful.
objdump
Displays an object file or binary executable in either hexadecimal form or as a disassembled listing
(with the −d option).
bash$ objdump −d /bin/ls
/bin/ls: file format elf32−i386
Disassembly of section .init:
080490bc :
80490bc: 55 push %ebp
80490bd: 89 e5 mov %esp,%ebp
. . .
mcookie
This command generates a "magic cookie", a 128−bit (32−character) pseudorandom hexadecimal
number, normally used as an authorization "signature" by the X server. This also available for use in a
script as a "quick 'n dirty" random number.
random000=`mcookie | sed −e '2p'`
# Uses 'sed' to strip off extraneous characters.
Of course, a script could use md5 for the same purpose.
# Generate md5 checksum on the script itself.
random001=`md5sum $0 | awk '{print $1}'`
Chapter 12. External Filters, Programs and Commands 211