- Logging File Accesses
Log all accesses to the files in /etc during the course of
a single day. This information should include the filename,
user name, and access time. If any alterations to the
files take place, that should be flagged. Write this data
as neatly formatted records in a logfile.
- Strip Comments
Strip all comments from a shell script whose name
is specified on the command line. Note that the "#!
line" must not be stripped out.
- HTML Conversion
Convert a given text file to HTML. This non-interactive
script automatically inserts all appropriate HTML tags into
a file specified as an argument.
- Strip HTML Tags
Strip all HTML tags from a specified HTML file, then
reformat it into lines between 60 and 75 characters
in length. Reset paragraph and block spacing, as
appropriate, and convert HTML tables to their approximate
text equivalent.
- XML Conversion
Convert an XML file to both HTML and text format.
- Chasing Spammers
Write a script that analyzes a spam e-mail by doing
DNS lookups on the IP addresses in the headers to identify
the relay hosts as well as the originating ISP. The
script will forward the unaltered spam message to the
responsible ISPs. Of course, it will be necessary to
filter out your own ISP's IP address,
so you don't end up complaining about yourself.
As necessary, use the appropriate network analysis commands.
- Morse Code
Convert a text file to Morse code. Each character of the
text file will be represented as a corresponding Morse
code group of dots and dashes (underscores), separated by
whitespace from the next. For example, "script"
===> "... _._. ._. .. .__. _".
- Hex Dump
Do a hex(adecimal) dump on a binary file
specified as an argument. The output should be in neat
tabular fields, with the first field showing the address,
each of the next 8 fields a 4-byte hex number, and the final
field the ASCII equivalent of the previous 8 fields.
- Emulating a Shift Register
Using Example 26-9 as an inspiration,
write a script that emulates a 64-bit shift register as
an array. Implement
functions to load the register,
shift left, and shift
right. Finally, write a function that
interprets the register contents as eight 8-bit ASCII
characters.
- Determinant
Solve a 4 x 4 determinant.
- Hidden Words
Write a "word-find" puzzle generator,
a script that hides 10 input words in a 10 x 10 matrix
of random letters. The words may be hidden across, down,
or diagonally.
- Anagramming
Anagram 4-letter input. For example, the
anagrams of word are:
do or rod row word. You may use
/usr/share/dict/linux.words as the
reference list.
- Fog Index
The "fog index" of a passage of text
estimates its reading difficulty, as a number corresponding
roughly to a school grade level. For example, a passage
with a fog index of 12 should be comprehensible to anyone
with 12 years of schooling.
The Gunning version of the fog index uses the following
algorithm.
Choose a section of the text at least
100 words in length.
Count the number of sentences (a portion of
a sentence truncated by the boundary of the text section
counts as one).
Find the average number of words per
sentence.
AVE_WDS_SEN = TOTAL_WORDS / SENTENCES
Count the number of "difficult"
words in the segment -- those containing at least
3 syllables. Divide this quantity by total words to
get the proportion of difficult words.
PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS
The Gunning fog index is the sum of the above two
quantities, multiplied by 0.4, then rounded to the
nearest integer.
G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )
Step 4 is by far the most difficult portion of the
exercise. There exist various algorithms for estimating
the syllable count of a word. A rule-of-thumb formula
might consider the number of letters in a word and the
vowel-consonant mix.
A strict interpretation of the Gunning Fog index does
not count compound words and proper nouns as
"difficult" words, but this would enormously
complicate the script.
- Calculating PI using Buffon's Needle
The Eighteenth Century French mathematician de Buffon
came up with a novel experiment. Repeatedly drop a
needle of length "n" onto a wooden floor
composed of long and narrow parallel boards. The cracks
separating the equal-width floorboards are a fixed distance
"d" apart. Keep track of the total drops and
the number of times the needle intersects a crack on the
floor. The ratio of these two quantities turns out to be
a fractional multiple of PI.
In the spirit of Example 12-35, write a
script that runs a Monte Carlo simulation of Buffon's
Needle. To simplify matters, set the needle length equal to
the distance between the cracks, n = d.
Hint: there are actually two critical variables:
the distance from the center of the needle to the nearest
crack to it, and the angle of the needle to that crack. You
may use bc to handle the
calculations.
- Playfair Cipher
Implement the Playfair (Wheatstone) Cipher in a
script.
The Playfair Cipher encrypts text by substitution
of each 2-letter "digram"
(grouping). Traditionally, one would use a 5 x 5 letter
scrambled alphabet code key square for the encryption
and decryption.
C O D E S
A B F G H
I K L M N
P Q R T U
V W X Y Z
Each letter of the alphabet appears once, except "I" also represents
"J". The arbitrarily chosen key word, "CODES" comes first, then all the
rest of the alphabet, skipping letters already used.
To encrypt, separate the plaintext message into digrams (2-letter
groups). If a group has two identical letters, delete the second, and
form a new group. If there is a single letter left over at the end,
insert a "null" character, typically an "X".
THIS IS A TOP SECRET MESSAGE
TH IS IS AT OP SE CR ET ME SA GE
For each digram, there are three possibilities.
----------------------------------------------
1) Both letters will be on the same row of the key square
For each letter, substitute the one immediately to the right, in that
row. If necessary, wrap around left to the beginning of the row.
or
2) Both letters will be in the same column of the key square
For each letter, substitute the one immediately below it, in that
row. If necessary, wrap around to the top of the column.
or
3) Both letters will form the corners of a rectangle within the key
square. For each letter, substitute the one on the other corner the
rectangle which lies on the same row.
The "TH" digram falls under case #3.
G H
M N
T U (Rectangle with "T" and "H" at corners)
T --> U
H --> G
The "SE" digram falls under case #1.
C O D E S (Row containing "S" and "E")
S --> C (wraps around left to beginning of row)
E --> S
=========================================================================
To decrypt encrypted text, reverse the above procedure under cases #1
and #2 (move in opposite direction for substitution). Under case #3,
just take the remaining two corners of the rectangle.
Helen Fouche Gaines' classic work, "Elementary Cryptoanalysis" (1939), gives a
fairly detailed rundown on the Playfair Cipher and its solution methods. |
This script will have three main sections
Generating the "key square",
based on a user-input keyword.
Encrypting a "plaintext"
message.
Decrypting encrypted
text.
The script will make extensive use of arrays and functions.
