writingscripts.html

一本完整的描述Unix Shell 编程的工具书的所有范例

CLASS="COMMAND"
>Testing Passwords</B
></DT
><DD
><P
>Write a script to check and validate passwords. The object
	      is to flag <SPAN
CLASS="QUOTE"
>"weak"</SPAN
> or easily guessed password
	      candidates.</P
><P
>A trial password will be input to the script as a
	       command line parameter. To be considered acceptable,
	       a password must meet the following minimum qualifications:

            <UL
><LI
><P
>Minimum length of 8 characters</P
></LI
><LI
><P
>Must contain at least one numeric character</P
></LI
><LI
><P
>Must contain at least one of the following
		  non-alphabetic characters: <SPAN
CLASS="TOKEN"
>@</SPAN
>,
		  <SPAN
CLASS="TOKEN"
>#</SPAN
>, <SPAN
CLASS="TOKEN"
>$</SPAN
>, <SPAN
CLASS="TOKEN"
>%</SPAN
>,
		  <SPAN
CLASS="TOKEN"
>&#38;</SPAN
>, <SPAN
CLASS="TOKEN"
>*</SPAN
>, <SPAN
CLASS="TOKEN"
>+</SPAN
>,
		  <SPAN
CLASS="TOKEN"
>-</SPAN
>, <SPAN
CLASS="TOKEN"
>=</SPAN
></P
></LI
></UL
></P
><P
>Optional:

            <UL
><LI
><P
>Do a dictionary check on every sequence of at least
		  four consecutive alphabetic characters in the password under
		  test. This will eliminate passwords containing embedded
		  <SPAN
CLASS="QUOTE"
>"words"</SPAN
> found in a standard dictionary.</P
></LI
><LI
><P
>Enable the script to check all the passwords on your
		  system. These may or may not reside in
		  <TT
CLASS="FILENAME"
>/etc/passwd</TT
>.</P
></LI
></UL
></P
><P
>This exercise tests mastery of <A
HREF="regexp.html#REGEXREF"
>Regular Expressions</A
>.</P
></DD
><DT
><B
CLASS="COMMAND"
>Logging File Accesses</B
></DT
><DD
><P
>Log all accesses to the files in <TT
CLASS="FILENAME"
>/etc</TT
> 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.</P
></DD
><DT
><B
CLASS="COMMAND"
>Monitoring Processes</B
></DT
><DD
><P
>Write a script to continually monitor all running
	      processes and to keep track of how many child processes each
	      parent spawns. If a process spawns more than five children,
	      then the script sends an e-mail to the system administrator
	      (or root) with all relevant information, including the
	      time, PID of the parent, PIDs of the children, etc. The
	      script writes a report to a log file every ten minutes.
	      </P
></DD
><DT
><B
CLASS="COMMAND"
>Strip Comments</B
></DT
><DD
><P
>Strip all comments from a shell script whose name
	      is specified on the command line. Note that the <SPAN
CLASS="QUOTE"
>"#!
	      line"</SPAN
> must not be stripped out.</P
></DD
><DT
><B
CLASS="COMMAND"
>HTML Conversion</B
></DT
><DD
><P
>Convert a given text file to HTML. This non-interactive
	      script automatically inserts all appropriate HTML tags into
	      a file specified as an argument.</P
></DD
><DT
><B
CLASS="COMMAND"
>Strip HTML Tags</B
></DT
><DD
><P
>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.</P
></DD
><DT
><B
CLASS="COMMAND"
>XML Conversion</B
></DT
><DD
><P
>Convert an XML file to both HTML and text format.</P
></DD
><DT
><A
NAME="CSPAMMERS"
></A
><B
CLASS="COMMAND"
>Chasing Spammers</B
></DT
><DD
><P
> 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 <I
CLASS="EMPHASIS"
>your own ISP's IP address</I
>,
	      so you don't end up complaining about yourself.</P
><P
>As necessary, use the appropriate <A
HREF="communications.html#COMMUNINFO1"
>network analysis commands</A
>.</P
><P
>For some ideas, see <A
HREF="communications.html#ISSPAMMER"
>Example 12-37</A
> and <A
HREF="contributed-scripts.html#ISSPAMMER2"
>Example A-27</A
>.</P
><P
>Optional: Write a script that searches through a batch of
	      e-mail messages and deletes the spam according to specified
	      filters.</P
></DD
><DT
><B
CLASS="COMMAND"
>Creating man pages</B
></DT
><DD
><P
>Write a script that automates the process of creating
	      <I
CLASS="EMPHASIS"
>man pages</I
>.</P
><P
>Given a text file which contains information to be
	      formatted into a <I
CLASS="EMPHASIS"
>man page</I
>, the
	      script will read the file, then invoke the appropriate
	      <A
HREF="textproc.html#GROFFREF"
>groff</A
> commands to
	      output the corresponding <I
CLASS="EMPHASIS"
>man page</I
>
	      to <TT
CLASS="FILENAME"
>stdout</TT
>. The text file contains
	      blocks of information under the standard <I
CLASS="EMPHASIS"
>man
	      page</I
> headings, i.e., <SPAN
CLASS="QUOTE"
>"NAME,"</SPAN
>
	      <SPAN
CLASS="QUOTE"
>"SYNOPSIS,"</SPAN
> <SPAN
CLASS="QUOTE"
>"DESCRIPTION,"</SPAN
>
	      etc.</P
><P
>See <A
HREF="textproc.html#MANVIEW"
>Example 12-26</A
>.</P
></DD
><DT
><B
CLASS="COMMAND"
>Morse Code</B
></DT
><DD
><P
>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, <SPAN
CLASS="QUOTE"
>"script"</SPAN
>
	      ===&#62; <SPAN
CLASS="QUOTE"
>"... _._. ._. .. .__. _"</SPAN
>.</P
></DD
><DT
><B
CLASS="COMMAND"
>Hex Dump</B
></DT
><DD
><P
>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.</P
></DD
><DT
><B
CLASS="COMMAND"
>Emulating a Shift Register</B
></DT
><DD
><P
>Using <A
HREF="arrays.html#STACKEX"
>Example 26-14</A
> as an inspiration,
	      write a script that emulates a 64-bit shift register as
	      an <A
HREF="arrays.html#ARRAYREF"
>array</A
>.  Implement
	      functions to <I
CLASS="EMPHASIS"
>load</I
> the register,
	      <I
CLASS="EMPHASIS"
>shift left</I
>, <I
CLASS="EMPHASIS"
>shift
	      right</I
>, and <I
CLASS="EMPHASIS"
>rotate</I
>
	      it. Finally, write a function that interprets the register
	      contents as eight 8-bit ASCII characters.</P
></DD
><DT
><B
CLASS="COMMAND"
>Determinant</B
></DT
><DD
><P
>Solve a 4 x 4 determinant.</P
></DD
><DT
><B
CLASS="COMMAND"
>Hidden Words</B
></DT
><DD
><P
>Write a <SPAN
CLASS="QUOTE"
>"word-find"</SPAN
> 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.</P
><P
>Optional: Write a script that <I
CLASS="EMPHASIS"
>solves</I
>
	      word-find puzzles. To keep this from becoming too difficult,
	      the solution script will find only horizontal and vertical
	      words. (Hint: Treat each row and column as a string, and
	      search for substrings.)</P
></DD
><DT
><B
CLASS="COMMAND"
>Anagramming</B
></DT
><DD
><P
> Anagram 4-letter input. For example, the
	      anagrams of <I
CLASS="EMPHASIS"
>word</I
> are:
	      <I
CLASS="EMPHASIS"
>do or rod row word</I
>. You may use
	      <TT
CLASS="FILENAME"
>/usr/share/dict/linux.words</TT
> as the
	      reference list.</P
></DD
><DT
><SPAN
CLASS="QUOTE"
>"<B
CLASS="COMMAND"
>Word Ladders</B
>"</SPAN
></DT
><DD
><P
>A <SPAN
CLASS="QUOTE"
>"word ladder"</SPAN
> is a sequence of words,
              with each successive word in the sequence differing from
              the previous one by a single letter.</P
><P
>For example, to <SPAN
CLASS="QUOTE"
>"ladder"</SPAN
> from
              <I
CLASS="EMPHASIS"
>mark</I
> to
              <I
CLASS="EMPHASIS"
>vase</I
>:</P
><P
>	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;mark --&#62; park --&#62; part --&#62; past --&#62; vast --&#62; vase</PRE
></TD
></TR
></TABLE
>
            </P
><P
>Write a script that solves <SPAN
CLASS="QUOTE"
>"word ladder"</SPAN
>
              puzzles. Given a starting and an ending word,
              the script will list all intermediate steps in the
              <SPAN
CLASS="QUOTE"
>"ladder"</SPAN
>. Note that <I
CLASS="EMPHASIS"
>all</I
>
              words in the sequence must be <SPAN
CLASS="QUOTE"
>"legal."</SPAN
></P
></DD
><DT
><B
CLASS="COMMAND"
>Fog Index</B
></DT
><DD
><P
>The <SPAN
CLASS="QUOTE"
>"fog index"</SPAN
> 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.</P
><P
>The Gunning version of the fog index uses the following
	      algorithm.</P
><OL
TYPE="1"
><LI
><P
>Choose a section of the text at least
                 100 words in length.</P
></LI
><LI
><P
>Count the number of sentences (a portion of
                 a sentence truncated by the boundary of the text section
                 counts as one).</P
></LI
><LI
><P
>Find the average number of words per
                   sentence.</P
><P
>AVE_WDS_SEN = TOTAL_WORDS / SENTENCES</P
></LI
><LI
><P
>Count the number of <SPAN
CLASS="QUOTE"
>"difficult"</SPAN
>
		   words in the segment -- those containing at least
		   3 syllables. Divide this quantity by total words to
		   get the proportion of difficult words.</P
><P
>PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS</P
></LI
><LI
><P
>The Gunning fog index is the sum of the above two
                   quantities, multiplied by 0.4, then rounded to the
                   nearest integer.</P
><P
>G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN  + PRO_DIFF_WORDS ) )</P
></LI
></OL
><P
>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.</P
><P
>A strict interpretation of the Gunning fog index does
	      not count compound words and proper nouns as
	      <SPAN
CLASS="QUOTE"
>"difficult"</SPAN
> words, but this would enormously
	      complicate the script.</P
></DD
><DT
><B
CLASS="COMMAND"
>Calculating PI using Buffon's Needle</B
></DT
><DD
><P
>The Eighteenth Century French mathematician de Buffon
	      came up with a novel experiment. Repeatedly drop a
	      needle of length <SPAN
CLASS="QUOTE"
>"n"</SPAN
> onto a wooden floor
	      composed of long and narrow parallel boards. The cracks
	      separating the equal-width floorboards are a fixed distance
	      <SPAN
CLASS="QUOTE"
>"d"</SPAN
> 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.</P
><P
>In the spirit of <A
HREF="mathc.html#CANNON"
>Example 12-45</A
>, 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, <I
CLASS="EMPHASIS"
>n = d</I
>.</P
><P
>Hint: there are actually two critical variables:
              the distance from the center of the needle to the crack
              nearest to it, and the angle of the needle to that
              crack. You may use <A
HREF="mathc.html#BCREF"
>bc</A
> to
              handle the calculations.</P
></DD
><DT
><B
CLASS="COMMAND"
>Playfair Cipher</B
></DT
><DD
><P
>Implement the Playfair (Wheatstone) Cipher in a
	      script.</P
><P
>The Playfair Cipher encrypts text by substitution
	      of <SPAN
CLASS="QUOTE"
>"digrams"</SPAN
> (2-letter groupings).  It is
	      traditional to use a 5 x 5 letter scrambled-alphabet
	      <I
CLASS="EMPHASIS"
>key square</I
> for the encryption and
	      decryption.</P
><P
>	     <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;   C O D E S
   2&nbsp;   A B F G H
   3&nbsp;   I K L M N
   4&nbsp;   P Q R T U
   5&nbsp;   V W X Y Z
   6&nbsp;
   7&nbsp;Each letter of the alphabet appears once, except "I" also represents
   8&nbsp;"J". The arbitrarily chosen key word, "CODES" comes first, then all
   9&nbsp;the rest of the alphabet, in order from left to right, skipping letters
  10&nbsp;already used.
  11&nbsp;
  12&nbsp;To encrypt, separate the plaintext message into digrams (2-letter
  13&nbsp;groups). If a group has two identical letters, delete the second, and
  14&nbsp;form a new group. If there is a single letter left over at the end,
  15&nbsp;insert a "null" character, typically an "X."
  16&nbsp;
  17&nbsp;THIS IS A TOP SECRET MESSAGE
  18&nbsp;
  19&nbsp;TH IS IS AT OP SE CR ET ME SA GE
  20&nbsp;
  21&nbsp;For each digram, there are three possibilities.
  22&nbsp;----------------------------------------------
  23&nbsp;1) Both letters will be on the same row of the key square
  24&nbsp;   For each letter, substitute the one immediately to the right, in that
  25&nbsp;   row. If necessary, wrap around left to the beginning of the row.
  26&nbsp;
  27&nbsp;or
  28&nbsp;
  29&nbsp;2) Both letters will be in the same column of the key square
  30&nbsp;   For each letter, substitute the one immediately below it, in that
  31&nbsp;   row. If necessary, wrap around to the top of the column.
  32&nbsp;
  33&nbsp;or
  34&nbsp;
  35&nbsp;3) Both letters will form the corners of a rectangle within the key
  36&nbsp;   square. For each letter, substitute the one on the other corner the
  37&nbsp;   rectangle which lies on the same row.
  38&nbsp;
  39&nbsp;
  40&nbsp;The "TH" digram falls under case #3.
  41&nbsp;G H
  42&nbsp;M N
  43&nbsp;T U           (Rectangle with "T" and "H" at corners)
  44&nbsp;
  45&nbsp;T --&#62; U
  46&nbsp;H --&#62; G
  47&nbsp;
  48&nbsp;
  49&nbsp;The "SE" digram falls under case #1.
  50&nbsp;C O D E S     (Row containing "S" and "E")
  51&nbsp;
  52&nbsp;S --&#62; C  (wraps around left to beginning of row)
  53&nbsp;E --&#62; S
  54&nbsp;
  55&nbsp;=========================================================================
  56&nbsp;
  57&nbsp;To decrypt encrypted text, reverse the above procedure under cases #1
  58&nbsp;and #2 (move in opposite direction for substitution). Under case #3,
  59&nbsp;just take the remaining two corners of the rectangle.
  60&nbsp;
  61&nbsp;
  62&nbsp;Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a
  63&nbsp;fairly detailed rundown on the Playfair Cipher and its solution methods.</PRE
></TD
></TR
></TABLE
>
             </P
><P
>This script will have three main sections</P
><OL
TYPE="I"
><LI
><P
>Generating the <SPAN
CLASS="QUOTE"
>"key square"</SPAN
>,
	         based on a user-input keyword.</P
></LI
><LI
><P
>Encrypting a <SPAN
CLASS="QUOTE"
>"plaintext"</SPAN
>
	         message.</P
></LI
><LI
><P
>Decrypting encrypted
	         text.</P
></LI
></OL
><P
>The script will make extensive use of <A
HREF="arrays.html#ARRAYREF"
>arrays</A
> and <A
HREF="functions.html#FUNCTIONREF"
>functions</A
>.</P
></DD
></DL
></DIV
><P
>--</P
><P
>Please do not send the author your solutions to these
            exercises. There are better ways to impress him with your
            cleverness, such as submitting bugfixes and suggestions for
            improving this book.</P
></DIV
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