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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="APPENDIX"
><H1
><A
NAME="EXERCISES"
></A
>Appendix O. Exercises</H1
><P
>The exercises that follow test and extend your knowledge
	of scripting. Think of them as a challenge, as an entertaining way
	to take you further along the stony path toward UNIX wizardry.</P
><P
>      <P
CLASS="LITERALLAYOUT"
>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;On&nbsp;a&nbsp;dingy&nbsp;side&nbsp;street&nbsp;in&nbsp;a&nbsp;run-down&nbsp;section&nbsp;of&nbsp;Hoboken,&nbsp;New&nbsp;Jersey,<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;there&nbsp;sits&nbsp;a&nbsp;nondescript&nbsp;squat&nbsp;two-story&nbsp;brick&nbsp;building&nbsp;with&nbsp;an&nbsp;inscription<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;incised&nbsp;on&nbsp;a&nbsp;marble&nbsp;plate&nbsp;in&nbsp;its&nbsp;wall:<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<TT
CLASS="COMPUTEROUTPUT"
>Bash Scripting Hall of Fame</TT
>.<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Inside,&nbsp;among&nbsp;various&nbsp;dusty&nbsp;uninteresting&nbsp;exhibits&nbsp;is&nbsp;a&nbsp;corroding,<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cobweb-festooned&nbsp;brass&nbsp;plaque&nbsp;inscribed&nbsp;with&nbsp;a&nbsp;short,&nbsp;very&nbsp;short<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;list&nbsp;of&nbsp;those&nbsp;few&nbsp;persons&nbsp;who&nbsp;have&nbsp;successfully&nbsp;mastered&nbsp;the&nbsp;material<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;in&nbsp;the&nbsp;<I
CLASS="FIRSTTERM"
>Advanced Bash Scripting Guide</I
>,&nbsp;as&nbsp;evidenced&nbsp;by&nbsp;their&nbsp;performance<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;on&nbsp;the&nbsp;following&nbsp;Exercise&nbsp;sections.<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(Alas,&nbsp;the&nbsp;author&nbsp;of&nbsp;the&nbsp;<I
CLASS="FIRSTTERM"
>ABS Guide</I
>&nbsp;is&nbsp;not&nbsp;represented&nbsp;among&nbsp;the&nbsp;exhibits.<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;This&nbsp;is&nbsp;possibly&nbsp;due&nbsp;to&nbsp;malicious&nbsp;rumors&nbsp;about&nbsp;<A
HREF="aboutauthor.html#NOCREDS"
>lack of credentials</A
>&nbsp;and<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<A
HREF="contributed-scripts.html#KTOUR0"
>deficient scripting skills</A
>.)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</P
>
      </P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="SCRIPTANALYSIS"
></A
>O.1. Analyzing Scripts</H1
><P
>Examine the following script. Run it, then explain what it
        does.  Annotate the script and rewrite it in a more compact and
        elegant manner.</P
><P
>       <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;MAX=10000
   4&nbsp;
   5&nbsp;
   6&nbsp;  for((nr=1; nr&#60;$MAX; nr++))
   7&nbsp;  do
   8&nbsp;
   9&nbsp;    let "t1 = nr % 5"
  10&nbsp;    if [ "$t1" -ne 3 ]
  11&nbsp;    then
  12&nbsp;      continue
  13&nbsp;    fi
  14&nbsp;
  15&nbsp;    let "t2 = nr % 7"
  16&nbsp;    if [ "$t2" -ne 4 ]
  17&nbsp;    then
  18&nbsp;      continue
  19&nbsp;    fi
  20&nbsp;
  21&nbsp;    let "t3 = nr % 9"
  22&nbsp;    if [ "$t3" -ne 5 ]
  23&nbsp;    then
  24&nbsp;      continue
  25&nbsp;    fi
  26&nbsp;
  27&nbsp;  break   # What happens when you comment out this line? Why?
  28&nbsp;
  29&nbsp;  done
  30&nbsp;
  31&nbsp;  echo "Number = $nr"
  32&nbsp;
  33&nbsp;
  34&nbsp;exit 0</PRE
></TD
></TR
></TABLE
>
       </P
><P
>---</P
><P
>Explain what the following script does. It is really just
        a parameterized command-line pipe.</P
><P
>        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;DIRNAME=/usr/bin
   4&nbsp;FILETYPE="shell script"
   5&nbsp;LOGFILE=logfile
   6&nbsp;
   7&nbsp;file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l
   8&nbsp;
   9&nbsp;exit 0</PRE
></TD
></TR
></TABLE
>
      </P
><P
>---</P
><P
>Examine and explain the following script. For hints, you
        might refer to the listings for <A
HREF="moreadv.html#FINDREF"
>find</A
> and <A
HREF="system.html#STATREF"
>stat</A
>.</P
><P
>        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Author:  Nathan Coulter
   4&nbsp;# This code is released to the public domain.
   5&nbsp;# The author gave permission to use this code snippet in the ABS Guide.
   6&nbsp;
   7&nbsp;find -maxdepth 1 -type f -printf '%f\000'  | {
   8&nbsp;   while read -d $'\000'; do
   9&nbsp;      mv "$REPLY" "$(date -d "$(stat -c '%y' "$REPLY") " '+%Y%m%d%H%M%S'
  10&nbsp;      )-$REPLY"
  11&nbsp;   done
  12&nbsp;}
  13&nbsp;
  14&nbsp;# Warning: Test-drive this script in a "scratch" directory.
  15&nbsp;# It will somehow affect all the files there.</PRE
></TD
></TR
></TABLE
>
      </P
><P
>---</P
><P
>A reader sent in the following code snippet.</P
><P
>         <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;while read LINE
   2&nbsp;do
   3&nbsp;  echo $LINE
   4&nbsp;done &#60; `tail -f /var/log/messages`</PRE
></TD
></TR
></TABLE
>
	 </P
><P
>He wished to write a script tracking changes to the system log
	   file, <TT
CLASS="FILENAME"
>/var/log/messages</TT
>. Unfortunately,
	   the above code block hangs and does nothing
	   useful. Why? Fix this so it does work. (Hint:
	   rather than <A
HREF="redircb.html#REDIRREF"
>redirecting the
	   <TT
CLASS="FILENAME"
>stdin</TT
> of the loop</A
>, try a <A
HREF="special-chars.html#PIPEREF"
>pipe</A
>.)</P
><P
>---</P
><P
>Analyze the following <SPAN
CLASS="QUOTE"
>"one-liner"</SPAN
> (here
        split into two lines for clarity) contributed by Rory
        Winston:</P
><P
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;export SUM=0; for f in $(find src -name "*.java");
   2&nbsp;do export SUM=$(($SUM + $(wc -l $f | awk '{ print $1 }'))); done; echo $SUM</PRE
></TD
></TR
></TABLE
>
	 </P
><P
>Hint: First, break the script up into bite-sized
	sections. Then, carefully examine its use of <A
HREF="dblparens.html"
>double-parentheses</A
> arithmetic,
	the <A
HREF="internal.html#EXPORTREF"
>export</A
> command,
	the <A
HREF="moreadv.html#FINDREF"
>find</A
> command, the
	<A
HREF="textproc.html#WCREF"
>wc</A
> command, and <A
HREF="awk.html#AWKREF"
>awk</A
>.</P
><P
>---</P
><P
>Analyze <A
HREF="contributed-scripts.html#LIFESLOW"
>Example A-10</A
>, and reorganize it in a
	simplified and more logical style. See how many of the variables
	can be eliminated, and try to optimize the script to speed up
	its execution time.</P
><P
>Alter the script so that it accepts any ordinary ASCII
	text file as input for its initial <SPAN
CLASS="QUOTE"
>"generation"</SPAN
>. The
	script will read the first <TT
CLASS="PARAMETER"
><I
>$ROW*$COL</I
></TT
>
	characters, and set the occurrences of vowels as
	<SPAN
CLASS="QUOTE"
>"living"</SPAN
> cells. Hint: be sure to translate the
	spaces in the input file to underscore characters.</P
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