Appendix M. Exercises

Examine the following script. Run it, then explain what itdoes. Annotate the script and rewrite it in a more compact andelegant manner.

   1 #!/bin/bash
   2
   3 MAX=10000
   4
   5
   6   for((nr=1; nr<$MAX; nr++))
   7   do
   8
   9     let "t1 = nr % 5"
  10     if [ "$t1" -ne 3 ]
  11     then
  12       continue
  13     fi
  14
  15     let "t2 = nr % 7"
  16     if [ "$t2" -ne 4 ]
  17     then
  18       continue
  19     fi
  20
  21     let "t3 = nr % 9"
  22     if [ "$t3" -ne 5 ]
  23     then
  24       continue
  25     fi
  26
  27   break   # What happens when you comment out this line? Why?
  28
  29   done
  30
  31   echo "Number = $nr"
  32
  33
  34 exit 0

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Explain what the following script does. It is really justa parameterized command-line pipe.

   1 #!/bin/bash
   2
   3 DIRNAME=/usr/bin
   4 FILETYPE="shell script"
   5 LOGFILE=logfile
   6
   7 file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l
   8
   9 exit 0

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A reader sent in the following code snippet.

   1 while read LINE
   2 do
   3   echo $LINE
   4 done < `tail -f /var/log/messages`

He wished to write a script tracking changes to the system logfile, /var/log/messages. Unfortunately,the above code block hangs and does nothinguseful. Why? Fix this so it does work. (Hint:rather than redirecting thestdinof the loop, try a pipe.)

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Analyze Example A-10, and reorganize it in asimplified and more logical style. See how many of the variablescan be eliminated, and try to optimize the script to speed upits execution time.

Alter the script so that it accepts any ordinary ASCIItext file as input for its initial "generation". Thescript will read the first $ROW*$COLcharacters, and set the occurrences of vowels as"living"cells. Hint: be sure to translate thespaces in the input file to underscore characters.