M.1. Analyzing Scripts

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

#!/bin/bash MAX=10000 for((nr=1; nr<$MAX; nr++)) do let "t1 = nr % 5" if [ "$t1" -ne 3 ] then continue fi let "t2 = nr % 7" if [ "$t2" -ne 4 ] then continue fi let "t3 = nr % 9" if [ "$t3" -ne 5 ] then continue fi break # What happens when you comment out this line? Why? done echo "Number = $nr" exit 0

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

#!/bin/bash DIRNAME=/usr/bin FILETYPE="shell script" LOGFILE=logfile file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l exit 0

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A reader sent in the following code snippet.

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

He wished to write a script tracking changes to the system log file, /var/log/messages. Unfortunately, the above code block hangs and does nothing useful. Why? Fix this so it does work. (Hint: rather than redirecting the stdin of the loop, try a pipe.)

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Analyze Example A-10, 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.

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