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Corning Community College
UNIX/Linux Fundamentals
Case Study 0xD: Analyzing Shell Script Logic
~~TOC~~
Objective
To gain additional familiarity with shell scripting.
Background
Shell scripting enables you countless possibilities for automating tasks, both menial and sophisticated in nature.
However, like any new tool or concept, one needs to spend time with, and play with things, to get a better understanding.
Exploration
Analyze, poke at, and get running the following script on Lab46:
#!/bin/bash month="`date +%m`" year="`date +%Y`" day="`date +%d`" today="${year}${month}${day}" csem="" for item in `wget -q -O - http://www.corning-cc.edu/future/acacalendar/ \ | grep 'Semester begins' | sed 's/\.\.*/:/g' \ | sed 's/<[/a-z0-9]*>//g' | sed 's/:Semester begins : /:/g' \ | sed 's/:First Summer Session begins : /:/g' \ | egrep -o '[FWS][a-z]* [0-9]{4} S[a-z]*:[A-Z][a-z]* [1-9][0-9]?' \ | tr 'WSF' 'wsf' | sed 's/ \([1-9][0-9][0-9][0-9]\) semester/\1/g' \ | sed 's/ /*/g'`; do sm="`echo $item | cut -d':' -f1`" out=$(date --date "`echo $item \ | sed 's/^[a-z]*\([0-9]*\):\([A-Za-z]*\)\*\([0-9]*\)$/\2 \3 \1/'`" \ +%Y%m%d) if [ "$today" -ge "$out" ]; then csem="$sm" fi done echo "$csem" if [ -z "$csem" ]; then status=1 else status=0 fi exit $status
Questions
- What is the purpose of this script?
- What is the general flow of operations that takes place in this script?
- Explain what today=“${year}${month}${day}“ does.
- What does sed 's/<[/a-z0-9]*>//g' appear to do?
- What are the raw $item values being iterated in the for loop?
- What is the purpose of the if statement?
- Any potential bugs that could effect proper operation?
- What is the significance of exiting with a value of 0 vs. that of 1?
Conclusions
All questions in this assignment require an action or response. Please organize your responses into an easily readable format and submit the final results to your instructor.
Your assignment is expected to be performed and submitted in a clear and organized fashion- messy or unorganized assignments may have points deducted. Be sure to adhere to the submission policy.
The successful results of the following actions will be considered for evaluation:
- your responses to questions submitted at the following form:
<html><center><a href=“http://lab46.corning-cc.edu/haas/content/unix/submit.php?csd”>http://lab46.corning-cc.edu/haas/content/unix/submit.php?csd</a></center></html>
- the response from the form (received via e-mail) saved as csd.txt to your ~/src/unix/ directory
- addition/commit of ~/src/unix/csd.txt into your repository (CS 0x0 sets you up to do this).
As always, the class mailing list and class IRC channel are available for assistance, but not answers.
