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COMP 2100/2500/6442: Lab 8 - Design Patterns, Bash

Pre-Lab Tasks [2 marks]

The first 10 minutes of the lab session time may be spent by your tutor to mark pre-lab tasks.

Bash basics

• Revise the lecture material on bash
• Write a bash script that outputs "Hello World"
• Write a bash script that outputs a greeting which is different for different names. Provide the name as a parameter. An example of how you would interact with your script would be:

% ./greeting EricHello Eric% ./greeting AlistairGreetings Professor Rendell% ./greeting AnishG'Day Anish

You only need to have a few different greetings for different names and one default greeting. Please keep all your greetings positive and friendly.

String Counting with Java

This program must be completed in order to start the in-class activities.

• Write a Java program called, StringCount that takes two string arguments from the command line and returns a count of how many times an occurrence of the first string can be found in the second

  $ java StringCount "aaa" "aaaabbb"2

• Occurrences must be contiguous. For example, ‘cat’ does not occur in ‘cxaxt’

• Occurrences may overlap. For example ‘wowow’ occurs twice in ‘wowowow’

(Hint - if the expression i == mainString.indexOf(subString, i) evaluates to true then the subString will be within the mainString starting at position i.)

Using the Facade Design Pattern

• Extend the StringCount program to support loading files and counting occrrences of a string within the a file

  $ echo "aaabbbaaa" >> TestFile$ java StringCount -f "TestFile" "aaa"2

• Use the facade design pattern to implement the file load function

In Class Activity - Writing a Test Harness in Bash [2 marks]

The objective is to write a Bash script to test the StringCount program with different string combinations. This may be done individually or in a small group of 2 or 3 students.

• Create a file called 'cases' to contain the test cases. Each line should contain two strings. Here are a few lines to get you started. Add some more of your own

  x xx xxwowow wowowowow

• The following simple shell script will read words from each line of its input and print those words out again

#!/bin/bashwhile read x ydo  echo ${x} ${y}done

The key feature of this script is the read command which reads a line of input and assigns the first word to the variable x and the rest of the line to the variable y. The read command gives an exit code of 0 (= success = True) if was was able to read the words, or 1 (= failure = False) if it encounters the end of its input. That makes it perfect for use in a while loop, as seen in this example. When the input runs out, it returns 1, and the loop terminates.

• Save this script in a file called tester
• Make it executable, and run it over your test input by typing ./tester < cases
• Modify tester so that for each input line of the form string1 string2, it outputs the number of occurrences of string1 that StringCount claims are in string2.

Note: This does not mean rewrite the Java program as a bash script. It means write a bash script that runs the Java program on each line of its input and formats the results.

• Modify your cases file so that each line now contains 3 things, the two words as before, plus the correct number of occurrences of the first word in the second. For example, your file will start:

  x x 1x xx 2wowow wowowowow 3

• Modify your tester script so that for each test it prints a line of the following form:

  Test i: string1 string2: Expected n, got m

  where i is the line number of this test, n is the correct number of occurrences of string1 in string2, and m is the number found by StringCount.

• Modify your tester so that when strcnt gets the right answer, it prints a line of form:

  Test i: Passed

  The key to this is to use an if statement in your shell script. Remember that the form of an if statement is as follows:

  if command-listthencommand-listelsecommand-listfi

  Note: The line breaks (or semicolons in their place) are required. You can leave out the else part if you don't need it.

Have a look at the manual page for the test command (by typing man test). That should tell you everything you need to know now to complete this exercise. The test command evaluates a boolean expression (as well as being able to test for all sorts of other conditions). It basically lets you program in Bash in a way very like what you would do in Java.

The test command is so commonly used in shell scripts that it has a short-hand notation in which [ stuff ] means the same as test stuff (those spaces inside the square brackets are required). Use this notation to simplify your solution.

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