Java 1A Practical Class

Java Tick 1*


Table of Contents

Introduction
Further Bash
A more advanced script

Introduction

In order to gain a star in the mark sheet you must complete this exercise. Completing the exercise does not gain you any credit in the examination. To get a star you need to produce two files

answers.txt

a text file with answers to all the questions shown in the question boxes;

make.sh

a shell script which conforms to the specification described below.

To submit your work you should construct a jar file called crsid-1star.jar where csrid is your username. The jar file should contain the two files answers.txt and make.sh inside the directory uk/ac/cam/crsid/tick1star. For example, if your username is arb33 the jar test routine should output the following:

crsid@machine:~> jar tf arb33-1star.jar
uk/ac/cam/arb33/tick1star/answers.txt
uk/ac/cam/arb33/tick1star/make.sh

To submit your work, email the jar file as an attachment to ticks1a-java@cl.cam.ac.uk. There are no automated tests associated with this Tick but you should receive confirmation of your submission.

Further Bash

In Workbook 1 you explored a few basic commands. Most Linux systems offer a much larger range of commands and programs. In this exercise you will explore some of these and write your own script which can be called from the command line. You will find the on-line Bash manual[1] and the Advanced Bash-Scripting Guide[2] useful since they provide more extensive documentation than man.

It can take some time and effort to write a complex expression in Bash and consequently you may wish to keep a record of what you've written for future reference. The best way to do this is to write a script which you can not only read but also execute directly, avoiding the need to type in the command a second time; instead you need only type in the name of the script file. For example, if you wish to save the command

crsid@machine:~> echo "Hello, world"

then you can create a file called hello.sh with the following contents:

#!/bin/bash
echo "Hello, world"

The first line of this file contains the special phrase #!/bin/bash which tells the operating system to use the program /bin/bash to execute the contents of the file; since the operator # in Bash turns the remainder of the line into a comment (try #echo "no output" in a shell) then this line is ignored, but the commands written into the rest of the file are, broadly speaking, executed as if you had typed them in at the prompt. You can execute the script hello.sh by making the file executable using the chmod command:

crsid@machine:~> chmod u+x hello.sh
crsid@machine:~> ./hello.sh
Hello, world
crsid@machine:~>

A more advanced script

To complete this Tick you will need to read and learn more about Bash. The introduction section of the Advanced Bash-Scripting Guide mentioned earlier is a good place to start, but there are many other good tutorials on the web. Whichever tutorial you use, to successfully gain this Tick you will probably find the following concepts in Bash helpful: creating and using variables, conditional and loop constructs, positional and special parameters, and shell parameter expansion. (Don't worry if you don't know what these terms mean yet, you probably won't! Use them as a way to steer your initial reading.) The Lecturer and Demonstrator will be able to offer some assistance if you get stuck.

To gain Tick 1* you must produce a Bash script called make.sh which conforms to the following specification. Your script should inspect the first argument passed to it when executed and use this value to determine what action to take. For example, if a user types in

crsid@machine:~> ./make.sh build

the script should perform the "build" action. Your script should support the following actions:

show

Print out a list of all files with the extension .java found in the current directory or any sub-directory of the current directory.

clean

Delete all files which end with the extension .class which can be found in the current directory or any sub-directory of the current directory.

build

Use the javac compiler to create a new .class file for only those files whose equivalent .java file exists and the .java file has a more recent modification time as recorded by the filesystem. For example, if there exists a file uk/ac/cam/arb33/Test.class which was created at 10:42 today, and there exists a file uk/ac/cam/arb33/Test.java which was modified more recently (e.g. 10:46), then the Java compiler should be invoked to build a new copy of Test.class. Your program should recompile all such .java files in the current directory and any sub-directories unless the compilation of a Java source file fails, in which case, after printing out the error message from the compiler, the script should terminate and not attempt to compile any further files.

If no command is given to make.sh, the program should perform the "show" action. If the command is not recognised, then the script should print an error message and terminate.

Note: You will discover in future weeks that javac supports some of the "build" features described above since javac checks for dependencies between Java sources and rebuilds any dependent class files. For example, if you invoke javac Test1.java and Test1.java uses data or methods provided by Test2.java then javac will recompile Test2.java if the timestamp of Test2.class is older than that of Test2.java. (A good submission for Tick 1* might take this into account, but this isn't required.)



Copyright Alastair R. Beresford and Andrew C. Rice 2008,2009