Assignment1 Solution

Introduction:





This assignment has you design a set of classes for maintaining a mock <ile system and interacting with it in a program that operates like a Unix shell. You must work with a partner. Submitting this alone is not an option; if you do, we will give you a mark of 0.




IMPORTANT:




1) Your Assignment1 SVN repo is of the following form:




https://142.1.44.22/svn/csc207h/groupzzz




NOTE: The three digit groupNumber i.e. zzz can be found by checking your email. I had emailed this out on 4th October evening.




This Assignment consists of two tasks. i.e. Task1 and Task2.



You are NOT REQUIRED to implement any commands in Assignment1 (except for the exit command). The implementation of these commands happens in Assignment2. YOU WILL BE SUBMITTING ONLY 1 JAVA FILE (I.E. JShell.java).



Remember, this assignment is out of 10 marks. The break up of 10 marks is:




8 of these marks are allocated towards Task1 and




2 of these marks are allocated towards Task2.




Hence spend more time towards Task1 in order to get a good mark for this assignment. The code that is expected from you is very minimal and clearly outlined in Task2 of this assignment. Again you are not asked to implement any of the commands except for the exit command.

What's a shell?




Before the GUI, there was the command line. All interactions with




the <ile system were done using commands, much like what we have been seeing in lectures (Lecture1, your instructor using the svn client via the terminal) and in your future labs. Rather than double-clicking




on windows containing pictures of the contents of a hard drive, people would type commands:










mv old8ile new8ile to rename a <ile or move it to a different directory



cd to change directories



cat 8ilename to display the contents of a <ile called 8ilename. 






















A command consists of the command name followed (optionally) by a space and then arguments for the command.
















A shell is just another program. Most are written in the programming language C. Shells interact with the operating system.




Java's virtual machine is like an operating system. You'll be writing a shell program for it, and you'll be writing the complete functional code (only in Assignment2 and not Assignment1) to manage <iles and directories. These will all be Java objects; you do not need




to save them to disk. The expected code from you for Assignment1 is outlined in Task2 of this assignment.









When a user runs your program, your program will start a shell that allows the user to interact with an initially empty <ile system. As long as your shell is running, your program will keep track of the <iles the user has created, allowing them to move around the directory structure they've built and allowing them to display the contents of any <iles they have created.




Your program needs to support the equivalents of these bash commands (italics indicate the names of <iles and directories or

something else that the user chooses). Everything is case sensitive.










exit




Quit the program.










mkdir DIR




Create a directory DIR, which may be relative to the current directory or may be a full path.





cd DIR




Change directory to DIR, which may be relative to the current directory or may be a full path. As with Unix, .. means a parent




directory and . means the current directory. The directory separator must be /, the forward slash. The root of the <ile system




is a single slash: /.


ls




Print the names of <iles and directories in the current directory, with a new line following each of them.










pwd




Print the current working directory (including the whole path).










mv OLDFILE NEWFILE




Move <ile OLDFILE to NEWFILE. Both OLDFILE and NEWFILE may be relative to the current directory or may be full paths.













cp OLDFILE NEWFILE




Like mv, but don't remove OLDFILE.










cat FILE




Display the contents of FILE in the shell.










get URL




URL is a web address. This command retrieves the <ile at that URL and adds it to the current directory.


echo STRING OUTFILE




Put STRING in <ile OUTFILE. STRING is a string of characters surrounded by quotation marks. This creates a new <ile if OUTFILE does not exist, and erases the old contents if OUTFILE already exists. In either case, the only thing in OUTFILE should be STRING.










echo STRING OUTFILE




Like the previous command, but appends instead of overwrites.





Task1: [ 8 marks]




Create a set of CRC cards that could be used to implement the shell




described. Instead of handing in your index cards, there is a directory named crcCards in your subversion repository. In crcCards, for each card that create, create a text <ile (with a .txt

suf<ix) with the name of the class. Each CRC Card (or each .txt <ile)




contains the following format:










Class name: Classname





Parent class (if any): Classname
 Subclasses (if any): List all the subclasses separated by a comma.




Responsibilities:





*g





*h





*i





Collaborators:





*j





*k





*l




Do a thorough job on the CRC analysis. There may be hidden responsibilities that are not listed, but are necessary in order to write the program.




Make sure your responsibilities are clearly written: the grader (one of the TAs) should easily be able to understand each of them.





Task2: [2 marks]




Before you begin Task2, I encourage you to watch this video of mine to understand and know what are minimum expectations of this task.




http://youtu.be/Mk5p4X_oho8




Write the start of the shell program. Call your main class JShell. You should not submit any other java <iles other than JShell. Your shell program shall:




Print a prompt: the full path for the current working directory (in Assignment1, your current working directory will always be the root or / ), followed by the # symbol, followed by a space.









Figure 1: When your JShell is <irst executed, it must show the above prompt to the user and wait for user input. I am running this example from command line hence have a black background. If you happen to run this via Eclipse, you may have a white (or some other color depending on your Eclipse setting) background.










Repeatedly read a command that the user types and print the following information on two separate lines:



The command name (for example, ls)




The rest of the line (if it contains any arguments), which may be empty.





Figure2: For example, if the user types ls, then the <irst line of output would contain ls and the second line would be empty; your shell will then print another prompt waiting for the user input.







Figure 3: Few more examples that the user may type on your JShell and the expected output of your Assignment1. In the above example, cp .ile1 is considered as invalid command because cp expects two arguments. Likewise mkdir dir1 dir2 is considered as invalid command in JShell because it expects one argument.





There may be one or more space between a command and its arguments. You should ignore all the extra spaces after the end of the command. If the user types only spaces and press enter, your JShell should print the prompt again.













Figure 4: Few more examples dealing with white spaces.


Note:




YOUR PROGRAM SHOULD NOT CRASH. IF THE USER ENTERS INVALID SEQUENCE OF CHARACTERS, PRINT THE FOLLOWING ERROR MESSAGE:









Invalid command, please try again










You do not need to make any of the commands work; this is only to get you started writing the shell before Assignment2 begins. Your main focus for this Assignment1 is towards the CRC cards.









Marking:




Please follow the grading scheme for Assignment1 on Blackboard.










Checklist:




Have you...




1) Committed your <iles?



2) Made sure all your changes were committed by checking out a new copy of your repository?



3) Thanked your assignment partner for their effort?



4) Have your team of 4 ready for Assignment2?