Description
Labs 1 and 2 will provide some background for this assignment.
The question is presented from a Linux point of view using the computer science server
mimi.cs.mcgill.ca, which you can reach remotely using ssh or putty from your laptop (see lab 1). If you
do this assignment from an MS Windows machine, then make sure to provide the DLL libraries your
program uses (if any) so that the TA can run it from their MS Windows machine. It is not the TA’s
responsibility to make your program run. The TAs will not debug your program.
I suggest you do this assignment directly from mimi.cs.mcgill.ca since these are the computers the TA
will be using.
You must write this assignment in the C Programming language.
Assignment Question: Building an OS Shell
Your source files:
Your entire application must be built from three c files: shell.c, interpreter.c, and shellmemory.c.
These three source files must be built using modular programming techniques (see lab 2). The
file shell.c is the primary programming file. It contains the main() and parse() functions. The file
interpreter.c contains the interpreter() function. Each command the interpreter() function
accepts has a corresponding function that implements the command’s functionality. Give the
function the same name as the command. The file shellmemory.c contains the private data
structures and public functions that implement the shell memory (lab 2 will also be helpful
here).
Compiling your shell:
Compile your application using gcc with the name mysh. To do modular programming, you must
compile your application in the following manner:
gcc -c shell.c interpreter.c shellmemory.c
gcc -o mysh shell.o interpreter.o shellmemory.o
Running your shell:
From the command line prompt type: ./mysh
What mysh displays to the user:
Welcome to the shell!
Version 1.0 Created January 2020
$
The above dollar sign is the prompt. The cursor flashes beside the prompt waiting for the user’s
input. From this prompt the user will type in their command and the shell will display the result
from that command on the screen (or an error message), and then the dollar sign is displayed
again prompting the next command. The user stays in this interface until they ask to quit.
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #1 Page 2 of 4
The command line syntax for your shell is: COMMAND ARGUMENTS.
Each command line command is a single line of text (a string) separated by spaces and
terminated by a carriage return. Some commands are a single word, while other commands
contain multiple words. Processes each command in a way that is similar to how it was
presented in class. You do not need to follow the class slides exactly.
The commands supported by mysh:
The commands are case sensitive.
COMMAND DESCRIPTION
help Displays all the commands
quit Exits / terminates the shell with “Bye!”
set VAR STRING Assigns a value to shell memory
print VAR Displays the STRING assigned to VAR
run SCRIPT.TXT Executes the file SCRIPT.TXT
There are no other commands (for now).
If the user inputs an unsupported command the shell displays “Unknown command”.
The command set VAR STRING first checks to see if VAR already exists. If it does exist,
STRING overwrites the previous value assigned to VAR. If VAR does not exist, then a new entry is
added to the shell memory where the variable name is VAR and the contents of the variable is
STRING. For example: set x 10 creates a new variable x and assigns to it the string 10.
Another example: set name Bob creates a new variable called name with string value Bob.
Another example: set x Mary, replaced the value 10 with Mary. Implement the shell
memory as an array of struct, not as a linked list. Struct MEM { char *var; char *value; };
The command print VAR first checks to see if VAR exists. If it does not exist, then it displays
the error “Variable does not exist”. If VAR does exist, then it displays the STRING. For example:
print x from the above example will display Mary.
The command run SCRIPT.TXT assumes that a text file exists with the provided file name. It
opens that text file and then sends each line one at a time to the interpreter (as seen in class).
The interpreter treats each line of text as a command. Each line affects the shell and the UI. At
the end of the script, the file is closed, and the command line prompt is displayed once more.
While the script executes the command line prompt is not displayed. For example: run
test.txt will begin by opening the file test.txt. If that fails, then an error message is
displayed: “Script not found”. If the file is opened, then each line of the file is interpreted. At the
end, the file is closed, and the command line prompt is displayed. If an error occurs while
executing the script due a command syntax error, then the error is displayed and the script
stops executing.
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #1 Page 3 of 4
Testing your shell:
The TAs will use and modify the provided text file to test your shell. You can also use this file to
test your shell or you can test it the old fashion way by typing input from the keyboard. To use
the provided text file, you will need to run your program from the OS command line as follows:
$ ./mysh < testfile.txt
Each line from testfile.txt will be used as input for each prompt your program displays to the
user. Instead of typing the input from the keyboard the program will take the next line from the
file testfile.txt as the keyboard input.
Make sure your program works in the above way.
WHAT TO HAND IN
Your assignment has a due date plus two late days. If you choose to submit your assignment during the
late days, then your grade will be reduced by -5% per day. Submit your assignment to the assignment
#1 submission box in myCourses. You need to submit the following:
• A README.TXT file stating what OS you used: mimi.cs.mcgill.ca or MS Windows and any other
special instructions you think the TA needs to know to run your program.
• Your version of TESTFILE.TXT. This will be you telling the TA that you know for sure that your
program can at least do the following. The TA will run your program with this file and they will
also run it with their own version.
• Submit shell.c, interpreter.c and shellmemory.c (you may want to create .h files, if so, please
hand those in as well)
• Submit the executable (compiled on the appropriate machine).
• If you used MS Windows and you used a DLL then upload those as well.
You must submit your own work. You can speak to each other for help but copied code will be handled
as to McGill regulations.
HOW IT WILL BE GRADED
Your assignment is graded out of 20 points and it will follow this rubric:
• The student is responsible to provide a working solution for every requirement.
• The TA grades each requirement proportionally. This means, if the requirement is only 40%
correct (for instance), then the student receives only 40% of the points assigned to that
requirement.
• Your program must run to be graded. If it does not run, then the student receives zero for the
entire assignment. If your program only runs partially or sometimes, you should still hand it in.
You will receive partial points.
• The TA looks at your source code only if the program runs (correctly or not). The TA looks at
your code to verify that you (A) implemented the requirement as requested, and (B) to check if
the submission was copied.
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #1 Page 4 of 4
• Mark breakdown:
o 1 point – Source file names. The TA must verify that the student created the three
source files shell.c, interpreter.c and shellmemory.c as the only source files in the
application. In addition the source files must be populated with at least what was
specified in the assignment description for each file. The student is permitted to supply
additional helper functions as they see fit, as long as it does not hinder the assignment
requirements.
o 2 points – Modular programming. If the student wrote their application using modular
programming techniques as described in lab 2 (or seen from another course) then they
receive these points.
o 1 point – Executable named mysh. The students compiled program must be named
mysh.
o 2 points – Shell UI as specified. See the assignment specification for the opening UI
screen layout, the error messages, and the general flow of the application as described.
For example, prompts are displayed continually until the user exits the application using
the quit command.
o 1 points – The help command. Displays all the command and description as seen in the
assignment description.
o 1 points – The quit command. Terminates the application. Prints “Bye!” and returns the
user to their OS.
o 3 points – The set command. Creates or overwrites a variable with the new string.
o 3 points – The print command. Displays the string or the error message.
o 4 points – The run command. Executes a text file of valid or invalid commands.
o 2 points – Program can be tested with the TESTFILE.TXT file
