The Operating System Shell

Description

1. Preliminary Notes

1. • The Week 1 and Week 2 in-class exercise sessions, as well as the C Labs, will provide some important background for this assignment.

1. • 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 (explained in the Week 1 in-class exercise session). We 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.

2. Assignment Question: Building an OS Shell

2.1 Your source files:

Your application must be built from three C files:

• shell.c : This is the primary programming file. It contains the main() and parse() functions.

• interpreter.c : 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.

• shellmemory.c : The file shellmemory.c contains the private data structures and public functions that implement the shell memory (The Week 2 Monday/Wednesday exercise session will also be helpful here).

These three source files must be built using modular programming techniques that we discussed in the in-class exercises.

2.2 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

2.3 Running your shell:

From the command line prompt type: ./mysh 2.4 What mysh displays to the user:

Welcome to the <your name goes here> shell!

Version 1.0 Created January 2020

$

Operating Systems COMP 310 – ECSE 427 McGill University

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.

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.

2.5 Shell Interface:

The shell needs to support the following commands:

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

• The commands are case sensitive.

• There are no other commands (for now).

• If the user inputs an unsupported command the shell displays “Unknown command”.

More notes on command behavior:

• 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:

15. set x 10 creates a new variable x and assigns to it the string 10.

15. set name Bob creates a new variable called name with string value Bob. o set x Mary, replaced the value 10 with Mary.

Important: Implement the shell memory as an array of struct, not as a linked list.

Struct MEM { char *var; char *value; };

• 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.

• 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

Operating Systems COMP 310 – ECSE 427 McGill University

3. WHAT TO HAND IN

Your assignment has a due date on January 29, 2021 at 23:55 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 any 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).

• The SYSTEMCALLS.TXT file where you report the measurements from Section 2.7.

Note: You must submit your own work. You can speak to each other for help but copied code will be handled as to McGill regulations. Submissions are automatically checked via plagiarism detection tools.

4. 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.

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 the in-class exercise sessions (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