Assignment 3 - Simple Shell with Pipes

Welcome to your third homework assignment.

Assignment Purpose and Learning Outcomes:

• Understanding processes and the use of fork, exec, and wait functions
• Understanding error handling
• Reading user input
• Tokenizing input
• Getting user input and processing end-of-file condition
• Understanding and using Vectors (arrays of pointers)
• Comprehensive use of redirection of standard input and output and the use of pipes

Let me remind you of the general expectations for all projects:

• All projects are done in C in the Ubuntu Linux Virtual Machine.
• Code must be neat, with proper and consistent indentation and well documented.
• Keep line to around 80 characters per line, and not line greater than 100 characters.
• Comments must describe the logic and reason for the code and not repeat the code.
• Variables must have meaningful names and be in a consistent format (I do not care if you use camelCase or under_scores in variables but be consistent.
• You must use make to compile the program.
• You must use make run (sometimes with RUNOPTIONS) to execute the program.
• In addition, each file must have the standard header as defined below.
• All source files and writeup must be in the main branch of the github.

All primary filenames should be <lastname>_<firstname>_HW<#>_<component>.<proper extension>, except Makefile.

Each .c and .h file must have a standard header as defined below. Make sure to put in your section number (replace the #), your name, your student ID, a proper project name, GitHub name, filename, and description of the project. Do not alter the number of asterisks and ensure that the header starts on line 1 of the file.

/**************************************************************
* Class::  CSC-415-0# Summer 2024
* Name::
* Student ID::
* GitHub-Name::
* Project:: Assignment 3 – Simple Shell with Pipes
*
* File:: <name of this file>
*
* Description::
*
**************************************************************/

This is an INDIVIDUAL assignment. You can (and should) work in groups to research how to do the assignment, but each person should code their own version and make their own submission.

In our slides we talked about a simple shell. For this assignment you will implement your own shell that runs on top of the regular command-line interpreter for Linux.

Your shell should read lines of user input, then parse and execute the commands by forking/creating new processes. For each command, your shell should call fork() followed by execvp(). Following each command, your shell should wait for its child process to complete, and then print the child PID and the return result from the child process. The user should be able to specify the command to execute by giving a path to the executable file (e.g. /bin/ls) or by using path expansion to locate the executable file (i.e. searching each directory in the PATH environment variable). (Note that the execvp() function perform this processing automatically; you do not need to program it yourself.)

If your shell encounters an error while reading a line of input it should report the error and exit. If your shell encounters EOF while reading a line of input, it should exit gracefully without reporting an error. Ensure that you do not overflow a 159 byte buffer when fetching the line of input (functions that do not accept the size of your buffer are not able to prevent overflows whereas functions that do accept a size generally do; be sure to check the manpage of any function you use carefully). You do not need to report an error if the user's input line is larger than the 159 byte buffer; just use the truncated input as the command.

You can test the EOF by running make run < commands.txt or from the prompt enter CTRL-D

Before your shell forks a new process to call execvp(), it should parse the input string and separate it into a collection of substrings representing the executable file and any command-line arguments. If the user entered an empty line, report an error and fetch a new line of input. Your code must handle at least four command-line arguments (in addition to the name of the executable file itself) for each command.

You should store pointers to the substrings in an array (similar to the “argv” array passed to main()) and pass this array of arguments to execvp(). Note that the number of command-line arguments is variable; this is indicated in the array by including a NULL pointer in the array after the last substring. (This means that if the user specifies N substrings, your array must hold N + 1 pointers where the last pointer is NULL.) If the user enters the exit command, your shell should terminate (returning to the regular shell).

Note: your shell does not need to support cd (change directory).

Piping - You shell must also support piping. An example of using pipes in the Linux shell would be cat myfile.txt | wc -l which would copy the file myfile.txt to standard output which is redirected as the standard input to wc (wordcount) which will then display how many lines were in the file myfile.txt. The pipe character | seperates different commands and the output (stdout) of the program on the left becomes the input (stdin) for the program on the right. There is no limit to the number of commands that can be piped together. (Just limited by the command line length specified above).

Hint: get your shell working without pipes first, then go back and add pipes. While this may require a little more work to change your code to support pipes, it ensures you do the rest of the shell correctly so you are not trying to debug multiple things at once. Some functions you will need include pipe and dup2.

Your program must also accept a command line argument which is the prefix prompt. If no value is specified use “> ” as the prompt.

Here is a sample execution:

 student@student-VirtualBox:~/CSC415/assignment3$ ./assn3 prompt$
prompt$ ls -l
total 20
-rwxr-xr-x 1 student student 13216 Feb 23 13:44 assn3
-rw-r--r-- 1 student student  1583 Feb 23 13:44 assn3.c
Child 2124, exited with 0
prompt$ ls foo
ls: cannot access 'foo': No such file or directory
Child 2125, exited with 2
prompt$ exit
student@student-VirtualBox:~/CSC415/assignment3$

You should submit your source code file(s), and Makefile along with a writeup in PDF format (using the template) that includes a description of what you did issues you had and resolutions and the compilation and execution output (screen shots) from your program to GitHub and just the PDF also to Canvas. Your execution output should include commands with command-line arguments. Then use the exit command to exit your program and show the output of the same commands in the regular command-line interpreter for that machine to ensure they match.

All filenames should be <lastname>_<firstname>_HW<#>_<optional>.<proper extension>

Rubric

Grade Item	Grade Points
Standard Header	2
Command Processing	35
Handeling of Pipes	15
Custom Prompt	3
Output Status	10
Code Comments	5
Writeup	10 (Description, Compilation, Sample Output)