Shell Implementation in C

This project is a simple shell implementation written in C, designed to provide basic shell functionality such as parsing commands, searching for executables in the system's PATH, handling built-in commands (cd, exit), and executing external programs. The shell provides a controlled environment for process management and command execution.

Table of Contents

1. Overview
2. Features
3. File Structure
4. Compilation and Execution
5. Usage
6. Code Highlights
7. Future Improvements
8. Testing
9. Acknowledgments
10. Author

Overview

The Shell Implementation in C provides fundamental features for executing user commands. It includes:

• Parsing user input into arguments.
• Identifying commands (both built-in and external).
• Handling the PATH environment variable to locate executables.
• Executing commands in a child process while the parent process waits.
• Memory management and error handling to avoid leaks and crashes.
• Support for built-in commands like cd and exit.
• A simple interactive shell interface for user input and output.
• A Makefile for easy compilation and testing.
• Unit tests to verify the shell's functionality.
• Compatibility with GNU/Linux systems.

Features

1. Command Parsing: The shell parses user input into tokens (arguments) and constructs a command structure.
2. Built-in Commands:
   • cd: Changes the current working directory.
   • exit: Exits the shell.
3. External Command Execution: Supports executing external programs by searching for their full paths using the PATH environment variable.
4. Memory Management: Implements dynamic memory allocation using new and delete, ensuring no memory leaks or dangling pointers.
5. Error Handling: Comprehensive error handling for memory allocation, invalid commands, and execution errors.

File Structure

• shell.h: Header file containing definitions, constants, and function prototypes.
• shell.c: Main implementation of the shell logic.
• Makefile: File for building the project using make.
• tests.cpp: Unit tests to verify the shell's functionality (optional, if included).

Compilation and Execution

Requirements

• Operating System: Must be GNU/Linux system (might be incompatible with Windows or macOS).
• C Compiler: GCC or Clang (with C++ compatibility if using new/delete).
• Make: Build system utility.
• Valgrind (optional): For memory leak detection.

Steps to Compile and Run

1. Clone or download the project to your local machine.
2. Navigate to the project directory.
3. Compile the code:

   make

4. Run the shell:

   ./main

Running with Valgrind

To ensure there are no memory leaks:

make valgrind

Usage

1. Start the shell:

   ./main

2. Enter a command:
   • Example of a built-in command:

     cd /home/user

     or

     pwd

   • Example of an external command:

     ls -l

3. Exit the shell:

   exit

Code Highlights

1. Command Parsing

The parse function takes a user input string, tokenizes it into arguments, and constructs a command structure:

command* parse(char* line) {
    // Clone input to avoid modifying original
    char* line_copy = new char[strlen(line) + 1];
    strcpy(line_copy, line);

    // Tokenize input and count arguments
    int argc = 0;
    char* currentToken = strtok(line_copy, " \t\n");
    while (currentToken != NULL) {
        argc++;
        currentToken = strtok(NULL, " \t\n");
    }

    // Create command structure
    command* cmd = create_command(argc);
    if (!cmd) return NULL;

    // Populate arguments
    strcpy(line_copy, line); // Reset line_copy
    currentToken = strtok(line_copy, " \t\n");
    for (int i = 0; i < argc && currentToken != NULL; i++) {
        strncpy(cmd->argv[i], currentToken, MAX_ARG_LEN - 1);
        cmd->argv[i][MAX_ARG_LEN - 1] = '\0';
        currentToken = strtok(NULL, " \t\n");
    }

    delete[] line_copy;
    return cmd;
}

2. Built-in Commands

Support for built-in commands cd and exit is implemented in the do_builtin function:

int do_builtin(command* cmd) {
    if (strcmp(cmd->argv[0], "exit") == 0) {
        exit(SUCCESS);
    }
    if (cmd->argc == 1) {
        return chdir(getenv("HOME")); // cd with no arguments
    } else if (cmd->argc == 2) {
        return chdir(cmd->argv[1]); // cd with one argument
    } else {
        fprintf(stderr, "cd: Too many arguments\n");
        return ERROR;
    }
}

3. External Command Execution

The shell uses execv to execute external commands, following a search in the PATH environment variable:

bool find_full_path(command* cmd) {
    char* path_env = getenv("PATH");
    if (!path_env) return false;

    char* path_copy = new char[strlen(path_env) + 1];
    strcpy(path_copy, path_env);

    char* currentToken = strtok(path_copy, ":");
    while (currentToken != NULL) {
        char full_path[MAX_LINE_SIZE];
        snprintf(full_path, sizeof(full_path), "%s/%s", currentToken, cmd->argv[0]);

        struct stat buffer;
        if (stat(full_path, &buffer) == 0 && S_ISREG(buffer.st_mode)) {
            delete[] cmd->argv[0];
            cmd->argv[0] = new char[strlen(full_path) + 1];
            strcpy(cmd->argv[0], full_path);

            delete[] path_copy;
            return true;
        }
        currentToken = strtok(NULL, ":");
    }

    delete[] path_copy;
    return false;
}

4. Memory Management

Memory is managed using new and delete throughout the project, ensuring no leaks.

Future Improvements

1. Enhanced Built-in Commands: Add more built-ins like history or export.
2. Background Processing: Add support for background tasks.
3. Advanced Parsing: Handle pipes (|), redirections (>, <), and quotes.
4. Interactive Features: Improve user experience with command auto-completion and history navigation.
5. Error Reporting: Provide more descriptive error messages for better debugging.

Testing

Unit tests are provided in the tests.cpp file to verify the shell's functionality. The tests cover parsing, built-in commands, external command execution, and memory management. To run the tests, compile the project with the tests target:

make tests

Tests are provided using the Google Test framework. You might need to install the Google Test library to run the tests.

Acknowledgments

This shell was built as part of a project to understand and implement fundamental system programming concepts in C. It incorporates concepts like dynamic memory allocation, process creation, and system calls, inspired by Unix-based shell functionality.

Author

Son Nguyen
GitHub: hoangsonww

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Created with ❤️ by Son Nguyen in 2024.