Linux Shell

Overview

The Linux Shell is a custom Unix-like shell written in C as part of a university project for COMP 3659 – Operating Systems. It supports essential shell features such as command execution, I/O redirection, pipelines, signal handling, and a custom memory allocator. This project explores key operating system concepts including process management, inter-process communication, and signal control.

Features

• Command Execution: Launch any executable with arguments
• I/O Redirection: Support for input (<) and output (>) redirection
• Pipelines: Execute chained commands using pipes (|)
• Background Execution: Run processes in the background using &
• Built-in Commands: Includes cd for directory navigation and exit to quit
• Signal Handling: Gracefully handles SIGINT (Ctrl+C) without killing the shell
• Custom Memory Management: Lightweight heap allocator for internal operations

Code Structure

• shell.c: Main shell loop and command parsing
• job.c / job.h: Job and pipeline management
• str_lib.c / str_lib.h: Utility functions for string operations
• mylib.c / mylib.h: Custom heap allocator implementation
• signal_handle.c / signal_handle.h: Custom signal handling logic
• struct.h: Data structure definitions
• constants.h: Configuration values and limits

Build Instructions

Use the provided Makefile to build the project.

make        # Build the shell
make clean  # Remove compiled files

This will generate an executable named shell.

Usage

Run the shell:

./shell

Examples

Basic Command

mysh$ /bin/ls

I/O Redirection

mysh$ /bin/ls > output.txt
mysh$ /bin/cat < input.txt

Pipelines

mysh$ /bin/ls -l | /usr/bin/grep .c

Background Processes

mysh$ /bin/sleep 10 &

Built-in Commands

mysh$ cd /tmp
mysh$ exit

Testing

A test script test_cases.txt is included with sample inputs to validate core functionality.

To test the memory management module:

make test_mylib
./test_mylib

Limitations

• No support for environment variables or wildcard expansion
• Argument count and pipeline depth are limited by MAX_ARGS and MAX_PIPELINE_LEN
• No support for freeing individual allocations (entire heap reset only)

Implementation Notes

Custom Memory Management

MyShell uses a fixed-size heap with custom functions:

• my_alloc(size_t size): Allocates memory
• my_free_all(): Resets the heap

This approach reduces fragmentation and simplifies cleanup, but lacks granularity.

Job Execution Flow

1. Parse input into a job_t structure
2. Set up pipes (if pipeline is present)
3. Fork processes for each stage
4. Redirect input/output as required
5. Execute using execve
6. Wait for child processes unless running in background

Signal Handling

Custom SIGINT handler allows Ctrl+C to interrupt foreground jobs without killing the shell process itself.

Authors

• Abiola Raji
• Jacky On
• Cameron Goit

Class: COMP 3659 – Operating Systems