ShellGood - A Mini Shell in C

Overview

ShellGood is a Unix-like shell written in C with support for pipelines, redirection, and full job control. The shell implements POSIX process control primitives including process groups, signals, and terminal management.

Features

• Executes programs using fork + execvp
• Pipelines with multiple processes (|)
• Input/Output redirection (<,>)
• Background execution (&)
• Job control (jobs, fg, bg)
• Terminal control using tcsetpgrp
• Signal handling:
  • SIGCHLD for async child reaping
  • Proper handling of SIGINT and SIGTSTP

Architecture

The shell is structured into separate components:

• Parser: tokenizes inputs and builds process structures
• Commands: built-in shell commands
• Signals: signal handlers
• Execution: pipeline creation, process groups, job tracking and launching

Challenges/Highlights

• Race condition with SIGCHLD:

  • Ensured correct behavior by blocking signals during job creation and foreground job execution
  • Used a self-pipe to communicate between signal handler and main loop

• Foreground signal race handling:

  • Ensured signals like SIGINT/SIGTSTP are not lost when delivered before the shell transfers terminal control

• Correct job control implementation:

  • Used process groups so pipelines behave as single jobs and receive signals together

• Terminal ownership:

  • Transferred terminal control with tcsetpgrp so Ctrl+C/Z are delivered directly to the foreground job

• Pipeline execution:

  • Built a loop that correctly wires pipes and assigns a shared process group to all processes in a pipeline

Technical Details

• Uses setpgid to group processes into jobs and ensure correct signal delivery
• Transfers terminal control using tcsetpgrp for proper foreground execution
• Implements a self-pipe mechanism for safe communication from signal handlers
• Avoids race conditions by blocking SIGCHLD during critical sections
• Tracks individual process IDs within jobs for precise lifecycle management
• Uses waitpid with WUNTRACED to correctly detect stopped processes

Design Decisions

• Separation of built-in commands:
  • Built-ins are executed in the shell process and cannot be mixed with normal commands to avoid subshell semantics
• Parser-level validation:
  • Invalid jobs (e.g. mixing shell built-ins in pipelines) are rejected during parsing

Building and Running

to build: make

to run: ./ShellGood

Notes

• This project was made as a learning exercise in systems programming and not as a replacement for production shells like bash
• Parsing is intentionally minimal (no quoting or advanced shell syntax)
• Focus is on correct process, signals, and job control behavior