shell

This project was completed as part of CSCI0330: Introduction to Computer Systems at Brown University. It implements a custom Unix-style shell in C that supports command execution, job control, signal handling, and I/O redirection.

Note: Due to course policies, the full codebase cannot be shared publicly. Please contact me (Brendan Rathier) directly if you would like access for educational purposes.

Overview

This shell replicates essential functionality found in traditional Unix shells. It supports both built-in and external commands, background and foreground job execution, terminal signal control, and input/output redirection. The shell is designed to read user input in a continuous REPL, and includes extensive error checking to handle improper arguments or syntax.

Core Features

• Execution of external programs with argument parsing
• Built-in commands:
  • cd: Change directory
  • rm: Remove file
  • ln: Create hard link
  • exit: Exit the shell
  • jobs: List background jobs
  • fg <JID>: Resume a background job in the foreground
  • bg <JID>: Resume a stopped job in the background
• Input/output redirection using < and >
• Foreground and background process execution (&)
• Job list management with globally tracked JIDs
• Signal handling (e.g., SIGINT, SIGTSTP, SIGCHLD) and proper terminal control
• Error handling for incorrect command syntax, missing arguments, and invalid job IDs

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Implementation Highlights

• Command Parsing: Supports parsing of commands with arguments, redirection symbols, and background execution indicators.
• Built-in Handling: The shell detects and executes built-in commands without forking, managing edge cases such as invalid paths or missing arguments.
• Forking and Execution: For non-built-in commands, the shell forks child processes. It assigns foreground jobs terminal control and tracks background jobs in a global job list.
• Job Control: Maintains a list of background jobs with unique job IDs. Allows users to resume jobs using fg or bg.
• Signal Handling:
  • SIGCHLD: Reaps terminated or stopped background processes
  • SIGINT / SIGTSTP: Properly forwarded to foreground processes
• Reaping Logic:
  • reap_foreground: Waits on a foreground job and handles any state transitions
  • reap_background: Iterates through jobs at the start of each REPL cycle to reap exited or stopped background processes