Shell

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🌐 On Github

About

This project is a fully functional Unix-like shell written in C. It supports process execution, signal handling, and input/output redirection — all constructed from scratch. Inspired by classic CLI environments, it replicates much of the behavior seen in shells like bash and sh, while adding custom features like PID expansion and foreground-only toggling.

My main goal with this project was to gain low-level experience working with the POSIX API, including process control, signals, and file descriptors. Writing everything manually — from memory management to command tokenization — helped me deeply understand how shells work under the hood.

How I Built It

• Language & Tools: C (with POSIX headers), gcc, valgrind for memory safety, and a terminal for testing.
• Custom Lexer & Parser: I developed a lexer and parser by hand. The lexer handles quotation marks and filters I/O redirection, pipe, and background operators. The parser processes the output from the lexer and creates an Abstract Syntax Tree (AST), allowing efficient and organized execution of the user’s command.
• Signal Handling:
  • Used sigaction() to handle SIGTSTP (Ctrl+Z) for toggling foreground-only mode.
  • Foreground processes are interruptible via SIGINT (Ctrl+C), while the shell ignores it.
• Execution Model:
  • Forks child processes for external commands.
  • Handles input/output redirection using dup2() and open().
  • Tracks background processes in a linked list and reaps zombies with waitpid() using WNOHANG.

Features

• Built-in Commands: Supports cd, status, and exit internally.
• PID Expansion: Substitutes $$ with the current shell’s process ID.
• Foreground/Background Execution: Uses & to send jobs to the background; toggle-safe with SIGTSTP.
• Signal-Safe Behavior: Shell and children handle SIGINT/SIGTSTP differently for robust control.
• Redirection Support: Clean redirection of STDIN and STDOUT with file fallback for background jobs.
• Memory Management: Manual malloc() and free() for every structure, with clean shutdown logic.
• Zombie Reaping: Non-blocking cleanup of completed background jobs to avoid defunct processes.