Full-Stack Interactive Unix Command Tool

Welcome to my handcrafted Unix shell, a deep dive into systems programming that simulates how real-world shells like bash and csh operate under the hood.

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What’s a Shell?

A shell is a command-line interface between users and the operating system. It interprets user commands and delegates tasks to the system, such as executing programs, managing files, and handling processes.

This project reimagines that core functionality — building shell features from scratch and enhancing them with interactive capabilities, including command history, environment variable expansion, wildcard matching, and autocompletion.

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Features Implemented

1. Command Parsing & Execution

• Built a custom shell grammar using Flex and Bison (Lex & Yacc).
• Executed commands via fork() and execvp().
• Supported input/output redirection (>, <, 2>, >>, >>&) using dup2().
• Implemented piping (|) for inter-process communication.

Skills Applied: IPC, process control, parsing, file descriptor manipulation.

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2. Signal Handling

• Implemented graceful interruption via SIGINT (Ctrl+C).
• Cleaned up zombie processes using SIGCHLD and waitpid().

Skills Applied: Signal handling, asynchronous processing.

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3. Built-in Commands

• Implemented cd, exit, printenv, setenv, and unsetenv.
• Enabled environment manipulation and directory navigation.

Skills Applied: String handling, shell state management, memory safety.

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4. Environment Variable Expansion

• Supports ${VAR}, ${$}, ${?}, ${!}, ${_}.
• Dynamically substitutes variables during runtime.

Skills Applied: String parsing, runtime state tracking.

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5. Wildcard Expansion

• Supports * and ? for filename pattern matching.
• Uses regex and filesystem traversal (opendir()/readdir()).

Skills Applied: Regular expressions, file system programming.

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6. Line Editing

• Wrote a custom line editor in raw terminal mode.
• Supports cursor movement, backspace/delete, and home/end keys.
• Enabled command history navigation with arrow keys.

Skills Applied: Terminal control, user interaction design.

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7. Path Completion

• Implemented autocompletion using the Tab key.
• Dynamically completes file and directory paths.

Skills Applied: Real-time input parsing, filesystem introspection.

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Features In Progress

• Process Substitution (e.g., <(cmd)): Work in progress. Will use named pipes and advanced redirection handling.

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Extra Touches

• Custom Prompt: Supports configuration via $PROMPT.
• Error Feedback: Displays messages using ON_ERROR for failed commands.

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Manual Testing

• Verified all line editing functionality and cursor navigation.
• Tested wildcard edge cases and autocompletion under nested directories.

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Skills Demonstrated

Domain	Skills Applied
Systems Programming	Fork/exec, signals, redirection, zombie cleanup
Language Parsing	Command grammar via Lex/Yacc
Memory Management	Manual malloc/free, Valgrind for leak checking
CLI/UX Design	Custom line editor, autocompletion, command history
Filesystem Handling	Directory traversal, regex-based matching

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Tech Stack

• C — Core implementation
• Flex & Bison — Parsing logic
• Valgrind — Memory safety validation
• Makefile — Build automation

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Final Thoughts

This shell project offered an immersive experience in systems-level thinking. From managing processes to creating a smooth command-line interface, each feature helped deepen my understanding of how operating systems work behind the scenes.

“Why use someone else’s shell when you can build your own?”

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Feel free to explore the codebase or reach out with questions.