Niki's Shell

A POSIX-compliant shell implementation in Rust. This project was originally built as part of the CodeCrafters Shell Challenge and has been extended with additional features and improvements.

Features

Built-in Commands

• exit - Exit the shell with history persistence
• echo <text> - Print text to stdout
• pwd - Print current working directory
• cd <path> - Change directory (supports ~ for home directory)
• type <command> - Show command type (builtin vs external program location)
• history [options] - Command history management
  • history - Show all history
  • history <n> - Show last n commands
  • history -r <file> - Read history from file
  • history -w <file> - Write history to file
  • history -a <file> - Append new history to file

Interactive Features

• Command Line Editing
  • Arrow key navigation (↑/↓ for history browsing)
  • Backspace for character deletion
  • Raw mode terminal input with proper cleanup
• Tab Completion
  • Auto-completion for commands and executables in PATH
  • Common prefix completion for multiple matches
  • Press tab twice to show all available completions
• History Management
  • Persistent history across sessions (via HISTFILE environment variable)
  • In-memory history with navigation
  • History file operations (read/write/append)

Command Processing

• Shell Word Splitting with proper quote handling:
  • Single quotes ('text') - literal strings
  • Double quotes ("text") - variable expansion context
  • Backslash escaping for special characters
• Pipeline Support - Full pipe chains: cmd1 | cmd2 | cmd3
• I/O Redirection:
  • Output redirection: >, 1>, >>
  • Error redirection: 2>, 2>>
  • Append mode: >>, 1>>, 2>>
  • Mixed redirections: cmd >stdout.txt 2>stderr.txt

External Program Execution

• PATH Resolution - Automatic lookup of executables in system PATH
• Process Management - Proper spawning and waiting for child processes
• Signal Handling - Ctrl+C and Ctrl+D support

Architecture

The shell is organized into focused modules:

• main.rs - Core execution pipeline and process management
• readline.rs - Interactive terminal input with editing capabilities
• tokenize.rs - Command parsing and token generation
• words.rs - Shell word splitting with quote/escape handling
• cmd.rs - Built-in command implementations
• history.rs - Command history management and persistence
• paths.rs - PATH environment variable processing
• cmp.rs - Tab completion using trie data structure
• errors.rs - Comprehensive error handling

Usage

Installation and Usage

# Install from source
git clone https://github.com/nikiyerm/nikis_shell
cd nikis_shell
cargo install --path .

# Run the shell
nikis_shell

# Or compile and run directly
cargo run

Example Session

$ echo "Hello, World!"
Hello, World!
$ pwd
/home/user/projects
$ cd /tmp
$ ls -la | grep txt > text_files.txt
$ history 5
    1  echo "Hello, World!"
    2  pwd
    3  cd /tmp
    4  ls -la | grep txt > text_files.txt
    5  history 5
$ type ls
ls is /usr/bin/ls
$ exit

Implementation Highlights

• Memory Safety: Built with Rust's ownership system for safe memory management
• Concurrent Access: Thread-safe history and completion using LazyLock and Mutex
• Error Handling: Comprehensive error types for different failure modes
• Testing: Extensive unit tests for tokenization and word splitting
• Performance: Efficient trie-based completion and lazy initialization of system paths

TODO - Basic Missing Shell Features

The following core shell features are not yet implemented:

• Variable Expansion - $VAR, ${VAR}, environment variable substitution
• Globbing/Wildcards - *.txt, file?.log, [a-z]* pattern matching
• Input Redirection - < for reading from files
• Command Substitution - `command` and $(command)
• Background Jobs - command & and basic job control
• Quoting Edge Cases - Better handling of complex quote combinations
• Exit Codes - Proper exit code propagation from commands
• Signal Handling - Improved Ctrl+C, Ctrl+Z behavior
• More Built-ins - alias, unalias, export, unset, which