🧶 Kord

A lightweight, feature-rich POSIX shell implemented in C

Kord is a Unix shell that bridges user commands with kernel operations through system calls, providing efficient process management, I/O redirection, and a modern interactive terminal experience.

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🌟 Features

Core Shell Capabilities

• Command Execution: Execute external programs via fork() and execvp()
• Pipeline Support: Chain multiple commands with | operator
• I/O Redirection: Full support for <, >, and >> operators
• Built-in Commands: cd, pwd, echo, exit, help, and more
• Signal Handling: Proper Ctrl+C (SIGINT) management for shell and child processes

Advanced Features

• Shell Variables: Define and expand variables with $VAR syntax
• Environment Export: export variables to child processes
• Command Aliases: Create shortcuts for frequently used commands
• Persistent History: Commands saved to ~/.kord_history with navigation via arrow keys

Interactive Terminal

• Raw Mode Input: Custom terminal handling with ANSI escape sequences
• Line Editing:
  • Cursor navigation with arrow keys (←, →, Home, End)
  • Word-level navigation (Ctrl+←, Ctrl+→)
  • Character/word deletion (Backspace, Delete, Ctrl+W, Ctrl+Backspace)
• Tab Completion: Intelligent file/directory completion with case-insensitive matching
• History Navigation: Browse previous commands with ↑/↓ arrow keys
• Multi-line Support: Insert characters anywhere in the input line

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🔧 Terminal Architecture

Kord implements a sophisticated terminal I/O system that directly interfaces with the TTY subsystem:

TTY Subsystem & Raw Mode

The shell operates in raw mode by manipulating terminal attributes through termios:

// Disable canonical mode, echo, and signal generation
tcgetattr(STDIN_FILENO, &original_termios);
raw.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
tcsetattr(STDIN_FILENO, TCSAFLUSH, &raw);

Raw mode allows Kord to:

• Read input byte-by-byte without waiting for newlines
• Handle ANSI escape sequences (arrow keys, function keys)
• Implement custom line editing with immediate visual feedback
• Control cursor position and text rendering directly

PTY Master-Slave Architecture

While Kord primarily operates on a standard TTY, it's designed to work seamlessly in PTY (pseudo-terminal) environments:

• PTY Master: Owned by terminal emulators (e.g., xterm, gnome-terminal)

  • Handles rendering and user input events
  • Manages terminal window properties (size, colors)

• PTY Slave: Connected to shell processes like Kord

  • Receives input from master via kernel TTY driver
  • Sends output back through the TTY line discipline

• TTY Line Discipline: Kernel layer that processes terminal I/O

  • In cooked mode: handles backspace, Ctrl+C, line buffering
  • In raw mode (Kord's default): passes all input directly to the application

When a child process is spawned, Kord temporarily disables raw mode to ensure the child inherits standard terminal behavior, then re-enables it afterward for continued interactive control.

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📁 Project Structure

kord-sh/
├── src/
│   ├── main.c          # Entry point and main loop
│   ├── raw_input.c     # Raw mode terminal I/O and line editing
│   ├── parser.c        # Command parsing and variable expansion
│   ├── executor.c      # Process execution, pipes, and I/O redirection
│   ├── builtins.c      # Built-in command implementations
│   ├── prompt.c        # Dynamic shell prompt rendering
│   ├── history.c       # Command history management
│   ├── variables.c     # Shell variable storage
│   └── aliases.c       # Alias management
├── include/            # Header files
├── Makefile            # Build configuration
└── LICENSE             # MIT License

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🚀 Getting Started

Prerequisites

• GCC or any C compiler
• POSIX-compliant system (Linux, macOS, BSD)
• Make build tool

Installation

# Clone the repository
git clone https://github.com/yourusername/kord-sh.git
cd kord-sh

# Build the shell
make

# Run Kord
make run
# or directly
./bin/main

Usage

# Basic commands
$ pwd
$ ls -la
$ echo "Hello from Kord!"

# Pipelines
$ cat file.txt | grep "pattern" | sort

# I/O Redirection
$ echo "Log entry" >> log.txt
$ cat < input.txt > output.txt

# Variables
$ set USERNAME=kord
$ echo $USERNAME
$ export PATH=$PATH:/custom/bin

# Aliases
$ alias ll="ls -lah"
$ ll

# History
$ history
$ <press ↑ to navigate history>

# Tab completion
$ cd /usr/loc<TAB>  # Completes to /usr/local/

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🎯 Built-in Commands

Command	Description	Usage
cd	Change directory	cd [path]
pwd	Print working directory	pwd
echo	Print arguments	echo [args...]
exit	Exit shell	exit [code]
set	Set shell variable	set VAR=value
export	Export environment variable	export VAR=value
unset	Remove variable	unset VAR
alias	Define command alias	alias name='command'
unalias	Remove alias	unalias name
history	Show command history	history
help	Display help information	help [command]

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🔍 Implementation Highlights

Process Management

• fork(): Creates child processes by duplicating the parent's memory space
• exec() family: Replaces child process image with new program via execvp()
• Parent ignores SIGINT while child processes run
• Proper cleanup with waitpid() and signal restoration

Pipeline Implementation

// Create pipes between commands
pipe(fd_pipe);
// Child 1: stdout → pipe write end
// Child 2: pipe read end → stdin

Variable Expansion

• Parses $VAR syntax during command parsing
• Supports both shell-local and environment variables
• Quote handling preserves variable expansion: "$VAR" expands, '$VAR' literal

Terminal Control

• ANSI escape sequences for cursor movement: \033[C, \033[D
• Immediate visual feedback on edits via dup2() and terminal redraw
• Context-aware tab completion with directory traversal

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📝 Configuration Files

• ~/.kord_history: Stores last 50 commands, automatically loaded on shell startup and saved on exit. Supports up/down arrow navigation through command history.

• ~/.kordrc: Alias configuration file loaded at startup. Define persistent aliases that survive shell restarts.

  Note: Aliases defined with the alias command during runtime are not persisted to .kordrc. To make aliases permanent, manually edit this file.

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🛠️ Development

Cleaning Build Artifacts

make clean

Debugging

To debug with gdb:

gdb ./bin/main

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📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

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🐛 Known Limitations

• No command substitution (`command` or $(command))
• No globbing/wildcards (*.txt)
• Tab completion limited to files/directories (no command/variable completion)

Future enhancements welcome via pull requests!

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Built with ❤️ in C • Kord - Your friendly neighborhood shell 🧶