Project 11: Line Editor (Mini-Readline)

A readline-like library that provides line editing (arrow keys, Home/End, Ctrl+A/E), history navigation, and a pleasant interactive experience—all without using the readline library.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages Rust, Zig, Go
Difficulty Level 4: Expert (The Systems Architect)
Time Estimate 2 weeks
Knowledge Area Terminal Programming / TUI
Tooling GNU Readline
Prerequisites Projects 1-8, understanding of terminal control

What You Will Build

A readline-like library that provides line editing (arrow keys, Home/End, Ctrl+A/E), history navigation, and a pleasant interactive experience—all without using the readline library.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • Raw mode (disabling canonical mode and echo) → maps to terminal control
  • Reading escape sequences (arrow keys send \x1b[A etc.) → maps to input parsing
  • Cursor management (knowing where cursor is, moving it) → maps to terminal state
  • Redrawing the line (handling insertions/deletions in the middle) → maps to screen updates
  • Terminal width handling (wrapping, resizing) → maps to responsive design

Key Concepts

  • Terminal I/O: “The Linux Programming Interface” Chapter 62 - Kerrisk
  • Terminal raw mode: “Advanced Programming in the UNIX Environment” Chapter 18 - Stevens
  • ANSI escape codes: XTerm Control Sequences documentation
  • linenoise as reference: Salvatore Sanfilippo’s minimal readline alternative (GitHub)

Real-World Outcome

$ ./mysh
mysh> hello world    # Type, then press left arrow 6 times
mysh> hello█world    # Cursor is now before 'w'
mysh> hello beautiful world   # Type 'beautiful ', text inserted
mysh> ^A             # Ctrl+A moves to start
mysh> ^E             # Ctrl+E moves to end
mysh> ^W             # Ctrl+W deletes word backward
mysh> ^K             # Ctrl+K kills to end of line
# Press up arrow to get previous command
# Press Tab for completion (if integrated)

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: SHELL_INTERNALS_DEEP_DIVE_PROJECTS.md
  • “The Linux Programming Interface” by Michael Kerrisk