Project 15: POSIX-Compliant Shell

A fully POSIX-compliant shell that passes the POSIX conformance tests—a complete, production-quality shell implementation that could theoretically replace /bin/sh.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages Rust
Difficulty Level 5: Master (The First-Principles Wizard)
Time Estimate 2-3 months
Knowledge Area Operating Systems / Standards Compliance
Tooling POSIX Shell
Prerequisites Projects 1-14

What You Will Build

A fully POSIX-compliant shell that passes the POSIX conformance tests—a complete, production-quality shell implementation that could theoretically replace /bin/sh.

Why It Matters

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

Core Challenges

  • Full grammar (all POSIX syntax including edge cases) → maps to standards compliance
  • Subshells ((commands) execute in child process) → maps to execution contexts
  • Command substitution ($(command) captures output) → maps to advanced features
  • Traps (trap ‘handler’ SIGNAL) → maps to signal customization
  • Special built-ins (break, continue, return, set) → maps to shell control

Key Concepts

  • POSIX Shell Standard: “Shell Command Language” - The Open Group (pubs.opengroup.org)
  • Test suites: “POSIX conformance testing” - various open-source test suites
  • Reference implementations: dash (Debian Almquist Shell) source code

Real-World Outcome

$ ./mysh
mysh> result=$(echo hello | tr a-z A-Z)
mysh> echo $result
HELLO
mysh> (cd /tmp; pwd); pwd
/tmp
/home/douglas                # Subshell didn't affect parent
mysh> trap 'echo Caught!' INT
mysh> sleep 100
^CCaught!
mysh> set -e                 # Exit on error
mysh> false
$                           # Shell exited due to -e
$ ./run_posix_tests.sh ./mysh
PASS: 1247/1250 tests passed

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
  • “POSIX Shell Specification” by The Open Group