Project 14: Script Interpreter

A shell scripting interpreter supporting if/then/else/fi, while/do/done, for/in/do/done, case/esac, functions, local variables, and return/exit.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages Rust, Go, OCaml
Difficulty Level 4: Expert (The Systems Architect)
Time Estimate 2-3 weeks
Knowledge Area Interpreters / Language Design
Tooling Shell Scripting
Prerequisites Projects 1-7, understanding of interpreters

What You Will Build

A shell scripting interpreter supporting if/then/else/fi, while/do/done, for/in/do/done, case/esac, functions, local variables, and return/exit.

Why It Matters

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

Core Challenges

  • Control flow parsing (if/while/for/case grammar) → maps to language design
  • Condition evaluation (exit status determines truth) → maps to shell semantics
  • Function definition/call (name() { body; }) → maps to subroutines
  • Variable scope (local keyword, positional parameters) → maps to scoping
  • Reading scripts (shebang, sourcing vs executing) → maps to execution modes

Key Concepts

  • Shell compound commands: POSIX Shell Specification Section 2.9.4 - The Open Group
  • Shell functions: POSIX Shell Specification Section 2.9.5 - The Open Group
  • Interpreter patterns: “Language Implementation Patterns” Chapter 8 - Parr

Real-World Outcome

$ cat test.sh
#!/path/to/mysh

greet() {
    local name="$1"
    echo "Hello, $name!"
}

for i in 1 2 3; do
    greet "User$i"
done

if [ -f /etc/passwd ]; then
    echo "System file exists"
else
    echo "Not a Unix system"
fi

count=0
while [ $count -lt 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

$ ./mysh test.sh
Hello, User1!
Hello, User2!
Hello, User3!
System file exists
Count: 0
Count: 1
Count: 2
Count: 3
Count: 4

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
  • “Language Implementation Patterns” by Terence Parr