Chapter 8: Computation & Data Extraction Workflow

"If it's math, it belongs in a script. Period."

What You'll Build

By the end of this chapter, you'll have a personal toolbox of reusable utilities:

# Your workflow by chapter end:
find receipts/ -name "*.txt" | xargs cat | add-up
# Extracts dollar amounts from messy text, sums them with 100% accuracy

You'll transform from someone who asks AI to "do math in your head" (risky!) to someone who builds and verifies calculation tools that work every time.

Prerequisites

From Chapter 3 (Seven Principles):

• You understand ALL Seven Principles conceptually
• You know why "Bash is the Key" matters (Principle 1)
• You know why "Verification as Core Step" prevents failures (Principle 3)

From Chapter 6 (Business Workflows):

• You can navigate directories (cd, ls, pwd)
• You've run basic Bash commands
• You understand the pipe operator (|) conceptually

Technical Requirements:

• Python 3.x installed (type python3 --version to check)
• Unix-like terminal (macOS, Linux, or WSL on Windows)
• Access to Claude Code or similar AI assistant

Chapter Structure

Lesson	Title	Duration	Key Skill
1	The Accuracy Gap	20 min	Recognize when Bash math fails
2	Single-Purpose Utility	25 min	Build stdin-reading Python script
3	The Testing Loop	30 min	Verify with exit codes and test data
4	Personal Toolbox	20 min	Create persistent aliases
5	Data Wrangling	35 min	Extract data with regex
6	Capstone: Digital Shoebox	40 min	Orchestrate complete workflow

Total Duration: 170 minutes (~3 hours)

Seven Principles in Action

This chapter applies the principles you learned in Chapter 3:

Principle	How You'll Apply It
P1: Bash is the Key	Use cat, find, xargs, pipes as your foundation
P2: Code as Universal Interface	Python scripts as reusable components
P3: Verification as Core Step	Zero-trust debugging with exit codes
P4: Small, Reversible Decomposition	Each lesson builds one composable skill
P5: Persisting State in Files	Aliases and scripts as persistent tools
P6: Constraints and Safety	Test data prevents production errors
P7: Observability	Exit codes make failures visible

The Journey

Lesson 1-2: Foundation

• Discover why Bash arithmetic fails with decimals
• Build a Python script that reads from stdin and calculates sums

Lesson 3-4: Verification & Persistence

• Learn zero-trust debugging with exit codes
• Transform scripts into personal commands via aliases

Lesson 5: Data Extraction

• Use regex to extract dollar amounts from messy text
• Process multiple files with find and xargs

Lesson 6: Capstone

• Orchestrate everything into a real-world "Digital Shoebox" workflow
• Calculate totals from a folder of receipt files

Quick Start for Chapter 6 Graduates

Already comfortable with terminal basics? Here's what's new:

# Bash math FAILS with decimals (you'll discover why)
echo $((1.2 + 2.3))  # Error!

# Python script reads from stdin (you'll build this)
echo -e "1.2\n2.3" | python calc.py
# Output: Total: 3.50

# Regex extracts amounts from messy text (you'll learn this)
echo "Lunch: $14.50, Tip: $3.00" | python calc.py
# Output: Total: $17.50

Ready to Start?

Begin with Lesson 1: The Accuracy Gap to discover why precision matters.