Claude Code Fundamentals

What is Claude Code?

Claude Code is an agentic coding tool from Anthropic that lives in your terminal. It understands your codebase, helps you edit files, and executes terminal commands—all within a chat-based interface.

Requirements

Installation

Install Claude Code globally using npm:

# Install Claude Code globally
npm install -g @anthropic-ai/claude-code

# Verify installation
claude --version

# Start Claude Code
claude

API Key Setup

Claude Code needs your Anthropic API key. You can set it up in two ways:

# Start Claude Code and follow login prompts
claude

# Add to ~/.zshrc or ~/.bashrc
export ANTHROPIC_API_KEY="sk-ant-..."

# Reload your shell
source ~/.zshrc

Your First Run

Navigate to a project directory and start Claude Code:

# Navigate to your project
cd /path/to/your/project

# Start Claude Code
claude

# Try these commands:
/init           # Analyze project and create CLAUDE.md
/help           # Show all available commands
/status         # Show current context and settings

/init - Project Initialization

The /init command analyzes your project structure and generates a CLAUDE.md file containing project context.

# Run this at the start of any new project
/init

# Claude will analyze:
# - File structure
# - Dependencies (package.json, requirements.txt, etc.)
# - Existing documentation
# - Code patterns and conventions

CLAUDE.md - Project Memory

The CLAUDE.md file stores project-specific context that persists across sessions. It has three levels:

# - Add to Memory

Use the # prefix to add information directly to CLAUDE.md. This is useful for recording learnings or project conventions.

# When you discover important patterns:
# Always use TypeScript strict mode in this project

# Record learned lessons:
# FileSystemContext requires FileSystemProvider wrapper

# Document conventions:
# API routes follow /api/v1/resource pattern

@ - File References

Use @ to reference specific files in your prompts. This adds the file content to context.

# Reference a specific file
@src/components/Button.tsx add a loading state

# Reference multiple files
@package.json @tsconfig.json explain the build setup

# Reference with context
Based on @types/user.ts create a user form component

What is Plan Mode?

Plan Mode lets Claude analyze and plan without executing code changes. It's essential for complex tasks where you want to understand the approach before implementation.

# Press Shift+Tab twice to toggle Plan Mode
Shift + Tab + Tab

# Or use the command
/plan

When to Use Plan Mode

Plan Mode Workflow

# 1. Enter Plan Mode
[Plan Mode ON]

# 2. Ask Claude to analyze
Analyze how authentication works in this project

# 3. Claude explores without changing files
# - Reads relevant files
# - Identifies patterns
# - Creates a plan

# 4. Review the plan
# 5. Exit Plan Mode and execute
[Plan Mode OFF]
Implement the plan

Extended Thinking

Claude can use extended thinking for complex problems. Use keywords in your prompt to trigger deeper analysis.

When to Use Extended Thinking

# Debugging complex issues
ultrathink about why this race condition occurs

# Architecture decisions
think more about the trade-offs between these approaches

# Security analysis
ultrathink about potential vulnerabilities in this code

# Performance optimization
think more about how to optimize this database query

Why Context Management Matters

Claude has a context window limit. As you work, the conversation accumulates tokens. Managing context helps maintain performance and relevance.

/clear - Reset Conversation

Completely resets the conversation. Use when switching to a completely different task.

# Clear entire conversation
/clear

# When to use:
# - Switching to unrelated work
# - After completing a major task
# - When context becomes cluttered

/compact - Compress Context

Summarizes and compresses the conversation while retaining key information. Use to continue work with less token usage.

# Compress conversation
/compact

# When to use:
# - Long sessions approaching token limit
# - Before adding large file references
# - To maintain context across breaks