Claude Code Overview#

Claude Code is Anthropic’s official agentic CLI. It runs Claude directly in your terminal, reads and edits files, runs commands, manages git, and integrates with your development workflow.

Install#

npm install -g @anthropic-ai/claude-code

Output:

added 1 package, and audited 1 package in 3s
found 0 vulnerabilities

[!NOTE] Requires Node.js 18+. Claude Code is also available as a VS Code extension, JetBrains plugin, and via claude.ai/code.

Set your API key#

export ANTHROPIC_API_KEY="sk-ant-..."

Or run claude and it will prompt you to authenticate on first launch.

Launch interactive mode#

claude

Output:

╭─────────────────────────────────────────────╮
│ ✻ Welcome to Claude Code research preview!  │
│                                             │
│   /help for help, /exit to quit             │
╰─────────────────────────────────────────────╯

?  What would you like to do?

Type your request in natural language. Claude reads the current directory as context.

> refactor the auth module to use JWT instead of session tokens

Claude will plan the changes, show you a diff, and ask for confirmation before writing files.

Key interactive concepts#

Permission model#

Claude Code asks before taking potentially destructive actions (file edits, shell commands, git operations). You can:

• Allow once — approve this specific action
• Allow always (for this session) — auto-approve this tool type
• Deny — block the action and let Claude adjust

Context window management#

Use /compact when the conversation grows long. Claude summarizes prior context and continues with a clean window.

/clear vs /compact#

• /clear — discard all conversation history; start fresh
• /compact — summarize history and continue; context is preserved as a condensed form

Headless (non-interactive) mode#

Run Claude as a scripted command without the interactive UI. Useful for CI, automation, and piping output.

# Single prompt, non-interactive
claude --print "List all TODO comments in this repo"

# Short form
claude -p "What does this project do?"

Output:

This project is an Astro-based static site for developer cheat sheets and code snippets...

Pipe input to Claude#

cat error.log | claude -p "What is causing this error?"

Output:

The error is a NullPointerException at UserService.java:142. The `currentUser` field
is null when `getPreferences()` is called — likely because `authenticate()` was not
called before accessing the user object.

Output format flags#

# Plain text (default for pipes)
claude -p "Summarize this file" < README.md

# JSON output (useful for scripting)
claude --output-format json -p "List the exports in this file" < src/utils.ts

Output (JSON):

{"type":"result","result":"The file exports: formatDate, parseISO, addDays, diffDays","session_id":"sess_01...","cost_usd":0.0012}

Allowed tools flag#

Restrict which tools Claude can use in a session:

# Read-only — no edits or commands
claude --allowedTools "Read,Bash(git log:git diff:git status)" -p "Summarize recent changes"

# Allow specific bash subcommands only
claude --allowedTools "Bash(npm test)" -p "Run the tests and tell me what failed"

CLAUDE.md — persistent instructions#

CLAUDE.md files give Claude persistent context and standing instructions. Claude reads them automatically at the start of every session.

Hierarchy#

Claude merges all applicable CLAUDE.md files (global → project → subdir) before starting. More specific files take priority on conflicts.

Example project CLAUDE.md#

# Project: Acme API

## Stack
- FastAPI + SQLAlchemy 2.0
- Postgres 16, Alembic for migrations
- pytest + pytest-asyncio for tests

## Commands
- Run tests: `pytest tests/ -x`
- Start dev server: `uvicorn app.main:app --reload`
- Lint: `ruff check . && ruff format --check .`

## Conventions
- Use `async def` for all route handlers
- No `print()` statements — use `structlog`
- All new endpoints need a test in `tests/api/`
- Never edit `alembic/versions/` directly

## Files to avoid
- `config/secrets.yaml` — not in git; local only

Example global CLAUDE.md (~/.claude/CLAUDE.md)#

# Global preferences

- I prefer concise explanations — skip restating what I already know
- Default to Python 3.12+ syntax
- Use `ruff` for formatting, not `black`
- Never add emoji to code comments

[!TIP] Keep CLAUDE.md files short and machine-parseable. Long prose slows Claude’s context load. Use bullet points and code blocks. Think of it as a compact README for Claude.

IDE integration#

VS Code#

Install the Claude Code extension from the VS Code Marketplace. Adds:

• Inline diff review pane
• Chat panel docked to the sidebar
• Keyboard shortcut Cmd+Esc (Mac) / Ctrl+Esc (Windows/Linux) to open Claude

JetBrains#

Install from the JetBrains Marketplace. Same features; accessible via Tools → Claude Code.

Session management#

# List recent sessions
claude sessions list

# Resume a previous session
claude sessions resume <session-id>

# Clear all local session history
claude sessions clear

Useful startup flags#

First-run checklist#

☐ Node 18+ installed (node --version)
☐ npm install -g @anthropic-ai/claude-code
☐ ANTHROPIC_API_KEY set in shell profile
☐ Run `claude` in a project directory
☐ Create a CLAUDE.md at project root with key conventions
☐ Add CLAUDE.md to .gitignore if it contains personal notes, or commit it for team use