Day 2

Essential Commands

Learn the core ways to invoke Claude — REPL mode, one-shot, piping, and flags.

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Three main modes

Claude CLI has three key invocation patterns: interactive REPL (`claude`), one-shot (`claude -p "…"`), and piped input (`cat file | claude -p "explain"`). Knowing when to use each makes you dramatically more efficient.

One-shot vs REPL

bash

# One-shot: great for scripting
claude -p "Write a bash function to find duplicate files"

# REPL: great for iterative work
claude
> Scaffold a FastAPI app with user auth
> Now add a password reset endpoint
> Generate tests for both endpoints

Piping content

bash

# Explain a file
cat main.py | claude -p "Summarize what this does"

# Fix a bug from stderr
python app.py 2>&1 | claude -p "What is causing this error?"

# Review a git diff
git diff | claude -p "Review this change for issues"

Useful flags

bash

# Continue last conversation
claude --continue

# JSON output for scripting
claude -p "List 5 Python tips" --output-format json

# Choose a model
claude --model claude-opus-4-8

# Non-interactive (for scripts)
claude --no-interactive -p "Generate changelog"

Practice Tasks

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One-shot a code snippet easy

Use `claude -p` to generate a Python function that reverses a string. Observe how it formats the output.

Use REPL mode easy

Start `claude` and have a multi-turn conversation: ask it to write a function, then ask it to add error handling, then ask it to write a test.

Pipe a file easy

Pick any code file you have. Run `cat yourfile | claude -p "What does this do and how could it be improved?"` and read the response.

Pipe stderr medium

Intentionally break a small script (remove a colon, etc.) and pipe the error output to Claude asking for a fix.

JSON output mode medium

Run `claude -p "List 3 best practices for REST APIs" --output-format json` and pipe the result to `jq .`

Continue a conversation medium

Run a one-shot query, then immediately run `claude --continue` and ask a follow-up question that builds on the first answer.

← Day 1: Installation & First Steps ↑ Module overview Day 3: Working with Files →