Codex CLI | LLM CLI Training

What is Codex CLI?

Codex CLI is OpenAI’s command-line coding agent. It’s designed for autonomous task execution — you assign it a task, and it plans, implements, tests, and iterates until the job is done.

Key features:

Autonomous multi-step task execution
Can create branches, make changes, and iterate
Sandboxed execution for safety
Built on OpenAI’s latest models
Open source

Installation

Step 1: Install Codex CLI

npm install -g @openai/codex

Verify:

codex --version

Step 2: Set up your API key

Go to platform.openai.com
Create an account or sign in
Navigate to API Keys
Create a new secret key
Set it as an environment variable:

export OPENAI_API_KEY="your-key-here"

To make this permanent, add the export line to your shell config:

echo 'export OPENAI_API_KEY="your-key-here"' >> ~/.bashrc
source ~/.bashrc

(If your shell is zsh — check with echo $SHELL — use ~/.zshrc instead.)

⚠Keep your API key safe

Never commit your API key to a git repository or share it publicly. Treat it like a password. If you accidentally expose it, revoke it immediately from the OpenAI dashboard and create a new one.

Your first interaction

Step 1: Create a project folder

mkdir hello-codex && cd hello-codex

Step 2: Start Codex CLI

codex

Step 3: Give it a task

Create a simple to-do list app in a single HTML file. It should:
- Let users type a task and press Enter to add it
- Show tasks in a list with checkboxes to mark as done
- Have a "Clear completed" button
- Dark theme, clean design
- Save tasks to localStorage so they persist on refresh

Codex will:

Plan the implementation
Create the file
Write the code
Verify it meets requirements

Step 4: Check the result

ls
cat index.html

Open in your browser to test the todo list functionality.

Codex CLI modes

Codex CLI offers different autonomy levels:

Mode	What it does	Best for
Suggest	Shows proposed changes, asks for approval	Learning, careful work
Auto-edit	Makes file changes automatically, asks before commands	General use
Full auto	Executes everything autonomously	When you trust the task

Start with Suggest mode while learning, then move to Auto-edit as you get comfortable.

Tips

Be specific about requirements. Codex excels when you give it a clear, well-defined task. The more specific your instructions, the better the result.
Start with small tasks. Build confidence with simple, single-file tasks before trying multi-file projects.
Use the sandbox. Codex runs in a sandboxed environment for safety. This means it can’t accidentally harm your system, making it safe to experiment with.
Review before accepting. In Suggest mode, Codex shows you what it wants to do before doing it. Use this to learn what good code looks like.