name: Alice
age: 30
email: alice@example.com
Nested (use 2-space indent — NEVER tabs)
address:
street: 123 Main St
city: Brussels
country: Belgium

Rules: space after colon, consistent indentation, no tabs.

# Block style
fruits:
  - apple
  - banana
  - cherry
Inline style
fruits: [apple, banana, cherry]
List of objects
users:

name: Alice
role: admin
name: Bob
role: user

# Inline object (flow style)
person: {name: Alice, age: 30}
Inline list
colors: [red, green, blue]
Mixed
servers:

{host: web1, port: 80}
{host: web2, port: 80}

# Strings (quotes optional for simple strings)
name: Alice
name: "Alice"
name: 'Alice'
Numbers
count: 42
price: 19.99
hex: 0xFF
octal: 0o77
scientific: 1.2e+5
Booleans
active: true
active: false
Also: yes/no, on/off (YAML 1.1 — avoid these!)
Null
value: null
value: ~
value:          # Empty value = null

YAML auto-interprets values. This causes bugs.

# ❌ These are NOT strings — they're booleans!
country: NO        # false (Norway!)
answer: yes        # true
flag: on           # true
flag: off          # false
✅ Quote them
country: “NO”
answer: “yes”
❌ These are NOT strings — they’re numbers!
version: 1.0       # float 1.0
zipcode: 01onal    # octal in some parsers
✅ Quote them
version: “1.0”
zipcode: “01onal”
❌ Colon in value breaks things
message: Error: file not found    # YAML error!
✅ Quote it
message: “Error: file not found”

Rule of thumb: when in doubt, quote it.

# | = literal block (preserves newlines)
description: |
  This is line one.
  This is line two.
This is after a blank line.
Result: “This is line one.\nThis is line two.\n\nThis is after a blank line.\n”
> = folded block (newlines become spaces)
description: >
This is a long
paragraph that will
be joined into one line.
Result: “This is a long paragraph that will be joined into one line.\n”
Chomp indicators (trailing newline control)
text: |     # Keep trailing newline (default)
text: |-    # Strip trailing newline
text: |+    # Keep all trailing newlines
Most common: |- for clean strings without trailing newline
script: |-
#!/bin/bash
echo “Hello”
echo “World”

Reuse values to avoid repetition.

# Define with & (anchor), reference with * (alias)
defaults: &defaults
  timeout: 30
  retries: 3
  region: eu-west-1
production:
<<: *defaults          # Merge all defaults
timeout: 60            # Override one value
staging:
<<: *defaults
region: us-east-1
Simple value reuse
db_host: &db_host “db.example.com”
services:
api:
database: *db_host
worker:
database: *db_host

<<: is the merge key — it merges the anchored mapping into the current one.

# One file, multiple documents (common in Kubernetes)
---
apiVersion: v1
kind: Service
metadata:
  name: web
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: web

--- starts a new document. ... optionally ends one.

# Full line comment
name: Alice  # Inline comment
YAML only has single-line comments
There is no multi-line comment syntax
Just use multiple # lines

services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
    volumes:
      - ./html:/usr/share/nginx/html
    environment:
      - NODE_ENV=production
    depends_on:
      - db
db:
image: postgres:16
environment:
POSTGRES_PASSWORD: secret
volumes:
- db_data:/var/lib/postgresql/data
volumes:
db_data:

name: CI
on:
  push:
    branches: [main]
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: actions/setup-node@v4
with:
node-version: 20
- run: npm ci
- run: npm test

# Python
python -c "import yaml; yaml.safe_load(open('file.yaml'))"
Node.js
node -e “const y=require(‘yaml’);console.log(y.parse(require(‘fs’).readFileSync(‘file.yaml’,‘utf8’)))“
Online: yamllint.com
Install yamllint
pip install yamllint
yamllint file.yaml