YAML Syntax Guide for DevOps

string: hello world
quoted: "hello world"
number: 42
float: 3.14
boolean: true
null_value: null
date: 2026-04-10

# Block style
fruits:
  - apple
  - banana
  - cherry

# Inline style
fruits: [apple, banana, cherry]

# Block style
person:
  name: Alice
  age: 30
  role: engineer

# Inline style
person: {name: Alice, age: 30, role: engineer}

servers:
  - name: web-1
    ip: 10.0.1.10
    roles:
      - nginx
      - certbot
    ports:
      - 80
      - 443

  - name: db-1
    ip: 10.0.1.20
    roles:
      - postgresql
    ports:
      - 5432

script: |
  #!/bin/bash
  echo "Hello"
  echo "World"
  exit 0

description: >
  This is a long description
  that spans multiple lines
  but becomes one paragraph.

keep_newline: |+
  text
  
strip_newline: |-
  text

defaults: &defaults
  timeout: 30
  retries: 3
  region: eu-west-1

production:
  <<: *defaults
  timeout: 60  # Override just this

staging:
  <<: *defaults
  # Inherits timeout: 30, retries: 3, region: eu-west-1

# These are all booleans, not strings!
country: NO      # false
answer: yes      # true
flag: on         # true
switch: off      # false

# Fix: quote them
country: "NO"
answer: "yes"

# Breaks
url: http://example.com:8080

# Fix: quote it
url: "http://example.com:8080"

# YAML does not allow tabs! Only spaces.
# Most editors convert tabs, but verify your settings.
good:
  key: value    # 2 spaces ✅

bad:
	key: value    # tab ❌ (will fail to parse)

# These become numbers
version: 1.0     # float 1.0
port: 8080       # integer 8080

# Force string
version: "1.0"
zip_code: "01onal"

# These are all null
key1:
key2: null
key3: ~

# Empty string requires quotes
key4: ""

---
- name: Deploy application
  hosts: webservers
  become: true
  vars:
    app_version: "2.1.0"
    env_vars:
      DB_HOST: db.internal
      DB_PORT: "5432"       # Quote to keep as string
      DEBUG: "false"        # Quote to keep as string

  tasks:
    - name: Create config file
      copy:
        content: |
          APP_VERSION={{ app_version }}
          {% for key, value in env_vars.items() %}
          {{ key }}={{ value }}
          {% endfor %}
        dest: /etc/app/config

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
  labels:
    app: web
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    spec:
      containers:
        - name: web
          image: nginx:1.27
          ports:
            - containerPort: 80
          env:
            - name: NODE_ENV
              value: "production"    # Always quote env values
          resources:
            requests:
              cpu: 100m
              memory: 128Mi
            limits:
              cpu: 500m
              memory: 256Mi

name: CI
on:
  push:
    branches: [main]
  pull_request:

jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        node: [20, 22]
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ matrix.node }}
      - run: npm ci
      - run: npm test

# Python
pip install yamllint
yamllint myfile.yml

# Node
npx yaml-lint myfile.yml

# Online
# yaml-online-parser.appspot.com