YAML Syntax Guide

What is YAML?

YAML (YAML Ain't Markup Language) is a human-readable data serialization format. It's widely used for configuration files in Docker, Kubernetes, CI/CD pipelines, and application settings.

Basic Syntax

# Key-value pairs
name: Alice
age: 30

# Nested objects (use 2-space indent)
address:
  city: London
  country: UK

# Lists
hobbies:
  - reading
  - cycling
  - cooking

# Inline list
colors: [red, green, blue]

Data Types

string: "hello"          # Quoted string
unquoted: hello           # Also a string
number: 42                # Integer
float: 3.14               # Float
boolean: true             # Boolean (true/false, yes/no)
null_value: null           # Null (null or ~)
date: 2024-01-15          # Date
multiline: |              # Block literal (preserves newlines)
  Line one
  Line two

Anchors & Aliases

# Define anchor with &, reference with *
defaults: &defaults
  timeout: 30
  retries: 3

production:
  <<: *defaults       # Merge anchor
  timeout: 60         # Override specific values

Common Mistakes

Tabs instead of spaces: YAML requires spaces for indentation, never tabs.
Unquoted special values: yes, no, on, off are interpreted as booleans. Quote them if you mean strings.
Inconsistent indentation: pick 2 spaces and stick to it throughout the file.
Colon in values: key: value: here breaks. Quote the value: key: "value: here".

Convert and validate YAML with our YAML to JSON and YAML Validator tools.

FAQ

YAML vs JSON, when to use which?

YAML is better for human-edited config files (comments, readability). JSON is better for machine-to-machine data exchange (strict, widely supported).

Can YAML have comments?

Yes! Use # for comments. This is a major advantage over JSON which has no comment syntax.