YAML (YAML Ain't Markup Language) is a human-readable data serialisation format. It uses indentation (spaces, not tabs) to represent hierarchy. YAML is a superset of JSON — all valid JSON is valid YAML. It is widely used in configuration files for Docker, Kubernetes, GitHub Actions, and Ansible.

What is the difference between YAML and JSON?

YAML is more human-readable — it uses indentation instead of braces/brackets and supports comments. JSON is stricter and better for machine-to-machine data exchange. YAML is preferred for config files humans edit; JSON is preferred for API responses.

Can I use tabs in YAML?

No. YAML explicitly forbids tab characters for indentation. Tabs cause parse errors in all YAML parsers. Use spaces only — typically 2 spaces per indentation level.

What is YAML?

YAML (YAML Ain't Markup Language) is a human-readable serialisation format. It represents structured data using indentation rather than brackets or tags, making it easy to read and write by hand.

Syntax Basics

# A YAML config example
server:
  host: example.com
  port: 8080
  tls: true

allowed_ips:
  - 10.0.0.1
  - 10.0.0.2

Key-value pairs — key: value
Nested objects — indented with 2 spaces (never tabs)
Lists — items prefixed with -
Comments — lines starting with #
Strings — usually unquoted; use quotes if the value contains special characters

Where YAML Is Used

Docker Compose — docker-compose.yml defines services
Kubernetes — all resource manifests are YAML
GitHub Actions — workflow files in .github/workflows/
Ansible — playbooks and inventory files
Helm charts — values and templates

Common Gotchas

No tabs — tab characters cause parse errors; use spaces only
Norway problem — bare NO is parsed as false in some parsers (YAML 1.1); use quotes: "NO"
Octal numbers — 0777 parses as an octal integer in YAML 1.1; quote if you mean a string
Multiline strings — use | to preserve newlines or > to fold them

Convert YAML to JSON or JSON to YAML: Open JSON ↔ YAML Converter →

Frequently Asked Questions

Is YAML a superset of JSON?

Yes — every valid JSON document is also valid YAML 1.2. This means you can use JSON syntax inside a YAML file (braces, brackets, quoted strings). In practice YAML files use the indentation style for readability.

What is a YAML anchor?

Anchors (&name) and aliases (*name) allow you to reuse a node in multiple places. Define once with &, reference elsewhere with *. Useful for DRY config in long Kubernetes manifests.

Related Terms

JSON — The stricter, more machine-friendly sibling of YAML.
XML — An older structured format, more verbose than YAML.