What is a YAML file?

YAML stands for YAML Ain’t Markup Language. YAML is a data serialization format similar to XML and JSON. However, it is more human-readable.

YAML is a case-sensitive data format. It uses spaces ( ) to define document structure. Tabs (\t) are not allowed in YAML.

Usually, YAML files are used to store configuration data and are commonly named with either .yml or .yaml extensions, e.g., config.yml.

A YAML file contains a stream of data that may contain multiple documents.

Different documents are separated using three dashes (---).

If you want to mark the end of a document without starting a new one, you may use three dots (...). Let’s look at the example below to understand this better.

Example: Block style

---
# list of markup languages 
Formats: 
- XML: eXtensible Markup Language, 
- JSON: JavaScript Object Notation, 
- CSV: Comma-separated Values
---
# list of encodings
Encodings:
 - Unicode
 - ASCII
 - UTF8
---
 # list of programming languages 
Language: 
 - Java
 - C++
 - Python
...

The above style where hyphens (-) and spaces ( ) are used to specify a list of items is known as Block Style.

Example: Flow style

We can also represent the above document in a more compact notation known as Flow Style. It uses inline JSON to represent lists and key-value pairs. This makes YAML a superset of JSON.

Below is the same document represented using Flow Style notation.

---
 # list of markup languages
Formats: [{ "XML": "eXtensible Markup Language"}, {
"JSON": "JavaScript Object Notation" }, {"CSV": "Comma-separated Values"}]
---
# list of encodings
Encodings: [ Unicode, ASCII, UTF8]
---
# list of programming 
Languages: [Java, C++, Ruby]
...

Learn more about the syntax, data types, and various key concepts related to the YAML language through this mini-course: Introduction to YAML.