Basic YAML Syntax

Introduction

YAML, which stands for "YAML Ain't Markup Language," is a human-readable data serialization standard that is commonly used for configuration files and data exchange between languages with different data structures. In this tutorial, we'll cover the basics of YAML syntax, providing you with the foundation to start working with YAML in the context of tools like Ansible.

Basic Structure

YAML files use indentation to indicate structure. Indentation is done using spaces, and the level of indentation indicates the level of nesting. Here is an example of a basic YAML structure:

key: value
another_key:
  sub_key: sub_value
                

In this example, key has a value of value, and another_key contains a nested key sub_key with a value of sub_value.

Lists

YAML supports lists, which are indicated by a dash followed by a space. Here is an example of a list in YAML:

items:
  - item1
  - item2
  - item3
                

In this example, items is a list containing three items: item1, item2, and item3.

Dictionaries

YAML also supports dictionaries (or maps), which are collections of key-value pairs. Here's an example:

person:
  name: John Doe
  age: 30
  address:
    street: 123 Main St
    city: Anytown
                

In this example, person is a dictionary with keys name, age, and address. The address key is itself a dictionary containing street and city.

Comments

Comments in YAML are denoted by the # character. Everything on a line after a # is ignored by the parser. Here's an example:

# This is a comment
key: value  # This is an inline comment
                

Multi-line Strings

YAML supports multi-line strings using the pipe | or greater-than > characters. The pipe character preserves line breaks, while the greater-than character folds line breaks into spaces. Here are examples of both:

# Using pipe
description: |
  This is a multi-line
  string that preserves
  line breaks.

# Using greater-than
summary: >
  This is a multi-line
  string that folds
  line breaks into spaces.
                

Anchors and Aliases

YAML allows you to define reusable content with anchors and aliases. An anchor is defined using the & character, and an alias is used with the * character. Here is an example:

defaults: &default_values
  name: Default Name
  age: Default Age

person1:
  <<: *default_values
  name: John Doe

person2:
  <<: *default_values
  age: 25
                

In this example, &default_values defines an anchor with default values. The alias *default_values is used to merge these default values into person1 and person2, with individual overrides for specific keys.