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What is YAML?

YAML stands for YAML Ain’t Markup Language according to the homepage.

Or Yet Another Markup Language, from an older page of the same site, from 2001.

It is a convenient way of storing data, typically for config files. YAML is supported in many programming languages - the homepage linked below covers libraries you can use.

The extension is either:

  • .yml
  • .yaml

Some use-cases include storing data for pages on a site like title and description, describing a CI workflow or setting up containers in a docker compose file.

YAML vs JSON

YAML is a superset if JSON, so you can use JSON within YAML. But YAML is lighter in syntax making it easier to read and write. Especially for non-developers. It is light because you don’t need quotes for keys or values, you don’t need commas to separate lines and indentation is used for nested data like arrays or dictionaries, without having to use square or curly brackets.

From history in the docs:

Yet Another Markup Language, abbreviated YAML, describes a class of data objects called YAML documents and partially describes the behavior of computer programs that process them.

YAML documents encode into a serialized form information having a recursive null, scalar, map, or list structure. YAML also includes a method to encode references. At its core, a YAML document consists of a sequence of characters, some of which are considered part of the document’s content, and others that are used to indicate structure within the information stream.

Syntax

Sample content, going through data types:

  • foo.yml 
      my_bool: true
    
  my_int: 123
  my_float: 123.4
    
  my_string_unquoted: ABC DEF
  my_string_quoted: "ABC DEF"
  my_string_quoted: 'ABC DEF'
  # Escape characters like single quotes using quotes.
  my_string_escaped: "That's my code"
    
  int_as_string: '123'
  bool_as_string: 'true'
    
  my_multi_line_string: |
    abc
    def

  my_array:
    - a
    - b
    - c
  # OR
  my_array: [ a, b, c]
  # OR
  my_array: [ "a", "b", "c" ]

  my_hash:
    a: 1
    b: 2
    c: 3
  # OR
  my_hash: { "a": 1, "b": 2, "c": 3 
    
  my_array_of_hashes:
    - a: 1
      b: 2
      c: 3
    - a: 100
      b: 200
    - a: 1000
      
  my_date: 2018-30-09
  my_datetime: 2020-10-01 09:10:30

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