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title date icon background tags categories intro
YAML
2020-12-29 10:26:55 -0800
icon-yaml
bg-[#b42e28]
config
format
Programming
This is a quick reference cheat sheet for understanding and writing YAML format configuration files.

Getting started {.cols-3}

Introduction

YAML is a data serialisation language designed to be directly writable and readable by humans

  • YAML does not allow the use of tabs
  • Must be space between the element parts
  • YAML is CASE sensitive
  • End your YAML file with the .yaml or .yml extension
  • YAML is a superset of JSON
  • Ansible playbooks are YAML files {.style-round}

Scalar types {.row-span-2}

n1: 1            # integer          
n2: 1.234        # float      

s1: 'abc'        # string        
s2: "abc"        # string           
s3: abc          # string           

b: false         # boolean type 

d: 2015-04-05    # date type

↓ Equivalent JSON

{
  "n1": 1,
  "n2": 1.234,
  "s1": "abc",
  "s2": "abc",
  "s3": "abc",
  "b": false,
  "d": "2015-04-05"
}

Use spaces to indent. There must be space between the element parts.

Variables

some_thing: &VAR_NAME foobar
other_thing: *VAR_NAME

↓ Equivalent JSON

{
  "some_thing": "foobar",
  "other_thing": "foobar"
}

Comments

# A single line comment example

# block level comment example
# comment line 1
# comment line 2
# comment line 3

Multiline strings

description: |
  hello
  world

↓ Equivalent JSON

{"description": "hello\nworld\n"}

Inheritance {.row-span-2}

parent: &defaults
  a: 2
  b: 3

child:
  <<: *defaults
  b: 4

↓ Equivalent JSON

{
  "parent": {
    "a": 2,
    "b": 3
  },
  "child": {
    "a": 2,
    "b": 4
  }
}

Reference {.row-span-2}

values: &ref
  - Will be
  - reused below
  
other_values:
  i_am_ref: *ref

↓ Equivalent JSON

{
  "values": [
    "Will be",
    "reused below"
  ],
  "other_values": {
    "i_am_ref": [
      "Will be",
      "reused below"
    ]
  }
}

Folded strings

description: >
  hello
  world

↓ Equivalent JSON

{"description": "hello world\n"}

Two Documents

---
document: this is doc 1
---
document: this is doc 2

YAML uses --- to separate directives from document content.

YAML Collections {.cols-3}

Sequence

- Mark McGwire
- Sammy Sosa
- Ken Griffey

↓ Equivalent JSON

[
  "Mark McGwire",
  "Sammy Sosa",
  "Ken Griffey"
]

Mapping

hr:  65       # Home runs
avg: 0.278    # Batting average
rbi: 147      # Runs Batted In

↓ Equivalent JSON

{
  "hr": 65,
  "avg": 0.278,
  "rbi": 147
}

Mapping to Sequences

attributes:
  - a1
  - a2
methods: [getter, setter]

↓ Equivalent JSON

{
  "attributes": ["a1", "a2"],
  "methods": ["getter", "setter"]
}

Sequence of Mappings

children:
  - name: Jimmy Smith
    age: 15
  - name: Jimmy Smith
    age: 15
  -
    name: Sammy Sosa
    age: 12

↓ Equivalent JSON

{
  "children": [
    {"name": "Jimmy Smith", "age": 15},
    {"name": "Jimmy Smith", "age": 15},
    {"name": "Sammy Sosa", "age": 12}
  ]
}

Sequence of Sequences

my_sequences:
  - [1, 2, 3]
  - [4, 5, 6]
  -  
    - 7
    - 8
    - 9
    - 0

↓ Equivalent JSON

{
  "my_sequences": [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9, 0]
  ]
}

Mapping of Mappings

Mark McGwire: {hr: 65, avg: 0.278}
Sammy Sosa: {
    hr: 63,
    avg: 0.288
  }

↓ Equivalent JSON

{
  "Mark McGwire": {
    "hr": 65,
    "avg": 0.278
  },
  "Sammy Sosa": {
    "hr": 63,
    "avg": 0.288
  }
}

Nested Collections

Jack:
  id: 1
  name: Franc
  salary: 25000
  hobby:
    - a
    - b
  location: {country: "A", city: "A-A"}

↓ Equivalent JSON

{
  "Jack": {
    "id": 1,
    "name": "Franc",
    "salary": 25000,
    "hobby": ["a", "b"],
    "location": {
        "country": "A", "city": "A-A"
    }
  }
}

Unordered Sets

set1: !!set
  ? one
  ? two
set2: !!set {'one', "two"}

↓ Equivalent JSON

{
  "set1": {"one": null, "two": null},
  "set2": {"one": null, "two": null}
}

Sets are represented as a Mapping where each key is associated with a null value

Ordered Mappings

ordered: !!omap
- Mark McGwire: 65
- Sammy Sosa: 63
- Ken Griffy: 58

↓ Equivalent JSON

{
  "ordered": [
     {"Mark McGwire": 65},
     {"Sammy Sosa": 63},
     {"Ken Griffy": 58}
  ]
}

YAML Reference {.cols-3}

Terms

  • Sequences aka arrays or lists
  • Scalars aka strings or numbers
  • Mappings aka hashes or dictionaries {.style-round}

Based on the YAML.org refcard.

Document indicators
% Directive indicator
--- Document header
... Document terminator

Collection indicators {.row-span-2}
? Key indicator
: Value indicator
- Nested series entry indicator
, Separate in-line branch entries
[] Surround in-line series branch
{} Surround in-line keyed branch

Alias indicators
& Anchor property
* Alias indicator

Special keys
= Default "value" mapping key
<< Merge keys from another mapping

Scalar indicators
'' Surround in-line unescaped scalar
" Surround in-line escaped scalar
` `
> Folded scalar indicator
- Strip chomp modifier (`
+ Keep chomp modifier (`
1-9 Explicit indentation modifier (`

Tag Property (usually unspecified) {.col-span-2}
none Unspecified tag (automatically resolved by application)
! Non-specific tag (by default, !!map/!!seq/!!str)
!foo Primary (by convention, means a local !foo tag)
!!foo Secondary (by convention, means tag:yaml.org,2002:foo)
!h!foo Requires %TAG !h! <prefix> (and then means <prefix>foo)
!<foo> Verbatim tag (always means foo)

Misc indicators
# Throwaway comment indicator
`@ Both reserved for future use

Core types (default automatic tags) {.row-span-2}
!!map {Hash table, dictionary, mapping}
!!seq {List, array, tuple, vector, sequence}
!!str Unicode string

Escape Codes {.row-span-3}

Numeric

  • \x12 (8-bit)
  • \u1234 (16-bit)
  • \U00102030 (32-bit) {.cols-2 .style-none}

Protective

  • \\ (\)
  • \" (")
  • \ ( )
  • \<TAB> (TAB) {.cols-3 .style-none}

C

  • \0 (NUL)
  • \a (BEL)
  • \b (BS)
  • \f (FF)
  • \n (LF)
  • \r (CR)
  • \t (TAB)
  • \v (VTAB) {.cols-3 .style-none}

Additional

  • \e (ESC)
  • \_ (NBSP)
  • \N (NEL)
  • \L (LS)
  • \P (PS) {.cols-3 .style-none}

More types
!!set {cherries, plums, apples}
!!omap [one: 1, two: 2]

Language Independent Scalar Types {.col-span-2}
{~, null} Null (no value).
[1234, 0x4D2, 02333] [Decimal int, Hexadecimal int, Octal int]
[1_230.15, 12.3015e+02] [Fixed float, Exponential float]
[.inf, -.Inf, .NAN] [Infinity (float), Negative, Not a number]
{Y, true, Yes, ON} Boolean true
{n, FALSE, No, off} Boolean false

Also see