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Basics

  • YAML stands for YAML Ain’t Markup Language.
  • It is similar to XML or JSON.
  • YAML files usually store configuration data.
  • To define a YAML file we use either .yml or .yaml as file extension.
  • YAML is case sensitive.
  • Key-value pairs are represented using the following syntax: <key>: <value>
  • YAML stream is a collection of zero or more documents.
    • An empty stream contains no documents.
    • A single document may or may not be marked with ---.
    • Different documents are separated using three dashes ---.
    • Documents could end with ....
  • Items of a list in YAML are represented by preceding it with - (hyphen).
  • Any text after # is considered a comment.
    • YAML does not support block or multi-line comments.

Data types in YAML

  • A name followed by a colon (:) and a single space ( ) defines a variable.
  • Variables can also be referred to as scalars.
  • The following is the list of supported data types:
    • Boolean
    • Numbers (integer, and float)
    • Strings
    • Timestamp
    • Null
  • YAML can auto-detect the data types, but users can also specify the type they need. To force a type, you can prefix the type with a !! symbol.
# STRING
list of texts:
  - Hello World!
  - 'single quoted string'
  - " double quoted strings"
text 0: | 
  Every line in this text 
  will be stored 
  as separate lines.
text 1: > 
  This text will
  be wrapped into
  a single paragraph
name: !!str "James"
message: !!str "This is a \n multiline text"

#
# INTEGER
#
negative: !!int -12 
zero: !!int 0
positive: !!int 23
binary: !!int 0b101010
octal: !!int 01672
hexadecimal: !!int 0x1C7A
number: !!int +687_456
sexagesimal: 180:30:20 # base 60

#
# FLOAT
#
negative: !!float -1.23
zero: !!float 0.0
positive: !!float 2.3e4
infinity: !!float .inf
not a number: !!float .nan

#
# BOOLEAN
#
married: !!bool true
odd: !!bool false

#
# NULL
#
manager: !!null null
tilde: ~
title: null
~: null key

#
# TIMESTAMP
#
time: 2020-12-07T01:02:59:34.02Z
timestamp: 2020-12-07T01:02:59:34.02 +05:30
datetime: 2020-12-07T01:02:59:34.02+05:30
notimezone: 2020-12-07T01:02:59:34.02
date: 2020-12-07

String (!!str)

  • Strings can be written without quotes or enclosed in ''(quotes) or "" (double quotes).
  • A | character denotes a string with newlines; indentations and spaces preserved.
  • A > character denotes a string with newlines folded.

NULL (~)

  • Null represents a lack of value.
  • You can also use a tilde ~, null in lowercase, NULL in uppercase, or Null in camelCase.

Timestamp

  • It uses the notation form ISO8601.
  • If no time zone is added, then it is assumed to be in UTC.

Data Type

Array

  • Arrays or Lists are used to represent a list of items or values. They are also referred to as sequences in YAML.

Block Sequence

  • The block sequence style of YAML represents Arrays by using hyphens (-) followed by white space ( ).
colors: 
  - red
  - green
  - blue
  - orange

Flow Sequence

  • The flow sequence style of YAML uses brackets and commas to represent arrays.
colors: [red, green, blue, orange]

Sequence (!!seq)

  • Sequence represents a collection indexed by sequential integers.
  • Sequences and Mappings in YAML can be nested within each other.
# Ordered sequence of nodes
## normal version
normal: !!seq
  - red
  - orange
  - yellow

## compact version
compact: [red, orange, yellow]

Sparse Sequence

  • A sequence where not all the keys have values
sparse: 
  - red
  - ~
  - blue
  - Null
  - NULL
  - orange

Nested sequence

  • A nested sequence represents a sequence of items and sub-items.
sparse: 
  - red
  - blue
    - brown
    - NULL

Mappings (!!map)

  • Mappings are used to store a name-value pair.

Nested Mappings

  • In a nested mapping, a value in a mapping can be another mapping.
  • There is also a compact notation using {} for maps.
# normal version
a: 1
b: 
  c: 3
  d: 4

# compact version
a: 1
b: {c: 3, d: 4}

Pairs (!!pairs)

  • Pairs are ordered lists of named values that allow duplicates.
Block tasks: 
  - meeting: standup
  - meeting: demo
  - break: lunch
  - meeting: all hands

# flow style and explicit type
Flow tasks: !!pairs [ meeting: standup, meeting: lunch ]

Set (!!set)

  • A set is an unordered collection of nodes with distinct values.
players:
  ? Mark
  ? Steve
  ? Smith

# flow style and explicit type
soccer teams: !!set { Chelsea, Arsenal, Liverpool }

Dictionary (!!omap)

  • A dictionary is an ordered sequence of name value pairs where a value in the mapping can be a sequence.
  • List members are denoted by a leading hyphen (-).
Fruits:
  - Apple:  
      calories: 120
      fat: 0.2 g
      carbs: 35 g
  - Banana: 
      calories: 105
      fat: 0.4 g
      carbs: 27 g

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