YAML Syntax

This page provides a basic overview of correct YAML syntax, which is how Ansibleplaybooks (our configuration management language) are expressed.

We use YAML because it is easier for humans to read and write than other commondata formats like XML or JSON. Further, there are libraries available in mostprogramming languages for working with YAML.

You may also wish to read Working With Playbooks at the same time to see how thisis used in practice.

YAML Basics

For Ansible, nearly every YAML file starts with a list.Each item in the list is a list of key/value pairs, commonlycalled a “hash” or a “dictionary”. So, we need to know howto write lists and dictionaries in YAML.

There’s another small quirk to YAML. All YAML files (regardless of their association with Ansible or not) can optionallybegin with —- and end with …. This is part of the YAML format and indicates the start and end of a document.

All members of a list are lines beginning at the same indentation level starting with a "- " (a dash and a space):

  1. ---
  2. # A list of tasty fruits
  3. fruits:
  4.     - Apple
  5.     - Orange
  6.     - Strawberry
  7.     - Mango
  8. ...

A dictionary is represented in a simple key: value form (the colon must be followed by a space):

  1. # An employee record
  2. martin:
  3.     name: Martin D'vloper
  4.     job: Developer
  5.     skill: Elite

More complicated data structures are possible, such as lists of dictionaries, dictionaries whose values are lists or a mix of both:

  1. # Employee records
  2. -  martin:
  3.     name: Martin D'vloper
  4.     job: Developer
  5.     skills:
  6.       - python
  7.       - perl
  8.       - pascal
  9. -  tabitha:
  10.     name: Tabitha Bitumen
  11.     job: Developer
  12.     skills:
  13.       - lisp
  14.       - fortran
  15.       - erlang

Dictionaries and lists can also be represented in an abbreviated form if you really want to:

  1. ---
  2. martin: {name: Martin D'vloper, job: Developer, skill: Elite}
  3. fruits: ['Apple', 'Orange', 'Strawberry', 'Mango']

These are called “Flow collections”.

Ansible doesn’t really use these too much, but you can also specify a boolean value (true/false) in several forms:

  1. create_key: yes
  2. needs_agent: no
  3. knows_oop: True
  4. likes_emacs: TRUE
  5. uses_cvs: false

Values can span multiple lines using | or >. Spanning multiple lines using a “Literal Block Scalar” | will include the newlines and any trailing spaces.Using a “Folded Block Scalar” > will fold newlines to spaces; it’s used to make what would otherwise be a very long line easier to read and edit.In either case the indentation will be ignored.Examples are:

  1. include_newlines: |
  2.             exactly as you see
  3.             will appear these three
  4.             lines of poetry
  5.  
  6. fold_newlines: >
  7.             this is really a
  8.             single line of text
  9.             despite appearances

While in the above > example all newlines are folded into spaces, there are two ways to enforce a newline to be kept:

  1. fold_some_newlines: >
  2.     a
  3.     b
  4.  
  5.     c
  6.     d
  7.       e
  8.     f
  9. same_as: "a b\nc d\n  e\nf\n"

Let’s combine what we learned so far in an arbitrary YAML example.This really has nothing to do with Ansible, but will give you a feel for the format:

  1. ---
  2. # An employee record
  3. name: Martin D'vloper
  4. job: Developer
  5. skill: Elite
  6. employed: True
  7. foods:
  8.     - Apple
  9.     - Orange
  10.     - Strawberry
  11.     - Mango
  12. languages:
  13.     perl: Elite
  14.     python: Elite
  15.     pascal: Lame
  16. education: |
  17.     4 GCSEs
  18.     3 A-Levels
  19.     BSc in the Internet of Things

That’s all you really need to know about YAML to start writing Ansible playbooks.

Gotchas

While you can put just about anything into an unquoted scalar, there are some exceptions.A colon followed by a space (or newline) : is an indicator for a mapping. A space followed by the pound sign # starts a comment.

Because of this, the following is going to result in a YAML syntax error:

  1. foo: somebody said I should put a colon here: so I did
  2.  
  3. windows_drive: c:

…but this will work:

  1. windows_path: c:\windows

You will want to quote hash values using colons followed by a space or the end of the line:

  1. foo: 'somebody said I should put a colon here: so I did'
  2.  
  3. windows_drive: 'c:'

…and then the colon will be preserved.

Alternatively, you can use double quotes:

  1. foo: "somebody said I should put a colon here: so I did"
  2.  
  3. windows_drive: "c:"

The difference between single quotes and double quotes is that in double quotesyou can use escapes:

  1. foo: "a \t TAB and a \n NEWLINE"

The list of allowed escapes can be found in the YAML Specification under “Escape Sequences” (YAML 1.1) or “Escape Characters” (YAML 1.2).

The following is invalid YAML:

  1. foo: "an escaped \' single quote"

Further, Ansible uses “{{ var }}” for variables. If a value after a colon startswith a “{“, YAML will think it is a dictionary, so you must quote it, like so:

  1. foo: "{{ variable }}"

If your value starts with a quote the entire value must be quoted, not just part of it. Here are some additional examples of how to properly quote things:

  1. foo: "{{ variable }}/additional/string/literal"
  2. foo2: "{{ variable }}\\backslashes\\are\\also\\special\\characters"
  3. foo3: "even if it's just a string literal it must all be quoted"

Not valid:

  1. foo: "E:\\path\\"rest\\of\\path

In addition to ' and " there are a number of characters that are special (or reserved) and cannot be usedas the first character of an unquoted scalar: [] {} > | * & ! % # ` @ ,.

You should also be aware of ? : -. In YAML, they are allowed at the beginning of a string if a non-spacecharacter follows, but YAML processor implementations differ, so it’s better to use quotes.

In Flow Collections, the rules are a bit more strict:

  1. a scalar in block mapping: this } is [ all , valid
  2.  
  3. flow mapping: { key: "you { should [ use , quotes here" }

Boolean conversion is helpful, but this can be a problem when you want a literal yes or other boolean values as a string.In these cases just use quotes:

  1. non_boolean: "yes"
  2. other_string: "False"

YAML converts certain strings into floating-point values, such as the string1.0. If you need to specify a version number (in a requirements.yml file, forexample), you will need to quote the value if it looks like a floating-pointvalue:

  1. version: "1.0"

See also