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diff --git a/docs/docsite/rst/reference_appendices/YAMLSyntax.rst b/docs/docsite/rst/reference_appendices/YAMLSyntax.rst new file mode 100644 index 00000000..7d439664 --- /dev/null +++ b/docs/docsite/rst/reference_appendices/YAMLSyntax.rst @@ -0,0 +1,242 @@ +.. _yaml_syntax: + + +YAML Syntax +=========== + +This page provides a basic overview of correct YAML syntax, which is how Ansible +playbooks (our configuration management language) are expressed. + +We use YAML because it is easier for humans to read and write than other common +data formats like XML or JSON. Further, there are libraries available in most +programming languages for working with YAML. + +You may also wish to read :ref:`working_with_playbooks` at the same time to see how this +is 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, commonly +called a "hash" or a "dictionary". So, we need to know how +to 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 optionally +begin 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):: + + --- + # A list of tasty fruits + - Apple + - Orange + - Strawberry + - Mango + ... + +A dictionary is represented in a simple ``key: value`` form (the colon must be followed by a space):: + + # An employee record + martin: + name: Martin D'vloper + job: Developer + skill: Elite + +More complicated data structures are possible, such as lists of dictionaries, dictionaries whose values are lists or a mix of both:: + + # Employee records + - martin: + name: Martin D'vloper + job: Developer + skills: + - python + - perl + - pascal + - tabitha: + name: Tabitha Bitumen + job: Developer + skills: + - lisp + - fortran + - erlang + +Dictionaries and lists can also be represented in an abbreviated form if you really want to:: + + --- + martin: {name: Martin D'vloper, job: Developer, skill: Elite} + ['Apple', 'Orange', 'Strawberry', 'Mango'] + +These are called "Flow collections". + +.. _truthiness: + +Ansible doesn't really use these too much, but you can also specify a boolean value (true/false) in several forms:: + + create_key: yes + needs_agent: no + knows_oop: True + likes_emacs: TRUE + uses_cvs: false + +Use lowercase 'true' or 'false' for boolean values in dictionaries if you want to be compatible with default yamllint options. + +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:: + + include_newlines: | + exactly as you see + will appear these three + lines of poetry + + fold_newlines: > + this is really a + single line of text + despite appearances + +While in the above ``>`` example all newlines are folded into spaces, there are two ways to enforce a newline to be kept:: + + fold_some_newlines: > + a + b + + c + d + e + f + 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:: + + --- + # An employee record + name: Martin D'vloper + job: Developer + skill: Elite + employed: True + foods: + - Apple + - Orange + - Strawberry + - Mango + languages: + perl: Elite + python: Elite + pascal: Lame + education: | + 4 GCSEs + 3 A-Levels + 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:: + + foo: somebody said I should put a colon here: so I did + + windows_drive: c: + +...but this will work:: + + windows_path: c:\windows + +You will want to quote hash values using colons followed by a space or the end of the line:: + + foo: 'somebody said I should put a colon here: so I did' + + windows_drive: 'c:' + +...and then the colon will be preserved. + +Alternatively, you can use double quotes:: + + foo: "somebody said I should put a colon here: so I did" + + windows_drive: "c:" + +The difference between single quotes and double quotes is that in double quotes +you can use escapes:: + + 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: + +.. code-block:: text + + foo: "an escaped \' single quote" + + +Further, Ansible uses "{{ var }}" for variables. If a value after a colon starts +with a "{", YAML will think it is a dictionary, so you must quote it, like so:: + + 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:: + + foo: "{{ variable }}/additional/string/literal" + foo2: "{{ variable }}\\backslashes\\are\\also\\special\\characters" + foo3: "even if it's just a string literal it must all be quoted" + +Not valid:: + + foo: "E:\\path\\"rest\\of\\path + +In addition to ``'`` and ``"`` there are a number of characters that are special (or reserved) and cannot be used +as 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-space +character follows, but YAML processor implementations differ, so it's better to use quotes. + +In Flow Collections, the rules are a bit more strict:: + + a scalar in block mapping: this } is [ all , valid + + 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:: + + non_boolean: "yes" + other_string: "False" + + +YAML converts certain strings into floating-point values, such as the string +`1.0`. If you need to specify a version number (in a requirements.yml file, for +example), you will need to quote the value if it looks like a floating-point +value:: + + version: "1.0" + + +.. seealso:: + + :ref:`working_with_playbooks` + Learn what playbooks can do and how to write/run them. + `YAMLLint <http://yamllint.com/>`_ + YAML Lint (online) helps you debug YAML syntax if you are having problems + `GitHub examples directory <https://github.com/ansible/ansible-examples>`_ + Complete playbook files from the github project source + `Wikipedia YAML syntax reference <https://en.wikipedia.org/wiki/YAML>`_ + A good guide to YAML syntax + `Mailing List <https://groups.google.com/group/ansible-project>`_ + Questions? Help? Ideas? Stop by the list on Google Groups + `irc.freenode.net <http://irc.freenode.net>`_ + #ansible IRC chat channel and #yaml for YAML specific questions + `YAML 1.1 Specification <https://yaml.org/spec/1.1/>`_ + The Specification for YAML 1.1, which PyYAML and libyaml are currently + implementing + `YAML 1.2 Specification <https://yaml.org/spec/1.2/spec.html>`_ + For completeness, YAML 1.2 is the successor of 1.1 |