Yamllint

A linter for YAML files. yamllint does not only check for syntax validity, but for weirdness like key repetition and cosmetic problems such as lines length, trailing spaces, indentation, etc.

Use an online validator like https://www.yamllint.com/ or https://jsonformatter.org/yaml-validator

Of course, you should have one in your local tools and CI pipeline to ensure that your Yaml is always correct. With yamllint, there is both a script and a Python module; meaning you can write your own linting tool in Python by invoking (importing) the yamllint module. See the caveat section below about "using the right tool for the job" - meaning use the right linter for the language/project you are linting.

Manually fix errors

Unfortunately, yamllint does not fix your file for you. There could be ambiguities about the proper fix, so you need to do the fix(es) yourself.

VSCode settings

I had a problem with VSCode automatically re-introducing spaces around curly braces - causing the very same yamllint errors I was trying to fix. Despite trying various linters (aka formatters), I ultimately found that the only way to be able to save the correct formatting was to disable formatOnSave in my user settings. This was because the "default settings" (/defaultSettings.json) was read-only so I could not change it, AND it was configured to formatOnSave - using the kennylong linter which is apparently bad for yaml outside kubernetes contexts.

In VSCode, use Ctrl+Shift+P to open the preferences dialog and type 'Preferences: Open User Settings' then open User Settings (JSON) settings. Ensure you have lines like this (especially the editor.formatOnSave line because I tried other linters like "redhat.vscode-yaml" without success):

	"[yaml]": {
		"editor.defaultFormatter": "kennylong.kubernetes-yaml-formatter",
		"editor.autoIndent": "advanced",
		"editor.formatOnSave": false
	},

Cheatsheet

You can use comment directives to control the behavior of yamllint

With disable-line you put the directive in-line, or on the line above.

# The following mapping contains the same key twice,
# but I know what I'm doing:
key: value 1
# yamllint disable-line rule:key-duplicates
key: value 2

# yamllint disable-line rule:line-length
- This line is waaaaaaaaaay too long but yamllint will not report anything about it.
  This line will be checked by yamllint.

With disable you can disable multiple rules for the entire file.

# yamllint disable rule:hyphens rule:commas rule:indentation

Or even use disable-file

# yamllint disable-file
# This file is not valid YAML because it is a Jinja template

--no-warnings will suppress the warnings so you can focus only on errors.

--list-files will show you the list of files that yamllint finds (and would otherwise lint).

--format (or -f) gives you options for how you want the output to display.

Yaml multiline

When writing YAML in your Ansible playbooks, you run across values that 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 is used to make what would otherwise be a very long line easier to read and edit. In either case the indentation (leading spaces) will be ignored.

from YAML in Ansible (Community Documentation)

Interactive demonstration of Block scalars and Flow scalars https://yaml-multiline.info/

The SO answer that describes them all

Many linters

You can't just use "one" solution either. The leading GPL linter is based on Python, so depending on your code repo, you may instead want to use a JavaScript or PHP implementation. Thus, tools like wp:Grunt (software) may be used to automate JSHint linting in JavaScript projects.

Source, Docs and Reading

• https://github.com/adrienverge/yamllint
• https://yamllint.readthedocs.io/en/stable/
• https://www.redhat.com/sysadmin/check-yaml-yamllint
• https://www.redhat.com/en/blog/ansible-lists-dictionaries-yaml
• YAML in Symphony

• YAML 1.2.2 spec on scalars
• YAML 1.2.2 spec on block chomping

JavaScript Object Notation = A data representation format in JavaScript

General Public License = A sane copyright license

PHP is the HyperText Preprocessor = the most popular programming language for the web