What is the YAML file format?
.yaml is a text file format used for exchanging data between various applications. YAML - short for *YAML Ain't Markup Language* - is a standard for organizing information in a human-readable, platform-independent format using indentation-based key: value pairs encoded in UTF-8. Parsing .yaml files requires a YAML library to be installed; popular libraries include PyYAML (Python), js-yaml (JavaScript), and SnakeYAML (Java). .yaml files are sometimes saved with the .yml extension, which is functionally identical. Beyond data interchange, .yaml files are typically used for serializing data and for writing configuration files in tools such as Docker Compose, Kubernetes, and GitHub Actions.
Serialization
Serialization is a term used in computer science. Serialization is a process of translating data or objects into a format that can be stored, transmitted, and reconstructed later. For example, two institutions exchanging some data in a standardized format using different applications must serialize that data before sending it. Serialized data can also be stored in a file - .yaml, .xml, .json, .toml, etc. - and then sent to the recipients.
A .yaml document may optionally begin with the --- document-start marker and end with ...; this also allows multiple documents inside a single file. Since YAML 1.2 (2009, revised 2021), every valid JSON file is also valid YAML.
Security & safety
RISK: MEDIUMA YAML file is plain text and doesn't execute on its own, so reading one is safe. Two real risks: (1) config YAML (CI pipelines, Compose, K8s, Ansible) frequently contains secrets - tokens, passwords, keys - so don't paste it into untrusted online validators or commit it publicly. (2) Programmatic danger: loading untrusted YAML with an unsafe parser (e.g. PyYAML's yaml.load instead of yaml.safe_load) can execute arbitrary objects/code - always use the safe loader for files you don't trust. Editing risk: a tab or a misaligned space silently changes meaning, so validate after editing.
Format details
in a nutshellFULL NAMEYAML Ain't Markup Language file
DEVELOPERClark Evans, Ingy döt Net, Oren Ben-Kiki (YAML spec)since 2001 (YAML 1.0); YAML 1.1 (2005); current YAML 1.2 (2009, revised 2021)
CATEGORYDeveloper Files
MIME TYPEapplication/yaml
TYPEHuman-readable data-serialization text (indentation-based, UTF-8)
STANDARDOpen · royalty-free
Programs that open YAML files
Windows4 apps
Sublime Text
Freemium Open the .yaml for fast editing with YAML highlighting; configure 'translate tabs to spaces' so indentation stays valid. 
Windows Notepad
Built-in Opens YAML as text for a quick look - but it doesn't show whitespace/tabs, so it's risky for editing indentation-sensitive YAML. 
Notepad
Open-source Open with YAML syntax highlighting; turn on View > Show Symbol > Show White Space to keep indentation correct. 
Visual Studio Code
Free Open the file; the built-in/Red Hat YAML extension gives highlighting, indentation guides, schema validation and error squiggles. Best choice for YAML. 
macOS3 apps
TextEdit
Built-in Format > Make Plain Text first, and disable smart quotes - but it won't show whitespace, so prefer a code editor for editing.
Visual Studio Code
Free Open and edit with the YAML extension (highlighting, schema validation). Recommended over TextEdit. 
Sublime Text / BBEdit
Freemium Open the .yaml in a code editor for proper YAML highlighting and space-based indentation. 
Linux3 apps
GNOME Text Editor / gedit
Open-source Open with YAML highlighting; set it to insert spaces (not tabs) before editing.
vim / nano
Open-source Edit in the terminal; in Vim use ':set expandtab' so indentation uses spaces. Standard for server config files.
Web1 app
YAML Lint / online YAML validators
Free Paste the file to validate syntax and pretty-print it in the browser. Don't paste secrets - config YAML often holds credentials. Technical details
deep spec| File extension | .yaml (also .yml) |
| MIME type | application/yaml |
| Encoding | UTF-8 plain text; no binary header or magic bytes |
| Structure | Indentation-based key: value pairs; indentation uses spaces only - tabs are forbidden by the spec |
| Document markers | --- marks document start, ... marks document end; multiple documents can coexist in one file |
| Native data types | Strings, integers, floats, booleans, null, sequences (lists), mappings (dictionaries), timestamps |
| Spec version | YAML 1.2 (2009, revised 2021); supersedes YAML 1.1 (2005) and YAML 1.0 (2001) |
| JSON compatibility | Since YAML 1.2, every valid JSON document is also valid YAML - JSON is a strict subset |
| Comments | Single-line comments introduced by # ; may appear on their own line or inline after a value |
| Multiline strings | Literal block scalar (|) preserves newlines; folded scalar (>) converts newlines to spaces |
| Anchors and aliases | & defines a named anchor and * references (aliases) it, enabling value reuse without duplication |
| Developers | Clark Evans, Ingy döt Net, Oren Ben-Kiki |
| Common use cases | Configuration files (Kubernetes, Docker Compose, GitHub Actions, Ansible), data serialization, OpenAPI definitions |
| Quoting | Unquoted, single-quoted ('...'), and double-quoted ("...") strings all supported; double-quoted allows escape sequences |
| Released | 2001 (YAML 1.0); YAML 1.1 (2005); current YAML 1.2 (2009, revised 2021) |
| Open standard | Yes · royalty-free |
| Specification | yaml.org |
YAML conversions
Possible
.YAML .JSONOnline converter
Yes, YAML converts to JSON without data loss -- the quickest method on any platform is one command: yq…
How to convert
.YAML .XMLOnline converter
Yes, YAML converts to XML -- the fastest command-line method is yq piped through xq: cat file.yaml | xq -x >…
How to convert
.YAML .TOMLGUIDE
Yes, YAML can be converted to TOML -- the fastest method is one yq command: yq -o=toml file.yaml > file.toml.
How to convert
.YAML .CSVGUIDE
Yes, when the YAML holds a flat list of records -- the fastest route is a one-liner with yq or a short Python…
How to convert
.YAML .YMLGUIDE
.yaml and .yml are identical - rename the file and you are done.
How to convert
Not possible
Community Q&A
asked by users
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Frequently asked questions
What's the difference between .yaml and .yml?
None - they're the same YAML format with two extensions. .yml is the older 3-letter form. Use whichever the tool expects; you can simply rename the file.
How do I open a .yaml file?
With any code/text editor. VS Code (with the YAML extension) is best because it highlights structure and flags indentation errors; Notepad++ and Sublime work too. It's plain text, so Notepad opens it - but it won't show whitespace, which YAML cares about.
Why is my YAML giving an indentation/parse error?
YAML uses spaces for structure and forbids tabs, so a stray tab or a misaligned line breaks it. Open it in a YAML-aware editor (show whitespace, convert tabs to spaces) or paste it into an online YAML validator to find the exact line.
How do I convert YAML to JSON?
Use 'yq -o=json file.yaml', a VS Code command, or an online converter. It's essentially lossless since JSON is a subset of YAML - only comments are dropped.
Can I use tabs in YAML?
No. YAML only allows spaces for indentation; tabs cause a parse error. Set your editor to insert spaces (e.g. ':set expandtab' in Vim, or 'translate tabs to spaces' in Sublime).
Is YAML only for Python?
No - that's a common misconception. YAML is language-independent and is used everywhere: Docker Compose, Kubernetes, GitHub Actions, Ansible, Go/Java/JS apps and more. Python just happens to have a popular YAML library (PyYAML).
References
1YAML 1.2.2 Specification (yaml.org)yaml.org
2yaml.org - official site & resourcesyaml.org
