YML vs YAML: File Extension Difference Explained

Quick Answer

There is no functional difference between .yml and .yaml file extensions. Both refer to the same YAML (YAML Ain't Markup Language) format. The choice is purely a matter of preference and convention.

Detailed Explanation

Historical Context

In the early days of computing, file systems like MS-DOS had a limitation of three-character file extensions (the "8.3" filename format). This led to the adoption of .yml as a shortened version of .yaml.

Modern operating systems no longer have this restriction, so both .yml and .yaml are equally valid and widely supported.

Technical Perspective

• Both extensions are recognized by all YAML parsers and tools
• The file content and syntax are identical
• No performance difference between the two
• MIME type is the same: text/yaml or text/x-yaml

Industry Usage Patterns

Projects Using .yml

✓ Docker Compose: docker-compose.yml
✓ Kubernetes: deployment.yml
✓ GitHub Actions: .github/workflows/*.yml
✓ Ruby on Rails: config/database.yml
✓ CircleCI: .circleci/config.yml

Projects Using .yaml

✓ Ansible: playbook.yaml
✓ Home Assistant: configuration.yaml
✓ OpenAPI: openapi.yaml
✓ ESLint: .eslintrc.yaml
✓ Azure Pipelines: azure-pipelines.yaml

Observation: .yml appears to be slightly more popular in practice, but .yaml is the official file extension according to the YAML specification.

Usage Statistics

.yml usage ~60%

.yaml usage ~40%

Based on GitHub repository analysis

Which Should You Use?

1. Follow Project Conventions

The most important rule: be consistent within your project.

✓ If your project already uses .yml, stick with .yml
✓ If your project uses .yaml, stick with .yaml

2. Check Tool Requirements

Some tools have specific naming conventions:

# Docker Compose expects this exact name
docker-compose.yml

# GitHub Actions expects files in this location
.github/workflows/build.yml

# Home Assistant expects this name
configuration.yaml

3. Consider Team Preference

If starting a new project, discuss with your team and establish a convention. Document this choice in your project's style guide.

Official Recommendation

The YAML specification officially recommends .yaml as the preferred extension, but acknowledges that .yml is widely used and equally valid.

Practical Examples

Both Extensions Work Identically

config.yml

app:
  name: MyApp
  version: 1.0.0
  port: 3000

config.yaml

app:
  name: MyApp
  version: 1.0.0
  port: 3000

Both files contain identical content and will be parsed exactly the same way by any YAML parser.

Programming Language Support

Python (PyYAML):

import yaml

# Both work
with open('config.yml') as f:
    data = yaml.safe_load(f)

with open('config.yaml') as f:
    data = yaml.safe_load(f)

JavaScript (js-yaml):

const yaml = require('js-yaml');
const fs = require('fs');

// Both work
const doc1 = yaml.load(fs.readFileSync('config.yml', 'utf8'));
const doc2 = yaml.load(fs.readFileSync('config.yaml', 'utf8'));

Common Misconceptions

✗ Myth: .yml is faster to parse

False. The file extension has no impact on parsing speed. Both are processed identically.

✗ Myth: .yaml is more "official"

Partially true. While the spec recommends .yaml, .yml is equally valid and widely used.

✗ Myth: Some features only work with one extension

False. All YAML features work with both extensions. The file extension doesn't affect syntax or capabilities.

Conclusion

The choice between .yml and .yaml is a matter of personal or organizational preference. Both extensions are:

✓ Functionally identical
✓ Widely supported by all tools and parsers
✓ Equally valid according to the YAML specification

The key is to be consistent within your project and follow established conventions in your ecosystem.

YML vs YAML: What's the Difference?