XDG Base Directory Specification

Implement standardized directory paths for configuration, data, cache, and state files following the XDG Base Directory Specification.

When to Use This Skill

Use this skill when:

• Creating CLI tools or applications that store user-specific files
• Implementing configuration file management for Linux/Unix applications
• Building cross-platform Python applications requiring standardized directory paths
• Migrating legacy applications from ~/.appname to XDG-compliant paths
• Designing file storage architecture for Python packages
• Implementing proper environment variable handling for user directories
• Writing applications that respect user-configured directory preferences
• Testing applications with custom XDG directory overrides

Core Specification Rules

The XDG Base Directory Specification (version 0.8, May 2021) defines these critical requirements:

1. All Paths Must Be Absolute

Environment variable values MUST be absolute paths. Relative paths are invalid and MUST be ignored.

def validate_xdg_path(path_str: str | None) -> Path | None:
    """Validate XDG path is absolute per specification."""
    if not path_str:
        return None
    path = Path(path_str)
    if not path.is_absolute():
        return None  # Ignore relative paths per spec
    return path

2. Empty Variables Use Defaults

When an environment variable is unset or empty, use the specification-defined default value.

3. Priority Order for Search Paths

• User-specific directories ($XDG_CONFIG_HOME, $XDG_DATA_HOME) take precedence
• System directories ($XDG_CONFIG_DIRS, $XDG_DATA_DIRS) searched in order
• First match wins

4. Colon Separator for Search Paths

$XDG_DATA_DIRS and $XDG_CONFIG_DIRS use colon (:) as path separator, similar to $PATH.

Environment Variables

User-Specific Directories (Single Path)

Variable	Purpose	Default	Use For
$XDG_CONFIG_HOME	User configuration files	$HOME/.config	Settings, preferences, user configs
$XDG_DATA_HOME	User data files	$HOME/.local/share	Databases, generated content, persistent data
$XDG_STATE_HOME	User state files	$HOME/.local/state	Logs, history, undo buffers, recent files
$XDG_CACHE_HOME	User cache files	$HOME/.cache	Temporary data, downloaded files, build artifacts
$XDG_RUNTIME_DIR	User runtime files	System-set (/run/user/$UID)	Sockets, pipes, lock files, IPC

System-Wide Directories (Search Paths)

Variable	Purpose	Default
$XDG_DATA_DIRS	System data search path	/usr/local/share/:/usr/share/
$XDG_CONFIG_DIRS	System config search path	/etc/xdg

User Executables (Convention, Not XDG)

Path	Purpose
$HOME/.local/bin	User-specific executable files

Note: Distributions should ensure $HOME/.local/bin appears in $PATH.

Directory Selection Guide

Choose the appropriate directory based on data characteristics:

Use $XDG_CONFIG_HOME for:

• Application settings and preferences
• User-specific configuration files
• TOML/YAML/JSON configuration
• Example: ~/.config/myapp/config.toml

Use $XDG_DATA_HOME for:

• Persistent application data
• User-generated content
• Downloaded models or assets
• Database files
• Example: ~/.local/share/myapp/models/model.gguf

Use $XDG_STATE_HOME for:

• Action history (command history, undo buffers)
• Application state that can be regenerated
• Recently used files lists
• Log files specific to user actions
• Example: ~/.local/state/myapp/history.log

Use $XDG_CACHE_HOME for:

• Temporary files safe to delete
• Downloaded data that can be re-fetched
• Build artifacts and compiled files
• Thumbnail caches
• Example: ~/.cache/myapp/downloads/

Use $XDG_RUNTIME_DIR for:

• Unix domain sockets
• Named pipes (FIFOs)
• Lock files
• Temporary IPC files
• Warning: Often tmpfs-mounted with size limits; avoid large files
• Warning: Cleaned on logout; do not store persistent data
• Example: /run/user/1000/myapp/socket

Python Implementation Patterns

Stdlib-Only Implementation

Use for applications targeting only Linux/Unix systems or requiring no dependencies.

"""XDG Base Directory compliant path management using stdlib only."""
from pathlib import Path
import os


def get_config_home() -> Path:
    """Get XDG_CONFIG_HOME path.

    Returns user-specific configuration directory.
    Falls back to $HOME/.config if XDG_CONFIG_HOME unset or relative.
    """
    xdg = os.environ.get('XDG_CONFIG_HOME')
    if xdg and Path(xdg).is_absolute():
        return Path(xdg)
    return Path.home() / '.config'


def get_data_home() -> Path:
    """Get XDG_DATA_HOME path.

    Returns user-specific data directory.
    Falls back to $HOME/.local/share if XDG_DATA_HOME unset or relative.
    """
    xdg = os.environ.get('XDG_DATA_HOME')
    if xdg and Path(xdg).is_absolute():
        return Path(xdg)
    return Path.home() / '.local' / 'share'


def get_state_home() -> Path:
    """Get XDG_STATE_HOME path.

    Returns user-specific state directory.
    Falls back to $HOME/.local/state if XDG_STATE_HOME unset or relative.
    """
    xdg = os.environ.get('XDG_STATE_HOME')
    if xdg and Path(xdg).is_absolute():
        return Path(xdg)
    return Path.home() / '.local' / 'state'


def get_cache_home() -> Path:
    """Get XDG_CACHE_HOME path.

    Returns user-specific cache directory.
    Falls back to $HOME/.cache if XDG_CACHE_HOME unset or relative.
    """
    xdg = os.environ.get('XDG_CACHE_HOME')
    if xdg and Path(xdg).is_absolute():
        return Path(xdg)
    return Path.home() / '.cache'


def get_runtime_dir() -> Path | None:
    """Get XDG_RUNTIME_DIR path.

    Returns user-specific runtime directory, or None if not set.
    This variable has no default - it must be set by the system.
    """
    xdg = os.environ.get('XDG_RUNTIME_DIR')
    if xdg and Path(xdg).is_absolute():
        return Path(xdg)
    return None


def get_config_dirs() -> list[Path]:
    """Get XDG_CONFIG_DIRS as list of paths.

    Returns preference-ordered list of system configuration directories.
    Falls back to [Path('/etc/xdg')] if XDG_CONFIG_DIRS unset.
    """
    xdg = os.environ.get('XDG_CONFIG_DIRS')
    if xdg:
        paths = [Path(p) for p in xdg.split(':') if p and Path(p).is_absolute()]
        if paths:
            return paths
    return [Path('/etc/xdg')]


def get_data_dirs() -> list[Path]:
    """Get XDG_DATA_DIRS as list of paths.

    Returns preference-ordered list of system data directories.
    Falls back to [Path('/usr/local/share'), Path('/usr/share')] if unset.
    """
    xdg = os.environ.get('XDG_DATA_DIRS')
    if xdg:
        paths = [Path(p) for p in xdg.split(':') if p and Path(p).is_absolute()]
        if paths:
            return paths
    return [Path('/usr/local/share'), Path('/usr/share')]

Application-Specific Path Module

Create a dedicated paths module for your application:

"""Path management for myapp following XDG specification."""
from pathlib import Path
import os

APP_NAME = 'myapp'


def get_config_dir() -> Path:
    """Get myapp config directory."""
    xdg = os.environ.get('XDG_CONFIG_HOME')
    base = Path(xdg) if xdg and Path(xdg).is_absolute() else Path.home() / '.config'
    return base / APP_NAME


def get_config_file() -> Path:
    """Get myapp config file path."""
    return get_config_dir() / 'config.toml'


def get_data_dir() -> Path:
    """Get myapp data directory."""
    xdg = os.environ.get('XDG_DATA_HOME')
    base = Path(xdg) if xdg and Path(xdg).is_absolute() else Path.home() / '.local' / 'share'
    return base / APP_NAME


def get_cache_dir() -> Path:
    """Get myapp cache directory."""
    xdg = os.environ.get('XDG_CACHE_HOME')
    base = Path(xdg) if xdg and Path(xdg).is_absolute() else Path.home() / '.cache'
    return base / APP_NAME


def get_state_dir() -> Path:
    """Get myapp state directory."""
    xdg = os.environ.get('XDG_STATE_HOME')
    base = Path(xdg) if xdg and Path(xdg).is_absolute() else Path.home() / '.local' / 'state'
    return base / APP_NAME


def ensure_directories() -> None:
    """Create all required directories with appropriate permissions."""
    for directory in [get_config_dir(), get_data_dir(), get_cache_dir(), get_state_dir()]:
        directory.mkdir(parents=True, exist_ok=True)

Cross-Platform Implementation with platformdirs

For applications targeting Linux, macOS, and Windows, use the platformdirs library:

"""Cross-platform path management using platformdirs."""
from platformdirs import user_config_dir, user_data_dir, user_cache_dir, user_state_dir
from pathlib import Path

APP_NAME = 'myapp'
APP_AUTHOR = 'myapp'  # Used on Windows

# Get platform-appropriate directories
# Linux: Uses XDG specification
# macOS: Uses ~/Library/Application Support, ~/Library/Caches
# Windows: Uses %APPDATA%, %LOCALAPPDATA%

config_dir = Path(user_config_dir(APP_NAME, APP_AUTHOR))
data_dir = Path(user_data_dir(APP_NAME, APP_AUTHOR))
cache_dir = Path(user_cache_dir(APP_NAME, APP_AUTHOR))
state_dir = Path(user_state_dir(APP_NAME, APP_AUTHOR))

# Auto-create directories
config_dir = Path(user_config_dir(APP_NAME, APP_AUTHOR, ensure_exists=True))

Platform-specific paths:

Platform	Config	Data	Cache	State
Linux	~/.config/myapp	~/.local/share/myapp	~/.cache/myapp	~/.local/state/myapp
macOS	~/Library/Application Support/myapp	~/Library/Application Support/myapp	~/Library/Caches/myapp	~/Library/Application Support/myapp
Windows	C:\Users\<user>\AppData\Local\myapp\myapp	C:\Users\<user>\AppData\Local\myapp\myapp	C:\Users\<user>\AppData\Local\myapp\myapp\Cache	C:\Users\<user>\AppData\Local\myapp\myapp

When to use platformdirs:

• Application targets multiple operating systems
• Need native platform conventions (macOS ~/Library, Windows %APPDATA%)
• Want automatic directory creation with ensure_exists=True

When to use stdlib-only:

• Application targets only Linux/Unix
• Minimize dependencies
• Need complete control over path resolution logic

Common Anti-Patterns

1. Using ~/.appname (Legacy Pattern)

Problem: Violates XDG specification, clutters home directory.

# ❌ WRONG
config_file = Path.home() / '.myapp' / 'config.toml'

Solution: Use ~/.config/appname/

# ✅ CORRECT
def get_config_dir() -> Path:
    xdg = os.environ.get('XDG_CONFIG_HOME')
    base = Path(xdg) if xdg and Path(xdg).is_absolute() else Path.home() / '.config'
    return base / 'myapp'

config_file = get_config_dir() / 'config.toml'

2. Ignoring Environment Variables

Problem: Hardcoded paths prevent user customization.

# ❌ WRONG
config_dir = Path.home() / '.config' / 'myapp'

Solution: Always check environment variables first.

# ✅ CORRECT
xdg = os.environ.get('XDG_CONFIG_HOME')
base = Path(xdg) if xdg and Path(xdg).is_absolute() else Path.home() / '.config'
config_dir = base / 'myapp'

3. Accepting Relative Paths

Problem: XDG specification mandates absolute paths only.

# ❌ WRONG
xdg = os.environ.get('XDG_CONFIG_HOME', str(Path.home() / '.config'))
return Path(xdg)  # Accepts relative paths

Solution: Validate absolute paths, fall back to default for relative.

# ✅ CORRECT
xdg = os.environ.get('XDG_CONFIG_HOME')
if xdg and Path(xdg).is_absolute():
    return Path(xdg)
return Path.home() / '.config'  # Default for unset or relative

4. Missing Directory Creation

Problem: Writing files without ensuring parent directories exist.

# ❌ WRONG
config_file = get_config_dir() / 'config.toml'
config_file.write_text(data)  # Fails if directory doesn't exist

Solution: Create directories before writing files.

# ✅ CORRECT
config_file = get_config_dir() / 'config.toml'
config_file.parent.mkdir(parents=True, exist_ok=True)
config_file.write_text(data)

5. Storing Cache in Config Directory

Problem: Mixing regenerable data with configuration.

# ❌ WRONG
cache_file = get_config_dir() / 'download_cache.json'

Solution: Use $XDG_CACHE_HOME for regenerable data.

# ✅ CORRECT
cache_file = get_cache_dir() / 'download_cache.json'

6. Large Files in Runtime Directory

Problem: $XDG_RUNTIME_DIR often tmpfs-mounted with size limits.

# ❌ WRONG
large_file = get_runtime_dir() / 'model.gguf'  # May be 1GB+

Solution: Use $XDG_DATA_HOME for large persistent files.

# ✅ CORRECT
large_file = get_data_dir() / 'models' / 'model.gguf'

7. Not Handling Unset XDG_RUNTIME_DIR

Problem: $XDG_RUNTIME_DIR has no default value.

# ❌ WRONG
runtime_dir = get_runtime_dir()
socket_path = runtime_dir / 'socket'  # Fails if None

Solution: Check for None before using.

# ✅ CORRECT
runtime_dir = get_runtime_dir()
if runtime_dir is None:
    raise RuntimeError("XDG_RUNTIME_DIR not set by system")
socket_path = runtime_dir / 'socket'

Testing XDG Compliance

Manual Testing with Environment Variables

# Test with custom XDG directories
export XDG_CONFIG_HOME=/tmp/test-config
export XDG_DATA_HOME=/tmp/test-data
export XDG_CACHE_HOME=/tmp/test-cache
export XDG_STATE_HOME=/tmp/test-state

# Run application
myapp --help

# Verify files are in correct locations
ls -la /tmp/test-config/myapp/
ls -la /tmp/test-data/myapp/
ls -la /tmp/test-cache/myapp/
ls -la /tmp/test-state/myapp/

# Test with unset variables (should use defaults)
unset XDG_CONFIG_HOME XDG_DATA_HOME XDG_CACHE_HOME XDG_STATE_HOME
myapp --help
ls -la ~/.config/myapp/
ls -la ~/.local/share/myapp/
ls -la ~/.cache/myapp/
ls -la ~/.local/state/myapp/

Automated Testing

"""Test XDG compliance."""
import os
import tempfile
from pathlib import Path
import pytest


def test_xdg_config_home_override(monkeypatch):
    """XDG_CONFIG_HOME environment variable is respected."""
    with tempfile.TemporaryDirectory() as tmpdir:
        monkeypatch.setenv('XDG_CONFIG_HOME', tmpdir)
        config_dir = get_config_dir()
        assert config_dir == Path(tmpdir) / 'myapp'


def test_xdg_config_home_default(monkeypatch):
    """Default used when XDG_CONFIG_HOME unset."""
    monkeypatch.delenv('XDG_CONFIG_HOME', raising=False)
    config_dir = get_config_dir()
    assert config_dir == Path.home() / '.config' / 'myapp'


def test_relative_path_ignored(monkeypatch):
    """Relative paths in XDG variables are ignored per specification."""
    monkeypatch.setenv('XDG_CONFIG_HOME', 'relative/path')
    config_dir = get_config_dir()
    # Should fall back to default, not use relative path
    assert config_dir == Path.home() / '.config' / 'myapp'


def test_empty_string_uses_default(monkeypatch):
    """Empty string in XDG variable uses default."""
    monkeypatch.setenv('XDG_CONFIG_HOME', '')
    config_dir = get_config_dir()
    assert config_dir == Path.home() / '.config' / 'myapp'


def test_runtime_dir_none_when_unset(monkeypatch):
    """XDG_RUNTIME_DIR returns None when unset (no default)."""
    monkeypatch.delenv('XDG_RUNTIME_DIR', raising=False)
    runtime_dir = get_runtime_dir()
    assert runtime_dir is None


def test_config_dirs_search_path(monkeypatch):
    """XDG_CONFIG_DIRS parsed as colon-separated search path."""
    monkeypatch.setenv('XDG_CONFIG_DIRS', '/etc/xdg:/opt/config')
    dirs = get_config_dirs()
    assert dirs == [Path('/etc/xdg'), Path('/opt/config')]


def test_data_dirs_ignores_relative_paths(monkeypatch):
    """XDG_DATA_DIRS filters out relative paths."""
    monkeypatch.setenv('XDG_DATA_DIRS', '/usr/share:relative/path:/opt/data')
    dirs = get_data_dirs()
    assert dirs == [Path('/usr/share'), Path('/opt/data')]

Example Directory Structure

~/.config/myapp/              # XDG_CONFIG_HOME
    config.toml               # Main configuration
    credentials.json          # User credentials

~/.local/share/myapp/         # XDG_DATA_HOME
    models/                   # Downloaded models
        model-v1.gguf
    databases/
        user.db

~/.local/state/myapp/         # XDG_STATE_HOME
    history.log               # Command history
    recent-files.json         # Recently used files

~/.local/bin/                 # User binaries (convention)
    myapp                     # Application binary

~/.cache/myapp/               # XDG_CACHE_HOME
    downloads/                # Downloaded temporary files
    build/                    # Build artifacts

Configuration File Loading Pattern

For TOML configuration files with XDG support, activate the toml-python skill:

Skill(skill: "python3-development:toml-python")

The toml-python skill provides comprehensive guidance on TOML parsing with tomllib (Python 3.11+) and tomli (backport), including validation with Pydantic models.

Related Skills

Activate these skills for related functionality:

• toml-python - TOML configuration file parsing and validation
• python3-development - Modern Python development patterns and best practices
• uv - Python package and project management

References

Official Specification

• XDG Base Directory Specification v0.8 - Primary authoritative source (accessed 2025-01-15)
• Freedesktop.org Specifications Index - All freedesktop specifications
• XDG Specs Git Repository - Source repository

Implementation Guides

• ArchWiki: XDG Base Directory - Comprehensive implementation guide (accessed 2025-01-15)
• platformdirs Python Library - Cross-platform Python implementation (accessed 2025-01-15)

Community Resources

• XDG Mailing List - Official discussion forum
• Freedesktop.org Wiki - Additional documentation

Key Principles

1. Absolute Paths Only: Validate all XDG paths are absolute; ignore relative paths
2. Environment Variable Priority: Always check XDG environment variables before defaults
3. Specification Compliance: Follow XDG Base Directory Specification v0.8 exactly
4. Directory Creation: Create parent directories before writing files
5. Appropriate Storage: Use correct directory for data type (config vs data vs cache vs state)
6. Runtime Directory Limits: Avoid large files in $XDG_RUNTIME_DIR (tmpfs limits)
7. Cross-Platform Awareness: Use platformdirs for macOS/Windows support
8. Search Path Handling: Parse colon-separated search paths correctly
9. No Default for Runtime: $XDG_RUNTIME_DIR has no default; check for None
10. Testing: Validate XDG compliance with environment variable overrides