Agent skill

env-var-mapper

Generate environment variable to CLI argument mapping with prefix support, type conversion, and fallback chains for configuration.

Stars 514
Forks 31

Install this agent skill to your Project

npx add-skill https://github.com/a5c-ai/babysitter/tree/main/library/specializations/cli-mcp-development/skills/env-var-mapper

SKILL.md

Environment Variable Mapper

Generate environment variable to CLI argument mapping for flexible configuration.

Capabilities

  • Generate env var to argument mapping logic
  • Create prefix-based environment loading
  • Set up type conversion for env vars
  • Implement fallback chains (env -> config -> default)
  • Configure .env file support
  • Generate documentation for env vars

Usage

Invoke this skill when you need to:

  • Map environment variables to CLI arguments
  • Create prefix-based config loading
  • Implement configuration fallback chains
  • Support .env files in CLI applications

Inputs

Parameter Type Required Description
language string Yes Target language (typescript, python, go)
prefix string No Env var prefix (e.g., MYAPP_)
mappings array Yes Environment variable mappings
dotenvSupport boolean No Enable .env file support (default: true)

Mapping Structure

json
{
  "mappings": [
    {
      "envVar": "PORT",
      "argument": "port",
      "type": "number",
      "default": 3000,
      "description": "Server port"
    },
    {
      "envVar": "DATABASE_URL",
      "argument": "database-url",
      "type": "string",
      "required": true,
      "sensitive": true
    },
    {
      "envVar": "DEBUG",
      "argument": "debug",
      "type": "boolean",
      "default": false
    }
  ]
}

Generated Code Patterns

TypeScript Mapper

typescript
import { config } from 'dotenv';
import { z } from 'zod';

// Load .env file
config();

const ENV_PREFIX = 'MYAPP_';

// Environment schema
const envSchema = z.object({
  PORT: z.coerce.number().default(3000),
  DATABASE_URL: z.string().min(1),
  DEBUG: z.coerce.boolean().default(false),
  LOG_LEVEL: z.enum(['debug', 'info', 'warn', 'error']).default('info'),
});

// Get prefixed env var
function getEnv(name: string): string | undefined {
  return process.env[`${ENV_PREFIX}${name}`] ?? process.env[name];
}

// Build env object with prefix support
function buildEnvObject(): Record<string, string | undefined> {
  return {
    PORT: getEnv('PORT'),
    DATABASE_URL: getEnv('DATABASE_URL'),
    DEBUG: getEnv('DEBUG'),
    LOG_LEVEL: getEnv('LOG_LEVEL'),
  };
}

// Parse and validate environment
export function loadEnvironment() {
  const env = buildEnvObject();
  return envSchema.parse(env);
}

// Map env vars to CLI arguments
export function envToArgs(): string[] {
  const env = loadEnvironment();
  const args: string[] = [];

  if (env.PORT !== 3000) {
    args.push('--port', String(env.PORT));
  }
  if (env.DATABASE_URL) {
    args.push('--database-url', env.DATABASE_URL);
  }
  if (env.DEBUG) {
    args.push('--debug');
  }
  if (env.LOG_LEVEL !== 'info') {
    args.push('--log-level', env.LOG_LEVEL);
  }

  return args;
}

// Generate documentation
export const ENV_DOCS = `
Environment Variables:

  ${ENV_PREFIX}PORT          Server port (default: 3000)
  ${ENV_PREFIX}DATABASE_URL  Database connection URL (required)
  ${ENV_PREFIX}DEBUG         Enable debug mode (default: false)
  ${ENV_PREFIX}LOG_LEVEL     Log level: debug|info|warn|error (default: info)
`;

Python Mapper

python
import os
from dataclasses import dataclass
from typing import Optional
from dotenv import load_dotenv

# Load .env file
load_dotenv()

ENV_PREFIX = 'MYAPP_'

@dataclass
class Environment:
    port: int = 3000
    database_url: str = ''
    debug: bool = False
    log_level: str = 'info'

def get_env(name: str) -> Optional[str]:
    """Get env var with prefix fallback."""
    return os.getenv(f'{ENV_PREFIX}{name}') or os.getenv(name)

def parse_bool(value: Optional[str]) -> bool:
    """Parse boolean from environment variable."""
    if value is None:
        return False
    return value.lower() in ('true', '1', 'yes', 'on')

def load_environment() -> Environment:
    """Load and validate environment variables."""
    env = Environment()

    if port := get_env('PORT'):
        env.port = int(port)

    if database_url := get_env('DATABASE_URL'):
        env.database_url = database_url
    else:
        raise ValueError('DATABASE_URL is required')

    env.debug = parse_bool(get_env('DEBUG'))

    if log_level := get_env('LOG_LEVEL'):
        if log_level not in ('debug', 'info', 'warn', 'error'):
            raise ValueError(f'Invalid LOG_LEVEL: {log_level}')
        env.log_level = log_level

    return env

def env_to_args() -> list[str]:
    """Convert environment to CLI arguments."""
    env = load_environment()
    args = []

    if env.port != 3000:
        args.extend(['--port', str(env.port)])
    if env.database_url:
        args.extend(['--database-url', env.database_url])
    if env.debug:
        args.append('--debug')
    if env.log_level != 'info':
        args.extend(['--log-level', env.log_level])

    return args

ENV_DOCS = f'''
Environment Variables:

  {ENV_PREFIX}PORT          Server port (default: 3000)
  {ENV_PREFIX}DATABASE_URL  Database connection URL (required)
  {ENV_PREFIX}DEBUG         Enable debug mode (default: false)
  {ENV_PREFIX}LOG_LEVEL     Log level: debug|info|warn|error (default: info)
'''

Go Mapper

go
package config

import (
    "fmt"
    "os"
    "strconv"
    "strings"

    "github.com/joho/godotenv"
)

const EnvPrefix = "MYAPP_"

type Environment struct {
    Port        int
    DatabaseURL string
    Debug       bool
    LogLevel    string
}

func init() {
    // Load .env file if present
    godotenv.Load()
}

func getEnv(name string) string {
    if val := os.Getenv(EnvPrefix + name); val != "" {
        return val
    }
    return os.Getenv(name)
}

func parseBool(value string) bool {
    lower := strings.ToLower(value)
    return lower == "true" || lower == "1" || lower == "yes" || lower == "on"
}

func LoadEnvironment() (*Environment, error) {
    env := &Environment{
        Port:     3000,
        Debug:    false,
        LogLevel: "info",
    }

    if port := getEnv("PORT"); port != "" {
        p, err := strconv.Atoi(port)
        if err != nil {
            return nil, fmt.Errorf("invalid PORT: %s", port)
        }
        env.Port = p
    }

    env.DatabaseURL = getEnv("DATABASE_URL")
    if env.DatabaseURL == "" {
        return nil, fmt.Errorf("DATABASE_URL is required")
    }

    env.Debug = parseBool(getEnv("DEBUG"))

    if logLevel := getEnv("LOG_LEVEL"); logLevel != "" {
        valid := []string{"debug", "info", "warn", "error"}
        found := false
        for _, v := range valid {
            if v == logLevel {
                found = true
                break
            }
        }
        if !found {
            return nil, fmt.Errorf("invalid LOG_LEVEL: %s", logLevel)
        }
        env.LogLevel = logLevel
    }

    return env, nil
}

Workflow

  1. Define mappings - Specify env vars and arguments
  2. Configure prefix - Set application prefix
  3. Generate loader - Create env loading code
  4. Add type conversion - Handle type coercion
  5. Generate docs - Create env var documentation
  6. Add validation - Required and constraints

Best Practices Applied

  • Prefix support for namespacing
  • Fallback from prefixed to non-prefixed
  • Type-safe env var parsing
  • Boolean string parsing
  • .env file support
  • Auto-generated documentation

Target Processes

  • argument-parser-setup
  • configuration-management-system
  • cli-application-bootstrap

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