/**
 * Substitutes environment variables in a string.
 * Replaces ${VAR_NAME} patterns with their corresponding values from the env object.
 * Supports default values with ${VAR_NAME:-default} syntax.
 * If a variable is not found in env and no default is provided, the pattern is left as-is.
 *
 * @param str - The string containing environment variable patterns
 * @param env - A record of environment variable names to their values
 * @returns The string with all ${VAR_NAME} patterns replaced
 *
 * @example
 * substituteEnvVars("Hello ${USER}!", { USER: "Alice" })
 * // Returns: "Hello Alice!"
 *
 * @example
 * substituteEnvVars("Path: ${HOME}/docs", { HOME: "/Users/alice" })
 * // Returns: "Path: /Users/alice/docs"
 *
 * @example
 * substituteEnvVars("Missing: ${UNKNOWN}", {})
 * // Returns: "Missing: ${UNKNOWN}"
 *
 * @example
 * substituteEnvVars("URL: ${API_URL:-https://api.example.com}", {})
 * // Returns: "URL: https://api.example.com"
 *
 * @example
 * substituteEnvVars("URL: ${API_URL:-https://api.example.com}", { API_URL: "https://custom.com" })
 * // Returns: "URL: https://custom.com"
 */
export declare function substituteEnvVars(str: string, env: Record<string, string | undefined>): string;