envfmt

A lightweight Rust crate for expanding environment-style variables in strings, similar to shell expansion.

This crate provides a simple and efficient way to substitute variables in a string, using either the process environment or a custom context like a HashMap.

• Overview
• Getting Started
• Use HashMap as Data Source
• Provide Default Values
• Escape a Dollar Sign
• Implement the Context Trait

Features

• Zero Dependencies: envfmt is built with only the Rust standard library (plus thiserror for convenience).
• Familiar Syntax: Supports common shell variable expansion patterns.
  • Simple variables: $VAR
  • Braced variables: ${VAR}
  • Default values for unset variables: ${VAR:-default}
  • Escaping: $$ to get a literal $
• Flexible & Testable: Use the primary envfmt::format() for environment variables, or envfmt::format_with() with a HashMap for easy testing and custom data sources.
• Ergonomic: Works with HashMap<String, _> and HashMap<&str, _> keys out of the box.

Getting Started

Add envfmt to your project's dependencies:

cargo add envfmt

Usage

This is the most common use case. First, ensure an environment variable is set in your shell:

export ETHERSCAN_API_KEY="key"

Then, use envfmt::format() to expand it:

let key = envfmt::format("${ETHERSCAN_API_KEY}").unwrap();
assert_eq!(key, "key");

If a variable might be missing, you can provide a fallback default value:

let log_config = "${LOG_LEVEL:-INFO}";
let log_level = envfmt::format(log_config).unwrap();
assert_eq!(log_level, "INFO");

For advance usage, or if your variables come from a source other than the environment, use envfmt::format_with(). This is fast, safe, and doesn't require modifying the global environment.

let mut context = HashMap::new();
context.insert("user", "Alice");

let template = "Hello $user!";
let message = envfmt::format_with(template, &context)?;
assert_eq!(message, "Hello Alice!");

License

This project is licensed under the MIT License.