temp-env

April 3, 2025 ยท View on GitHub

Set environment variables temporarily.

This crate is useful for testing with different environment variables that should not interfere when running cargo test.

This code started as a small test helper written by @fabian-braun and @nbaztec and published by @fabian-braun on StackOverflow. @vmx found it useful and took the time to make it a proper crate.

Examples

temp_env::with_var("MY_ENV_VAR", Some("production"), || {
    // Run some code where `MY_ENV_VAR` is set to `production`.
});


temp_env::with_vars(
    [
        ("FIRST", Some("Hello")),
        ("SECOND", Some("World!")),
    ],
    || {
        // Run some code where `FIRST` is set to `Hello` and `SECOND` is set to `World!`.
    }
);

temp_env::with_vars(
    [
        ("FIRST", Some("Hello")),
        ("SECOND", None),
    ],
    || {
        // Run some code where `FIRST` is set to `Hello` and `SECOND` is unset (even if
        // it was set before)
    }
);

let value = temp_env::with_var("MY_ENV_VAR", Some("production"), || {
    // Compute a value in a closure while `MY_ENV_VAR` is set to `production`.
    let envvar = env::var("MY_ENV_VAR").unwrap();
    if envvar == "production" {
        true
    } else {
        false
    }
});

How does it work?

This crate sets and unsets environment variables for the currently running (Rust) process. It leverages std::env::set_var.

The provided functions temp_env::with_* provide the following features:

  • Avoid interference when running concurrently
  • Reset previously set env variables back to their original value upon completion, also in case of panic
  • Temporarily unsetting env variables

Note that the crate makes use of a singleton mutex to avoid side effects between concurrently running tests. This may impact the degree of concurrency in your test execution.

Features

  • async_closure: When enabled you can use async_with_var() with async closures. This feature needs at least Rust version 1.64.

Noteworthy

As an alternative to using this crate, you might consider migrating your testing to cargo-nextest. cargo-nextest runs each test in a separate process, which makes it unnecessary to synchronize the altering of environment variables as described here.

License

This project is licensed under either of

at your option.