Mini Post: Parsing Environment Variables Safely in Rust

bloom ⋄ posted about 2 years ago ⋄ last updated about 2 years ago ⋄ 2 min read

Almost every app I write nowadays uses an environment file, and therefore I always have to read things from the environment and parse them into different forms. Doing this safely with defaults in Rust was a bit tricky for me at first, but I’ve landed on a solution I like.

Optional Numbers#

The reason I was looking for code like this was to try and get a port for a database from the environment with a reasonable fallback. Here’s what I landed on:

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use std::env;
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use dotenv::dotenv;
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fn main() {
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    dotenv().ok();
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    let count = env::var("COUNT")
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        // convert to Option<String>
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        .ok()
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        // convert to Option<u64> via parse
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        .and_then(|value| value.parse::<u64>().ok())
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        // Use 5 as a default if either the value was not found or it could
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        // not be converted.
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        .unwrap_or(5)
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    println!("{}", count * 5);
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}

Default Strings#

Of course, you can always just get a required value like this:

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std::env;
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use dotenv::dotenv;
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fn main() {
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    dotenv().ok();
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    let test = env::var("TEST")
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        .unwrap_or(String::from("Test"));
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    println!("{}", test);
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}

Required Environment Variables#

And of course, you can make them required by just unwraping them, but that will crash your program:

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std::env;
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use dotenv::dotenv;
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fn main() {
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    dotenv().ok();
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    let test = env::var("TEST").unwrap();
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    println!("{}", test);
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}

thread 'main' panicked at src\main.rs:6:35:
called `Result::unwrap()` on an `Err` value: NotPresent

You can instead print a helpful message:

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std::env;
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use dotenv::dotenv;
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fn main() {
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    dotenv().ok();
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    let test = env::var("TEST");
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    match test {
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        Ok(test) => {
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            println!("{}", test)
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        }
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        Err(_) => {
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            eprintln!("Missing required environment variable 'TEST'");
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            return;
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        }
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    }
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}

Missing required environment variable 'TEST'