How to read environment variables in Rust

Runtime configuration often reaches a Rust program through the process environment rather than command-line arguments or checked-in files. Reading a variable with the standard library keeps deployment-specific values outside the binary while still letting the program choose a fallback when the variable is absent.

std::env::var fetches one key from the current process and returns Result<String, VarError>. Match on that result when a missing value should mean default configuration but invalid data should stop the program.

APP_ENV is a neutral application mode name for the runtime check. Replace it with the variable your application reads, and use command-scoped values while testing so the host shell profile and long-lived session environment stay unchanged.

1. Open a terminal in the Rust binary package root.

   The directory should contain the Cargo.toml file for the package. Create a package first if needed.
   Related: How to create a Rust project with Cargo

2. Add an environment-variable reader to src/main.rs.

   src/main.rs

   use std::env;
    
   fn main() {
       match env::var("APP_ENV") {
           Ok(value) => println!("APP_ENV={value}"),
           Err(env::VarError::NotPresent) => {
               println!("APP_ENV is not set; using development")
           }
           Err(env::VarError::NotUnicode(_)) => {
               eprintln!("APP_ENV is not valid Unicode");
               std::process::exit(1);
           }
       }
   }

   std::env::var reads runtime values. Use std::env::var_os when the value may contain non-Unicode data, and use env! or option_env! only for values read while compiling the binary.

3. Check that the source compiles.

   $ cargo check
       Checking demo-cli v0.1.0 (/work/demo-cli)
       Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s

   Related: How to check Rust code with Cargo

4. Run the package with the variable set for this command.

   $ APP_ENV=production cargo run
      Compiling demo-cli v0.1.0 (/work/demo-cli)
       Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.13s
        Running `target/debug/demo-cli`
   APP_ENV=production

   The prefix assignment sets APP_ENV only for this cargo run command in POSIX shells. In PowerShell, set $Env:APP_ENV = 'production' before running Cargo and remove it after testing.

5. Run the package with the variable unset.

   $ env -u APP_ENV cargo run
       Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
        Running `target/debug/demo-cli`
   APP_ENV is not set; using development

   env -u removes APP_ENV only from the child process. The fallback output comes from the VarError::NotPresent branch.