The issue you're encountering is due to how the shell interprets the command string. When you use std::process::Command::new(prog), it does not invoke a shell by default. This means environment variables like $USER are not expanded, and the command is treated literally.

Why it happens

• echo $USER is passed as-is to the echo command, so $USER is not expanded to your username.
• The shell (e.g., bash or zsh) is responsible for expanding environment variables, but std::process::Command does not use a shell by default.

Solutions

1. Use sh -c to invoke a shell

You can explicitly invoke a shell (e.g., /bin/sh) and pass your command as a string to be interpreted by the shell:

fn main() {
    let command = "echo $USER";
    let output = std::process::Command::new("/bin/sh")
        .arg("-c")
        .arg(command)
        .output()
        .unwrap();

    println!("stdout: {}|", String::from_utf8_lossy(&output.stdout));
}

2. Expand the variable manually

If you know the variable name, you can expand it manually using Rust's std::env module:

fn main() {
    let user = std::env::var("USER").unwrap_or_else(|_| "unknown".to_string());
    println!("stdout: {}|", user);
}

3. Use the shell-words crate (for complex commands)

If you need to split a command string into arguments safely, you can use the shell-words crate, but this still won't expand environment variables. You'd still need a shell for that.

Recommendation

For your use case, Solution 1 (sh -c) is the simplest and most direct way to get the expected behavior. It ensures the command is interpreted by a shell, allowing environment variable expansion.