use std::error::Error;
use std::f64;
use std::process::Command;

fn main() {
    println!("{}", value("echo 5"));
}

fn value(cmd: &str) -> f64 {
    value_inner(cmd).unwrap_or(f64::NAN)
}

fn value_inner(cmd: &str) -> Result<f64, Box<dyn Error>> {
    let stdout = Command::new("sh").args(&["-c", cmd]).output()?.stdout;
    let string = String::from_utf8(stdout)?;
    let string = string.trim();
    Ok(string.parse()?)
}