structopt 0.3.14

Parse command line argument by defining a struct.
Documentation 
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//! How to use environment variable fallback an how it
//! interacts with `default_value`.

use structopt::StructOpt;

/// Example for allowing to specify options via environment variables.
#[derive(StructOpt, Debug)]
#[structopt(name = "env")]
struct Opt {
    // Use `env` to enable specifying the option with an environment
    // variable. Command line arguments take precedence over env.
    /// URL for the API server
    #[structopt(long, env = "API_URL")]
    api_url: String,

    // The default value is used if neither argument nor environment
    // variable is specified.
    /// Number of retries
    #[structopt(long, env = "RETRIES", default_value = "5")]
    retries: u32,
}

fn main() {
    let opt = Opt::from_args();
    println!("{:#?}", opt);
}