yes

yes.rs

use std::iter::FromIterator;
use std::io::{Write, Result};

const BUF_SIZE: usize = 8192;

fn main() {
    if let Some(message) = message() {
        let buf = prepare_buf(&message.into_bytes());
        let mut stdout = std::io::stdout();
        while let Ok(written) = write_all(&mut stdout, &buf) {
            if written != buf.len() {
                break;
            }
        }
    } else {
        usage();
    }
}

fn message() -> Option<String> {
    let params = Vec::from_iter(std::env::args().skip(1));
    if (params.len() == 1) && (params[0] == "--help") {
        return None;
    }
    let first_index = if (params.len() > 0) && (params[0] == "--") { 1 } else { 0 };
    if params.len() == first_index {
        Some(String::from("y\n"))
    } else {
        let mut msg = params[first_index..].join(" ");
        msg.push('\n');
        Some(msg)
    }
}

fn usage() {
    println!("Usage: {} [--help|<messages...>]", std::env::args().next().unwrap());
}

fn prepare_buf(message: &[u8]) -> Vec<u8> {
    let nr_repeats = std::cmp::max(1, BUF_SIZE / message.len());
    let mut buf = vec![];
    for _ in 0 .. nr_repeats {
        buf.extend_from_slice(message);
    }
    return buf;
}

fn write_all(out: &mut Write, buf: &[u8]) -> Result<usize> {
    let mut total_written = 0usize;
    while total_written < buf.len() {
        match out.write(&buf[total_written..]) {
            Ok(size) => total_written += size,
            Err(e)   => return if total_written == 0 { Err(e) } else { Ok(total_written) },
        }
    }
    Ok(total_written)
}

yes2.rs

use std::iter::FromIterator;
use std::io::Write;

const BUF_SIZE: usize = 8192;

fn main() {
    if let Some(message) = message() {
        let buf = prepare_buf(&message.into_bytes());
        let mut stdout = std::io::stdout();
        while let Ok(_) = stdout.write_all(&buf) {
        }
    } else {
        usage();
    }
}

fn message() -> Option<String> {
    let params = Vec::from_iter(std::env::args().skip(1));
    if (params.len() == 1) && (params[0] == "--help") {
        return None;
    }
    let first_index = if (params.len() > 0) && (params[0] == "--") { 1 } else { 0 };
    if params.len() == first_index {
        Some(String::from("y\n"))
    } else {
        let mut msg = params[first_index..].join(" ");
        msg.push('\n');
        Some(msg)
    }
}

fn usage() {
    println!("Usage: {} [--help|<messages...>]", std::env::args().next().unwrap());
}

fn prepare_buf(message: &[u8]) -> Vec<u8> {
    let nr_repeats = std::cmp::max(1, BUF_SIZE / message.len());
    let mut buf = vec![];
    for _ in 0 .. nr_repeats {
        buf.extend_from_slice(message);
    }
    return buf;
}