main.rs

use clap::{arg, App};
use std::fs::File;
use std::io::{self, BufRead, BufReader};

fn main() {
    let matches = App::new("cat")
        .about("Rust version cat")
        .arg(arg!([file] ...))
        .arg(arg!(number: -n "Number the output lines"))
        .get_matches();

    let number_lines: bool = matches.is_present("number");

    match matches.values_of("file") {
        Some(files) => {
            for filename in files {
                match File::open(filename) {
                    Ok(file) => {
                        for (num, line_result) in BufReader::new(file).lines().enumerate() {
                            let line = line_result.unwrap();
                            if number_lines {
                                println!("{:>6}\t{}", num + 1, line);
                            } else {
                                println!("{}", line);
                            }
                        }
                    }
                    Err(err) => eprintln!("Failed to open {}: {}", filename, err),
                }
            }
        }
        None => {
            for (num, line_result) in BufReader::new(io::stdin()).lines().enumerate() {
                let line = line_result.unwrap();
                if number_lines {
                    println!("{:>6}\t{}", num + 1, line)
                } else {
                    println!("{}", line)
                }
            }
        }
    }
}