As part of my journey in learning the Rust programming language, I decided to make a miniature cat clone (catty) in it. The following is my code, which depends on clap for argument parsing (see below). It currently only supports concatenating 1 file with possibly numbered lines (-n/--number). I tried to be as close as possible to the actual cat program for this:

#[macro_use] extern crate clap;

use std::{io, error, env, fs::read_to_string, path::PathBuf, process};

fn main() {
        if let Err(err) = cli(env::args().collect::<Vec<_>>()) {
            // CLI parsing errors
            if let Some(clap_err) = err.downcast_ref::<clap::Error>() {
                eprint!("{}", clap_err);
            } else {
                eprintln!("{}", err);
        } else {

fn cli(cli_args: Vec<String>) -> Result<(), Box<error::Error>> {
    let matches = clap::App::new("catty")
        .about("A minimal clone of the linux utility cat.")
            .help("The file to concatenate to standard output")
            .help("Numbers all output lines"))

    let file_contents = get_file_contents(matches.value_of("FILE").unwrap())?;
    let file_contents: Vec<&str> = file_contents.split("\n").collect();

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

    for (i, line) in file_contents.iter().enumerate() {
        let formatted_line = if number_lines {
            format!("{:>6}  {}", i + 1, line)
        } else {

        if i == file_contents.len() - 1 && line.len() > 0 {
            print!("{}", formatted_line);
        } else if !(i == file_contents.len() - 1 && line.len() == 0) {
            println!("{}", formatted_line);


fn get_file_contents(passed_argument: &str) -> Result<String, Box<error::Error>> {
    let mut resolved_path = PathBuf::from(passed_argument);

    if !resolved_path.exists() || !resolved_path.is_file() {
        resolved_path = PathBuf::from(env::current_dir()?);

        if !resolved_path.exists() || !resolved_path.is_file() {
            return Err(io::Error::new(io::ErrorKind::NotFound, "The passed file is either not a file or does not exist!").into());


My Cargo.toml is as follows:

name = "catty"
version = "0.1.0"
authors = ["My Name <[email protected]>"]


version = "2.32"
default-features = false
features = ["suggestions"]

Here is what I want to know from this code review:

  1. Is my code idiomatic rust (i.e good error handling, not overly verbose, etc)?
  2. Is my code performant or can it be improved in some way?

1 Answer 1


4: use std::{env, error, fs::read_to_string, io, path::PathBuf, process};

Just a personal taste, but I would do

use std::{env, error, io, process};
use std::fs::read_to_string;
use std::path::PathBuf;

Yes, nested includes are nice and easy, but hard to extend. It's up to you.

35: let file_contents: Vec<&str> = file_contents.split('\n').collect();

I would suggest using lines instead.

Also, you can omit &str or change the line completly.


let file_contents: Vec<_> = file_contents.lines().collect();


let file_contents = file_contents.lines().collect::<Vec<_>>();

46/48: line.len() > 0 / line.len() == 0

Replace that by !line.is_empty() and line.is_empty().

60: resolved_path = PathBuf::from(env::current_dir()?);

Remove PathBuf::from completly, because current_dir is already a PathBuf

  • \$\begingroup\$ Also, is there an equivalent of enumerate for an iterator (so I can avoid collecting into a vector)? \$\endgroup\$ Commented Nov 26, 2018 at 20:31
  • 1
    \$\begingroup\$ Yes, you can call enumerate on every iterator. The thing is, that you don't know the length of your iterator, which is the reason why I didn't mentioned it. If you are willing to use println! every iteration, you can simplify your code/loop a lot. \$\endgroup\$
    – hellow
    Commented Nov 27, 2018 at 7:03

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