Building a Shell - Part 1 https://nixpulvis.com/ramblings/2018-07-11-building-a-shell-part-1

shell.rb

# Commands to be executed directly by this shell.
BUILTINS = {}

# The builtin `cd` for changing the shell's working directory.
BUILTINS['cd'] = lambda do |args|
  # Change to the home directory by default.
  args << ENV['HOME'] if args.empty?

  # Read the destination path, doing very basic path expansion.
  dest = args.pop.gsub(/~/, ENV['HOME'])

  # Try to change this shell's working directory.
  begin
    Dir.chdir(dest)
  rescue Exception
    puts("no such directory: #{dest}")
  end
end

# The builtin to exit the shell.
BUILTINS['exit'] = lambda do |args|
  # Exit with a status of 0 by default.
  args << 0 if args.empty?

  # Exit the shell.
  exit args.pop.to_i
end

# Print a prompt, then read a line from STDIN.
def read
  print('$ ')

  # Try to read a line, exiting if there's no more lines.
  line = STDIN.gets
  if line
    line.chomp.split(' ')
  else
    exit
  end
end

# Run the given command in a subprocess.
def evaluate(argv)
  if BUILTINS[argv[0]]
    BUILTINS[argv[0]].call(argv[1..-1])
  else
    success = system(*argv)
    unless success
      puts("unknown command '#{argv[0]}'")
    end
  end
end

# Don't exit on `SIGINT`, instead simply return a new prompt.
trap('INT') { print("\n$ ") }

# The glorious REPL itself.
loop { evaluate(read) }

shell.rs

use std::io::{self, Read, Write};
use std::process::{Command, Output};
use std::str;

// Our shell, for the greater good. Ready and waiting.
fn main() {
    // Standard input file descriptor (0), used for user input from the user
    // of the shell.
    let mut stdin = io::stdin();

    // Standard output file descriptor (1), used to display program output to
    // the user of the shell.
    let mut stdout = io::stdout();

    // STDIN, input buffer, used to `read` text into for further processing.
    // TODO: Full fledged parser will be neato.
    let mut input = [0; 24];

    loop {
        // XXX: Blindly drop the contents of input, again this will be better
        // with a real parser.
        input = [0; 24];

        // Print a boring static prompt.
        print!("oursh> ");
        stdout.lock().flush();

        loop {
            // TODO: Enable raw access to STDIN, so we can read as the user
            // types. By default the underlying file is line buffered. This
            // will allow us to process history, syntax, and more!

            // Read what's avalible to us.
            stdin.read(&mut input).expect("error reading STDIN");

            // Once we've read a complete "program" (§2.10.2) we handle it,
            // until then we keep reading. Once we have a proper parser this
            // wont look like a huge hack.
            let vec: Vec<&[u8]> = input.splitn(2, |b| *b == '\n' as u8).collect();
            match &vec[..] {
                [line, rest] => {
                    let program = str::from_utf8(&line).expect("error reading utf8");
                    let mut output = handle_program(program);
                    stdout.write(&output.stdout).expect("error writing to STDOUT");
                    break
                }
                _ => {},
            }
        }
    }
}

// Obviously very wrong. Most notably this blocks until the command completes.
fn handle_program(program: &str) -> Output {
    Command::new(program)
        .output()
        .expect("error executing program")
}

Rust / Ruby Filesize equivalent.

oursh progress

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Language                     files          blank        comment           code
-------------------------------------------------------------------------------
Rust                             7            100            478            594
-------------------------------------------------------------------------------

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Language                     files          blank        comment           code
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Rust                            30            296            788           2257