iamjose

tiny shell

Tue, 28 Jul 2026 00:00:00 GMT

A shell is the best first systems project I know: read a line, split it into words, spawn a process, wait for it. Every piece is small enough to hold in your head, and every milestone below ends with something you can actually run.

Follow along one milestone at a time — each chapter states what we are building, walks through it, and sometimes leaves you a challenge to try before the next one picks up.

tiny shell · A prompt that reads

Tue, 28 Jul 2026 00:00:00 GMT

Everything a shell does starts with one loop: print a prompt, read a line, do something with it. Milestone one is that loop, honest and unadorned.

use std::io::{self, Write};

fn main() {
    loop {
        print!("tiny$ ");
        io::stdout().flush().expect("flush failed");

        let mut line = String::new();
        match io::stdin().read_line(&mut line) {
            Ok(0) => break, // EOF: ctrl-d ends the session
            Ok(_) => println!("you typed: {}", line.trim_end()),
            Err(err) => eprintln!("read error: {err}"),
        }
    }
}

read_line returning Ok(0) is how end-of-input shows up — that is your ctrl-d exit. Everything else just echoes for now; the next milestone starts making sense of the line.

tiny shell · Splitting the command line

Tue, 28 Jul 2026 00:00:00 GMT

Before we can run ls -la, we need to see it as a command and its arguments. The obvious tokenizer is split_whitespace, and for milestone two the obvious tokenizer is the right one:

fn tokenize(line: &str) -> Vec<String> {
    line.split_whitespace().map(str::to_string).collect()
}

Wire it into the loop from milestone one and print the pieces. The first word is the program; the rest are its arguments.

challenge: handle quoted arguments

split_whitespace breaks echo "hello world" into three tokens, but a real shell yields two. Extend tokenize so double-quoted spans keep their spaces. Keep the signature; you will want to walk the characters yourself.

solution

Walk the characters with a flag for whether you are inside quotes:

fn tokenize(line: &str) -> Vec<String> {
    let mut tokens = Vec::new();
    let mut current = String::new();
    let mut in_quotes = false;

    for c in line.chars() {
        match c {
            '"' => in_quotes = !in_quotes,
            c if c.is_whitespace() && !in_quotes => {
                if !current.is_empty() {
                    tokens.push(std::mem::take(&mut current));
                }
            }
            c => current.push(c),
        }
    }
    if !current.is_empty() {
        tokens.push(current);
    }
    tokens
}

Unterminated quotes silently close at end of line — a real shell would keep reading. That wart becomes a milestone of its own later.

Reading borrow checker errors as a conversation

Fri, 24 Jul 2026 00:00:00 GMT

rustc's diagnostics are not obstacles. They are the most patient code reviewer you will ever work with.

A blog you can type into

Mon, 20 Jul 2026 00:00:00 GMT

Most technical writing about a language asks you to leave it. You read a snippet, nod, and then — if you are conscientious — open a separate tab, paste the code somewhere, and find out whether it actually compiles. That context switch is where most of the learning quietly leaks away.

So this site has a playground attached to it. Press any printable key, right now, anywhere on this page. The sidebar opens, the editor takes focus, and the character you typed is already sitting inside main. No button to hunt for.

The smallest possible hello

fn main() {
    println!("hello");
}

That runs on play.rust-lang.org, the same service that backs the online Rust Book. There is no backend here — this site is a pile of static files on a CDN, and your code goes straight from your browser to the playground’s public API.

Why the trigger is a keystroke

The header has a [playground] link, because discoverability matters and not everyone guesses that a blog will respond to typing. But the keystroke is the real interface. It costs nothing to reach for, and it means the distance between “I wonder if that works” and a running program is a single character.

The tradeoff is that every printable key is now spoken for. That is why the theme toggle is a link rather than a shortcut: d belongs to the editor.

What it does not do

No crates beyond the standard library. No multi-file projects. No saving to a gist. If you want those, the real playground is one click away and does all of them better. This one exists for the thirty-second experiment — the kind you would otherwise skip.

iamjose

tiny shell

tiny shell · A prompt that reads

tiny shell · Splitting the command line

Reading borrow checker errors as a conversation

A blog you can type into

The smallest possible hello#

Why the trigger is a keystroke#

What it does not do#

The smallest possible hello

Why the trigger is a keystroke

What it does not do