Hello, world!

Now that we have Rust installed, let’s write our first Rust program. It’s traditional to make our first program in any new language one that prints the text “Hello, world!” to the screen. The nice thing about starting with such a simple program is that we can verify that our compiler isn’t just installed, but also working properly. And printing information to the screen is a pretty common thing to do.

The first thing that we need to do is make a file to put our code in. I like to make a projects directory in my home directory, and keep all my projects there. Rust doesn't care where our code lives.

This actually leads to one other concern we should address: this guide will assume that we have basic familiarity with the command line. Rust itself makes no specific demands on our editing tooling, or where our code lives. If we prefer an IDE to the command line, we may want to check out SolidOak, or wherever plugins are for our favorite IDE. There are a number of extensions of varying quality in development by the community. The Rust team also ships plugins for various editors. Configuring our editor or IDE is out of the scope of this tutorial, so check the documentation for our setup, specifically.

With that said, let’s make a directory in our projects directory.

$ mkdir ~/projects
$ cd ~/projects
$ mkdir hello_world
$ cd hello_world

If we’re on Windows and not using PowerShell, the ~ may not work. Consult the documentation for our shell for more details.

Let’s make a new source file next. We’ll call our file main.rs. Rust files always end in a .rs extension, and if we’re using more than one word in a Rust filename, we use an underscore: for example, linked_list.rs, not linkedlist.rs or LinkedList.rs.

Now that we’ve got our file open, type this in:

fn main() { println!("Hello, world!"); }
fn main() {
    println!("Hello, world!");
}

Save the file, and then type this into our terminal window:

$ rustc main.rs
$ ./main # or main.exe on Windows
Hello, world!

Success! Let’s go over what just happened in detail.

fn main() { }
fn main() {

}

These lines define a function in Rust. The main function is special: it's the beginning of every Rust program. The first line says "I’m declaring a function named main which takes no arguments and returns nothing." If there were arguments, they would go inside the parentheses (( and )), and because we aren’t returning anything from this function, we can omit the return type entirely. We’ll get to it later.

You’ll also note that the function is wrapped in curly braces ({ and }). Rust requires these around all function bodies. It is also considered good style to put the opening curly brace on the same line as the function declaration, with one space in between.

Next up is this line:

fn main() { println!("Hello, world!"); }
    println!("Hello, world!");

This line does all of the work in our little program. There are a number of details that are important here. The first is that it’s indented with four spaces, not tabs. Please configure your editor of choice to insert four spaces with the tab key. We provide some sample configurations for various editors.

The second point is the println!() part. This is calling a Rust macro, which is how metaprogramming is done in Rust. If it were a function instead, it would look like this: println(). For our purposes, we don’t need to worry about this difference. Just know that sometimes, we’ll see a !, and that means that we’re calling a macro instead of a normal function. Rust implements println! as a macro rather than a function for good reasons, but that's an advanced topic. One last thing to mention: Rust’s macros are significantly different from C macros, if you’ve used those. Don’t be scared of using macros. We’ll get to the details eventually, you’ll just have to take it on trust for now.

Next, "Hello, world!" is a ‘string’. Strings are a surprisingly complicated topic in a systems programming language, and this is a ‘statically allocated’ string. If you want to read further about allocation, check out the stack and the heap, but you don’t need to right now if you don’t want to. We pass this string as an argument to println!, which prints the string to the screen. Easy enough!

Finally, the line ends with a semicolon (;). Rust is an ‘expression oriented’ language, which means that most things are expressions, rather than statements. The ; is used to indicate that this expression is over, and the next one is ready to begin. Most lines of Rust code end with a ;.

Finally, actually compiling and running our program. We can compile with our compiler, rustc, by passing it the name of our source file:

$ rustc main.rs

This is similar to gcc or clang, if you come from a C or C++ background. Rust will output a binary executable. We can see it with ls:

$ ls
main  main.rs

Or on Windows:

$ dir
main.exe  main.rs

There are now two files: our source code, with the .rs extension, and the executable (main.exe on Windows, main everywhere else).

$ ./main  # or main.exe on Windows

This prints out our Hello, world! text to our terminal.

If you come from a dynamic language like Ruby, Python, or JavaScript, you may not be used to these two steps being separate. Rust is an ‘ahead-of-time compiled language’, which means that we can compile a program, give it to someone else, and they don't need to have Rust installed. If we give someone a .rb or .py or .js file, they need to have a Ruby/Python/JavaScript implementation installed, but we just need one command to both compile and run our program. Everything is a tradeoff in language design, and Rust has made its choice.

Congratulations! You have officially written a Rust program. That makes you a Rust programmer! Welcome. 🎊🎉👍

Next, I'd like to introduce you to another tool, Cargo, which is used to write real-world Rust programs. Just using rustc is nice for simple things, but as our project grows, we'll want something to help us manage all of the options that it has, and to make it easy to share our code with other people and projects.