path: root/src/doc/trpl/hello-world.md

% 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][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][plugins]. Configuring our
editor or IDE is out of the scope of this tutorial, so check the documentation
for our setup, specifically.

[solidoak]: https://github.com/oakes/SolidOak
[plugins]: https://github.com/rust-lang/rust/blob/master/src/etc/CONFIGS.md

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

```bash
$ 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:

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

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

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

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

```rust
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:

```rust
    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][configs].

[configs]: https://github.com/rust-lang/rust/tree/master/src/etc/CONFIGS.md

The second point is the `println!()` part. This is calling a Rust
[macro][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.

[macro]: macros.html

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][allocation], 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!

[allocation]: the-stack-and-the-heap.html

Finally, the line ends with a semicolon (`;`). Rust is an [‘expression oriented’
language][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 `;`.

[expression-oriented language]: glossary.html#expression-oriented-language

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

```bash
$ 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`:

```bash
$ ls
main  main.rs
```

Or on Windows:

```bash
$ 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).

```bash
$ ./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.