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Hello, world!

After you install the MAX SDK or Mojo SDK or you can use the Mojo CLI to build and run Mojo programs. So let's create the classic starter program that prints "Hello, world!", in three different ways.

Before you start

Before you start, make sure the MODULAR_HOME and PATH environment variables are set, as described in the install procedure, so you can run the mojo command:

mojo --version

If you have other issues during install, check our known issues.

1. Run code in the REPL

First, let's use the Mojo REPL, which allows you to write and run Mojo code in a command prompt:

  1. To start a REPL session, type mojo in your terminal and press Enter.

  2. Then type print("Hello, world!") and press Enter twice (a blank line is required to indicate the end of an expression).

That's it! For example:

$ mojo
Welcome to Mojo! 🔥

Expressions are delimited by a blank line.
Type `:quit` to exit the REPL and `:mojo help repl` for further assistance.

1> print("Hello, world!")
Hello, world!

You can write as much code as you want in the REPL. You can press Enter to start a new line and continue writing code, and when you want Mojo to evaluate the code, press Enter twice. If there's something to print, Mojo prints it and then returns the prompt to you.

The REPL is primarily useful for short experiments because the code isn't saved. So when you want to write a real program, you need to write the code in a .mojo source file.

2. Run a Mojo file

Now let's write the code in a Mojo source file and run it with the mojo command:

  1. Create a file named hello.mojo (or hello.🔥) and add the following code:

    fn main():
    print("Hello, world!")

    That's all you need. Save the file and return to your terminal.

  2. Now run it with the mojo command:

    mojo hello.mojo

    It should immediately print the message:

    Hello, world!

If this didn't work for you, double-check your code looks exactly like the code in step 1, and make sure you correctly installed Mojo.

3. Build an executable binary

Finally, let's build and run that same code as an executable:

  1. Create an executable file with the build command:

    mojo build hello.mojo

    The executable file uses the same name as the .mojo file, but you can change that with the -o option.

  2. Then run the executable:


This creates a statically compiled binary file, so it contains all the code and libraries it needs to run.

Next steps

  • If you're new to Mojo, we suggest you continue to the next section about language basics.

  • If you want to experiment with some code, clone the Mojo repo to try our code examples:

    git clone

    In addition to several .mojo examples, the repo includes Jupyter notebooks that teach advanced Mojo features.

  • To see all the available Mojo APIs, check out the Mojo standard library reference.