Hello World in Go

Your First Program

Traditionally the first program you write in any programming language is called a “Hello World” program – a program that simply outputs Hello World to your terminal. Let's write one using Go.

First create a new folder where we can store our program. Create a folder named ~/src/go_example. (Where ~ means your home directory) From the terminal you can do this by entering the following commands:

mkdir src/go_example
touch src/go_example/main.go

Using your text editor type in the following in the main.go:

package main

import "fmt"

// this is a comment

func main() {
    fmt.Println("Hello World")
}

Open up a new terminal and type in the following:

cd src/go_example
go run main.go

You should see Hello World displayed in your terminal. The go run command takes the subsequent files (separated by spaces), compiles them into an executable saved in a temporary directory and then runs the program. 

How to Read a Go Program

Let's look at this program in more detail. Go programs are read top to bottom, left to right. (like a book) The first line says this:

package main

This is known as a “package declaration”. Every Go program must start with a package declaration. Packages are Go's way of organising and reusing code. There are two types of Go programs: executable and libraries. 

Then we see this:

import "fmt"

The import keyword is how we include code from other packages to use with our program. The fmt package (shorthand for format) implements formatting for input and output. 

After this you see a function declaration:

func main() {
    fmt.Println("Hello World")
}

Functions are the building blocks of a Go program. They have inputs, outputs and a series of steps called statements which are executed in order. All functions start with the keyword funcfollowed by the name of the function (main in this case), a list of zero or more “parameters” surrounded by parentheses, an optional return type and a “body” which is surrounded by curly braces. This function has no parameters, doesn't return anything and has only one statement. The name main is special because it's the function that gets called when you execute the program.

The final piece of our program is this line:

    fmt.Println("Hello World")

This statement is made of three components. First we access another function inside of the fmtpackage called Println (that's the fmt.Println piece, Println means Print Line). Then we create a new string that contains Hello World and invoke (also known as call or execute) that function with the string as the first and only argument.