In Go, every Go source file (even the main.go file) must declare a package at the top of the file. This is a requirement of the Go language. The package declaration indicates which package the file belongs to and helps organize the code.

• Create a main.go file

touch main.go

• Declare a package

package main

import "fmt"

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

• Run the program

go run main.go