Hello, World and the go Command

One tool for everything

The go command builds, runs, tests and manages dependencies: go mod init example.com/app creates a module, go run . compiles and runs the package in the current directory, go build produces a binary, go test ./... runs tests, go vet finds suspicious code and gofmt formats it. A program starts in package main at func main(). Imports must be used, and unused variables are compile errors, which keeps code clean.

A first program, run

I ran this with Go 1.27.1 (go run . in a module named demo, standard library only). fmt.Printf formats with verbs such as %s for strings and %d for integers.

package main

import "fmt"

func main() {
	name := "Gopher"
	fmt.Printf("Hello, %s! You have %d new messages.\n", name, 3)
}

Output:

Hello, Gopher! You have 3 new messages.

Use go run while learning

go run . compiles and runs in one step; use go build when you need the binary.

Quick check: Where does a Go program start running?

• func main() in package main
• The first function in any file
• init.go
• A class with a static main method