Your First Program

Scope: starting a Go module, writing a program, running it two ways, and reading the files involved. All commands below were run and verified.

Make a project

mkdir hello
cd hello
go mod init hello

Check: go mod init prints go: creating new go.mod: module hello and creates a file called go.mod:

module hello

go 1.25

go.mod marks this folder as a Go project (a module). The module name (hello here) is just a label for now; for published libraries it becomes the import path.

Write the program

Create main.go:

package main

import "fmt"

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

Three things to notice:

package main — every runnable program starts with this line. It says "this file belongs to the main package," which is Go's word for "this builds into a program, not a library."
import "fmt" — fmt is the formatting/printing library, part of Go's standard library. Unused imports are a compile error in Go — the compiler makes you clean up after yourself.
func main() — where the program starts. Capital Println because exported (public) names start uppercase — more on that in Functions.

Run it

go run .

Check: prints Hello, world!

The . means "the package in this folder." go run compiles to a temporary file, runs it, and throws it away — perfect while developing.

Build it

go build .

Check: a file named hello (or hello.exe on Windows) appears in the folder. Run it directly:

./hello        # Mac/Linux
.\hello.exe    # Windows PowerShell

That's the single binary Go is known for: one file, no interpreter, no dependencies to install alongside it. Copy it to another machine with the same OS and it just runs.

Change it and re-run

package main

import "fmt"

func main() {
    fmt.Println("Hello, world!")
    fmt.Println("Go builds fast.")
}

Run go run . again.

Check: both lines print. Notice the speed — Go compiles small programs in well under a second. That fast cycle is a deliberate feature, not an accident.

gofmt: the formatter

Go has one official code style, enforced by a tool:

gofmt -l .

If it prints filenames, those files aren't formatted. Fix them:

gofmt -w .

Most editors run this on save automatically. Don't argue with the formatter — the entire Go world looks the same, which means any Go code you read feels familiar.

Check

[ ] go mod init, go run ., and go build . all work
[ ] You can say what go.mod is for
[ ] You can explain why import "fmt" is there and what happens if you don't use it

Next: Variables and Types.