Go file not running which is not in main package

Question

I have a very simple Go project setup. At root directory I have go.mod file and main.go and a folder called main2. Inside main2 folder there is main2.go file.

/
|_ go.mod
|_ main.go
|_ main2
   |_ main2.go

From root directory I am trying to run go run command

go run main2/main2.go

and it throws error:

package command-line-arguments is not a main package

Can someone help?

Answer 1

The package of your main2.go file must be main. When there is a main package, and a function main in your project, the compiler knows it will be compiled as a executable, and not as a library.

So try to change package command-line-arguments to package main inside the main2/main2.go file.

Answer 2

Imagine a golang executable as a house with only a front door and many different rooms inside it. You can go through any door you want once you are in the house, but to get inside you have to go through the front door first. That front door is the main() function.

Golang's entry point into an executable is through main(). If you want to run different logic paths for a single executable, you can use main() as a routing function to the other packages using command line arguments:

package main

import (
    "os"
    "otherpackage"
    // Your child packages get imported here.
)

func main() {

    // The first argument
    // is always program name
    // So os.Args[1] is the first dynamic argument
    arg1 := os.Args[1]

    // use arg1 to decide which packages to call
    if arg1 == "option1" {
        // option1 code executes here.
        otherpackage.DoThis()
    }
    if arg1 == "option2" {
        // option2 code executes here.
        otherpackage.DoThat()
    }
}

Then you can run your program with something like:

go run main.go option1

From golang documentation:

Program execution A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name main and declare a function main that takes no arguments and returns no value.