go-cmd-sample

This repo shows how to put your "main" package code into a "cmd" folder to make things easier to manage, when you have multiple main.go files.
This seems a common pattern with go projects, especially those with multiple entrypoints.

This is a bit unusual for me as a long time Java dev, as creating multiple Java classes, each with a 'main' method, does not cause any problems.
The difference is, when running Java you usually specify the entrypoint class, whereas when running a binary built with Go you do not - the entrypoint is fixed at compile time.

Step 1 - when you have a single "cmd/main.go" and no other .go files

(see "step_1" branch for complete code)

1.1. We place our "main.go" into the "cmd" directory.
See the "step_1" branch for the code

1.2. Use this command to build the binary:

go build -o go-cmd-sample-app ./...

Notice the triple dot, which is a bit unusual. In Go, "x/..." means "every path under x/"

1.3. Run your app with:

./go-cmd-sample-app

References:

• go build: here

Step 2 - you then want more packages to add more code, and call those packages from your "main.go"

(see "step_2" branch for complete code)

2.1. We now introduce a new file "useful.go" in the root repo dir and use it from our "main.go"

2.2. We modified main.go to reference package "useful" and call a function from it, and print the result.
The new lines in main.go look like this:

usefulResult := useful.MyUsefulFunction()
fmt.Printf("usefulResult: %s\n", usefulResult)

Note: The above code uses the word "useful" to refer to the useful package, not the name of the useful.go file.

Note that main.go also needs an extra import line near the top:

import (
	"fmt"
	useful "go-cmd-sample"
)

2.3. Try to build using the same command as in Step 1

go build -o go-cmd-sample-app ./...

You'll see an error like this:

go: cannot write multiple packages to non-directory go-cmd-sample-app

This seems to be a quirk of Go when using a wildcard, so instead we'll use a specific name on the build command, as follows:

go build -o go-cmd-sample-app cmd/main.go

2.4. Run your app with:

./go-cmd-sample-app

You'll see the output, including the string returned from the call out to the function in the 'useful' package.