Unit-testing in Go is simple and a pleasure. The minimum structure required to do unit-tests is scarcely more than that required to write any kind of code. In fact, most of the time it is so easy that you are almost, arguably, guaranteed to waste time doing any debugging at all before you have written unit-tests.

However, it may take a little more thought to test your executables. Even though you can still have a unit-testing source-file (“*_test.go”) and you can call your main() to do something, it’s non-trivial to capture your output and/or pass arguments:

• You might end-up using os.Pipe() to hook stdout/stderr and launching a goroutine to read from the other end, but you might have issues.
• Your test might call back into the execute in os.Args[0] (the tests run from a test-specific binary generated by the testing process), but this won’t accept the arbitrary command-line arguments required by your application.
• You might wrap a call to “go test” and try to pass “-args ” (“-args” is like “–” for tests, where all following arguments are passed verbatim), but I have had issues with this.

Naturally, you want to avoid having to kick-off a build of your application at the top of the tests in order to have something to test against.

You can use “go run” with exec.Command (in os/exec) to easily accomplish all of this while still avoiding a manual build. You can even provide it alternative io.Writer instances in order to capture stdout/stderr output.

Example:

package main

import (
    "testing"
    "os"
    "path"
    "bytes"
    "fmt"

    "os/exec"
)

var (
    assetsPath = ""
    appFilepath = ""
)

func TestMain(t *testing.T) {
    imageFilepath := path.Join(assetsPath, "NDM_8901.jpg")

    cmd := exec.Command(
            "go", "run", appFilepath,
            "-filepath", imageFilepath)

    b := new(bytes.Buffer)
    cmd.Stdout = b
    cmd.Stderr = b

    err := cmd.Run()
    actual := b.String()

    if err != nil {
        fmt.Printf(actual)
        panic(err)
    }

    expected := `IFD=[IfdIdentity] ID=(0x010f) NAME=[Make] COUNT=(6) TYPE=[ASCII] VALUE=[Canon]
IFD=[IfdIdentity] ID=(0x0110) NAME=[Model] COUNT=(22) TYPE=[ASCII] VALUE=[Canon EOS 5D Mark III]
IFD=[IfdIdentity] ID=(0x0112) NAME=[Orientation] COUNT=(1) TYPE=[SHORT] VALUE=[1]
IFD=[IfdIdentity] ID=(0x011a) NAME=[XResolution] COUNT=(1) TYPE=[RATIONAL] VALUE=[72/1]
IFD=[IfdIdentity] ID=(0x011b) NAME=[YResolution] COUNT=(1) TYPE=[RATIONAL] VALUE=[72/1]
IFD=[IfdIdentity] ID=(0x0128) NAME=[ResolutionUnit] COUNT=(1) TYPE=[SHORT] VALUE=[2]
...
IFD=[IfdIdentity] ID=(0x0128) NAME=[ResolutionUnit] COUNT=(1) TYPE=[SHORT] VALUE=[2]
IFD=[IfdIdentity] ID=(0x0201) NAME=[JPEGInterchangeFormat] COUNT=(1) TYPE=[LONG] VALUE=[11444]
IFD=[IfdIdentity] ID=(0x0202) NAME=[JPEGInterchangeFormatLength] COUNT=(1) TYPE=[LONG] VALUE=[21491]
`

    if actual != expected {
        t.Fatalf("Output not as expected:\n%s", actual)
    }
}

func init() {
    goPath := os.Getenv("GOPATH")

    assetsPath = path.Join(goPath, "src", "github.com", "dsoprea", "go-exif", "assets")
    appFilepath = path.Join(goPath, "src", "github.com", "dsoprea", "go-exif", "exif-read-tool", "main.go")
}