This is not a direct answer to the OP's question and I'm in general agreement with prior answers and comments urging that main should be mostly a caller of packaged functions. That being said, here's an approach I'm finding useful for testing executables. It makes use of log.Fataln and exec.Command.
- Write
main.go with a deferred function that calls log.Fatalln() to write a message to stderr before returning.
- In
main_test.go, use exec.Command(...) and cmd.CombinedOutput() to run your program with arguments chosen to test for some expected outcome.
For example:
func main() {
// Ensure we exit with an error code and log message
// when needed after deferred cleanups have run.
// Credit: https://medium.com/@matryer/golang-advent-calendar-day-three-fatally-exiting-a-command-line-tool-with-grace-874befeb64a4
var err error
defer func() {
if err != nil {
log.Fatalln(err)
}
}()
// Initialize and do stuff
// check for errors in the usual way
err = somefunc()
if err != nil {
err = fmt.Errorf("somefunc failed : %v", err)
return
}
// do more stuff ...
}
In main_test.go,a test for, say, bad arguments that should cause somefunc to fail could look like:
func TestBadArgs(t *testing.T) {
var err error
cmd := exec.Command(yourprogname, "some", "bad", "args")
out, err := cmd.CombinedOutput()
sout := string(out) // because out is []byte
if err != nil && !strings.Contains(sout, "somefunc failed") {
fmt.Println(sout) // so we can see the full output
t.Errorf("%v", err)
}
}
Note that err from CombinedOutput() is the non-zero exit code from log.Fatalln's under-the-hood call to os.Exit(1). That's why we need to use out to extract the error message from somefunc.
The exec package also provides cmd.Run and cmd.Output. These may be more appropriate than cmd.CombinedOutput for some tests. I also find it useful to have a TestMain(m *testing.M) function that does setup and cleanup before and after running the tests.
func TestMain(m *testing.M) {
// call flag.Parse() here if TestMain uses flags
os.Mkdir("test", 0777) // set up a temporary dir for generate files
// Create whatever testfiles are needed in test/
// Run all tests and clean up
exitcode := m.Run()
os.RemoveAll("test") // remove the directory and its contents.
os.Exit(exitcode)
main.gobegins with a lowercase "f" making it private, so I don't think your test can call it directly, as it would not have access. – Thoughtful