Today I Learned: collection of notes, tips and tricks and stuff I learn from day to day working with computers and technology as an open source contributor and product manager

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Test Main Function

I picked up this practice from this marvelous blog post by Johannes Malsam.

The idea is to move the main code out of the main function and use the main function as a wrapper for something you can test.


package main
import (
func main() {
    // I'm ok with not testing this call
func realMain() int {
    name := flag.String("name", "", "Your Name")
    if *name == "" {
        fmt.Printf("Missing flag -name\n")
        return 1
    fmt.Printf("Hi %v\n", *name)
    return 0

In the above example, the realMain function wraps main.

realMain returns integers, which are then handles as exit codes.

The test file (main_test.go) could look as follows:

package main

import (

func TestFlags(T *testing.T) {
    // We manipuate the Args to set them up for the testcases
    // After this test we restore the initial args
    oldArgs := os.Args
    defer func() { os.Args = oldArgs }()

    cases := []struct {
        Name           string
        Args           []string
        ExpectedExit   int
        ExpectedOutput string
        {"flags set", []string{"-name", "johannes"}, 0, "Hi johannes\n"},
        {"flags not set", []string{"test"}, 1, "Missing flag -name\n"},

    for _, tc := range cases {
        // this call is required because otherwise flags panics,
        // if args are set between flag.Parse call
        flag.CommandLine = flag.NewFlagSet(tc.Name, flag.ExitOnError)
        // we need a value to set Args[0] to cause flag begins parsing at Args[1]
        os.Args = append([]string{tc.Name}, tc.Args...)
        actualExit := realMain()
        if tc.ExpectedExit != actualExit {
            T.Errorf("Wrong exit code for args: %v, expected: %v, got: %v",
                tc.Args, tc.ExpectedExit, actualExit)


I have used this for a few of my small projects for learning Go:

I have not yet cracked the nut of how to test reading from STDIN, so I am pondering using Bats

Resources and References