How to Correctly Test Argument Passing in Go?

Testing Argument Passing in Golang

Consider the following code:

package main

import (
    "flag"
    "fmt"
)

func main() {
    passArguments()
}

func passArguments() string {
    username := flag.String("user", "root", "Username for this server")
    flag.Parse()
    fmt.Printf("Your username is %q.", *username)

    usernameToString := *username
    return usernameToString
}

We pass an argument to the compiled code using the command:

./args -user=bla

This displays the following output:

Your username is "bla"

Testing Approach

To avoid manual compilation and execution, we aim to write a test that verifies argument passing. We create a test function:

func TestArgs(t *testing.T) {
    expected := "bla"
    os.Args = []string{"-user=bla"}

    actual := passArguments()

    if actual != expected {
        t.Errorf("Test failed, expected: '%s', got:  '%s'", expected, actual)
    }
}

Problem

When we run the test, we encounter an unexpected outcome:

Your username is "root".Your username is "root".--- FAIL: TestArgs (0.00s)
    args_test.go:15: Test failed, expected: 'bla', got:  'root'
FAIL
coverage: 87.5% of statements
FAIL    tool    0.008s

The argument "bla" is not passed to the function, resulting in "root" as the displayed username.

Solution

The issue lies in setting os.Args. The first argument in os.Args is the path to the executable itself. To fix this, we modify it as follows:

os.Args = []string{"cmd", "-user=bla"}

Additionally, it's good practice to save the original os.Args state and restore it after the test for other tests that rely on the correct arguments.

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