How to Test Go Applications That Read from Stdin: A Guide with Mock Readers and Writers

Testing Go Applications that Write to Stdin

This guide demonstrates how to write Go test cases that interact with stdin-reading applications. Consider the example application below:

<code class="go">package main

import (
    "bufio"
    "fmt"
    "io"
    "os"
)

func main() {
    reader := bufio.NewReader(os.Stdin)
    for {
        fmt.Print("> ")
        bytes, _, err := reader.ReadLine()
        if err == io.EOF {
            os.Exit(0)
        }
        fmt.Println(string(bytes))
    }
}</code>

Creating a Test Case

To test this application's stdin functionality, we define a separate function that reads from an io.Reader and writes to an io.Writer:

<code class="go">func testFunction(input io.Reader, output io.Writer) {
    // Your test logic here...
}</code>

Modifying the main function

In the main function, we call the testFunction with stdin and stdout as arguments:

<code class="go">func main() {
    testFunction(os.Stdin, os.Stdout)
}</code>

Writing the Test Case

In our test case, we can now directly test the testFunction using a mock io.Reader and io.Writer:

<code class="go">func TestInput(t *testing.T) {
    input := "abc\n"
    output := &bytes.Buffer{}

    inputReader := bytes.NewReader([]byte(input))
    testFunction(inputReader, output)

    if got := output.String(); got != input {
        t.Errorf("Wanted: %v, Got: %v", input, got)
    }
}</code>

By using this approach, you can effectively test applications that write to stdin, isolating the testing logic from the intricacies of stdin and stdout management in the main function.

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