How to Test Go Code that Reads from Stdin and Writes to Stdout?

Writing Go Tests for Code that Writes to Stdin

To write a Go test for code that reads from stdin and echoes it back to stdout, it's advisable to isolate the functionality into a separate function that takes an io.Reader and an io.Writer as parameters.

Instead of performing all operations in the main function using stdin and stdout directly, your code should define a function for this specific purpose. For instance:

<code class="go">func echo(r io.Reader, w io.Writer) {
    reader := bufio.NewReader(r)
    for {
        fmt.Print("> ", w)
        bytes, _, err := reader.ReadLine()
        if err == io.EOF {
            os.Exit(0)
        }
        fmt.Println(string(bytes), w)
    }
}</code>

Within your main function, you can then call the echo function:

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

To test this function, you can create a test like this:

<code class="go">import (
    "bytes"
    "io"
    "testing"
)

func TestEcho(t *testing.T) {
    input := "abc\n"
    expected := "abc\n"

    r := bytes.NewBufferString(input)
    w := bytes.NewBufferString("")

    echo(r, w)

    if got := w.String(); got != expected {
        t.Errorf("Expected: %v, Got: %v", expected, got)
    }
}</code>

This test creates a buffered reader from the specified input string and a buffered writer to capture the output. It then calls the echo function, passing in the reader and writer. Finally, it compares the output with the expected result.

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