Add new CountingWriter struct

internal/command/readwriter/readwriter.go

 package readwriter
 
-import "io"
+import (
+	"io"
+)
 
 type ReadWriter struct {
 	Out    io.Writer
 	In     io.Reader
 	ErrOut io.Writer
 }
+
+// CountingWriter wraps an io.Writer and counts all the writes. Accessing
+// the count N is not thread-safe.
+type CountingWriter struct {
+	W io.Writer
+	N int64
+}
+
+func (cw *CountingWriter) Write(p []byte) (int, error) {
+	n, err := cw.W.Write(p)
+	cw.N += int64(n)
+	return n, err
+}

internal/command/readwriter/readwriter_test.go

+package readwriter
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCountingWriter_Write(t *testing.T) {
+	testString := []byte("test string")
+	buffer := &bytes.Buffer{}
+
+	cw := &CountingWriter{
+		W: buffer,
+	}
+
+	n, err := cw.Write(testString)
+
+	require.NoError(t, err)
+	require.Equal(t, 11, n)
+	require.Equal(t, int64(11), cw.N)
+
+	cw.Write(testString)
+	require.Equal(t, int64(22), cw.N)
+}