Golang example of an exec streaming the output

exec.go

package myexec

import (
	"bufio"
	"bytes"
	"context"
	"fmt"
	"io"
	"os"
	"os/exec"
	"sync"

	"github.com/sirupsen/logrus"
)

// CapturingPassThroughWriter is a writer that remembers
// data written to it and passes it to w
type CapturingPassThroughWriter struct {
	buf bytes.Buffer
	w   io.Writer
}

// NewCapturingPassThroughWriter creates new CapturingPassThroughWriter
func NewCapturingPassThroughWriter(w io.Writer) *CapturingPassThroughWriter {
	return &CapturingPassThroughWriter{
		w: w,
	}
}

func (w *CapturingPassThroughWriter) Write(d []byte) (int, error) {
	w.buf.Write(d)
	return w.w.Write(d)
}

// Bytes returns bytes written to the writer
func (w *CapturingPassThroughWriter) Bytes() []byte {
	return w.buf.Bytes()
}

// DoExec exec a command-line and send stdout to the writer `w`.
func DoExec(ctx context.Context, w io.Writer, command ...string) error {
	cmd := exec.CommandContext(ctx, command[0], command[1:]...)

	var errStdout, errStderr error
	stdoutIn, _ := cmd.StdoutPipe()
	stderrIn, _ := cmd.StderrPipe()
	//stdout := NewCapturingPassThroughWriter(os.Stdout)
	stdout := w
	stderr := NewCapturingPassThroughWriter(os.Stderr)
	err := cmd.Start()
	if err != nil {
		return fmt.Errorf("%w; cmd.Start() failed", err)
	}

	var wg sync.WaitGroup
	wg.Add(1)

	go func() {
		_, errStdout = io.Copy(stdout, stdoutIn)
		wg.Done()
	}()

	_, errStderr = io.Copy(stderr, stderrIn)
	wg.Wait()

	err = cmd.Wait()
	// NOTE: exec status != 0 sets err. The "diff" command succeeds with status=1 on a difference. We check that stderr has data before we return an error.
	if err != nil && stderr.buf.Len() > 0 {
		return fmt.Errorf("%w; cmd.Run() failed\n%s", err, stderr.buf.String())
	} else {
		err = nil
	}
	if errStdout != nil || errStderr != nil {
		err = fmt.Errorf("failed to capture stdout or stderr")
		if errStdout != nil {
			err = fmt.Errorf("%w; %s", errStdout, err.Error())
		}
		if errStderr != nil {
			err = fmt.Errorf("%w; %s", errStderr, err.Error())
		}
	}

	return err
}

func Example() {
	pr, pw := io.Pipe()
	go func() {
		if err := DoExec(context.Background(), pw, "diff", "/tmp/prev", "/tmp/cur"); err != nil {
			_ = pw.CloseWithError(err)
			logrus.WithError(err).Error("diff failed")
		} else {
			_ = pw.Close()
		}
	}()

	// print the output from the command
	sc := bufio.NewScanner(pr)
	for sc.Scan() {
		fmt.Println(sc.Text())
	}
}