Multiplexed writer in exec.Cmd

multiplexed_writer_cmd.go

package main

import (
    "bytes"
    "fmt"
    "log"
    "io"
    "os/exec"
)

func main() {
    command := `
echo stdout.
echo stdout..

>&2 echo stderr.
>&2 echo stderr..

echo stdout.
echo stdout..

>&2 echo stderr.
>&2 echo stderr..

echo stdout.
echo stdout..

>&2 echo stderr.
>&2 echo stderr..

sleep 1
`

    // Create the writer which will be used in stderr and stdout
    combinedWriter := &bytes.Buffer{}

    // Create multiplexed writer
    stdout := MultiplexedWriter{[]io.Writer{combinedWriter}}
    stderr := MultiplexedWriter{[]io.Writer{combinedWriter}}

    // Execute the command
    cmd := exec.Command("/bin/sh", "-c", command)
    cmd.Stdout = stdout
    cmd.Stderr = stderr

    if err := cmd.Run(); err != nil {
        log.Fatal(err)
    }

    fmt.Printf(combinedWriter.String())
}

// MultiplexedWriter is used to write into multiple writers,
type MultiplexedWriter struct {
    writers []io.Writer
}

func (w MultiplexedWriter) Write(p []byte) (n int, err error){
    for _, o := range w.writers {
        n, err = o.Write(p)
        if err != nil {
            return 0, fmt.Errorf("Error occured %s", err.Error())
        }
    }

    return n, nil
}

// Result
// stderr.
// stderr..
// stderr.
// stderr..
// stderr.
// stderr..
// stdout.
// stdout..
// stdout.
// stdout..
// stdout.
// stdout..