How to get the realtime output for a shell command in golang?

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Looks like ffmpeg sends all diagnostic messages (the “console output”)
to stderr instead of stdout. Below code works for me. 

package main

import (
    "bufio"
    "fmt"
    "os/exec"
    "strings"
)

func main() {
    args := "-i test.mp4 -acodec copy -vcodec copy -f flv rtmp://aaa/bbb"
    cmd := exec.Command("ffmpeg", strings.Split(args, " ")...)

    stderr, _ := cmd.StderrPipe()
    cmd.Start()

    scanner := bufio.NewScanner(stderr)
    scanner.Split(bufio.ScanWords)
    for scanner.Scan() {
        m := scanner.Text()
        fmt.Println(m)
    }
    cmd.Wait()
}

The version of ffmpeg is detailed as below. 

ffmpeg version 3.0.2 Copyright (c) 2000-2016 the FFmpeg developers
built with Apple LLVM version 7.3.0 (clang-703.0.29)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.0.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-vda
libavutil      55. 17.103 / 55. 17.103
libavcodec     57. 24.102 / 57. 24.102
libavformat    57. 25.100 / 57. 25.100
libavdevice    57.  0.101 / 57.  0.101
libavfilter     6. 31.100 /  6. 31.100
libavresample   3.  0.  0 /  3.  0.  0
libswscale      4.  0.100 /  4.  0.100
libswresample   2.  0.101 /  2.  0.101
libpostproc    54.  0.100 / 54.  0.100

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