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

Question

I am trying to call shell command with os/exec in golang, that command will take some time, so I would like to retrieve the reatime output and print the processed output (a progressing ratio number).

package main

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

func main() {
    cmdName := "ffmpeg -i t.webm  -acodec aac -vcodec libx264  cmd1.mp4"
    cmdArgs := strings.Fields(cmdName)

    cmd := exec.Command(cmdArgs[0], cmdArgs[1:len(cmdArgs)]...)
    stdout, _ := cmd.StdoutPipe()
    cmd.Start()
    go print(stdout)
    cmd.Wait()
}

// to print the processed information when stdout gets a new line
func print(stdout io.ReadCloser) {
    r := bufio.NewReader(stdout)
    line, _, err := r.ReadLine()
    fmt.Println("line: %s err %s", line, err)
}

I want to have a function where can update the screen when the command print something,

The ffmpeg command output is as follows:

frame=  101 fps=0.0 q=28.0 size=      91kB time=00:00:04.13 bitrate= 181.2kbits/
frame=  169 fps=168 q=28.0 size=     227kB time=00:00:06.82 bitrate= 272.6kbits/
frame=  231 fps=153 q=28.0 size=     348kB time=00:00:09.31 bitrate= 306.3kbits/
frame=  282 fps=140 q=28.0 size=     499kB time=00:00:11.33 bitrate= 360.8kbits/

in fact, the above 4 line is the last line of ffmpeg command output which keeps changing, I want to print that change out, like

18%
44%
69%
100%

how could I achieve this?

Answer 1

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

Answer 2

I do find icza's solution that he mentioned in that post is quite useful, however it didn't't solve my problem.

I did a little test as following:

1, I write a script which print some info every second for ten times, here is the script.sh

#!/bin/bash

for i in {1..10}
do
    echo "step " $i
    sleep 1s
done

2, read the stdout and extract the needed information from stdout and do some process to get the expected format, here is the code: package main

import (
    "fmt"
    "os/exec"
    "regexp"
    "strconv"
    "strings"
)

func getRatio(text string) float32 {
    re1, _ := regexp.Compile(`step[\s]+(\d+)`)
    result := re1.FindStringSubmatch(text)
    val, _ := strconv.Atoi(result[1])
    return float32(val) / 10
}

func main() {
    cmdName := "ffmpeg -i t.webm  -acodec aac -vcodec libx264  cmd1.mp4"
    //cmdName := "bash ./script.sh"
    cmdArgs := strings.Fields(cmdName)

    cmd := exec.Command(cmdArgs[0], cmdArgs[1:len(cmdArgs)]...)
    stdout, _ := cmd.StdoutPipe()
    cmd.Start()

    oneByte := make([]byte, 10)
    for {
        _, err := stdout.Read(oneByte)
        if err != nil {
            break
        }
        progressingRatio := getRatio(string(oneByte))
        fmt.Printf("progressing  ratio %v \n", progressingRatio)
    }
}

This does work for my script.sh test, but for the ffmpeg command it doesn't work, in ffmpeg's case, nothing get printed and the process get finished (not stuck), I guess the way of writing data to stdout for ffmpeg is a little special (maybe no newline character at all, and I tried icza's solution, but it still doesn't work).