How to Get Realtime Output from Shell Commands in Go?

How to Retrieve Realtime Output for Shell Commands in Go

Problem:

When executing a long-running shell command in Go using os/exec, how can you retrieve the real-time output and process it as it becomes available?

Solution:

1. Use stderr Instead of stdout:

   ffmpeg is known to output diagnostic messages to stderr instead of stdout. Therefore, connect to stderr when creating the command object instead of stdout.

2. Read Output from stderr Continuously:

   Create a bufio.Scanner for stderr and use the Scan method to read the output line by line or word by word.

3. Handle Output Line by Line:

   In the example code, a scanner.Split(bufio.ScanWords) is used to split the output on spaces. Adjust the split type based on your processing requirements.

4. Process Output:

   Manipulate the output in your desired manner, such as extracting and processing progress information.

Example Code:

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()
}

Output:

The code will continuously print the ffmpeg output messages, such as:

frame=  101 fps=0.0 q=28.0 size=      91kB time=00:00:04.13 bitrate= 181.2kbits/s

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