How to Stream the Progress of a Long-Running Command's Output in Go?

Streaming Command Output Progress

Question:

How can I stream output from a long-running command, providing intermittent updates rather than a final result?

Answer:

Utilizing the bufio.NewScanner() function with bufio.ScanLines() allows you to read the output of a command line-by-line, streaming the results as they become available. Below is a modified example based on your provided code:

func main() {
    cmd := exec.Command("go", "run", "child_process.go")

    stdout, _ := cmd.StdoutPipe()
    cmd.Start()

    scanner := bufio.NewScanner(stdout)
    for scanner.Scan() {
        fmt.Println(scanner.Text())
        log.Printf(scanner.Text())
    }

    cmd.Wait()
}

For this to work effectively, the executed command should output the progress using fmt.Println() or log.Printf().

Possible Pitfalls:

• If the command does not print newlines, ScanLines() will only yield output once the command finishes.
• By default, standard output and error streams are discarded unless explicitly connected to another process. This may cause blocking issues, so reading both streams is recommended.

Alternative Solutions:

In cases where newline characters are not reliable, alternative methods can be employed:

• ScanRunes: Read by individual characters using bufio.ScanRunes().
• Manual Reading: Byte-wise or rune-wise reading without relying on bufio.Scanner.

Caution:

Reading byte-wise or rune-wise may introduce issues with multi-byte UTF-8 encoded characters. A buffer large enough to capture UTF-8 runes should be used.

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