How to Stream Live Output from Executed Commands in Go?

Live Output of Executed Commands

This code snippet showcases a problem often encountered when running commands and retrieving their output in Go: the output becomes available only after the command has finished executing. To address this, a modified solution is provided that allows for live streaming of the command's output.

cmdParams := [][]string{
    {filepath.Join(dir, path), "npm", "install"},
    {filepath.Join(pdir, n.path), "gulp"},
}

for _, cp := range cmdParams {
    log.Printf("Starting %s in folder %s...", cp[1:], cp[0])
    cmd := exec.Command(cp[1], cp[2:]...)
    cmd.Dir = cp[0]

    stdout, err := cmd.StdoutPipe()
    if err != nil {
        log.Printf("%s cmd.StdoutPipe() error: %v\n", cp[1:], err)
        return
    }
    // Start command:
    if err = cmd.Start(); err != nil {
        log.Printf("%s start error: %v\n", cp[1:], err)
        return
    }

    // Stream command output:
    scanner := bufio.NewScanner(stdout)
    scanner.Split(bufio.ScanRunes)
    for scanner.Scan() {
        fmt.Print(scanner.Text())
    }
    if scanner.Err() != nil {
        log.Printf("Reading %s stdout error: %v\n", cp[1:], err)
        return
    }

    // Get execution success or failure:
    if err = cmd.Wait(); err != nil {
        log.Printf("Error running %s: %v\n", cp[1:], err)
        return
    }
    log.Printf("Finished %s", cp[1:])
}

Explanation:

• cmd.StdoutPipe() creates a pipe that allows us to read the command's standard output in real-time.
• bufio.NewScanner(stdout) creates a scanner that reads from the output pipe by runes (characters).
• scanner.Scan() reads and prints each character as it becomes available.
• The loop continues reading and printing characters until the command finishes or an error occurs.
• After the command finishes, the remaining output is printed, and the command's exit status is retrieved.

Additional Notes:

• This solution only streams standard output. Standard error can also be streamed using Command.StderrPipe().
• Some commands may not write everything to their standard output or error. In such cases, the output may not be streamed correctly.

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