How to Stream Command Output in Real-Time Using Go?

Streaming Command Output Online

Consider the following code that executes a command and captures its 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]
    // Wait to finish, get output:
    out, err := cmd.Output()
    if err != nil {
        log.Printf("Error running %s: %v\n", cp[1:], err)
        return
    }
    log.Println("Finished %s, output: %s", cp[1:], out)
}

This code captures the complete output of the command and prints it only after the command finishes execution. However, there is a need to stream the output in real-time.

To achieve live output streaming, we can modify the code as follows:

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:])
}

Here's an explanation of the changes:

• cmd.StdoutPipe() creates a pipe for reading the command's standard output.
• cmd.Start() starts the command in the background.
• bufio.NewScanner(stdout) creates a scanner for reading from the stdout pipe.
• scanner.Split(bufio.ScanRunes) sets the scanner to read by runes, providing live streaming.
• The scanner's Scan() loop continuously reads and prints each rune (character) produced by the command.
• After the loop finishes (the command writing to stdout has finished), scanner.Err() checks for any errors.
• cmd.Wait() waits for the command to finish and checks for errors.
• The final log.Println() statement logs the command's completion.

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