Reading Stdout from a subprocess

Question

I am attempting to spawn a subprocess from Golang. The goal is to read and process the input line-by-line. Here is what I am trying to get working:

func readStuff(scanner *bufio.Scanner) {
    for scanner.Scan() {
        fmt.Println("Performed Scan")
        fmt.Println(scanner.Text())
    }
    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "reading standard input:", err)
    }
}

func main() {
    cmd := exec.Command("/usr/local/bin/pocketsphinx_continuous", "-inmic", "yes")
    out, err := cmd.StdoutPipe()

    err = cmd.Start()
    checkError(err)

    scanner := bufio.NewScanner(out)
    fmt.Println("Scanner created")

    defer cmd.Wait()
    go readStuff(scanner)
}

In this example, "Scanner created" is printed, but nothing happens after that.

Running this command however does result in what I am expecting to be printed to :

/usr/local/bin/pocketsphinx_continuous -inmic yes 1>out.txt

And modifying the code to directly copy to stdout works as well:

cmd := exec.Command("/usr/local/bin/pocketsphinx_continuous", "-inmic", "yes")
cmd.Stdout = os.Stdout

What am I missing that is keeping me from reading the output?

Answer 1

This seems to work fine, and it works with go readStuff(scanner) and also with just readStuff(scanner) - I don't think that this usage actually calls for a goroutine, but don't know the actual context.:

package main

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

func readStuff(scanner *bufio.Scanner) {
    for scanner.Scan() {
        fmt.Println("Performed Scan")
        fmt.Println(scanner.Text())
    }
    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "reading standard input:", err)
    }
}

func main() {
    cmd := exec.Command("/Users/rfay/bin/junk.sh")
    out, err := cmd.StdoutPipe()

    err = cmd.Start()
    if err != nil {
        fmt.Fprintf(os.Stderr, "Failed to start err=%v", err)
        os.Exit(1)
    }

    scanner := bufio.NewScanner(out)
    fmt.Println("Scanner created")

    defer cmd.Wait()

    go readStuff(scanner)
}

This is the junk.sh I used for testing.

#!/bin/bash
for i in `seq 1 10`;
do
  echo $i
  sleep 1
done    

Answer 2

Seems you need not

go readStuff(scanner)

because of

cmd.Start()

do system fork itself so just

readStuff(scanner)

would be enough to my mind (not spawning gorouting for that)