如何正确捕获所有标准输出/标准错误

Question

I want to capture and save to a file all the stdout and stderr

For testing purposes I am printing only what I capture using this:

package main

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

func main() {
    cmd := exec.Command("/tmp/stdout")
    stdout := new(bytes.Buffer)
    stderr := new(bytes.Buffer)

    cmd.Stdout = stdout
    cmd.Stderr = stderr

    if err := cmd.Start(); err != nil {
        panic(err)
    }

    if err := cmd.Wait(); err != nil {
        panic(err)
    }

    in := bufio.NewScanner(io.MultiReader(stdout, stderr))
    for in.Scan() {
        fmt.Println(in.Text())
    }

}

The /tmp/stdout command can be build using this code:

package main

import (
    "fmt"
    "os"
    "time"
)

func main() {

    for i := 1; i < 1000; i++ {
        if i%3 == 0 {
            fmt.Fprintf(os.Stderr, "STDERR i: %d
", i)
        } else {
            fmt.Printf("STDOUT i: %d
", i)
        }
        time.Sleep(1 * time.Second)
    }

}

For some reason I am not been available to capture anything from the output, If I run the /tmp/stdout command I get this:

$ /tmp/stdout 
STDOUT i: 1
STDOUT i: 2
STDERR i: 3
STDOUT i: 4
STDOUT i: 5
STDERR i: 6
STDOUT i: 7

I was expecting to be available to get the same output while calling it from go with the previous code, the strange thing is that if I change to command to be something like id, whoami uname I do get the result and can printed, therefore wondering what could be wrong.

Any ideas ?

UPDATE

Found that I have to wait the program to finish, as suggested in the comments in order to get the output, but in case I would like to get the output in realtime how could I achieve this, what could be the best way of doing it, either io.Copy an os.Pipe, etc ?

Answer 1

but in case I would like to get the output in realtime

One way to do that is to attach os.Stdout to cmd.Stdout

oCmd := exec.Command(bin, cmdArgs...)
oCmd.Stdout = os.Stdout
oCmd.Stderr = os.Stderr

err := oCmd.Run()

Then you can use a File instead with f,_ := os.Create("file").

If you want to write it to both a file and to the terminal at same time, i suspect (I have not done that yet), that you need to use io.Mutiwriters

f, _ := os.Create("file")
cmd.Stdout = io.MultiWriter(os.Stdout, f)