Running external python in Golang, Catching continuous exec.Command Stdout

So my go script will call an external python like this

cmd = exec.Command("python","game.py")
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
go func(){
	err := cmd.Run()
	if err != nil{
	panic(err)
	}
}()

It runs my python script concurrently which is awesome.
But now the problem is, my python script will run infinitely and it will print out some information from time to time. I want to "catch" these Stdout and print them out on my golang terminal. How do I do it concurrently (without waiting my python script to exit)?

Use cmd.Start() and cmd.Wait() instead of cmd.Run().

https://golang.org/pkg/os/exec/#Cmd.Run

> Run starts the specified command and waits for it to complete.
>
> Start starts the specified command but does not wait for it to complete.
>
> Wait waits for the command to exit. It must have been started by Start.

And if you want to capture stdout/stderr concurrently, use cmd.StdoutPipe() / cmd.StderrPipe() and read it by bufio.NewScanner()

package main

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

func main() {
    cmd := exec.Command("python", "game.py")
    stdout, err := cmd.StdoutPipe()
    if err != nil {
        panic(err)
    }
    stderr, err := cmd.StderrPipe()
    if err != nil {
        panic(err)
    }
    err = cmd.Start()
    if err != nil {
        panic(err)
    }

    go copyOutput(stdout)
    go copyOutput(stderr)
    cmd.Wait()
}

func copyOutput(r io.Reader) {
    scanner := bufio.NewScanner(r)
    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }
}

The following is a sample python code for reproducing real-time output. The stdout may be buffered in Python. Explicit flush may be required.

import time
import sys

while True:
    print "Hello"
    sys.stdout.flush()
    time.sleep(1)

If you want to capture stdout and stderr concurrently, indeed you need to use cmd.StdoutPipe(), cmd.StderrPipe(), cmd.Start() and cmd.Wait().

But as mentioned in the documentation (https://pkg.go.dev/os/exec#Cmd.StdoutPipe):

> It is thus incorrect to call Wait before all reads from the pipe have completed.

You'll need to wait for you goroutines to finish before calling cmd.Wait() (see the following thread: https://github.com/golang/go/issues/38268)

You can use add the flag -u to turn off buffering,

cmd := exec.Command("python", "-u", "game.py")

https://docs.python.org/3/using/cmdline.html#cmdoption-u

https://bugs.python.org/issue526382