Can't read from Pipe using exec.Command in Go

Question content

I am writing a go program that sends data to another program via stdin and reads the response via stdout.

This is a script that acts as an "echo server" of sorts:

import sys

if __name__=='__main__':
    for line in sys.stdin:
        print("Hello", line.strip())

When I try to communicate with the program in Go, it hangs on buf.ReadLine(). This is my Go code:

package main

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

func main() {
    cmd := exec.Command("python3", "app.py")
    stdout, _ := cmd.StdoutPipe()
    stdin, _ := cmd.StdinPipe()

    cmd.Start()

    stdin.Write([]byte("Bob\n"))

    buf := bufio.NewReader(stdout)
    buf.ReadLine()
    log.Println(buf)
}

Write() The function does not return an error. However, when I try ReadLine(), the program hangs. What did i do wrong?

Correct answer

This is not a problem with the Go code; your Python program is buffering output because its stdout is a pipe and not a terminal, so there is nothing wrong with Go readable, and you'll get a deadlock, with both processes waiting for input, and neither process producing any output.

See How to Flush the Print Function or Disable Output Buffering for ways to handle it in Python - the first applies to a single print statement, the second to the entire program . For your simple example they are all the same, but in other cases they may be different, so it's worth knowing the options.

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