Access StdIn of Exec'd process with Golang

dummyprog.go

package main

import (
    "fmt"
)

func main() {
    fmt.Println("Hello, What's your favorite number?")
    var i int
    fmt.Scanf("%d\n", &i)
    fmt.Println("Ah I like ", i, " too.")
}

wrapperprog.go

package main

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

func main() {
    subProcess := exec.Command("go", "run", "./helper/main.go") //Just for testing, replace with your subProcess

    stdin, err := subProcess.StdinPipe()
    if err != nil {
        fmt.Println(err) //replace with logger, or anything you want
    }
    defer stdin.Close() // the doc says subProcess.Wait will close it, but I'm not sure, so I kept this line

    subProcess.Stdout = os.Stdout
    subProcess.Stderr = os.Stderr

    fmt.Println("START") //for debug
    if err = subProcess.Start(); err != nil { //Use start, not run
        fmt.Println("An error occured: ", err) //replace with logger, or anything you want
    }

    io.WriteString(stdin, "4\n")
    subProcess.Wait()
    fmt.Println("END") //for debug
}

i find this simpler

func callCmd(ctx context.Context, data []byte) ([]byte, error) {
    cmd := exec.CommandContext(ctx, "cmd", "params")
    cmd.Stdin = bytes.NewReader(data)

    // CombinedOutput will wait, then return stdout+stderr in one buffer
    out, err := cmd.CombinedOutput()
    if err != nil {
        return nil, fmt.Errorf("Failed: %w; output: %s", err, string(out))
    }
    return out, nil
}

You can pass data into stdin and retrieve stdout