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
}

for some subprocess you need to close stdin to indicate all data has been send.

for example if you use ubuntu base64 tool in line 11 and you want to pipe some data to it you need to change line 27-29 like this:

...
 subProcess := exec.Command("/usr/bin/base64")
 ...
 io.WriteString(stdin,`multiline
 text
 file
 content`)
 stdin.Close() // <--- indicate that we piped all data we want!
 subProcess.Wait()
 fmt.Println("END") 

this will behave exactly like cat myfile.txt | base64 with the given file content.