golang exec.Command read std input - go

Question

I have a go program that should invoke a ruby script.
I have a runCommand function:
func runCommand(cmdName string, arg ...string) {
cmd := exec.Command(cmdName, arg...)
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
cmd.Stdin = os.Stdin
err = cmd.Run()
if err != nil {
fmt.Printf("Failed to start Ruby. %s\n", err.Error())
os.Exit(1)
}
}
I invoke it like this:
runCommand("ruby", "-e", "require 'foo'")
It works for most cases, except if there is a gets or any similar operation in the child process that needs to pause for an input.
I have tried setting cmd.Stdin = os.Stdin, but it does not wait for input.
What am I doing wrong?

Answer 1

The following program seems do what you ask for (my runCommand is almost identical to yours. I just changed the = to := for the err line.) Are you doing something differently?
package main
import (
"fmt"
"os"
"os/exec"
)
func main() {
runCommand("ruby", "-e", `puts "Running"; $in = gets; puts "You said #{$in}"`)
}
func runCommand(cmdName string, arg ...string) {
cmd := exec.Command(cmdName, arg...)
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
cmd.Stdin = os.Stdin
err := cmd.Run()
if err != nil {
fmt.Printf("Failed to start Ruby. %s\n", err.Error())
os.Exit(1)
}
}

Answer 2

You might need to use a pseudoterminal. You can do this in go with this library: github.com/kr/pty:
package main
import (
"bufio"
"io"
"log"
"os"
"os/exec"
"github.com/kr/pty"
)
func runCommand(cmdName string, arg ...string) {
cmd := exec.Command(cmdName, arg...)
tty, err := pty.Start(cmd)
if err != nil {
log.Fatalln(err)
}
defer tty.Close()
go func() {
scanner := bufio.NewScanner(tty)
for scanner.Scan() {
log.Println("[" + cmdName + "] " + scanner.Text())
}
}()
go func() {
io.Copy(tty, os.Stdin)
}()
err = cmd.Wait()
if err != nil {
log.Fatalln(err)
}
}
func main() {
log.SetFlags(0)
runCommand("ruby", "-e", `
puts "Enter some text"
text = gets
puts text
`)
}