Go の `exec.Command()` でコマンド出力を適切にパイプする方法は?

out, err := exec.Command("ps", "cax").Output() // Works and prints command output

out, err := exec.Command("ps", "cax | grep myapp").Output() // Fails

package main

import (
    "fmt"
    "os/exec"
)

func main() {
    grep := exec.Command("grep", "redis")
    ps := exec.Command("ps", "cax")

    // Connect ps's stdout to grep's stdin.
    pipe, _ := ps.StdoutPipe()
    defer pipe.Close()
    grep.Stdin = pipe

    // Start ps first.
    ps.Start()

    // Run and get the output of grep.
    res, _ := grep.Output()

    fmt.Println(string(res))
}