exec.go

package main

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

type Data struct {
	output []byte
	error  error
}

func runCommand(ch chan<- Data) {
	cmd := exec.Command("ls", "-la")
	data, err := cmd.CombinedOutput()
	ch <- Data{
		error:  err,
		output: data,
	}
}

func main() {
	c := make(chan Data)

	// This will work in background
	go runCommand(c)

	// Do other things here

	// When everything is done, you can check your background process result
	res := <-c
	if res.error != nil {
		fmt.Println("Failed to execute command: ", res.error)
	} else {
		// You will be here, runCommand has finish successfuly
		f, err := os.Create("output.txt")
		if err != nil {
			log.Fatal(err)
		}

		_, err = f.Write(res.output)
		if err != nil {
			log.Fatal(err)
		}
	}
}