62_spawning_processes.go

package gobyexample

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

// SpawningProcessesDemo - sometimes our Go programs need to spawn
// other non-Go processes.
func SpawningProcessesDemo() {
	// We'll start with a simple command that takes no arguments or input and
	// just prints something to stdout. The `exec.Command` helper creates an object
	// to represent this external process.
	dateCmd := exec.Command("date")

	// `.Output` is another helper that handles the common case of running a command,
	// waiting for it to finish, and collecting its output. If there were no errors,
	// `dateOut` will hold bytes with the date info.
	dateOut, err := dateCmd.Output()
	if err != nil {
		panic(err)
	}
	fmt.Println("> date")
	fmt.Println(string(dateOut))

	// Next we'll look at a slightly more involved case where we pipe data to the external
	// process on its stdin and collect the results from its stdout.
	grepCmd := exec.Command("grep", "hello")

	// Here we explicitly grab input/output pipes, start the process, write some input to it,
	// read the resulting output, and finally wait for the process to exit.
	grepIn, _ := grepCmd.StdinPipe()
	grepOut, _ := grepCmd.StdoutPipe()
	grepCmd.Start()
	grepIn.Write([]byte("hello grep\ngoodbye grep"))
	grepIn.Close()
	grepBytes, _ := ioutil.ReadAll(grepOut)
	grepCmd.Wait()

	// We omitted error checks in the above example, but we could use the usual `if err != nil`
	// pattern for all of them. We also only collect the StdoutPipe results, but we could collect
	// the StderrPipe in exactly the same way.
	fmt.Println("> grep hello")
	fmt.Println(string(grepBytes))

	// Note that when spawning commands we need to provide an explicitly delineated command and
	// argument array, vs being able to just pass in one command-line string. If we want to
	// spawn a full command with a string, we can use bash's -c option:
	lsCmd := exec.Command("bash", "-c", "ls -a -l -h")
	lsOut, err := lsCmd.Output()
	if err != nil {
		panic(err)
	}
	fmt.Println("> ls -a -l -h")
	fmt.Println(string(lsOut))
}