Go by Example: Spawning Processes

Sometimes our Go programs need to spawn other, non-Go processes. For example, the syntax highlighting on this site is implemented by spawning a `pygmentize` process from a Go program. Let’s look at a few examples of spawning processes from Go.
	package main
	import "fmt" import "io/ioutil" import "os/exec"
	func main() {
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 than 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 ommited error checks in the above example, but you could use the usual `if err != nil` pattern for all of them. We also only collect the `StdoutPipe` results, but you 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 you want to spawn a full command with a string, you 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)) }

The spawned programs return output that is the same as if we had run them directly from the command-line.

$ go run spawning-processes.go 
> date
Wed Oct 10 09:53:11 PDT 2012

> grep hello
hello grep

> ls -a -l -h
drwxr-xr-x  4 mark 136B Oct 3 16:29 .
drwxr-xr-x 91 mark 3.0K Oct 3 12:50 ..
-rw-r--r--  1 mark 1.3K Oct 3 16:28 spawning-processes.go

Previous example: Environment Variables.

Next example: Exec'ing Processes.