Is there a way to store stdout from a bash script into a variable within GO?

Question

I'm running a really simple bash script which just echo's some data within Go. I've placed this into a wrapper and used the exec package to execute this. This works nicely by outputting to my terminal, however, I can't find any way to actually store this into a variable in Go.

I'm new to Go, so my debugging skills aren't amazing. However, I've placed some basic logging outputs to try to narrow down where exactly I need to get the output from, but to no avail.

The two functions which run bash:

func main(){
    _, result,_ := runBash()
    log.Printf("Result: ", result)
}


func runBash()(bool, string, string){
    cmd := exec.Command("/bin/sh", "-s")
    cmd.Stdin = strings.NewReader(replicateRouter())
    return finishRunning(cmd)
}

func finishRunning(cmd *exec.Cmd) (bool, string, string) {
    log.Printf("Running: ")
    stdout, stderr := bytes.NewBuffer(nil), bytes.NewBuffer(nil)
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr
    done := make(chan struct{})
    defer close(done)
    go func() {
        for {
            select {
            case <-done:
                return
            case s := <-signals:
                cmd.Process.Signal(s)
            }
        }
    }()
    log.Printf("Flag 1")
    if err := cmd.Run(); err != nil {
        log.Printf("Error running %v", err)
        return false, string(stdout.Bytes()), string(stderr.Bytes())
    }
    log.Printf("Flag 2")
    return true, string(stdout.Bytes()), ""
}

This is the function to fake test my bash script:

func replicateRouter() string{
    return `echo <HOSTNAME> <IP> <MACADDRESS>`
}

The echo happens between flag 1 & 2 and at any point when I try to log any values from cmd/stdout I get empty strings. Within the main function, the result variable produces: 2020/06/19 18:17:14 Result: %!(EXTRA string=)

So I suppose my first question is why isn't result (which is in theory string(stdout.Bytes())) not producing the echo? & Secondly, where/how can I save the output to a variable?

Thanks & feel free to ping me if I've missed any questions &/or need more details

--Edit: Also forgot to mention, the code was heavily inspired by this Kubernetes go script. If there are any recommendations/criticism to doing it this way, I'd be really happy to hear/learn :)

Typo? Use stderr and stdout instead of os.Stderr and os.Stdin. — user5728991
– user5728991, Commented Jun 19, 2020 at 18:11
Yeah that's right. As soon as I removed os.Stderr/&Stdin and replaced them it worked straight away. I thought using os.stderr/in was how to actually print it to the terminal without using logging/fmt in Go. But we learn something new everyday :) — Tom Bradely
– Tom Bradely, Commented Jun 19, 2020 at 18:22

Ярослав Рахматуллин · Accepted Answer · 2020-06-19 17:42:23Z

1

Question 1:

stdout and stderr are never assigned to cmd in your code.

try this:

cmd.Stdout = stdout
cmd.Stderr = stderr

alternatively, try this simpler version:

  func main() {
      out, err := exec.Command("date").Output()
      if err != nil {
          log.Fatal(err)
      }
      fmt.Printf("The date is %s\n", out)
  }

Finally, you can "proxy" to os.Stdout

package main

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


func main() {
    stdout := new(bytes.Buffer)
    stderr := new(bytes.Buffer)
    _, _ = io.Copy(os.Stdout, stdout)
    _, _ = io.Copy(os.Stderr, stderr)

    cmd  := exec.Command("date")
    cmd.Stdout = stdout
    cmd.Stderr = stderr
    cmd.Run()



    output := string(stdout.Bytes())
    fmt.Printf("The date is %s\n", output)
}

answered Jun 19, 2020 at 17:42

Ярослав Рахматуллин

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1 Comment

Tom Bradely Over a year ago

Thanks so much, you were right I forgot to assign stdout & stderr anywhere in my code. Now that I did, I was able to place that succesfully into Result. I'll give a try the alternative method. Again thanks, this was killing me!

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