We explored how to create child processes in the previous chapter. Go makes it possible for you to easily check child exit codes, however, it's not straightforward because there is a field in the exec.Cmd struct that has an os.ProcessState attribute.

The os.ProcessState attribute has a Sys method that returns an interface. Its value is a syscall.WaitStatus struct in Unix, which makes it possible to access the exit code using the ExitCode method. This is demonstrated using the following code:

package main

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

func exitStatus(state *os.ProcessState) int {
    status, ok := state.Sys().(syscall.WaitStatus)
    if !ok {
        return -1
    }
    return status.ExitStatus()
}

func main() {
    cmd := exec.Command("ls", "__a__")
    if err := cmd.Run(); err != nil {
        if status := exitStatus(cmd.ProcessState); status == -1 {
            fmt.Println(err)
        } else {
            fmt.Println("Status:", status)
        }
    }
}

If the command variable cannot be accessed, then the error returned is exec.ExitError and this wraps the os.ProcessState attribute, as follows:

func processState(e error) *os.ProcessState {
    err, ok := e.(*exec.ExitError)
    if !ok {
        return nil
    }
    return err.ProcessState
}

We can see that obtaining the exit code is not straightforward and requires some typecasting.