CODEWITHSUNDEEP
linuxprocessgo
Khash
Khash

Reputation: 2570

Start a process in Go and detach from it

I need to start a new process in Go with the following requirements:

Here is an attempt:

var attr = os.ProcAttr {
Dir: "/bin",
Env: os.Environ(),
Files: []*os.File{
    os.Stdin,
    "stdout.log",
    "stderr.log",
  },
}
process, err := os.StartProcess("sleep", []string{"1"}, &attr)

This works fine but has the following shortcomings from the requirements:

This needs to run on Linux only if that simplifies things.

Upvotes: 21

Views: 27212

Answers (2)

jhaynie
jhaynie

Reputation: 2368

What I have found that seems to work cross-platform is to re-run the program with a special flag. In your main program, check for this flag. If present on startup, you're in the "fork". If not present, re-run the command with the flag.

func rerunDetached() error {
    cwd, err := os.Getwd()
    if err != nil {
       return err
    }
    args := append(os.Args, "--detached")
    cmd := exec.Command(args[0], args[1:]...)
    cmd.Dir = cwd
    err = cmd.Start()
    if err != nil {
       return err
    }
    cmd.Process.Release()
    return nil
}

This will simply re-run your process with the exact parameters and append --detached to the arguments. When your program starts, check for the --detached flag to know if you need to call rerunDetached or not. This is sort of like a poor mans fork() which will work across different OS.

Upvotes: 2

bieffe62
bieffe62

Reputation: 474

  1. You can use process.Release to detach the child process from the parent one and make it survive after parent death
  2. Look at the definition of *os.ProcAttr.Sys.Credentials attribute : it looks like using the attribute you can set process user and group ID.

Here is a working version of your example (I did not check if process ID's where actually the one set )

package main

import "fmt"
import "os"
import "syscall"

const (
    UID = 501
    GUID = 100
    )


func main() {
    // The Credential fields are used to set UID, GID and attitional GIDS of the process
    // You need to run the program as  root to do this
        var cred =  &syscall.Credential{ UID, GUID, []uint32{} }
    // the Noctty flag is used to detach the process from parent tty
    var sysproc = &syscall.SysProcAttr{  Credential:cred, Noctty:true }
    var attr = os.ProcAttr{
        Dir: ".",
        Env: os.Environ(),
        Files: []*os.File{
            os.Stdin,
            nil,
            nil,
        },
            Sys:sysproc,

    }
    process, err := os.StartProcess("/bin/sleep", []string{"/bin/sleep", "100"}, &attr)
    if err == nil {

        // It is not clear from docs, but Realease actually detaches the process
        err = process.Release();
        if err != nil {
            fmt.Println(err.Error())
        }

    } else {
        fmt.Println(err.Error())
    }
}

Upvotes: 28

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