golang exec错误行为

Question

I'm using following code segment to get the XML definition of a virtual machine running on XEN Hypervisor. The code is trying to execute the command virsh dumpxml Ubutnu14 which will give the XML of the VM named Ubuntu14

virshCmd := exec.Command("virsh", "dumpxml", "Ubuntu14")

var virshCmdOutput bytes.Buffer
var stderr bytes.Buffer
virshCmd.Stdout = &virshCmdOutput
virshCmd.Stderr = &stderr
err := virshCmd.Run()
if err != nil {
    fmt.Println(err)
    fmt.Println(stderr.String())
}

fmt.Println(virshCmdOutput.String())

This code always goes into the error condition for the given domain name and I get the following output.

exit status 1
error: failed to get domain 'Ubuntu14'
error: Domain not found: no domain with matching name 'Ubuntu14'

But if I run the standalone command virsh dumpxml Ubuntu14, I get the correct XML definition.

I would appreciate if someone could give me some hints on what I'm doing wrong. My host machine is Ubuntu-16.04 and golang version is go1.6.2 linux/amd64

Answer 1

I expect you are running virsh as a different user in these two scenarios, and since you don't provide any URI, it is connecting to a different libvirtd instance. If you run virsh as non-root, then it'll usually connect to qemu:///session, but if you run virsh as root, then it'll usually connect to qemu:///system. VMs registered against one URI, will not be visible when connecting to the other URI.

BTW, if you're using go, you'd be much better off using the native Go library bindings for libvirt instead of exec'ing virsh. Your "virsh dumpxml" invokation is pretty much equivalent to this:

   import (
      "github.com/libvirt/libvirt-go"
   )

   conn, err := libvirt.NewConnect("qemu:///system")

   dom, err := conn.LookupDomainByName("Ubuntu14")

   xml, err := dom.GetXMLDesc(0)

(obviously do error handling too)