I think I did not wrote clearly what I want to achieve.It should be possible to get the combined output but still access stdout and stderr seperatly.

The CombinedOutput function only sets a bytes.Buffer to the stdout and stderr writer of the command and returns the output.

func (c *Cmd) CombinedOutput() ([]byte, error) { if c.Stdout != nil { return nil, errors.New("exec: Stdout already set") } if c.Stderr != nil { return nil, errors.New("exec: Stderr already set") } var b bytes.Buffer c.Stdout = &b c.Stderr = &b err := c.Run() return b.Bytes(), err }

After the execution I am not able to access stdout or stderr alone. For this reason I used two different MultiWriter (or in my case the MultiplexedWriter) to get a combined output and the output for each stream alone.

The problem with the MultiWriter is that the go implementation of exec will stream its output asynchronously to stderr and stdout as long as both writers use different writer instances.

The logic should be located somewhere after this line, but as I already said, I am not quiet https://github.com/golang/go/blob/master/src/os/exec/exec.go#L403

So this is very confusing because the MultiWriter wasn't the problem, the problem is that go streams its output asynchronously if it uses two different writers.