New returns a new Interface which will os/exec to run commands.

A simple scripted Cmd type.

A simple scripted Interface type.

A simple fake ExitError type.

Cmd is an interface that presents an API that is very similar to Cmd from os/exec.

ExitError is an interface that presents an API similar to os.ProcessState, which is what ExitError from os/exec is.

Interface is an interface that presents a subset of the os/exec API.