Execute PIPED command in behave acceptance test

sample.py

from behave import *
from os.path import join
from subprocess import Popen
import os

@when(u'the requirements are generated for documentation')
def step_impl(context):
    cmd = " ".join(["my_fictional_command",
                    "input1",
                    context.requirements_filename,
                    ">",
                    context.output_filename,
            ])
    print(cmd)
    p = Popen(cmd, shell=True)
    os.waitpid(p.pid, 0)