使用Xvfb，让需要X service的程序在后台跑

If you are using Selenium web driver with Python,you can use PyVirtualDisplay, a Python wrapper for Xvfb and Xephyr.

PyVirtualDisplay needs Xvfb as a dependency. On Ubuntu, first install Xvfb:

sudo apt-get install xvfb

then install PyVirtualDisplay from Pypi:

pip install pyvirtualdisplay

Sample Selenium script in Python in a headless mode with PyVirtualDisplay:

    #!/usr/bin/env python

    from pyvirtualdisplay import Display
    from selenium import webdriver

    display = Display(visible=0, size=(800, 600))
    display.start()

    # now Firefox will run in a virtual display. 
    # you will not see the browser.
    browser = webdriver.Firefox()
    browser.get('http://www.google.com')
    print browser.title
    browser.quit()

    display.stop()

EDIT The initial answer was posted in 2014 and now we are at the cusp of 2018.Like everything else, browsers have also advanced. Chrome has a completely headless version now which eliminates the need to use any third party libraries to hide the UI window. Sample code is as follows:

    from selenium import webdriver
    from selenium.webdriver.chrome.options import Options

    CHROME_PATH = '/usr/bin/google-chrome'
    CHROMEDRIVER_PATH = '/usr/bin/chromedriver'
    WINDOW_SIZE = "1920,1080"

    chrome_options = Options()  
    chrome_options.add_argument("--headless")  
    chrome_options.add_argument("--window-size=%s" % WINDOW_SIZE)
    chrome_options.binary_location = CHROME_PATH

    driver = webdriver.Chrome(executable_path=CHROMEDRIVER_PATH,
                              chrome_options=chrome_options
                             )  
    driver.get("https://www.google.com")
    driver.get_screenshot_as_file("capture.png")
    driver.close()

There are a few ways, but it isn't a simple "set a configuration value". Unless you invest in a headless browser, which doesn't suit everyone's requirements, it is a little bit of a hack:

How to hide Firefox window (Selenium WebDriver)?

and

扫描二维码关注公众号，回复： 10275667 查看本文章

Is it possible to hide the browser in Selenium RC?

You can 'supposedly', pass in some parameters into Chrome, specifically: --no-startup-window

Note that for some browsers, especially IE, it will hurt your tests to not have it run in focus.

You can also hack about a bit with AutoIT, to hide the window once it's opened.