How do I start and stop a Linux program using the subprocess module in Python?

Question

I’m writing a web app that uses Selenium to screen-scrape another website. This screen-scraping only happens once a day, so I’d rather not leave Selenium and Xvfb running all the time.

I’m trying to figure out how to start Xvfb and Selenium from Python, and then stop them once the screen-scraping’s done.

If I was doing it manually, I’d start them at the command line, and hit CTRL C to stop them. I’m trying to do the same thing from Python.

I seem to be able to successfully start Xvfb like this:

xvfb = Popen('Xvfb :99 -nolisten tcp', shell=True)

But when I’ve tried to terminate it:

xvfb.terminate()

and then tried to start it again (by repeating my initial command), it tells me it’s already running.

Answer 1

I don't know why you want to run Xvfb as root. Your usual X server only needs to run as root (on many but not all unices) only so that it can access the video hardware; that's not an issue for Xvfb by definition.

tempdir = tempfile.mkdtemp()
xvfb = subprocess.Popen(['Xvfb', ':99', '-nolisten', 'tcp', '-fbdir', tempdir])

When you terminate the X server, you may see a zombie process. This is in fact not a process (it's dead), just an entry in the process table that goes away when the parent process either reads the child's exit status or itself dies. Zombies are mostly harmless, but it's cleaner to call wait to read the exit status.

xvfb.terminate()
# At this point, `ps -C Xvfb` may still show a running process
# (because signal delivery is asynchronous) or a zombie.
xvfb.wait()
# Now the child is dead and reaped (assuming it didn't catch SIGTERM).