How do you create a daemon in Python?
A reference implementation of PEP 3143 (Standard daemon process library) is now available as python-daemon.
Sander Marechal's code sample is superior to the original, which was originally posted in 2004. I once contributed a daemonizer for Pyro, but would probably use Sander's code if I had to do it over.
There are many fiddly things to take care of when becoming a well-behaved daemon process:
prevent core dumps (many daemons run as root, and core dumps can contain sensitive information)
behave correctly inside a
set UID, GID, working directory, umask, and other process parameters appropriately for the use case
close all open file descriptors, with exclusions depending on the use case
behave correctly if started inside an already-detached context, such as
set up signal handlers for sensible daemon behaviour, but also with specific handlers determined by the use case
redirect the standard streams
stderrsince a daemon process no longer has a controlling terminal
handle a PID file as a cooperative advisory lock, which is a whole can of worms in itself with many contradictory but valid ways to behave
allow proper cleanup when the process is terminated
actually become a daemon process without leading to zombies
Some of these are standard, as described in canonical Unix literature (Advanced Programming in the UNIX Environment, by the late W. Richard Stevens, Addison-Wesley, 1992). Others, such as stream redirection and PID file handling, are conventional behaviour most daemon users would expect but that are less standardised.
All of these are covered by the PEP 3143 “Standard daemon process library” specification. The python-daemon reference implementation works on Python 2.7 or later, and Python 3.2 or later.
Here's my basic 'Howdy World' Python daemon that I start with, when I'm developing a new daemon application.
#!/usr/bin/pythonimport timefrom daemon import runnerclass App(): def __init__(self): self.stdin_path = '/dev/null' self.stdout_path = '/dev/tty' self.stderr_path = '/dev/tty' self.pidfile_path = '/tmp/foo.pid' self.pidfile_timeout = 5 def run(self): while True: print("Howdy! Gig'em! Whoop!") time.sleep(10)app = App()daemon_runner = runner.DaemonRunner(app)daemon_runner.do_action()
Note that you'll need the
python-daemon library. You can install it by:
pip install python-daemon
Then just start it with
./howdy.py start, and stop it with