How do you create a daemon in Python?


Question

Searching on Google reveals x2 code snippets. The first result is to this code recipe which has a lot of documentation and explanation, along with some useful discussion underneath.

However, another code sample, whilst not containing so much documentation, includes sample code for passing commands such as start, stop and restart. It also creates a PID file which can be handy for checking if the daemon is already running etc.

These samples both explain how to create the daemon. Are there any additional things that need to be considered? Is one sample better than the other, and why?

1
234
2/6/2019 2:31:52 PM

Accepted Answer

Current solution

A reference implementation of PEP 3143 (Standard daemon process library) is now available as python-daemon.

Historical answer

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.

155
4/20/2018 8:38:19 AM

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 chroot gaol

  • set UID, GID, working directory, umask, and other process parameters appropriately for the use case

  • relinquish elevated suid, sgid privileges

  • close all open file descriptors, with exclusions depending on the use case

  • behave correctly if started inside an already-detached context, such as init, inetd, etc.

  • set up signal handlers for sensible daemon behaviour, but also with specific handlers determined by the use case

  • redirect the standard streams stdin, stdout, stderr since 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.


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