pydaemonize is a library of utilities for writing system daemons in Python. It is a direct port of the Haskell library hdaemonize. Its source code is available from GitHub at http://github.com/madhadron/pydaemonize.
pydaemonize is released under GNU General Public License version 3, or any later version. A copy is included in the source distribution as LICENSE.txt.
pydaemonize is distributed via the Python Package Index (PyPI). To install it, run
$ pip install pydaemonize
This will always be up to date with the master branch on GitHub. You can get the latest development source from the dev branch.
Subclass the Daemon class in pydaemonize, and override the action method to implement your daemon's behavior. The simplest possible such daemon is
import pydaemonize
class NopDaemon(pydaemonize.Daemon):
def action(self):
while True:
pass
if __name__=='__main__':
NoDaemon()When you run this script, it goes through all the steps to become a daemon on the system, sets up some basic signal handling, and then loops forever until you send it SIGTERM or SIGKILL (hitting Ctrl-C won't work since it ignores SIGINT).
Note that when writing daemons, you can't use stdin, stdout, or
stderr. They are set to /dev/null. Any other file
descriptors are closed when you call serviced (or
daemonize). If you need to log data, use the syslog
module. It is initialized for you by serviced, so you can use
it by importing syslog in your code, then calling
syslog.syslog(syslog.LOG_NOTICE, "message to send")You can also use syslog.LOG_ERR if you are reporting errors. On
Linux, these messages go to /var/log/messages or
/var/log/system.log. On MacOS X, they go to
/var/log/system.log. You can specify options to be passed when
initializing syslog by passing a mask as argument syslogoptions when initializing a daemon.
To add signal handling, override the method onsignal. Generally, you should handle the signals you are interested in for a daemon, and pass on all the others, as in
import signal
class NopDaemon(pydaemonize.Daemon):
...
def onsignal(self, sig, stackframe):
if sig == signal.SIGHUP:
# handle SIGHUP
elif sig == signal.SIGTERM:
# handle SIGTERM
else:
pass # Ignore all other signalsNote that when you start messing with signals, you will probably have to call os._exit instead of the usual exit function to exit your script. exit dispatches to SIGTERM, but if you have added your own handler, you need to deal with actually exiting yourself. os._exit is guaranteed to exit no matter what.
You may also want your daemon to start as a superuser in order to
connect to various services, then drop privileges and run thereafter
as a normal user. The Daemon base class provides a method
dropprivileges to do exactly that. You should pass it a username
and a groupname (either of which may be None or omitted). If the
user exists, it will try to change to it. Otherwise it tries to change
to a user with the same name as the daemon, and failing that to a user
called daemon. Groups go through the same process. Warning:
pydaemonize uses the pwd and grp modules to look up users
and groups, and these modules depend on /etc/passwd and
/etc/groups, respectively. If your system's authentication doesn't
go through these files for the users your daemon will use,
pydaemonize will fail.
The __init__ method of Daemon takes several options which may be useful:
detach: (default:True) Go through daemonization. For testing and debugging purposes you may want to have the daemon fully attached, in which case initialize the object withdetach=False.name: (default: the name of the script) The name of the daemon, used to provide a default user and group to try dropping privileges to, and as the label for messages in syslog.pidfilepath: (default:/var/run) The path to write a PID file to. If there is already a copy of the daemon running, it will refuse to start a new one. You can passpidfilepath=Noneto omit this step entirely.syslogoptions: (default: 0) A binary OR'd list of flags to syslog.