How to Make a Python Script Run Like a Service or Daemon in Linux

How to make a Python script run like a service or daemon in Linux

You have two options here.

1. Make a proper cron job that calls your script. Cron is a common name for a GNU/Linux daemon that periodically launches scripts according to a schedule you set. You add your script into a crontab or place a symlink to it into a special directory and the daemon handles the job of launching it in the background. You can read more at Wikipedia. There is a variety of different cron daemons, but your GNU/Linux system should have it already installed.

2. Use some kind of python approach (a library, for example) for your script to be able to daemonize itself. Yes, it will require a simple event loop (where your events are timer triggering, possibly, provided by sleep function).

I wouldn't recommend you to choose 2., because you would be, in fact, repeating cron functionality. The Linux system paradigm is to let multiple simple tools interact and solve your problems. Unless there are additional reasons why you should make a daemon (in addition to trigger periodically), choose the other approach.

Also, if you use daemonize with a loop and a crash happens, no one will check the mail after that (as pointed out by Ivan Nevostruev in comments to this answer). While if the script is added as a cron job, it will just trigger again.

How to make Python script run as service?

I use this code to daemonize my applications. It allows you start/stop/restart the script using the following commands.

python myscript.py start
python myscript.py stop
python myscript.py restart

In addition to this I also have an init.d script for controlling my service. This allows you to automatically start the service when your operating system boots-up.

Here is a simple example to get your going. Simply move your code inside a class, and call it from the run function inside MyDeamon.

import sys
import time

from daemon import Daemon


class YourCode(object):
    def run(self):
        while True:
            time.sleep(1)


class MyDaemon(Daemon):
    def run(self):
        # Or simply merge your code with MyDaemon.
        your_code = YourCode()
        your_code.run()


if __name__ == "__main__":
    daemon = MyDaemon('/tmp/daemon-example.pid')
    if len(sys.argv) == 2:
        if 'start' == sys.argv[1]:
            daemon.start()
        elif 'stop' == sys.argv[1]:
            daemon.stop()
        elif 'restart' == sys.argv[1]:
            daemon.restart()
        else:
            print "Unknown command"
            sys.exit(2)
        sys.exit(0)
    else:
        print "usage: %s start|stop|restart" % sys.argv[0]
        sys.exit(2)

Upstart

If you are running an operating system that is using Upstart (e.g. CentOS 6) - you can also use Upstart to manage the service. If you use Upstart you can keep your script as is, and simply add something like this under /etc/init/my-service.conf

start on started sshd
stop on runlevel [!2345]

exec /usr/bin/python /opt/my_service.py
respawn

You can then use start/stop/restart to manage your service.

e.g.

start my-service
stop my-service
restart my-service

A more detailed example of working with upstart is available here.

Systemd

If you are running an operating system that uses Systemd (e.g. CentOS 7) you can take a look at the following Stackoverflow answer.

How can I keep my python-daemon process running or restart it on fail?

If you really want to run a script 24/7 in background, the cleanest and easiest way to do it would surely be to create a systemd service.

There are already many descriptions of how to do that, for example here.

One of the advantages of systemd, in addition to being able to launch a service at startup, is to be able to restart it after failure.

Restart=on-failure

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If all you want to do is automatically restart the program after a crash, the easiest method would probably be to use a bash script.

You can use the until loop, which is used to execute a given set of commands as long as the given condition evaluates to false.

#!/bin/bash

until python /path/to/script.py; do
    echo "The program crashed at `date +%H:%M:%S`. Restarting the script..."
done

If the command returns a non zero exit-status, then the script is restarted.

Python script as linux service/daemon

Assuming your daemon has some way of continually running (some event loop, twisted, whatever), you can try to use upstart.

Here's an example upstart config for a hypothetical Python service:

description "My service"
author  "Some Dude <blah@foo.com>"

start on runlevel [234]
stop on runlevel [0156]

chdir /some/dir
exec /some/dir/script.py
respawn

If you save this as script.conf to /etc/init you simple do a one-time

$ sudo initctl reload-configuration
$ sudo start script

You can stop it with stop script. What the above upstart conf says is to start this service on reboots and also restart it if it dies.

As for signal handling - your process should naturally respond to SIGTERM. By default this should be handled unless you've specifically installed your own signal handler.

Can I run a Python script as a service?

You can make it a daemon. There is a PEP for a more complete solution, but I have found that this works well.

import os, sys

def become_daemon(our_home_dir='.', out_log='/dev/null', err_log='/dev/null', pidfile='/var/tmp/daemon.pid'):
    """ Make the current process a daemon.  """

    try:
        # First fork
        try:
            if os.fork() > 0:
                sys.exit(0)
        except OSError, e:
            sys.stderr.write('fork #1 failed" (%d) %s\n' % (e.errno, e.strerror))
            sys.exit(1)

        os.setsid()
        os.chdir(our_home_dir)
        os.umask(0)

        # Second fork
        try:
            pid = os.fork()
            if pid > 0:
                # You must write the pid file here.  After the exit()
                # the pid variable is gone.
                fpid = open(pidfile, 'wb')
                fpid.write(str(pid))
                fpid.close()
                sys.exit(0)
        except OSError, e:
            sys.stderr.write('fork #2 failed" (%d) %s\n' % (e.errno, e.strerror))
            sys.exit(1)

        si = open('/dev/null', 'r')
        so = open(out_log, 'a+', 0)
        se = open(err_log, 'a+', 0)
        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())
    except Exception, e:
        sys.stderr.write(str(e))

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