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

Question

I have written a Python script that checks a certain e-mail address and passes new e-mails to an external program   How can I get this script to execute 24 7  such as turning it into daemon or service in Linux   Would I also need a loop that never ends in the program  or can it be done by just having the code re executed multiple times

User · Answer

cron is clearly a great choice for many purposes   However it doesn t create a service or daemon as you requested in the OP   cron just runs jobs periodically  meaning the job starts and stops   and no more often than once   minute   There are issues with cron -- for example  if a prior instance of your script is still running the next time the cron schedule comes around and launches a new instance  is that OK   cron doesn t handle dependencies  it just tries to start a job when the schedule says to   If you find a situation where you truly need a daemon  a process that never stops running   take a look at supervisord   It provides a simple way to wrapper a normal  non-daemonized script or program and make it operate like a daemon   This is a much better way than creating a native Python daemon

User · Answer

You can use fork   to detach your script from the tty and have it continue to run  like so   import os  sys fpid   os fork   if fpid  0      Running as daemon now  PID is fpid   sys exit 0    Of course you also need to implement an endless loop  like  while 1    do your check     sleep 5    Hope this get s you started

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Ubuntu has a very simple way to manage a service  For python the difference is that ALL the dependencies  packages  have to be in the same directory  where the main file is run from  I just manage to create such a service to provide weather info to my clients  Steps   Create your python application project as you normally do   Install all dependencies locally like  sudo pip3 install package name -t    Create your command line variables and handle them in code  if you need any   Create the service file  Something  minimalist  like     Unit    Description 1Droid Weather meddleware provider     Service    Restart always   User root   WorkingDirectory  home ubuntu weather   ExecStart  usr bin python3  home ubuntu weather main py httpport 9570  provider OWMap     Install    WantedBy multi-user target   Save the file as myweather service  for example   Make sure that your app runs if started in the current directory   python3  main py httpport 9570  provider OWMap   The service file produced above  and named myweather service  important to have the extension  service  will be treated by the system as the name of your service   That is the name that you will use to interact with your service   Copy the service file    sudo cp myweather service  lib systemd system myweather service   Refresh demon registry    sudo systemctl daemon-reload   Stop the service  if it was running    sudo service myweatherr stop   Start the service    sudo service myweather start   Check the status  log file with where your print statements go     tail -f  var log syslog   Or check the status with    sudo service myweather status   Back to the start with another iteration if needed   This service is now running and even if you log out it will not be affected  And YES if the host is shutdown and restarted this service will be restarted   information for my mobile android app

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to creating some thing that is running like service you can use this thing    The first thing that you must do is installing the Cement framework  Cement frame work is a CLI frame work that you can deploy your application on it   command line interface of the app    interface py   from cement core foundation import CementApp  from cement core controller import CementBaseController  expose  from YourApp import yourApp   class Meta      label    base      description    your application description      arguments                -r     --run              dict action  store true   help  Run your application                -v    --version              dict action  version   version  Your app version                          -s    --stop              dict action  store true   help  Stop your application                     expose hide True      def default self           if self app pargs run               Start to running the your app from there               YourApp yourApp           if self app pargs stop               Stop your application             YourApp yourApp stop     class App CementApp          class Meta         label    Uptime         base controller    base         handlers    MyBaseController    with App   as app         app run     YourApp py class    import threading   class yourApp       def   init            self loger   log exception exception loger           thread   threading Thread target self start  args             thread daemon   True         thread start         def start self            Do every thing you want         pass      def stop self            Do some things to stop your application   Keep in mind that your app must run on a thread to be daemon  To run the app just do this in command line      python interface py --help

User · Answer

Here s a nice class that is taken from here        usr bin env python  import sys  os  time  atexit from signal import SIGTERM  class Daemon                      A generic daemon class           Usage  subclass the Daemon class and override the run   method                     def   init   self  pidfile  stdin   dev null   stdout   dev null   stderr   dev null                    self stdin   stdin                 self stdout   stdout                 self stderr   stderr                 self pidfile   pidfile          def daemonize self                                       do the UNIX double-fork magic  see Stevens   Advanced                 Programming in the UNIX Environment  for details  ISBN 0201563177                  http   www erlenstar demon co uk unix faq 2 html SEC16                                     try                          pid   os fork                           if pid  gt  0                                    exit first parent                                 sys exit 0                  except OSError  e                          sys stderr write  fork  1 failed   d   s  n     e errno  e strerror                           sys exit 1                     decouple from parent environment                 os chdir                      os setsid                   os umask 0                     do second fork                 try                          pid   os fork                           if pid  gt  0                                    exit from second parent                                 sys exit 0                  except OSError  e                          sys stderr write  fork  2 failed   d   s  n     e errno  e strerror                           sys exit 1                     redirect standard file descriptors                 sys stdout flush                   sys stderr flush                   si   file self stdin   r                   so   file self stdout   a                    se   file self stderr   a    0                  os dup2 si fileno    sys stdin fileno                    os dup2 so fileno    sys stdout fileno                    os dup2 se fileno    sys stderr fileno                       write pidfile                 atexit register self delpid                  pid   str os getpid                    file self pidfile  w    write   s n    pid           def delpid self                   os remove self pidfile           def start self                                       Start the daemon                                       Check for a pidfile to see if the daemon already runs                 try                          pf   file self pidfile  r                           pid   int pf read   strip                            pf close                   except IOError                          pid   None                  if pid                          message    pidfile  s already exist  Daemon already running  n                          sys stderr write message   self pidfile                          sys exit 1                     Start the daemon                 self daemonize                   self run            def stop self                                       Stop the daemon                                       Get the pid from the pidfile                 try                          pf   file self pidfile  r                           pid   int pf read   strip                            pf close                   except IOError                          pid   None                  if not pid                          message    pidfile  s does not exist  Daemon not running  n                          sys stderr write message   self pidfile                          return   not an error in a restart                    Try killing the daemon process                        try                          while 1                                  os kill pid  SIGTERM                                  time sleep 0 1                  except OSError  err                          err   str err                          if err find  No such process    gt  0                                  if os path exists self pidfile                                           os remove self pidfile                          else                                  print str err                                  sys exit 1           def restart self                                       Restart the daemon                                     self stop                   self start            def run self                                       You should override this method when you subclass Daemon  It will be called after the process has been                 daemonized by start   or restart

User · Answer

First  read up on mail aliases   A mail alias will do this inside the mail system without you having to fool around with daemons or services or anything of the sort   You can write a simple script that will be executed by sendmail each time a mail message is sent to a specific mailbox   See http   www feep net sendmail tutorial intro aliases html  If you really want to write a needlessly complex server  you can do this   nohup python myscript py  amp    That s all it takes   Your script simply loops and sleeps   import time def do the work          one round of polling -- checking email  whatever  while True      time sleep  600     10 min      try          do the work       except          pass

User · Answer

You have two options here    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  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

User · Answer

how about using  nohup command on linux   I use it for running my commands on my Bluehost server   Please advice if I am wrong

User · Answer

I would recommend this solution  You need to inherit and override method run   import sys import os from signal import SIGTERM from abc import ABCMeta  abstractmethod    class Daemon object         metaclass     ABCMeta       def   init   self  pidfile           self  pidfile   pidfile        abstractmethod     def run self           pass       def  daemonize self             decouple threads         pid   os fork              stop first thread         if pid  gt  0              sys exit 0             write pid into a pidfile         with open self  pidfile   w   as f              print  gt  gt  f  os getpid         def start self             if daemon is started throw an error         if os path exists self  pidfile               raise Exception  Daemon is already started              create and switch to daemon thread         self  daemonize              run the body of the daemon         self run         def stop self             check the pidfile existing         if os path exists self  pidfile                 read pid from the file             with open self  pidfile   r   as f                  pid   int f read   strip                   remove the pidfile             os remove self  pidfile                 kill daemon             os kill pid  SIGTERM           else              raise Exception  Daemon is not started         def restart self           self stop           self start

User · Answer

You can also make the python script run as a service using a shell script  First create a shell script to run the python script like this   scriptname arbitary name      bin sh script   home    full path to script   usr bin python  script  amp    now make a file in  etc init d scriptname       bin sh  PATH  bin  usr bin  sbin  usr sbin DAEMON  home    path to shell script scriptname created to run python script PIDFILE  var run scriptname pid  test -x  DAEMON    exit 0     lib lsb init-functions  case   1  in   start       log daemon msg  Starting feedparser       start daemon -p  PIDFILE  DAEMON      log end msg            stop       log daemon msg  Stopping feedparser       killproc -p  PIDFILE  DAEMON      PID  ps x  grep feed   head -1   awk   print  1         kill -9  PID             log end msg            force-reload restart        0 stop       0 start         status       status of proc -p  PIDFILE  DAEMON atd  amp  amp  exit 0    exit                 echo  Usage   etc init d atd  start stop restart force-reload status      exit 1      esac  exit 0   Now you can start and stop your python script using the command  etc init d scriptname start or stop

User · Answer

Assuming that you would really want your loop to run 24 7 as a background service For a solution that doesn t involve injecting your code with libraries  you can simply create a service template  since you are using linux   Unit  Description    lt Your service description here gt  After   network target   Assuming you want to start after network interfaces are made available    Service  Type   simple ExecStart   python  lt Path of the script you want to run gt  User     User to run the script as Group     Group to run the script as Restart   on-failure   Restart when there are errors SyslogIdentifier    lt Name of logs for the service gt  RestartSec   5 TimeoutStartSec   infinity    Install  WantedBy   multi-user target   Make it accessible to other users  Place that file in your daemon service folder  usually  etc systemd system    in a   service file  and install it using the following systemctl commands  will likely require sudo privileges   systemctl enable  lt service file name without  service extension gt   systemctl daemon-reload  systemctl start  lt service file name without  service extension gt   You can then check that your service is running by using the command  systemctl   grep running

User · Answer

A simple and supported version is Daemonize   Install it from Python Package Index  PyPI      pip install daemonize   and then use like       import os  sys from daemonize import Daemonize     def main           your code here  if   name         main             myname os path basename sys argv 0           pidfile   tmp  s    myname         any name         daemon   Daemonize app myname pid pidfile  action main          daemon start

User · Answer

Use whatever service manager your system offers - for example under Ubuntu use upstart  This will handle all the details for you such as start on boot  restart on crash  etc

User · Answer

You should use the python-daemon library  it takes care of everything   From PyPI  Library to implement a well-behaved Unix daemon process

User · Answer

If you are using terminal ssh or something  and you want to keep a long-time script working after you log out from the terminal  you can try this   screen  apt-get install screen  create a virtual terminal inside  namely abc   screen -dmS abc  now we connect to abc  screen -r abc  So  now we can run python script  python keep sending mails py  from now on  you can directly close your terminal  however  the python script will keep running rather than being shut down     Since this keep sending mails py s PID is a child process of the virtual screen rather than the   terminal ssh    If you want to go back check your script running status  you can use screen -r abc again

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

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