system_daemon_setup

# Setup a Script to Autorun using Linux Systemd

# (Step 1): Application - Write a sample application or service which is desired to run as a system daemon.
[For Example, A sample Python Flask application as below]

  #!/usr/bin/python3
  from flask import Flask  
  app = Flask(__name__)
  
  @app.route('/')
  def hello_world():
      return 'Hello, world!'
  
  if __name__ == '__main__':
      app.run(host='localhost', port=5000, debug=True)

Save the application/service as: 
  sudo nano /usr/bin/flask_app.py


# (Step 2): Service File - Create a service for system daemon.
The service file must have .service extension under /lib/systemd/system directory
  sudo nano /lib/systemd/system/flaskapp.service

Add the below content to '/lib/systemd/system/flaskapp.service'. 
  [Unit]
  Description=Flask Application 
  After=multi-user.target
  Conflicts=getty@tty1.service
  
  [Service]
  Type=simple
  ExecStart=/usr/bin/python3 /usr/bin/flask_app.py
  StandardInput=tty-force
  
  [Install]
  WantedBy=multi-user.target

In [Service]: Change Application filename, path of the application file accordingly. 
In [Unit]: Describe about the Application/Service


# (Step 3): Enable the Service - Reload the systemctl daemon to read newly added service
The system service has been added to the service. Reload the systemctl daemon to read newly added service. 

This daemon need to be reloaded each time after making any changes in the .service file.
  sudo systemctl daemon-reload

Enable & Start the service to start on system boot.
  sudo systemctl enable flaskapp.service
  sudo systemctl start flaskapp.service


# (Step 4): Check the Status of the system daemon
  sudo systemctl status flaskapp.service
  
 To Start/Stop/Restart the service:
   sudo systemctl start flaskapp.service
   sudo systemctl stop flaskapp.service
   sudo systemctl restart flaskapp.service