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Simple systemd on Linux

Simple systemd on Linux

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Below is a super simple guide to creating a systemd service on linux. Systemd is not yet supported on all the linux systems, but it's popular and easy to use.

What is systemd?

systemd is a Linux initialization system and service manager that includes features like on-demand starting of daemons, mount and automount point maintenance, snapshot support, and processes tracking using Linux control groups.

In simple terms, it helps us manage the lifecycle of services. You can add services to startup jobs, if your services die, it can auto restart and much more.

Making a simple flask application (our service)

Requirements:

  • python3 installed
  • pip3 installed

Install flask using pip3

sudo pip3 install flask

Make a basic flask application

Make a file main.py, place it in any folder ex - /home/mridul/main.py and add the following contents in it.

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello World!'

app.run(port=8000)

Run and test the flask app

To run out flask server -

/usr/bin/python3 /home/mridul/main.py

To test if our application is running -

curl http://localhost:8000

You should be able to see Hello World! in the curl output.

Making a systemd entry

To begin with the systemd service entry will be stored in /etc/systemd/system/ So, create a new file /etc/systemd/system/flaskapp.service

[Unit]
Description=Simple flask app service
After=network.target
StartLimitIntervalSec=0

[Service]
Type=simple
Restart=always
RestartSec=1
User=mridul
ExecStart=/usr/bin/python3 /home/mridul/main.py

[Install]
WantedBy=multi-user.target

Most of the above fields are easy to understand. The After field is used to start our service only after the network service runs. We can also put something else like mysql.target to run it after mysql service runs.

In the User field, we need to provice which user should the service be run from, for our example we can run it from my user i.e. mridul.

The ExecStart is where we give the command to run our service.

Restart is used to specify the behavior when our service dies due to some reason. setting it to always will restart our service everytime it dies, even if it exited with status code 0. To restart only in case of failure we can set it to on-failure.

Basic systemd commands

To start our service

sudo systemctl start flaskapp

To stop our service

sudo systemctl stop flaskapp

To restart our service

sudo systemctl restart flaskapp

To check status of our service

sudo systemctl status flaskapp

To start our service during system startup

sudo systemctl enable flaskapp
 
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