Lab 05 - Linux services

Linux Services

Introduction to Managing Services in Linux

Many services run continuously in the background on a Linux system, including network and system services. Services running on Linux are also known as daemons.

systemd is a suite of tools used by most modern Linux distributions. All system tasks can be controlled through systemd. You can start or stop processes with this tool and display all enabled and disabled services along with their details.

Listing services with systemctl

The simplest way to list services on a system that uses systemd is to run the systemctl command followed by list-units. You can add the --type=service option to restrict the results to services only.

systemctl list-units --type=service

To display all services, both active and inactive, use the --all option:

systemctl list-units --type=service --all

Starting a service

systemctl start [service_name]

Stopping a service

systemctl stop [service_name]

Enabling a service at boot

systemctl enable [service_name]

Disabling a service at boot

systemctl disable [service_name]

Checking service status

systemctl status [service_name]

Viewing logs produced by services

The logs produced by various services are stored by the journald daemon. To view the logs that journald has collected, use the journalctl command.

When the command is used on its own, every journal entry is displayed inside a pager (usually less). The oldest entries appear first:

journalctl

The journal may contain a significant number of messages—tens or even hundreds of thousands of lines—which shows how much information is stored in the database.

Showing logs from the current boot

Adding the -b flag to journalctl filters the logs so that only entries recorded since the last system reboot are shown:

journalctl -b

Viewing logs produced by a specific service

journalctl -u [service_name]

Creating Your Own Services in Linux

Sometimes you want to let systemd manage your application. In some cases it is useful to restart a script automatically if it stops for any reason. systemd is a good solution for these scenarios.

  1. Create a file named [service_name].service in /etc/systemd/system using the following template:

    [Unit]
Description=Service description

[Service]
User=username (for example, root)
ExecStart=/absolute/path/to/script (for example, /home/student/Desktop/script.sh)
Restart=always

[Install]
WantedBy=multi-user.target

  2. For Python projects that use a virtual environment, you can use:

    [Unit]
Description=Service description

[Service]
User=username (for example, root)
ExecStart=/home/user/virtualenv/bin/python main.py
Restart=always
# replace /home/user/virtualenv/bin/python with your virtualenv interpreter and main.py with your script

[Install]
WantedBy=multi-user.target

  3. Make your script executable:

    chmod +x script_name

  4. Reload the service files to include the new service:

    sudo systemctl daemon-reload

  5. Start your service:

    sudo systemctl start your-service.service

Example service and script:

my_service.service:

[Unit]
Description=Service description

[Service]
User=jakub
ExecStart=/usr/bin/python3 /home/jakub/Desktop/print_messages.py
Restart=always

[Install]
WantedBy=multi-user.target

print_messages.py:

#!/usr/bin/env python3
import time

if __name__ == "__main__":
    while True:
        print("Hello world")
        time.sleep(1)

or for Bash:

#!/bin/bash
while $(sleep 1);
do
    echo "hello world"
done

When building your own script, make sure to include a shebang line (the first line in the examples above).

🛠🔥 Task 1 🛠🔥

Create a Python script named print_messages.py. The file contents were shown earlier in this guide. Run the script from two different locations in the terminal:

🛠🔥 Task 2 🛠🔥

Create a my_service.service unit file. The file contents were shown earlier in this guide. Start the service.

🛠🔥 Task 3 🛠🔥

Write a Python application that uses the click library. The application’s CLI should accept a path to a file and an interval in seconds (call it n). The application should append a random number along with the current time to the file every n seconds.

Create a service file that runs your application. Add a few print statements to your code so that you can verify that the logs are collected correctly. Enable and start your new service. Check its status and logs. Restart your system; the service should start automatically.

You may want to create a virtual environment (venv) and install the click library inside it.

Finish the task by stopping and disabling the service. Remove the service file from /etc/systemd/system. Save your code - you will need it in future labs.

Sources