Linux Service

Creating and using services in Linux with systemd is an essential skill for managing applications and processes. Here’s an overview of everything you need to know, from basic concepts to advanced usage.
Author

Benedict Thekkel

What is systemd?

systemd is a system and service manager for Linux that initializes the system and manages services. It uses unit files to define services, targets, and other system components.

Key Concepts

  1. Unit Files:
    • The configuration files used by systemd.
    • Common types of unit files:
      • .service: Describes a service.
      • .target: Groups units into logical targets (e.g., multi-user.target).
      • .timer: Schedules tasks.
      • .mount: Manages filesystem mounts.
  2. Unit File Locations:
    • System-wide: /etc/systemd/system/
    • Default: /usr/lib/systemd/system/
    • User-specific: ~/.config/systemd/user/
  3. Service States:
    • active: Running.
    • inactive: Stopped.
    • failed: Encountered an error.

Creating a Service

1. Writing a Service File

Example: my-service.service

[Unit]
Description=My Custom Service
After=network.target

[Service]
Type=simple
ExecStart=/path/to/your/application --arg1 --arg2
WorkingDirectory=/path/to/working/directory
User=myuser
Restart=always
RestartSec=5

[Install]
WantedBy=multi-user.target

2. Key Sections in a Service File

  • [Unit]:
    • Describes the service and its dependencies.
    • Description: Short description of the service.
    • After: Specifies when the service should start (e.g., after network.target).
  • [Service]:
    • Describes how the service operates.
    • Type: Defines the service type:
      • simple: (default) Starts the process specified in ExecStart.
      • forking: Used for daemons that fork.
      • oneshot: For one-time tasks.
      • notify or dbus: For advanced use cases.
    • ExecStart: Command to start the service.
    • WorkingDirectory: Sets the working directory.
    • User: User account under which the service runs.
    • Restart: Specifies restart behavior (always, on-failure, etc.).
    • RestartSec: Delay between restart attempts.
  • [Install]:
    • Defines how and when the service is enabled.
    • WantedBy: Defines the target in which the service is included (e.g., multi-user.target for most services).

3. Steps to Enable and Start a Service

  1. Place the Service File:

    sudo cp my-service.service /etc/systemd/system/

  2. Reload systemd:

    sudo systemctl daemon-reload

  3. Enable the Service (Start at Boot):

    sudo systemctl enable my-service.service

  4. Start the Service:

    sudo systemctl start my-service.service

  5. Check Status:

    sudo systemctl status my-service.service

Managing Services

  1. Basic Commands:

    • Start a service:

      sudo systemctl start my-service

    • Stop a service:

      sudo systemctl stop my-service

    • Restart a service:

      sudo systemctl restart my-service

    • Reload service configuration without restarting:

      sudo systemctl reload my-service

    • Check service status:

      sudo systemctl status my-service
  2. Enable/Disable Services:

    • Enable a service (start at boot):

      sudo systemctl enable my-service

    • Disable a service:

      sudo systemctl disable my-service
  3. Inspect Logs:

    • View logs for a specific service:

      journalctl -u my-service

    • Tail logs in real-time:

      journalctl -u my-service -f
  4. Remove a Service:

    • Stop the service:

      sudo systemctl stop my-service

    • Disable the service:

      sudo systemctl disable my-service

    • Delete the service file:

      sudo rm /etc/systemd/system/my-service.service

    • Reload systemd:

      sudo systemctl daemon-reload

Advanced Topics

1. Environment Variables

You can pass environment variables to a service using Environment= or EnvironmentFile=.

Example:

Environment=MY_VAR=value
EnvironmentFile=/path/to/env/file

2. Dependency Management

  • Requires: Specifies hard dependencies. If the required unit fails, this unit also fails.
  • Wants: Specifies soft dependencies. If the dependency fails, this unit still runs.
  • Before/After: Specifies the order of startup.

Example:

[Unit]
Requires=network.target
After=network.target

3. Timer Units

Use .timer units to schedule tasks, similar to cron.

Example: my-service.timer

[Unit]
Description=Run My Service Every Day

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

4. User Services

Systemd also supports per-user services that don’t require root.

  1. Place the service file in ~/.config/systemd/user/.

  2. Reload user services:

    systemctl --user daemon-reload

  3. Enable and start the service:

    systemctl --user enable my-service
systemctl --user start my-service

5. Debugging Services

  • Check for syntax errors in the service file:

    sudo systemd-analyze verify /etc/systemd/system/my-service.service

  • Debug logs:

    sudo journalctl -xe

  • Test the service manually:

    sudo -u myuser /path/to/application --arg1 --arg2

Best Practices

  1. Use Absolute Paths: Systemd requires absolute paths for all file and directory references.

  2. Limit Privileges: Run services as non-root users wherever possible.

  3. Enable Restart Policies: Use Restart=always or Restart=on-failure for critical services.

  4. Use systemctl edit for Overrides: To modify system services without editing the main service file:

    sudo systemctl edit my-service

    This creates a drop-in configuration in /etc/systemd/system/my-service.d/.

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