These components form the foundation of the Systemd init system on Ubuntu, handling system initialization, service management, and process control:

• systemd Daemon: The central management process, PID 1, responsible for initializing the system, launching and supervising all other processes and services, and managing system states during boot, shutdown, and runtime.
• Unit Files: Declarative configuration files defining resources to be managed. Unit types include service, socket, target, timer, mount, device, and others, enabling flexible organization and control of system components.
• systemctl: The command-line utility used to interact with systemd. It controls the state of units, handles service start/stop/restart, and manages system states such as targets and dependencies.
• journald: The built-in logging system that collects and stores logs from the kernel, services, and applications, allowing centralized and structured access to system logs.
• cgroups (Control Groups): Integrated resource management feature that allows systemd to track and restrict resource usage (CPU, memory, IO) for each unit, improving system stability and performance.
• Targets: Special units that group other units, representing system states or milestones such as multi-user mode or graphical mode, replacing the traditional concept of runlevels.

This section covers frequently used commands for managing systemd units and services using systemctl on Ubuntu.

This section outlines the main directories where systemd stores unit files and runtime information on Ubuntu systems.

• /etc/systemd/system/ Directory for locally created or customized unit files and overrides. Units placed here take precedence over system-wide units.
• /lib/systemd/system/ Contains unit files provided by installed packages. These are the default service, target, and other unit definitions.
• /run/systemd/system/ Stores runtime unit files created dynamically during system operation. Content here is temporary and cleared at reboot.

Systemd provides several advanced features that enhance system management, improve performance, and offer fine-grained control over services and resources on Ubuntu systems.

• Dependency Management: Systemd models and manages service dependencies to ensure proper startup and shutdown order, supporting complex dependency trees and conditions.
• Parallel Service Startup: Services are started in parallel where possible, significantly reducing boot times compared to traditional sequential init systems.
• Resource Control with cgroups: Integrates Linux control groups (cgroups) to limit and track resource usage (CPU, memory, I/O) on a per-service basis, improving system stability and performance.
• Timers: Provides a native replacement for cron jobs with enhanced features such as calendar events, monotonic timers, and integration with service units.
• Socket Activation: Listens on sockets and launches services on demand when connections occur, optimizing resource usage and enabling faster startup of services.
• System State Snapshots: Captures and restores snapshots of the current system service states, useful for rollback or testing configuration changes.
• Centralized Logging with journald: Collects and stores logs from the kernel, system services, and user applications in a structured and centralized manner, supporting querying and filtering.

This section describes how to use systemd's built-in tools to diagnose issues and access logs effectively on Ubuntu systems.

• View all system logs: Use journalctl to display the complete system journal, including kernel, services, and application logs.

  journalctl

• Follow logs in real-time: Monitor live log output to observe system or service activity as it happens.

  journalctl -f

• View logs for a specific service: Restrict log output to a single unit or service by specifying the unit name.

  journalctl -u service_name

• Check recent boot logs: Show logs from the current or previous boot sessions, useful to analyze boot-related issues.

  journalctl -b          # Current boot
  journalctl -b -1       # Previous boot

• Limit log output by time: Filter logs starting from a specific date or time for targeted investigation.

  journalctl --since "YYYY-MM-DD HH:MM:SS"

• View log priority levels: Display logs filtered by priority (e.g., errors only) to focus on critical messages.

  journalctl -p err

• Verify unit state and logs simultaneously: Use systemctl status to check a service's current status and recent log entries.

  systemctl status service_name

This example demonstrates how to create and enable a simple custom service on Ubuntu using a systemd unit file.

1. Create the service unit file: Open a text editor and create a file at /etc/systemd/system/myapp.service with the following content:

   [Unit]
   Description=My Custom Application Service
   After=network.target
   
   [Service]
   Type=simple
   User=myuser
   ExecStart=/usr/local/bin/myapp
   
   [Install]
   WantedBy=multi-user.target
   

2. Reload systemd to recognize the new service:

   sudo systemctl daemon-reload

3. Enable the service to start automatically at boot:

   sudo systemctl enable myapp

4. Start the service immediately:

   sudo systemctl start myapp

5. Verify the service status:

   systemctl status myapp

This example sets up a basic service that runs the application located at /usr/local/bin/myapp under the user myuser, starting after the network is available. Adjust User and ExecStart as needed to match your environment and application.