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Introduction to Systemd and Init Systems

1. What Are Init Systems?​

An Init System is the first process that starts when a Linux system boots up and the last process that shuts down during system halt.

Key Role of Init Systems:​

  • Initializes the system by starting services (like network, logging, and databases).
  • Manages background processes (called daemons).
  • Controls system states like reboot or shutdown.

2. Types of Init Systems​

a. SysVinit (System V Init):​

  • Older system used in early Linux distributions.
  • Uses shell scripts stored in /etc/init.d/.
  • Executes tasks sequentially, one after another.

b. Upstart:​

  • Introduced as an improvement over SysVinit.
  • Uses event-based triggers (e.g., when hardware is added).
  • Faster than SysVinit but has been largely replaced by Systemd.

3. What Is Systemd?​

Systemd is the modern and default init system for most Linux distributions (e.g., Ubuntu, CentOS, Fedora).

Key Features of Systemd:​

  • Parallel Service Startup: Faster boot times by starting services in parallel instead of sequentially.

  • Dependency Management: Automatically starts services that depend on each other.

  • Unified Tools: Includes tools like systemctl for managing services.

  • Logging with Journald: Centralized logging through journalctl.

  • Cgroups Support: Controls resource usage (CPU, memory) for services.

Comparison Table: SysVinit vs Systemd​

FeatureSysVinitSystemd
Startup SpeedSequential (slower)Parallel (faster)
Dependency HandlingManual configurationAutomatic dependency detection
Service ManagementScript-based commandsUnified systemctl tool
LoggingPlain text logs in /var/logCentralized logs with journald
Resource ControlNo built-in supportSupports cgroups

4. Key Systemd Components​

a. Units:​

  • Configuration files for services, sockets, devices, and targets.
  • Stored in /etc/systemd/system/ and /usr/lib/systemd/system/.

b. Targets:**​

  • Groups of units that represent system states like multi-user or graphical mode.

Example Command:​

systemctl get-default

Displays the current target (e.g., multi-user or graphical mode) that the system boots into by default.

3. Services:​

  • Programs or daemons controlled by Systemd.

5. Systemd Unit Files​

Unit files define how a service behaves.

5. Example Unit File (nginx.service):​

[Unit]  
Description=NGINX Web Server  
After=network.target  

[Service]  
ExecStart=/usr/sbin/nginx  
ExecStop=/usr/sbin/nginx -s stop  
Restart=always  

[Install]  
WantedBy=multi-user.target
  • [Unit]: Provides metadata and dependencies.
  • [Service]: Defines commands for starting, stopping, and restarting services.
  • [Install]: Specifies how the service integrates with system targets.

6. Basic Systemd Commands​

a. Checking System Status​

systemctl is-system-running

Displays the overall health of the system (e.g., running, degraded, or maintenance mode).

b. List All Units​

systemctl list-units

Lists all active units, including services, devices, and sockets currently managed by Systemd.

c. Viewing Active Services​

systemctl list-units --type=service

Filters the list to show only active services, making it easier to monitor system activity.

d. Viewing Logs​

journalctl -u apache2.service

Displays logs specific to the Apache2 service, including error messages, start times, and warnings.

7. Switching Between Init Systems (Optional)​

  • Check Current Init System:
ps --pid 1

Checks which process is running as the first process (PID 1). If it’s systemd, then the system uses Systemd as the init system.

  • Switch to SysVinit (if supported):
sudo apt install sysvinit-core
sudo reboot

Installs and switches to SysVinit as the init system, then reboots the system to apply changes.

  • Revert to Systemd:
sudo apt install systemd
sudo reboot

Reinstalls Systemd and reboots the system, restoring it as the default init system.

8. Key Takeaways​

  • SysVinit and Upstart are older init systems used for managing Linux startup and processes.
  • Systemd is the modern and widely used init system with faster startup, automatic dependencies, and advanced logging features.
  • Logs and errors can be monitored using the journalctl command.
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