1. What is systemd? #

systemd is a system and service manager for Linux that replaces the traditional SysVinit and Upstart. It’s designed for faster boot times, better resource management, and centralized configuration.

Key components of systemd:

• systemd: The main daemon.
• journald: Centralized logging system.
• timedated, logind, networkd: Manage time, user sessions, and networking.
• Units: systemd uses “unit files” to define and control system components like services, sockets, devices, and more.

2. Basic Commands #

Service Management #

• Start a service:

  systemctl start <service>
  

• Stop a service:

  systemctl stop <service>
  

• Restart a service:

  systemctl restart <service>
  

• Enable a service (start at boot):

  systemctl enable <service>
  

• Disable a service (prevent from starting at boot):

  systemctl disable <service>
  

• Check the status of a service:

  systemctl status <service>
  

System State #

• Reboot the system:

  systemctl reboot
  

• Power off the system:

  systemctl poweroff
  

• View system logs:

  journalctl
  

3. Unit Files #

What are Unit Files? #

Unit files describe system resources and are located in /etc/systemd/system/ (custom), /lib/systemd/system/ (default), or /usr/lib/systemd/system/.

Types of unit files:

• .service: Services (e.g., web servers, databases).
• .socket: Sockets for communication.
• .timer: Timers (like cron jobs).
• .target: Group of units (like runlevels).

Viewing and Editing Unit Files #

• View a unit file:

  systemctl cat <unit>
  

• Edit a unit file:

  sudo systemctl edit <unit>
  

  This opens an override file for safe customization.

Reload systemd after editing: #

systemctl daemon-reload

4. Boot Targets #

Targets are similar to runlevels in SysVinit.

Common targets:

• graphical.target: Multi-user with a graphical interface.
• multi-user.target: Multi-user without GUI (default for servers).
• rescue.target: Single-user mode.
• emergency.target: Minimal system, no services.

Commands:

• Check current target:

  systemctl get-default
  

• Change default target:

  systemctl set-default <target>
  

• Switch to a target:

  systemctl isolate <target>
  

5. Logging with journalctl #

journalctl manages logs. It’s a central logging system for systemd.

• View logs:

  journalctl
  

• View logs for a service:

  journalctl -u <service>
  

• Follow logs in real-time:

  journalctl -f
  

• View logs since a specific time:

  journalctl --since "2025-01-01 00:00:00"
  

6. Creating a Custom Service #

1. Create the unit file:

   sudo nano /etc/systemd/system/myapp.service
   

   Example:

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

2. Reload systemd:

   systemctl daemon-reload
   

3. Enable and start the service:

   systemctl enable myapp.service
   systemctl start myapp.service
   

4. Check status:

   systemctl status myapp.service
   

7. Useful Tools #

• systemctl: Main tool for managing services and units.
• journalctl: Log management.
• systemd-analyze: Debug and analyze boot performance.

  systemd-analyze blame
  

8. Advanced Features #

• Timers: systemd alternative to cron. Example timer unit:

  [Unit]
  Description=Run MyApp Daily
  
  [Timer]
  OnCalendar=daily
  Persistent=true
  
  [Install]
  WantedBy=timers.target
  

• Socket Activation: Services start only when needed.

• Resource Management: Use systemd-cgls and systemd-cgtop to manage cgroups.

Mastering systemd makes you more efficient at managing Linux systems. It’s a powerful tool with many features to explore. Dive deeper into its documentation and practice to become a pro! 🚀