These are systemd unit types that manage different aspects of your system:

Services - Long-running programs (daemons). Example: nginx.service, postgresql.service. They start, run in background, and can be stopped/restarted.

Targets - Grouping/synchronization points, like runlevels. Example: multi-user.target (normal operation), graphical.target (with GUI). They bundle multiple units together.

Sockets - Manage IPC/network sockets for on-demand service activation. Example: docker.socket listens and only starts docker.service when something connects. Enables lazy-loading.

Timers - Scheduled tasks (like cron). Example: logrotate.timer triggers logrotate.service daily. More flexible than cron with dependency handling.

Paths - Monitor filesystem paths and activate services when files/directories change. Example: trigger a service when a file appears in /var/spool/.

Common systemctl Commands for Services

Here are the most frequently used systemctl commands:

Basic Service Control

sudo systemctl start [service]      # Start a service
sudo systemctl stop [service]       # Stop a service
sudo systemctl restart [service]    # Restart a service
sudo systemctl reload [service]     # Reload config without restart
sudo systemctl status [service]     # Check service status

Enable/Disable Services

sudo systemctl enable [service]     # Enable service at boot
sudo systemctl disable [service]    # Disable service at boot
sudo systemctl enable --now [service]   # Enable and start immediately
sudo systemctl disable --now [service]  # Disable and stop immediately

Service Information

systemctl is-active [service]       # Check if running
systemctl is-enabled [service]      # Check if enabled at boot
systemctl is-failed [service]       # Check if service failed
systemctl list-units --type=service # List all services
systemctl list-unit-files --type=service # List service states

Advanced Commands

sudo systemctl mask [service]       # Prevent service from starting
sudo systemctl unmask [service]     # Remove mask
sudo systemctl daemon-reload        # Reload systemd manager config
systemctl show [service]            # Show all properties
systemctl cat [service]             # Show service unit file

Examples

sudo systemctl restart nginx
sudo systemctl status apache2
sudo systemctl enable postgresql

Note: Most commands require sudo for system services, except status/information queries.