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In this lesson, you’ll learn how to control system services and targets (runlevels) using systemd, the modern init system for Linux. Systemd manages resources through units, each identified by a name, type, and configuration file. For instance, the Apache HTTP server appears as httpd.service on RHEL-based distributions and apache2.service on Debian-based systems.

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Understanding systemd Units

Systemd supports seven primary unit types:
Unit TypeDescription
serviceManages daemons and background services (e.g., nginx.service, httpd.service).
socketDefines file or network sockets and activates a service when a connection arrives.
deviceRepresents a kernel-recognized hardware device via a matching udev rule.
mountDescribes a filesystem mount point (equivalent to an entry in /etc/fstab).
automountMounts filesystems on first access; pairs with a mount unit for configuration.
targetGroups units for collective management (analogous to SysVinit runlevels).
snapshotCaptures the current state of systemd’s unit manager (not available on all distributions).
The image is a text-based explanation of systemd components, detailing various terms like service, socket, device, mount, automount, target, and snapshot, along with their definitions.

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Managing Units with systemctl

The systemctl command is your primary interface to start, stop, reload, and query units. Replace unit.service with your target unit name.

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Starting, Stopping, and Restarting Services

sudo systemctl start   unit.service   # Start the service
sudo systemctl stop    unit.service   # Stop the service
sudo systemctl restart unit.service   # Restart the service
sudo systemctl status  unit.service   # Show status and recent logs

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Checking Active State

systemctl is-active unit.service      # Outputs 'active' or 'inactive'

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Enabling and Disabling Services at Boot

sudo systemctl enable  unit.service   # Enable at boot
sudo systemctl disable unit.service   # Disable at boot
systemctl is-enabled unit.service     # Outputs 'enabled' or 'disabled'

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Working with Targets (Runlevels)

Systemd targets correspond to traditional runlevels. For example, multi-user.target is like runlevel 3, while graphical.target maps to runlevel 5. 
sudo systemctl isolate multi-user.target   # Switch immediately to runlevel 3
sudo systemctl set-default multi-user.target  # Persist default target across reboots
systemctl get-default                        # Display the current default target
Never set the default target to shutdown.target, as it will power off the system immediately after boot.

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Listing Unit Files and Active Units

  • All unit files and their boot-time enablement state: 
    systemctl list-unit-files
systemctl list-unit-files --type=service
systemctl list-unit-files --type=target
  • Currently active units in this session: 
    systemctl list-units
systemctl list-units --type=service
systemctl list-units --type=target

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Power Management with systemctl

Systemd can handle suspend and hibernate when no other power manager is running: 
sudo systemctl suspend    # Enter low-power mode; RAM remains powered
sudo systemctl hibernate  # Save RAM to disk, then power off
Power-related settings live in /etc/systemd/logind.conf and any drop-in files under /etc/systemd/logind.conf.d/. For finer control over ACPI events (lid close, battery thresholds, etc.), consider using the acpid daemon.

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