KodeKloud Notes

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.

Understanding systemd Units

Systemd supports seven primary unit types:

Unit Type	Description
service	Manages daemons and background services (e.g., `nginx.service`, `httpd.service`).
socket	Defines file or network sockets and activates a service when a connection arrives.
device	Represents a kernel-recognized hardware device via a matching udev rule.
mount	Describes a filesystem mount point (equivalent to an entry in `/etc/fstab`).
automount	Mounts filesystems on first access; pairs with a mount unit for configuration.
target	Groups units for collective management (analogous to SysVinit runlevels).
snapshot	Captures 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.

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.

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

Checking Active State

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

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'

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

Warning

Never set the default target to shutdown.target, as it will power off the system immediately after boot.

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

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

Note

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.

Links and References

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