Systemd Targets and Their Functionality

Systemd and Targets

  • The kernel has started and systemd is now starting, initiating the default target of the system.

Definition of a Target

  • A systemd target is a group of unit files (units).

  • Some targets are isolatable, establishing the final state that the system starts in, while others are not isolatable.

Examples of Targets

  • printer.target: A target that groups all units relating to printing.

  • isolatable targets: These are critical for defining how the system starts.

    • emergency.target: This is the most minimal system state.

    • rescue.target: Offers more functionality than emergency.target.

    • multi-user.target: A functional system state without a graphical user interface (GUI).

    • graphical.target: A fully functional system state with a graphical user interface (GUI).

Enabling Units with systemctl

  • When a unit is enabled using systemctl enable, the unit is associated with a specific target where it is added.

  • Command to show all targets: systemctl list-unit-files -t target

  • Example target to review: multi-user.target.

    • Command to view configuration: systemctl cat multi-user.target

    • Configuration includes:

    • Requires: basic.target (ensures this target is included when starting multi-user.target).

    • Allow isolatable: If true, indicates it can be used to start the system in a specific state.

Service Interaction with Targets

  • Example service: vsftpd.service.

    • Configuration can be viewed via: systemctl cat vsftpd.service

    • In this service's install section, it states it is wanted by: multi-user.target.

  • System behavior:

    • The service knows its target, but the target does not track which services it includes.

    • Upon disabling and then enabling the service, it creates a symbolic link at etc/systemd/system/multi-user.target.wants.

Dependencies of a Target

  • Command to list dependencies: systemctl list-dependencies multi-user.target

  • Outputs include the listed files and additional targets:

    • basic.target: Required for multi-user.target, ensuring it is always started alongside.

    • basic.target organizes boot process further by including other targets.

Booting with Specific Targets

  • At boot time, the target can be specified as a boot argument.

    • Pressing e on the default option allows you to modify boot parameters.

    • Use: systemd.unit=multi-user.target followed by pressing CTRL + X to boot into the multi-user target.

  • Troubleshooting targets such as emergency.target and rescue.target require a root password for access.

Entering Multi-User Target

  • Once logged in, the user is in text-only mode.

  • To switch back to graphical mode: sudo systemctl start graphical.target to resume the graphical user interface.