2.5 Systemd Boot Targets & Unit Management (101.3)

2.5 Systemd Boot Targets & Unit Management

Quick Summary: systemd is the standard initialization system and service manager for modern Linux distributions. Acting as PID 1, it replaces traditional SysVinit with parallel service activation, socket- and D-Bus-based activation, and declarative unit configuration. Operating states are organized into targets (e.g., multi-user.target, graphical.target), and all runtime service administration is performed through the systemctl command.

1. Systemd Architecture & Core Features

Unlike traditional sequential init scripts, systemd treats services and resources as structured objects called Units.

Core Systemd Capabilities

• Parallel Execution: Starts non-dependent services concurrently, drastically reducing boot times.
• Socket-Based Activation: Creates listening sockets (UNIX domain or TCP) before launching daemons. When a packet arrives, systemd launches the service instantly, eliminating startup race conditions.
• cgroups Process Tracking: Groups service child processes inside dedicated Control Groups (cgroups). When a service stops, systemd cleanly terminates all child processes, preventing orphaned daemons.
• Explicit Dependency Modeling: Uses unit directives (Requires=, Wants=, BindsTo=, Conflicts=, Before=, After=) to establish rigorous startup sequences.

2. The 11 Systemd Unit Types

Systemd organizes all system resources into 11 distinct unit types, distinguished by their filename extensions.

3. Unit File Locations & Configuration Precedence

Systemd loads unit files from three main directory locations. If unit files with identical names exist across multiple directories, systemd strictly follows a top-down priority hierarchy.

[1. /etc/systemd/system/]     ──► Priority 1 (Highest): Local Administrator Configuration
         │                        (Custom units, modified copies, enabled symlinks)
         ▼
[2. /run/systemd/system/]     ──► Priority 2 (Intermediate): Volatile Runtime Units
         │                        (Dynamically generated at boot in RAM)
         ▼
[3. /lib/systemd/system/]     ──► Priority 3 (Lowest): Vendor Package Defaults
    /usr/lib/systemd/system/      (Installed by rpm/dpkg packages; overwritten on updates)

Drop-In Configuration Overrides (.d/ Directories)

Instead of copying an entire 100-line vendor unit file to /etc/systemd/system/sshd.service, administrators can create a drop-in override directory named <unit>.d/:

# /etc/systemd/system/sshd.service.d/override.conf
[Service]
Restart=always
RestartSec=5s

After creating or modifying unit files, you must reload the systemd manager configuration:

sudo systemctl daemon-reload

Exam Trap — Unit Masking: Masking a unit renders it completely unstartable (manually or as a dependency). Systemd accomplishes this by creating a symbolic link pointing to /dev/null:

sudo systemctl mask apache2
# Creates: /etc/systemd/system/apache2.service -> /dev/null

4. Systemd Targets & Operating States

A target unit (.target) does not run a daemon directly. Instead, it groups other unit files together using dependency symlinks located in <target_name>.wants/ and <target_name>.requires/ directories.

Standard Systemd Targets

5. System Administration with systemctl

systemctl is the central CLI utility for inspecting, controlling, enabling, and diagnosing systemd units.

Service Lifecycle Management

• sudo systemctl start <unit>: Starts (activates) the unit immediately.
• sudo systemctl stop <unit>: Stops (deactivates) the unit immediately.
• sudo systemctl restart <unit>: Stops and restarts the unit.
• sudo systemctl reload <unit>: Instructs the service to reload its configuration files without dropping active connections (e.g., nginx -s reload).
• sudo systemctl reload-or-restart <unit>: Reloads configuration if supported; otherwise, restarts the service.

Boot Enablement & State Inspection

• sudo systemctl enable <unit>: Enables the unit to start at boot by creating a symlink in the target's .wants/ directory under /etc/systemd/system/.
• sudo systemctl disable <unit>: Disables automatic startup by deleting the .wants/ symlink.
• sudo systemctl enable --now <unit>: Enables and immediately starts the service in a single command.
• systemctl status <unit>: Displays unit state (active, inactive, failed), PID, memory usage, cgroup processes, and recent log snippets.
• systemctl is-active <unit>: Returns exit code 0 and prints active if running.
• systemctl is-enabled <unit>: Returns exit code 0 and prints enabled if configured for auto-start.
• systemctl is-failed <unit>: Checks if the unit crashed or entered an error state.
• systemctl list-units --type=service: Lists all currently loaded service units in memory.
• systemctl list-unit-files: Lists all unit files installed on disk and their enablement status.

Target Operations & System Control

• systemctl get-default: Queries and prints the current default boot target.
• sudo systemctl set-default graphical.target: Updates the /etc/systemd/system/default.target symlink.
• sudo systemctl isolate multi-user.target: Immediately switches the system state to multi-user.target, stopping all units not part of the new target.
• sudo systemctl reboot: Reboots the machine.
• sudo systemctl poweroff: Powers off the machine.