Service Management

systemd is the default init system and service manager in many modern Linux distributions. It is responsible for:

  • Initializing the system at boot.
  • Managing system and user services.
  • Handling system states and targets (boot modes) See System startup.
  • Logging via the system journal.

Unlike traditional init systems, systemd starts services in parallel, which can speed up boot times. It manages units, which are objects that describe system resources and how they should be handled.

Unit Types

A unit is a configuration object that represents something systemd can manage. Common unit types include:

  • Service units (.service) – background processes such as daemons or applications.
  • Target units (.target) – groupings of units representing a system state (e.g., graphical.target, multi-user.target).
  • Socket units (.socket) – network or IPC sockets that can trigger services on demand.
  • Device units (.device) – represent hardware devices recognized by the kernel.
  • Mount and automount units (.mount, .automount) – define and manage filesystem mounts.
  • Timer units (.timer) – scheduled tasks, similar to cron, but managed by systemd.

Unit Files and Directories

Unit definitions are stored in configuration files with a .unit extension (e.g., .service, .target).

systemd loads unit files from multiple directories in order of precedence:

  1. /etc/systemd/system/ – local administrator-defined units (highest priority).
  2. /run/systemd/system/ – runtime-generated units (cleared on reboot).
  3. /lib/systemd/system/ or /usr/lib/systemd/system/ – distribution-provided units (lowest priority).

Unit files are INI-style and generally have these sections:

[Unit]
Description=Description of the service
Documentation=URL or path to docs
After=other.service
Requires=dependency.service

[Service]
ExecStart=/path/to/command
User=username
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

How systemd Starts and Manages Services

  1. Boot process – The kernel starts systemd as the first user-space process (PID 1).
  2. Default target – systemd determines the default boot state (e.g., graphical.target).
  3. Dependency resolution – Units required by the target are started, respecting dependencies defined in Requires=, Wants=, and After=.
  4. Parallel execution – Independent units are started simultaneously.
  5. Ongoing management – Services are monitored and can be restarted automatically if they fail (Restart= option).

Managing Services with systemctl

systemctl is the primary command-line tool for interacting with systemd.

Service control

systemctl start <service>     # Start a service
systemctl stop <service>      # Stop a running service
systemctl restart <service>   # Restart a service
systemctl reload <service>    # Reload service configuration

Enable/disable on boot

systemctl enable <service>    # Start at boot
systemctl disable <service>   # Do not start at boot

Status and inspection

systemctl status <service>    # View service status and recent logs
systemctl list-units          # List active units
systemctl list-unit-files     # List all available units and enablement state

Reloading definitions

systemctl daemon-reload       # Reload unit files after editing

Logging with journalctl

systemd uses the journal to store logs for the entire system. Logs can be queried with journalctl.

Common options:

journalctl -b           # Logs from current boot
journalctl -u <unit>    # Logs for a specific service
journalctl --since "1 hour ago" # Filter by time