dbus-daemon

Run the daemon with a configuration file

$ dbus-daemon --config-file [path/to/file]

$ dbus-daemon --session

$ dbus-daemon --system

$ dbus-daemon --address [address]

$ dbus-daemon --print-pid

$ dbus-daemon --syslog

dbus-daemon [OPTIONS]

--system
    Run as the system-wide message bus daemon.

--session
    Run as a per-session message bus daemon (default if no other bus type is specified).

--config-file=FILE
    Use the specified configuration file instead of the default.

--fork
    Fork into the background after startup.

--nofork
    Do not fork into the background (this is the default behavior).

--print-address[=FILE]
    Print the bus address to standard output or a specified file.

--print-pid[=FILE]
    Print the process ID (PID) to standard output or a specified file.

--address=ADDRESS
    Listen on the specified D-Bus address instead of addresses from configuration.

--pidfile=FILE
    Write the daemon's PID to the specified file.

--nopidfile
    Do not write a PID file (overrides --pidfile option).

--version
    Display the version information and exit.

--help
    Display a help message and exit.

--system-d-activation
    Enable systemd service activation. This is typically used when launched by systemd itself.

--activation-only
    Only allow service activation, not direct messages. Useful for internal daemon startup.

The dbus-daemon is the core component of the D-Bus message bus system, a powerful and flexible mechanism for inter-process communication (IPC) on Linux and Unix-like operating systems. It acts as a central hub, enabling different applications and system services to communicate with each other by sending and receiving messages.

There are two primary types of D-Bus instances managed by dbus-daemon: the system bus and the session bus. The system bus provides a system-wide communication channel, typically used for low-level system services like hardware events, network configuration, and power management. In contrast, the session bus is launched per user login session, facilitating communication between desktop applications, such as a file manager notifying a text editor to open a file.

The daemon handles connection management, message routing, and enforces security policies defined in its configuration files. It also supports "service activation," where services are automatically launched on demand when a message is sent to them, improving system responsiveness and resource utilization.

The dbus-daemon is rarely started manually by users; it is typically launched automatically by system initialization systems like systemd for the system bus, or by display managers and desktop environments for the session bus.

Its behavior is heavily dependent on XML configuration files (e.g., /etc/dbus-1/system.conf and /usr/share/dbus-1/session.conf), which define crucial aspects like security policies (who can connect, send, and receive messages), service activation rules, and bus addresses. Misconfigurations can lead to security vulnerabilities or applications failing to communicate.

The behavior of dbus-daemon is primarily controlled by XML-based configuration files. For the system bus, the main configuration is typically found at /etc/dbus-1/system.conf, and for the session bus, a common path is /usr/share/dbus-1/session.conf or user-specific files in ~/.local/share/dbus-1/. These files define critical settings such as allowed connections, security policies (e.g., what messages can be sent or received by specific users or groups), and rules for service activation.

The system bus is a single instance running globally on the system, typically started by systemd. It handles communication for system-wide services (e.g., UDisks, NetworkManager, Polkit). The session bus is instantiated for each logged-in user session, managing communication between desktop applications belonging to that user (e.g., LibreOffice communicating with your desktop environment to open a file).

A key feature of D-Bus is service activation. Instead of services running continuously and consuming resources, dbus-daemon can activate a service only when an application attempts to communicate with it. This is managed through configuration files that map D-Bus service names to executable paths. When a message for a registered but inactive service arrives, dbus-daemon automatically launches the service, improving system efficiency.

D-Bus was initiated by freedesktop.org in the early 2000s as a standardized and simpler inter-process communication system. Its primary goal was to replace existing, more complex IPC mechanisms like CORBA, Bonobo, and DCOP that were prevalent in different desktop environments (GNOME and KDE, respectively). The development of dbus-daemon aimed to provide a unified solution for desktop applications to communicate with each other and with the underlying operating system. It quickly became a fundamental component for modern Linux desktop environments and has since expanded its role to include communication with various system services, making it an indispensable part of the Linux ecosystem.

dbus-send(1), dbus-monitor(1), dbus-launch(1), systemd(1), dbus-daemon(8)