What is the difference between exec and dbus-launch

d-busexec

If I start a process, particularly at session start, what is the difference between using exec p and dbus-launch p?

Best Answer

dbus-launch is DBus-related. exec is a shell built-in. See their manpages/help pages:

dbus-launch - Utility to start a message bus from a shell script.
exec - Replace the shell with the given command.

So... they basically do two different things... While both are expected to start program p passed as a parameter, you can read that in the case of dbus-launch, a few other things are done first:

You may specify a program to be run; in this case, dbus-launch will launch a session bus instance, set the appropriate environment variables so the specified program can find the bus, and then execute the specified program, with the specified arguments.

In this case, a DBus daemon instance is created prior to creating the process, and it should be running a secondary session bus, which is made available to the child process through environment variables.

Also note that in the case of exec, the current shell is replaced by a new program. dbus-launch will not replace the shell, and nor will the program you start alongside the daemon.

Related Solutions

KDE – Running a dbus Program in Crontab and Finding the SESSION ID

The problem is somewhat similar to accessing the X display and finding the location of the X cookie file. (Also, refer to these questions if you want to launch a GUI program on the user's display.)

Dbus stores the session address in a file in ~/.dbus/session-bus. The name of the file is $machine_id-$display_number, where $machine_id is a randomly generated number stored in /var/lib/dbus/machine-id and $display_number is the X display number ($DISPLAY is :$display_number or :$display_number.$screen_number). The file in ~/.dbus/session-bus is parseable by a shell and contains definitions for DBUS_SESSION_BUS_ADDRESS and DBUS_SESSION_BUS_PID.

dbus_session_file=~/.dbus/session-bus/$(cat /var/lib/dbus/machine-id)-0
if [ -e "$dbus_session_file" ]; then
  . "$dbus_session_file"
  export DBUS_SESSION_BUS_ADDRESS DBUS_SESSION_BUS_PID
  dbus-send …
fi

Beware that there's no guarantee that the dbus daemon is still available. The user may have logged out.

An alternative method is to find the PID of a process in the desktop session, and obtain the dbus address from its environment.

export $(</proc/$pid/environ tr \\0 \\n | grep -E '^DBUS_SESSION_BUS_ADDRESS=')

If the crontab is running as root and you want to communicate with the session of whatever user is logged in on the console, see Can I launch a graphical program on another user's desktop as root?

Linux – Trouble with dbus and systemd/user

If DBUS_SESSION_BUS_ADDRESS is not set for a process that tries to use D-Bus, by default that process will attempt to invoke dbus-launch with the --autolaunch option to start up a new session bus or find the existing bus address on the X display or in a file in ~/.dbus/session-bus/.

When dbus is not being used with Xorg, as much in the same manner as $DISPLAY works to use a different server, you can set this environment variable in order to tell dbus which bus session to use:

export DBUS_SESSION_BUS_ADDRESS=""

For more info, read http://dbus.freedesktop.org/doc/dbus-launch.1.html