dbus-launch [--version] [--sh-syntax] [--csh-syntax] [--auto-syntax] [--exit-with-session] [--autolaunch=MACHINEID] [--config-file=FILENAME] [PROGRAM] [ARGS...]
The dbus-launch command is used to start a session bus instance of dbus-daemon from a shell script. It would normally be called from a user's login scripts. Unlike the daemon itself, dbus-launch exits, so backticks or the $() construct can be used to read information from dbus-launch.
With no arguments, dbus-launch will launch a session bus instance and print the address and pid of that instance to standard output.
You may specify a program to be run; in this case, dbus-launch will launch a session bus instance, set the appropriate environment variables so the specified program can find the bus, and then execute the specified program, with the specified arguments. See below for examples.
If you launch a program, dbus-launch will not print the information about the new bus to standard output.
When dbus-launch prints bus information to standard output, by default it is in a simple key-value pairs format. However, you may request several alternate syntaxes using the --sh-syntax, --csh-syntax, --binary-syntax, or --auto-syntax options. Several of these cause dbus-launch to emit shell code to set up the environment.
With the --auto-syntax option, dbus-launch looks at the value of the SHELL environment variable to determine which shell syntax should be used. If SHELL ends in "csh", then csh-compatible code is emitted; otherwise Bourne shell code is emitted. Instead of passing --auto-syntax, you may explicity specify a particular one by using --sh-syntax for Bourne syntax, or --csh-syntax for csh syntax. In scripts, it's more robust to avoid --auto-syntax and you hopefully know which shell your script is written in.
See http://www.freedesktop.org/software/dbus/ for more information about D-Bus. See also the man page for dbus-daemon.
Here is an example of how to use dbus-launch with an sh-compatible shell to start the per-session bus daemon:
## test for an existing bus daemon, just to be safe
if test -z "$DBUS_SESSION_BUS_ADDRESS" ; then
## if not found, launch a new one
eval `dbus-launch --sh-syntax --exit-with-session`
echo "D-Bus per-session daemon address is: $DBUS_SESSION_BUS_ADDRESS"
fi
You might run something like that in your login scripts.
Another way to use dbus-launch is to run your main session program, like so:
dbus-launch gnome-sessionThe above would likely be appropriate for ~/.xsession or ~/.Xclients.
If DBUS_SESSION_BUS_ADDRESS is not set for a process that tries to use D-Bus, by default the 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/
Whenever an autolaunch occurs, the application that had to start a new bus will be in its own little world; it can effectively end up starting a whole new session if it tries to use a lot of bus services. This can be suboptimal or even totally broken, depending on the app and what it tries to do.
There are two common reasons for autolaunch. One is ssh to a remote machine. The ideal fix for that would be forwarding of DBUS_SESSION_BUS_ADDRESS in the same way that DISPLAY is forwarded. In the meantime, you can edit the session.conf config file to have your session bus listen on TCP, and manually set DBUS_SESSION_BUS_ADDRESS, if you like.
The second common reason for autolaunch is an su to another user, and display of X applications running as the second user on the display belonging to the first user. Perhaps the ideal fix in this case would be to allow the second user to connect to the session bus of the first user, just as they can connect to the first user's display. However, a mechanism for that has not been coded.
You can always avoid autolaunch by manually setting DBUS_SESSION_BUS_ADDRESS. Autolaunch happens because the default address if none is set is "autolaunch:", so if any other address is set there will be no autolaunch. You can however include autolaunch in an explicit session bus address as a fallback, for example DBUS_SESSION_BUS_ADDRESS="something:,autolaunch:" - in that case if the first address doesn't work, processes will autolaunch. (The bus address variable contains a comma-separated list of addresses to try.)
The --autolaunch option is considered an internal implementation detail of libdbus, and in fact there are plans to change it. There's no real reason to use it outside of the libdbus implementation anyhow.
--binary-syntax Write to stdout a nul-terminated bus address, then the bus PID as a binary integer of size sizeof(pid_t), then the bus X window ID as a binary integer of size sizeof(long). Integers are in the machine's byte order, not network byte order or any other canonical byte order.