How do I set an environment variable in a Unity session?

The easy way to set personal environment variables for Unity (or any other X session) is to add them to the file $HOME/.xsessionrc. These variables are seen by everything launched in the session, via menus, GUIs, terminal windows, etc. For example, put this text in $HOME/.xsessionrc:

# File sourced by /etc/X11/Xsession, for setting environment variables, etc.
export FOOVAR=testing

Log out and back in, then from a shell window:

my-pc> echo $FOOVAR
testing

Tested under 12.04, using Unity. But this feature is implemented by the /etc/X11/Xsession script used by many types of desktop session, so it should work with any session type. See the Xsession man page.

Create a file in /etc/X11/Xsession.d/ for it, e.g. /etc/X11/Xsession.d/98local (it has to be executed before 99x11-common_start).

If this doesn't work try to find out where the variable gets set. Something like

sudo grep -r IBUS_ENABLE_SYNC_MODE /etc/

may help here to find the file.