Updating shells in tmux for new X sessions, specifically GNOME shell

.tmux.conf

set -g update-environment 'DISPLAY SSH_ASKPASS SSH_AUTH_SOCK SSH_AGENT_PID SSH_CONNECTION WINDOWID XAUTHORITY DBUS_SESSION_BUS_ADDRESS'

precmd.zsh

precmd() {
        if [ -n "${TMUX}" ]; then
                local _tmux_env;
                _tmux_env=$(tmux show-environment);
                if [ "${_tmux_env}" != "${_expected_tmux_env}" ]; then
                        eval $(echo "${_tmux_env}" | sed -e '/^-/!{ s/=/="/; s/$/"/; s/^/export /; }' -e 's/^-/unset /' -e 's/$/;/');
                        _expected_tmux_env="${_tmux_env}";
                fi;
        fi;
}

# vim: filetype=zsh :

Just drop this function into your zsh infrastructure and get it loaded in your shells and ensure your tmux also updates DBUS_SESSION_ADDRESS.

This will automatically source all things you set in update-environment - especially DBUS_SESSION_BUS_ADDRESS - to update to your current runtime environment. So if you have to shoot your X session in the head and get a new one, just push enter in your shell to still be able to talk to your new ssh agent started with the D-Bus session.