Fcitx-dbus-status addon provides DBus interface such as org.fcitx.Fcitx.Status, and provides Get/Set/Changed methods on it. By accessing these interfaces, you can monitor or control status of input method with external command line tool such as dbus-send. As a typical use case, you can set "katakana" mode in the following example for fcitx-mozc.

dbus-send \ --dest=org.fcitx.Fcitx \ --type=method_call \ --print-reply \ /Status \ org.fcitx.Fcitx.Status.Set \ string:"mozc-composition-mode" \ string:"katakana"