How can I open the settings menu from bash on Xubuntu?
The main settings panel is
but, if you want to open specific settings dialog there are other commands to help you. Like
xfce4-mime-settings (opens mime editor) xfce4-session-settings (startup/auto-start settings) xfce4-accessibility-settings xfce4-mouse-settings xfce4-settings-editor xfce4-appearance-settings xfce4-notes-settings xfce4-notifyd-config (notification settings) xfce4-clipman-settings (default clip manager's) xfce4-power-manager-settings xfce4-display-settings xfce4-keyboard-settings
Other settings purpose can be easily derived from name.