xdg-settings

Manage settings of XDG-compatible desktop environments. More information: https://portland.freedesktop.org/doc/xdg-settings.html.

Print the default web browser:

xdg-settings get {{default-web-browser}}

Set the default web browser to Firefox:

xdg-settings set {{default-web-browser}} {{firefox.desktop}}

Set the default mail URL scheme handler to Evolution:

xdg-settings set {{default-url-scheme-handler}} {{mailto}} {{evolution.desktop}}

Set the default PDF document viewer:

xdg-settings set {{pdf-viewer.desktop}}

Display help:

xdg-settings --help