Search notes:

XDG

freedesktop.org / XDG @ XDG

XDG stands for X Desktop Group, which is the name by which freedesktop.org was formerly known.

Environment variables

$XDG_RUNTIME_DIR might point to /run/user/$uid.

XDG_CONFIG_DIRS, XDG_CONFIG_HOME, XDG_DATA_DIRS, XDG_DATA_HOME, XDG_CACHE_HOME

mimeapps.list

~/.config/mimeapps.list
/etc/xdg/mimeapps.list
~/.local/share/applications/mimeapps.list (deprecated)
/usr/local/share/applications/mimeapps.list
/usr/share/applications/mimeapps.list

xdg-utils

Utilities to manage MIME types.

xdg-mime, xdg-open

Debugging xdg-utils

$ export XDG_UTILS_DEBUG_LEVEL=10
$ xdg-open …

See also

Association between MIME type and application

https://cgit.freedesktop.org/xdg/shared-mime-info/