== How _xdg-mime_ searches for MIME type handlers (more or less)

XDG is what was once called the X Desktop Group and is now
[[freedesktop.org https://www.freedesktop.org/]]. Part of their
work is to create various cross-desktop and desktop agnostic standards
and tools for how to do things like determine what program should
handle a particular MIME type. This includes special 'scheme' MIME
types, like _x-scheme-handler/ftp_, which means 'whatever should
handle FTP' (technically this is 'FTP urls').
The XDG tool for mapping MIME types to programs (actually _.desktop_
files) is [[_xdg-mime_
https://portland.freedesktop.org/doc/xdg-mime.html]].  Like all of
the XDG tools, _xdg-mime_ uses a collection of environment variables,
which will normally have the default values covered in [[the XDG
base directory specification
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html]].

In theory there are two sorts of data that _xdg-mime_ looks at. The
first, found in files called _<desktop>-mimeapps.list_ and
_mimeapps.list_, is supposed to be a set of specifically chosen
applications (whether configured by the user or the system).  The
second is a general cache of MIME type mapping information based
on what program can handle what MIME types (as listed in each
program's _.desktop_ file); these are found in files called
_defaults.list_ and _mimeinfo.cache_. In practice, a system
_mimeapps.list_ file may have just been slapped together based on
what the distribution thought was a good idea, and it may not
correspond to what you want (or even what you have installed).

(Not all Linux distributions ship a system _mimeapps.list_.
Fedora does; Ubuntu doesn't.)

Xdg-mime searches for _mimeapps.list_ in any of (($XDG_CONFIG_HOME)),
(($XDG_CONFIG_DIRS)), and the '_applications_' subdirectories of
(($XDG_DATA_HOME)) and (($XDG_DATA_DIRS)). It searches for the other
two files only in the _applications_ subdirectories of (($XDG_DATA_HOME))
and (($XDG_DATA_DIRS)).

Now, we get to an important consequence of this. (($XDG_DATA_DIRS))
is normally two directories, _/usr/local/share_ and _/usr/share_,
each of which has MIME type information only for itself, and they
are checked in order. The normal result of this is that ~~things
installed into _/usr/local_ steal MIME types from things installed
in _/usr_~~ because _xdg-mime_ will check the
_/usr/local/share/applications_ files first.

(I discovered this when I installed Google Chrome on my laptop and
it promptly stole URL handling from Firefox, which I did not want,
because the RPM put bits into _/usr/local_ instead of _/usr_.
Actually finding the files that controlled this was remarkably
hard.)

Normally, nothing automatically generates or updates the system
_mimeapps.list_; on Fedora, it's part of the _shared-mime-info_ RPM
(and _kde-settings_ for the _kde-mimeapps.list_ version). The
_mimeinfo.cache_ files are maintained by [[_update-desktop-database_
http://manpages.ubuntu.com/manpages/precise/man1/update-desktop-database.1.html]],
which will normally be automatically run on package installs, removals,
and probably upgrades.

Now, _xdg-mime_ does not give you an actual program to run. What
it does is give you the name of a _.desktop_ file, eg _firefox.desktop_.
In order to use this to run a program, you must look through the
right magic places to find the _.desktop_ file and then parse it
to determine the command line to run. Probably you don't want to
do this yourself, but I don't know what your alternative is; as far
as I know, there is no XDG tool script to say 'run this _.desktop_
command with the following arguments'.

(Note that the _.desktop_ file is not necessarily in the same
directory as the MIME mapping file that caused it to be the handler
application. For example, your _$HOME/.local/share/applications_
might just have various MIME type overrides you've set for what
system application should handle what.)

This explanation is somewhat dependent on what exact version of the
XDG tools and scripts you have installed. It's also not necessarily
totally complete, because I am reading through undocumented shell
scripts here and I've left a few things out. See [[the Arch wiki
entry on default applications
https://wiki.archlinux.org/index.php/default_applications]] for
much more information.

PS: If you're trying to track down why _xdg-mime_ is giving you
some strange result, set (($XDG_UTILS_DEBUG_LEVEL)) to at least
'2'. This will tell you just what files it looked at when,
although I don't think it ever reports what directories it looked
at but didn't find any files in.