XDG MIME Applications

Applications describe what MIME types they can handle using デスクトップエントリ.
desktop-file-utils registers a pacman フック to build a cache database of MIME types handled by desktop entries, see update-desktop-database(1).
Applications can install new MIME types by placing XML files in /usr/share/mime/packages/.
shared-mime-info registers a pacman フック to build the Shared MIME-Info database cache, see update-mime-database(1).
デスクトップ環境s and users can change default applications and add or remove MIME type to application associations using mimeapps.list files.

Shared MIME database

The XDG Shared MIME-info Database specification facilitates a shared MIME database across desktop environments and allows applications to easily register new MIME types system-wide.

The database is built from the XML files installed by packages in /usr/share/mime/packages/ using the tools from shared-mime-info.

The files in /usr/share/mime/ should not be directly edited, however it is possible to maintain a separate database on a per-user basis in the ~/.local/share/mime/ tree.

"URI scheme handling [..] are handled through applications handling the x-scheme-handler/foo MIME type, where foo is the URI scheme in question."[1]

New MIME types

This example defines a new MIME type application/x-foobar and assigns it to any file with a name ending in .foo. Simply create the following file:

~/.local/share/mime/packages/application-x-foobar.xml

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
    <mime-type type="application/x-foobar">
        <comment>foo file</comment>
        <icon name="application-x-foobar"/>
        <glob-deleteall/>
        <glob pattern="*.foo"/>
    </mime-type>
</mime-info>

And then update the MIME database:

$ update-mime-database ~/.local/share/mime

Of course this will not have any effect if no desktop entries are associated with the MIME type. You may need to create new デスクトップエントリ or modify mimeapps.list.

mimeapps.list

The XDG standard is the most common for configuring desktop environments. Default applications for each MIME type are stored in mimeapps.list files, which can be stored in several locations. They are searched in the following order, with earlier associations taking precedence over later ones:

Path	Usage
`~/.config/mimeapps.list`	user overrides
`/etc/xdg/mimeapps.list`	system-wide overrides
`~/.local/share/applications/mimeapps.list`	(deprecated) user overrides
`/usr/local/share/applications/mimeapps.list` `/usr/share/applications/mimeapps.list`	distribution-provided defaults

Additionally, it is possible to define デスクトップ環境-specific default applications in a file named desktop-mimeapps.list where desktop is the name of the desktop environment (from the XDG_CURRENT_DESKTOP environment variable). For example, /etc/xdg/xfce-mimeapps.list defines system-wide default application overrides for Xfce. These desktop-specific overrides take precedence over the corresponding non-desktop-specific file. For example, /etc/xdg/xfce-mimeapps.list takes precedence over /etc/xdg/mimeapps.list but is still overridden by ~/.config/mimeapps.list.

ヒント: Although deprecated, several applications still read/write to ~/.local/share/applications/mimeapps.list. To simplify maintenance, simply symlink it to ~/.config/mimeapps.list:

$ ln -s ~/.config/mimeapps.list ~/.local/share/applications/mimeapps.list

ノート: You might also find files in these locations named defaults.list. This file is similar to mimeapps.list except it only lists default applications (not added/removed associations). It is now deprecated and should be manually merged with mimeapps.list.

To discover all the files that are scanned it is possible to enable debug mode by setting the environment variable XDG_UTILS_DEBUG_LEVEL=2: e.g. the xdg-mime query default <type> command will print each configuration file it is searchin for mime information.

Format

Consider the following example:

mimeapps.list

[Added Associations]
image/jpeg=bar.desktop;baz.desktop
video/H264=bar.desktop
[Removed Associations]
video/H264=baz.desktop
[Default Applications]
image/jpeg=foo.desktop

Each section assigns one or more desktop entries to MIME types.

Added Associations indicates that the applications support opening that MIME type. For example, bar.desktop and baz.desktop can open JPEG images. This might affect the application list you see when right-clicking a file in a file browser.
Removed Associations indicates that the applications do not support that MIME type. For example, baz.desktop cannot open H.264 video.
Default Applications indicates that the applications should be the default choice for opening that MIME type. For example, JPEG images should be opened with foo.desktop. This implicitly adds an association between the application and the MIME type. If there are multiple applications, they are tried in order.

Each section is optional and can be omitted if unneeded.

Utilities

While it is possible to configure default applications and MIME types by directly editing mimeapps.list and the shared MIME database, there are many tools that can simplify the process. These tools are also important because applications may delegate opening of files to these tools rather than trying to implement the MIME type standard themselves.

If you use a デスクトップ環境 you should first check if it provides its own utility. That should be preferred over these alternatives.

The official xdg-utils contain tools for managing MIME types and default applications according to the XDG standard (xdg-mime). Most importantly it provides xdg-open which many applications use to open a file with its default application.

lsdesktopf

lsdesktopf^AUR provides several methods of searching the MIME database and desktop MIME entries.

For example, to see all MIME extensions in the system's .desktop files that have MIME type video you can use lsdesktopf --gm -gx video or to search in the XML database files use lsdesktopf --gdx -gx video. To get a quick overview of how many and which .desktop files can be associated with a certain MIME type, use lsdesktopf --gen-mimeapps. To see all file name extensions in XML database files, use lsdesktopf --gdx -gfx.

selectdefaultapplication

selectdefaultapplication-git^AUR is GUI application that lists up all applications supporting various mimetypes and lets you quickly set it as default for all or some of the mimetypes it supports (by modifying mimeapps.list).

It shows the "readable" name and file extensions as well, so you do not need to remember the name of the mimetypes.

Troubleshooting

If a file is not being opened by your desired default application, there are several possible causes. You may need to check each case.

Missing desktop entry

A デスクトップエントリ is required in order to associate an application with a MIME type. Ensure that such an entry exists and can be used to (manually) open files in the application.

Missing association

If the application's desktop entry does not specify the MIME type under its MimeType key, it will not be considered when an application is needed to open that type. Edit mimeapps.list to add an association between the .desktop file and the MIME type.

Non-default application

If the desktop entry is associated with the MIME type, it may simply not be set as the default. Edit mimeapps.list to set the default association.

Nonstandard association

Applications are free to ignore or only partially implement the XDG standard. Check for usage of deprecated files such as ~/.local/share/applications/mimeapps.list and ~/.local/share/applications/defaults.list. If you are attempting to open the file from another application (e.g. a web browser or file manager) check if that application has its own method of selecting default applications.

Variables in .desktop files that affect application launch

Desktop environments and file managers supporting the specifications launch programs according to definition in the .desktop files. See デスクトップエントリ#アプリケーションエントリ.

Usually, configuration of the packaged .desktop files is not required, but it may not be bug-free. Even if an application containing necessary MIME type description in the .desktop file MimeType variable that is used for association, it can fail to start correctly, not start at all or start without opening a file.

This may happen, for example, if the Exec variable is missing internal options needed for how to open a file, or how the application is shown in the menu. The Exec variable usually begins with %; for its currently supported options, see exec-variables.

The following table lists the main variable entries of .desktop files that affect how an application starts, if it has a MIME type associated with it.

Variable names	Example 1 content	Example 2 content	Description
DBusActivatable	DBusActivatable=true	DBusActivatable=false	Application interact with D-Bus. See also configuration: D-Bus.
MimeType	MimeType=application/vnd.oasis.opendocument.text	MimeType=application/vnd.sun.xml.math	List of MIME types supported by application
StartupWMClass	StartupWMClass=google-chrome	StartupWMClass=xpad	Associate windows with the owning application
Terminal	Terminal=true	Terminal=false	Start in default terminal