How an application is chosen over others to open a particular filetype in Linux

file openingfreedesktopmime-types

Earlier my text/xml files were by default opened by firefox I think. Not sure of this because I do not remember exactly. Then I think I installed geany and then my text/xml files were always by default opened by Geany until I installed google-chrome. Now I see they are by default opened by google-chrome.

Until now I did not bother to check what are the default applications associated with a particular filetype.

Now when I check my /usr/share/applications/defaults.list file I see this:

text/html=firefox.desktop;google-chrome.desktop
text/xml=google-chrome.desktop;
application/xhtml_xml=google-chrome.desktop;

My default browser is still set to firefox. The first line above shows both Firefox and Chrome for text/html but for text/xml it is only Chrome.

I think earlier it was Geany for text/xml because that's the reason by default Geany used to open XML files. I'm not sure on this because I've never checked.

Questions

I'm wondering how it is set. How a particular application is chosen over another.
Also how the semicolon (";") is read in the file.

I'm using Fedora 20 – Mate desktop environment.

Query based on Answer by slm

Based on the Answer by @slm, I see my xml filetypes are associated with Geany.
But I see them opening with Chrome instead.

$ xdg-mime query default application/xml
geany.desktop
$

As I mentioned above my /usr/share/applications/defaults.list file shows

text/xml=google-chrome.desktop;

So what is the difference and which should override?

Best Answer

I usually use the command line tool xdg-mime to determine what applications are associated with a given MIME type.

$ xdg-mime query filetype IMAGE.PNG 
image/png

$ xdg-mime query default image/png
shotwell-viewer.desktop shutter.desktop

You can make this a single command like so:

$ xdg-mime query default $(xdg-mime query filetype IMAGE.PNG)
shotwell-viewer.desktop shutter.desktop

You can change the default using mimeopen like so:

$ mimeopen -d ~/test.pdf

Please choose a default application for files of type application/pdf

    1) E-book Viewer  (calibre-ebook-viewer)
    2) Document Viewer  (evince)
    3) Xournal  (xournal)
    4) GNU Image Manipulation Program  (gimp)
    5) Xpdf PDF Viewer  (xpdf)
    6) Print Preview  (evince-previewer)
    7) Inkscape  (inkscape)
    8) calibre  (calibre-gui)
    9) Other...

use application #2
Opening "/home/saml/Downloads/test.pdf" with Document Viewer  (application/pdf)

NOTE: By the way, the semi-colons are there because there can be multiple associations for a given MIME type. So text/xml could have google-chrome.desktop;firefox.desktop; as it's value would would indicate that there are 2 apps that can handle that particular type.

References

Related Solutions

Linux – How to Find Application for a MIME Type

The xdg-mime command is used to query or set file associations.

jinx:735 Z$ xdg-mime query default application/pdf  
evince.desktop

Linux File Opening – How Does Linux Choose Which Application to Open a File?

There may be different mechanisms to handle these default settings. However, other answers tend to focus on complete desktop environments, each of them with its own mechanism. Yet, these are not always installed on a system (I use OpenBox a lot), and in this case, tools such as xdg-open may be used.

Quoting the Arch Wiki:

xdg-open is a desktop-independent tool for configuring the default applications of a user. Many applications invoke the xdg-open command internally.

At this moment, I am using Ubuntu (12.04) and xdg-open is available. However, when you use a complete desktop environment such as GNOME, xdg-open acts as a simple forwarder, and relays the file requests to your DE, which is then free to handle it as it wants (see other answers for GNOME and Nautilus, for instance).

Inside a desktop environment (e.g. GNOME, KDE, or Xfce), xdg-open simply passes the arguments to that desktop environment's file-opener application (gvfs-open, kde-open, or exo-open, respectively), which means that the associations are left up to the desktop environment.

... which brings you back to the other answers in that case. Still, since this is Unix & Linux, and not Ask Ubuntu:

When no desktop environment is detected (for example when one runs a standalone window manager, e.g. Openbox), xdg-open will use its own configuration files.

All in all:

                            |-- no desktop env. > handle directly.
User Request > xdg-open > --|
                            |-- desktop env.    > pass information to the DE.

If the first case, you'll need to configure xdg-open directly, using the xdg-mime command (which will also allow you to see which application is supposed to handle which file). In the second case...

                        |-- GNOME? > gvfs-open handles the request.
                        |
Info. from xdg-open > --|-- KDE?   > kde-open handles the request.
                        |
                        |-- XFCE?  > exo-open handles the request.

... you'll need to configure the file-opener associated with your desktop environment. In some cases, configuration made through xdg-mime may be redirected to the proper configuration tool in your environment.