Where are file associations stored?

The file manager (Nautilus, by default) uses the MIME type of a file to determine which program to open it with. When an application is installed, it can specify what MIME types it can open and the command to use to open the files in the .desktop file which is placed in /usr/share/applications. This is the file used for menus, desktop shortcuts, etc.

For example, GIMP has the following .desktop file:

[Desktop Entry]
Version=1.0
Type=Application
Name=GNU Image Manipulation Program
GenericName=Image Editor
Comment=Create images and edit photographs
Exec=gimp-2.7 %U
TryExec=gimp-2.7
Icon=gimp
Terminal=false
Categories=Graphics;2DGraphics;RasterGraphics;GTK;
X-GNOME-Bugzilla-Bugzilla=GNOME
X-GNOME-Bugzilla-Product=GIMP
X-GNOME-Bugzilla-Component=General
X-GNOME-Bugzilla-Version=2.7.2
X-GNOME-Bugzilla-OtherBinaries=gimp-2.7
StartupNotify=true
MimeType=application/postscript;application/pdf;image/bmp;image/g3fax;image/gif;image/x-fits;image/pcx;image/x-portable-anymap;image/x-portable-bitmap;image/x-portable-graymap;image/x-portable-pixmap;image/x-psd;image/x-sgi;image/x-tga;image/x-xbitmap;image/x-xwindowdump;image/x-xcf;image/x-compressed-xcf;image/tiff;image/jpeg;image/x-psp;image/png;image/x-icon;image/x-xpixmap;image/svg+xml;application/pdf;image/x-wmf;image/jp2;image/jpeg2000;image/jpx;image/x-xcursor;

See the MimeType field - this lists the supported MIME types.The Exec field tells the system to use the command gimp-2.7 %U, replacing '%U' with the files to open. (Note GIMP 2.7 is a version I have installed from a PPA, so is higher than the current version in the Ubuntu repositories).

When the application package is installed, the system extracts this MIME type data into a more easily accessible database because looking in each .desktop file would take too long if it was done every time a file was opened.

This tells the system what applications can be used for that MIME type and provides the applications in the 'Open With' list. The default is defined elsewhere. The file /usr/share/applications/defaults.list provides the information for the system defaults. Unless you choose otherwise, these are the applications used when you 'Open' a file.

To supplement dv3500ea's excellent answer, I would like to add some information about what happens when you change your associations.

While the defaults.list (which you can find by typing 'locate defaults.list') provides the list of applications that are associated with each MIME type, any customizations that you make are stored in your home directory, in ~/.local/share/applications/mimeapps.list. So if you add or remove associations, or change the default association, this file is updated.

The GNOME Desktop System Administration Guide at http://library.gnome.org/admin/system-admin-guide/stable/ does not appear to discuss mimeapps.list, but I found the following description on http://live.gnome.org/SysAdminGuideUpdate:

mimeapps.list is located in $XDG_DATA_DIRS/applications. Its purpose is to add or remove mime associations from applications. nautilus writes ~/.local/share/applications/mimeapps.list when the user makes changes in the "Open With" dialog.

I found that on my system (Debian Jessie) there's also a ~/.config/mimeapps.list that had an association I accidentally created to open a specific file type with gedit. None of the standard ways of correcting this (default application settings, Thunar's open with property) reflected this, but gedit was the default application.

I was able to remove the line from ~/.config/mimeapps.list and now the correct application opens the file.

I was able to set an association like this:

xdg-mime default xnview.desktop image/jpeg

You can also remove associations and do other things: man xdg-mime

I did not need to run sudo update-desktop-database.

I found this link concerning default association, it might be helpful. https://wiki.archlinux.org/index.php/Default_Applications
for global association:

/usr/share/applications/mimeapps.list

per user association:

~/.local/share/applications/mimeapps.list

syntax is as follow:

[Added Associations]
mimetype=desktopfile1;desktopfile2;...;desktopfileN
...
[Removed Associations]
mimetype=desktopfile1;desktopfile2;...;desktopfileN

[Added Associations] section is used to specify preferred (default) applications in decreasing preference. which means desktopfile1 is the most preferred and desktopfileN is least preferred.
[Removed Associations] section is used to explicitly remove any previously inherited associations.

Create a file truecrypt.desktop in ~/.local/share/applications with the following lines:

[Desktop Entry]
Encoding=UTF-8
Type=Application
Icon=
Exec=/usr/bin/truecrypt %U
Name=truecrypt
Comment=manage truecrypt volumes

then edit the file ~/.local/share/applications/mimeapps.list and add the following line:

application/octet-stream=truecrypt.desktop

I found out the mime type of Truecrypt by following running this command:

file --mime-type -b <any truecrypt filename>

mimeapps.list search path

Others have mentioned this file, but none have explained it's location precisely.

The freedesktop.org standard says at https://specifications.freedesktop.org/mime-apps-spec/mime-apps-spec-1.0.html :

The lookup order for this file is as follows:

$XDG_CONFIG_HOME/$desktop-mimeapps.list user overrides, desktop-specific (for advanced users)
$XDG_CONFIG_HOME/mimeapps.list             user overrides (recommended location for user configuration GUIs)
$XDG_CONFIG_DIRS/$desktop-mimeapps.list sysadmin and ISV overrides, desktop-specific
$XDG_CONFIG_DIRS/mimeapps.list             sysadmin and ISV overrides
$XDG_DATA_HOME/applications/$desktop-mimeapps.list  for completeness, deprecated, desktop-specific
$XDG_DATA_HOME/applications/mimeapps.list             for compatibility, deprecated
$XDG_DATA_DIRS/applications/$desktop-mimeapps.list    distribution-provided defaults, desktop-specific
$XDG_DATA_DIRS/applications/mimeapps.list              distribution-provided defaults

and then the default values for those environment variables is given at https://specifications.freedesktop.org/basedir-spec/basedir-spec-0.6.html :

• $XDG_DATA_HOME: $HOME/.local/share
• $XDG_CONFIG_HOME: $HOME/.config
• $XDG_DATA_DIRS: none, appended to XDG_DATA_HOME
• $XDG_CONFIG_DIRS: none, appended to XDG_CONFIG_HOME

Older versions of Ubuntu may not check ~/.config, so make sure you try ~/.local/share as well.

Commands like:

 xdg-settings set default-web-browser chromium-browser.desktop
 xdg-settings get default-web-browser

will automatically edit those files.

On Ubuntu 18.04, I've found the order of precedence to be as follows, from strongest to weakest:

1. ~/.local/share/applications/defaults.list
2. /usr/share/applications/defaults.list
3. ~/.config/mimeapps.list ("Open With Other Application" will update this)
4. ~/.local/share/applications/mimeapps.list
5. The first encountered match in the mimeinfo.cache registry of compatible applications, which are generated from the .desktop files present in /usr/share/applications and /usr/local/share/applications, and can be regenerated using update-desktop-database.
6. (Never used) /etc/gnome/defaults.list

Freedesktop.org (previously called the X Desktop Group, thus the "XDG" abbreviation) is the group currently driving X standards like this (learn more at Wikipedia).

warvariuc's answer already mentions xdg-mime usage like:

xdg-mime default xnview.desktop image/jpeg

After lots of playing with image editors and viewers, I ended up with different viewers for different image types with almost no consistency, especially among the rarer image types.

I took the above command and assigned it to each of the MIME types controlled by the image viewers and then created a new image-viewer.desktop by cloning one of them (this is optional, you can merely reuse your favorite). Next, I manually verified that none of the MIME types would be incompatible with my new preferred application. Finally, I set that application to be the default for all of the other applications' MIME types:

sed '/^MimeType=/!d; s///; s/;/\n/g' \
  /usr/share/applications/{ristretto,geeqie,gthumb}.desktop |sort -u \
  |xargs xdg-mime default image-viewer.desktop

This examines the .desktop files for each of the viewers (Ristretto, Geeqie, and GThumb in my case), extracts the MIME definition (which is prefaced by MimeType= and then contains a semicolon-delimited list), removes the (previously matched) preface label, and splits the list into one entry per line. sort -u then removes the redundant lines, then they're handed to xdg-mime to set the new defaults.

(If you want to test this before running it live, change xargs to xargs echo and it will print the resulting command instead of running it.)

This technique should work for anything you want to reclaim for a target app. You don't even need to create a new .desktop file (you can use an existing one). I did that extra step so that when I inevitably change my allegiance between image previewers, I merely need to change that one file.

For all users:

/usr/share/applications/defaults.list

Each user has a config file for overriding these defaults:

~/.config/mimeapps.list

see also https://specifications.freedesktop.org/mime-apps-spec/mime-apps-spec-1.0.1.html for places that might be used for overriding defauts.

You must know the mime-type of the file you want to change it's association.

Also all associations can be handled-changed in command line using:

xdg-mime