The question remains: where did xdg-open get the idea that Mendeley should
be the default PDF viewer from?
This is an eminently reasonable question.
Here's a somewhat long answer in three parts.
Option 1: read the documentation
For example, the FreeDesktop standard
on mimetype associations has this to say:
Association between MIME types and applications
Users, system administrators, application vendors and distributions can
change associations between applications and mimetypes by writing into a
file called mimeapps.list.
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
In this table, $desktop is one of the names of the current desktop,
lowercase (for instance, kde, gnome, xfce, etc.)
Note that if the environment variables such as XDG_CONFIG_HOME and XDG_DATA_HOME are not set, they will revert to their default values.
$XDG_DATA_HOME defines the base directory relative to which user specific data files should be stored. If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.
$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.
This illustrates one of the trickiest aspects of mimetype associations:
they can be set in many different locations,
and those settings might be overridden in a different location.
However, ~/.config/mimeapps.list is the one that we should use to set our own associations.
This also matches the documentation for the GNOME desktop.
To override the system defaults for individual users, you need to create a
~/.config/mimeapps.list file with a list of MIME types for which you want
to override the default registered application.
There's also this helpful tidbit:
You can use the gio mime command to verify that the default registered
application has been set correctly:
$ gio mime text/html
Default application for “text/html”: myapplication1.desktop
Registered applications:
myapplication1.desktop
epiphany.desktop
Recommended applications:
myapplication1.desktop
epiphany.desktop
The cross-platform command to check mimetype associations is:
xdg-mime query default application/pdf
For GNOME, the command is:
gio mime application/pdf
For KDE Plasma the command is:
ktraderclient5 --mimetype application/pdf
When I look at my ~/.config/mimeapps.list file,
it looks something like this:
[Added Associations]
application/epub+zip=calibre-ebook-viewer.desktop;org.gnome.FileRoller.desktop;
<snip>
application/pdf=evince.desktop;qpdfview.desktop;okularApplication_pdf.desktop;<snip>
<snip>
[Default Applications]
application/epub+zip=calibre-ebook-viewer.desktop
<snip>
application/pdf=evince.desktop;
You can see there only one entry for application/pdf under [Default Applications];
so evince.desktop is the default handler for PDF files.
I don't have Mendeley installed, but one way to make it the default PDF handler
is to put its desktop file here instead of evince.desktop.
Notice we're trusting the documentation here that ~/.config/mimeapps.list
is the correct file; we don't actually know that for sure.
We'll come back to this in part 3.
Option 2: read the source code.
xdg-open is a shell script that behaves differently
depending on the value of $XDG_CURRENT_DESKTOP.
You can see how this works here:
if [ -n "${XDG_CURRENT_DESKTOP}" ]; then
case "${XDG_CURRENT_DESKTOP}" in
# only recently added to menu-spec, pre-spec X- still in use
Cinnamon|X-Cinnamon)
DE=cinnamon;
;;
ENLIGHTENMENT)
DE=enlightenment;
;;
# GNOME, GNOME-Classic:GNOME, or GNOME-Flashback:GNOME
GNOME*)
DE=gnome;
;;
KDE)
DE=kde;
;;
Since you are using i3,
the DE variable will be set to generic and the script will call
its open_generic() function,
which in turn will call either run-mailcap or mimeopen
depending on what is installed.
Note that you can get some extra information
by setting the XDG_UTILS_DEBUG_LEVEL, e.g.
XDG_UTILS_DEBUG_LEVEL=4 xdg-open ~/path/to/example.pdf
However, the debug information is not that informative for our purposes.
Option 3: trace the opened files.
From the previous investigations,
we know that mimetype associations are stored in files somewhere on the hard drive,
not e.g. as environment variables or dconf settings.
This means we don't have to rely on documentation,
we can use strace to determine what files the xdg-open command actually opens.
For the application/pdf mimetype, we can use this:
strace -f -e trace=open,openat,creat -o strace_log.txt xdg-open /path/to/example.pdf
The -f is to trace child processes since xdg-open doesn't do everything by itself.
The -e trace=open,openat,creat is to trace just the syscalls open, openat, and creat.
These are from the man page from man 2 open or online.
The -o strace_log.txt is to save to a log file to inspect later.
The output is somewhat voluminous,
but we can ignore the lines that say ENOENT (No such file or directory)
since these files do not exist.
You can also use other commands such as xdg-mime or gio mime.
I found that gio mime read these files in my home directory:
~/.local/share//mime/mime.cache~/.config/mimeapps.list~/.local/share/applications~/.local/share/applications/mimeapps.list~/.local/share/applications/defaults.list~/.local/share/applications/mimeinfo.cache
It also read these system-level files:
/usr/share/mime/mime.cache/usr/share/applications/defaults.list/usr/share/applications/mimeinfo.cache/var/lib/snapd/desktop/applications/var/lib/snapd/desktop/applications/mimeinfo.cache
To look for application/pdf associations, this should do the trick:
grep 'application/pdf' ~/.local/share//mime/mime.cache ~/.config/mimeapps.list ~/.local/share/applications ~/.local/share/applications/mimeapps.list ~/.local/share/applications/defaults.list ~/.local/share/applications/mimeinfo.cache /usr/share/mime/mime.cache /usr/share/applications/defaults.list /usr/share/applications/mimeinfo.cache /var/lib/snapd/desktop/applications /var/lib/snapd/desktop/applications/mimeinfo.cache | less
From here you can see where Mendeley's desktop file is getting added.
I have some applications (Calibre, texdoc) open PDFs with Mendeley. Opening
PDFs from Thunar, Thunderbird, Firefox etc. opens evince, the expected
default.
Firefox and Thunderbird have their own default application settings.
I believe texdoc relies on xdg-open.
I'm not sure about Thunar,
but I doubt it is relying on xdg-open.
So ultimately this is probably due to:
Addendum: xdg-open should not use the mimeinfo.cache file on i3,
but if you need to regenerate it, this is the command to use:
update-desktop-database ~/.local/share/applications
and here is the documentation:
Caching MIME Types
To make parsing of all the desktop files less costly, a
update-desktop-database program is provided that will generate a cache
file. The concept is identical to that of the 'update-mime-database' program
in that it lets applications avoid reading in (potentially) hundreds of
files. It will need to be run after every desktop file is installed. One
cache file is created for every directory in $XDG_DATA_DIRS/applications/,
and will create a file called $XDG_DATA_DIRS/applications/mimeinfo.cache.
https://specifications.freedesktop.org/desktop-entry-spec/0.9.5/ar01s07.html
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