Problem setting default application for Audio CD in Gnome 3
 

Objective: When I insert an audio CD in the drive, I want the system to automatically start a CD ripper of my choosing (ripper called abcde).

Already tried: I have successfully set up a desktop-accessible application by creating the file /usr/share/applications/abcde.desktop which is visible in the desktop GUI and works correctly when I click on the desktop icon.

Problem: The new application does not appear in the list of applications which may be chosen under System Settings...Details...Removable Media (Select how media should be handled)...CD audio. Therefore I cannot select the new application as the default application for removable media of the Audio CD type.

must have something to do with mimetypes and/or categories defined inside the .desktop file.
i suggest you look at one from an app that DOES show up, and copy these settings to your abcde.desktop.

Problem solved. Thank you for your help.

The problem was not due to the MimeType or Categories lines, which I had copied from another application to begin with. However, your suggestion encouraged me to look for other differences.

Eventually I found the problem in the Exec line. I have configured abcde so that it requires no arguments, but the application will not appear in the selection list unless a filename or device argument is provided. Here is the solution:

Before (application not visible in the selection list):
After (application visible, selectable, works fine):
or
Given my abcde configuration, the argument is ignored, and the program executes correctly.

Edit: One more detail: In order to make the selection persist beyond the current desktop login session, the new application must be added to the user's mimeapps.list, in its [Default Applications] section.

On my system, this is located at ~/.local/share/applications/mimeapps.list. However, I read that on some systems it may be found instead at ~/.config/mimeapps.list.