2020 Update
Some of the commands have been updated, but the overall idea and mechanism is the same. New commands:
$ gio mime x-scheme-handler/magnet
$ # create the file, new name should include reverse domain name
$ # so I went with se.wendt.magnet-thing.desktop
$ gio mime x-scheme-handler/magnet se.wendt.magnet-thing.desktop
2014 Original Post
The Linux world has agreed that xdg-open is the tool that handles launching applications as a response to a user interacting with a URL, typically clicking a link or double clicking an icon.
If Gnome is installed, and running for the logged in user, xdg-open hands over to gnome-open.
This tool uses couple of sources for deciding how to handle a URI.
I wanted my Gnome sessions to invoke a custom application I had built, when dealing with magnet URIs.
This is what I did:
gvfs-mime --query x-scheme-handler/magnet lists the current applications that can handle magnet URIs.
Getting gvfs to know about a new application typically means creating a Desktop Entry file.
I put mine in ~/.local/share/applications/magnet-thing.desktop, with the following content:
#!/usr/bin/env xdg-open
[Desktop Entry]
Version=1.0
Terminal=false
Type=Application
Name=Magnet Thing By Wendt
Exec=/usr/local/bin/magnet %U
Mimetype=x-scheme-handler/magnet
If the Mimetype isn’t enough, one can also run gvfs-mime --set x-scheme-handler/magnet magnet-thing.desktop to associate the application with the mimetype.