How to register custom URL schemes in Linux

First, create a desktop file in /usr/share/applications/

potato-handler.desktop

[Desktop Entry]
Name=Potato URL Handler
GenericName=Text Editor
Comment=Handle URL Scheme potato://
Exec=/usr/share/potato/potato-handler %u
Terminal=false
Type=Application
MimeType=x-scheme-handler/potato;
Icon=potato-icon
Categories=Development;Utility;
Name[en_US]=potato URL Handler

Then, update the MIME-types database, using:

sudo update-desktop-database

Check the mimeinfo.cache file. This file is the cache database created by update-desktop-database.

You will probably find multiple ones, under /applications of every directory in $XDG_DATA_DIRS.

In the potato example, you should find an entry x-scheme-handler/potato with the value potato-handler.desktop in the mimeinfo.cache of /usr/share/applications/, since we created the potato-handler.desktop file there.

Now, create the /usr/share/potato/potato-handler executable that we specified in the potato-handler.desktop's Exec field.

This executable should take exactly one command-line argument, which is the Potato URL starting with potato://, and launch your app with the parameters it needs.

You can make this executable in Bash, Python, or whatever you prefer, just make sure you specify in the Exec field of the potato-handler.desktop how to execute it.

Undoing what we just did

Just remove the directory /usr/share/potato/ with the potato-handler executable inside it, and remove the potato-handler.desktop file from /usr/share/applications/, then update the MIME-types database once more, using:

sudo update-desktop-database