different icons for generic source-code and other files

Question

I am trying to make a mimetype icon theme for gnome. I have created text-x-generic.svg for all unthemed filetype. But what I am looking for is different icons for unthemed source code (say, R, vala,awk etc) and plain-generic text file.

Is this possible?

N.B. I have tried text-x-generic.svg and text-generic.svg to differentiate the source code and plain text, but that is not what linux(gnome) understands.

reply for don_crissti's Comment Say, I have icons for ruby, and js, but not for C and R. So, file.c and file.r should show the icons for text-x-generic-unthemed-sourcecode icon. And file.dat and filename_without_extensions should show a different icons (text-generic-plaintext).

File-wise, they are different, as

$ file i.c
i.c: C source, ASCII text

$ file dos.dat
dos.dat: ASCII text

So, there should be some way.

Answer 1

The icons used by gnome are based on mime type hence your file.c will not be detected as text-x-generic-unthemed but as text-x-csrc and since you have no associated icon it will fallback to text-x-generic.
Ex: the icons used for .vala are: text-x-vala, text-x-generic but since there is no text-x-vala icon in my icon set, the system falls back to text-x-generic:

See the post here on how to determine the associated icons for a specific mime type.

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In order to use a different fallback icon (e.g. text-x-unthemed) one has to customize definitions for mime types that should fall back to this particular icon and add the relevant icon (text-x-unthemed) to the icon set under mimetypes .

1. Add the icon, then update the icon cache:

   gtk-update-icon-cache /path/to/icon_theme_directory
   

2. Add custom definitions for mime types that should use text-x-unthemed as generic (fallback) icon if they have no corresponding icons.
   Create unthemed.xml file under ~/.local/share/mime/packages e.g.:

   <?xml version="1.0"?>
    <mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
     <mime-type type="text/x-vala">
       <comment>Vala source code</comment>
       <glob pattern="*.vala" case-sensitive="true"/>
       <generic-icon name="text-x-unthemed"/>
     </mime-type>
    </mime-info>
   

   Add other mime types to the list if needed (use vala <mime-type> block as a template).

3. Rebuild the mime cache with:

   update-mime-database ~/.local/share/mime
   

The system should then fall back to text-x-unthemed icon for .vala files:

If a dedicated icon text-x-vala is added later on, the system will use that one, e.g.:

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Don't forget to update the icon and mime cache each time you add/remove icons or mime type definitions.

Answer 2

Thanks to @don_crissti for the details of how it works. I have gone an alternate way for solving the problem partially. I have put different icons for text-plain and text-x-generic and text-x-script. The text-plain incorporates all the files without extensions and like of .dat, .txt etc; where text-x-[generic,script] is the fallback for others and scripts.

I agree this is an ad-hoc solution, but it solves the current problem nonetheless. (The best solution is to make icons for each files in /usr/share/mime/*)

A screenshot is added to show how it looks with only text-x-[python,xml,scripts,generic,plain]