Ubuntu – How to change default program for files ending in one extension

file format

In Ubuntu 18.** how do I set a default program only for files with one specific file extension? For example if I use the graphical interface and use "Set as default" for a .txt file it also changes the default setting for files with many other extensions. I don't want this. How can I change the default program only for files with a specific extension?

Best Answer

This answer is adapted from Ondra Žižka's answer here: How do I set the default program?. Please upvote there as well if this helps you.

To change the default program for files with a specific file type, a mime type needs to be created for each file extension. This is because Ubuntu tracks this indirectly:

file extension -> MIME types
MIME type -> application(s) to open
Applications -> list of compatible MIME types

For Ubuntu (18.x), the following applies:

file extension -> MIME types

This is determined from files in /usr/share/mime/packages/. E.g.:

cat << EOF | sudo tee /usr/share/mime/packages/staruml.xml
<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
  <mime-type type="application/staruml-project">
    <comment>StarUML project</comment>
    <glob pattern="*.MDJ"/>
    <glob pattern="*.mdj"/>
  </mime-type>
</mime-info>
EOF

Other xml files are generated from these and placed in /usr/share/mime/. Sometimes these files may be found online, especially for open source software packaged for Ubuntu. For example, the mime files for KiCad can be found in the KiCad source code repository: https://gitlab.com/kicad/code/kicad/-/tree/master/resources/linux/mime

MIME type -> application(s) to open

This is driven by ~/.config/mimeapps.list or possibly ~/.local/share/applications/mimeapps.list. This should override /usr/share/applications/mimeapps.list. For example:

echo 'application/staruml=staruml.desktop' | tee -a ~/.local/share/applications/mimeapps.list

Or, to change for all users:

echo 'application/staruml=staruml.desktop' | sudo tee -a /usr/share/applications/mimeapps.list

Applications -> list of compatible MIME types

This is driven by the .desktop files in /usr/share/applications/. That file can contain a MimeType= entry with semicolon-separated list of MIME types. For instance, the EOG app has MimeType=image/bmp;image/gif;image/jpeg;image/jpg;....

cat << EOF | sudo tee /usr/share/applications/staruml.desktop
[Desktop Entry]
Name=StarUML
Comment=StarUML
GenericName=StarUML
Exec=/sw/prog/StarUML/StarUML-3.1.0-x86_64.AppImage %U
Icon=/sw/prog/StarUML/staruml.png
Type=Application
StartupNotify=false
Categories=Development;Programming;Modeling;UML
MimeType=application/staruml-project;
EOF

This makes it a full circle. The changes take effect after running

sudo update-mime-database /usr/share/mime

and, for icons,

sudo gtk-update-icon-cache /usr/share/icons/gnome -f

Or, you may need to logout/login.

Then, mimetypes for a given file extension can be checked using mimetype:

$ mimetype .mdj
.mdj:  application/staruml-project

References:

Credit: Ondra Žižka (Adapted)

Related Solutions

Ubuntu – How are file-extensions/mime-types/icons/default applications associated

You need to:

Add a new MIME type
Associate that MIME type with a program (see man xdg-mime for more details):
```
xdg-mime default APPLICATION MIMETYPE(S)
```

Ubuntu – How to change a document’s extension / file extension / file format

Forget about the concept of "file extensions" that thing is completely made up and useless. For example somebody could rename a music file on your system to garbage.gbr and since windows interprets extensions literally as file types it wouldn't even know that the file was a music file.

In Ubuntu on the other hand, you can name a file whatever you wish. You don't even have to choose an extension if you don't feel like it. Ubuntu will read the MIME type of the file and understand what type it is. Based on the file type it will associate it to the appropriate program to open it. You still have all the control you need. You can change the default program associated with a MIME type in several ways.

enter image description here

To change default program association of a file type, just right click on a file with that type and choose Properties and go to Open With tab.

Now that being said, I should mention that extensions are not completely useless in Ubuntu. They still provide a standard way to tag a file for easy identification of its type -- but they don't set the file type. The only exception to this is when saving a file in some applications that support saving in several different formats. When you provide a file name in those application it can read the extension you provided and based on that decides how to create and save your new file with the correct mime type. For example if you open a JPEG file in GIMP and then choose save as ... and enter image.png, GIMP opens the PNG file dialogue and lets you save the file with PNG MIME Type.