The pages listed in this category are meant to be user templates, including userboxes. This page is part of Wikipedia's administration and not part of the encyclopedia. Further template category notes This category contains pages in the template namespace. It should not be used to categorize articles or pages in other namespaces. To add a template to this category: If the template has a separate documentation page (usually called "Template:template name/doc"), add  [[Category:Operating system user templates/Linux]] to the <includeonly> section at the bottom of that page. Otherwise, add  <noinclude>[[Category:Operating system user templates/Linux]]</noinclude> to the end of the template code, making sure it starts on the same line as the code's last character.

Linux is not Unix, so it has its own subcategory. But you might want to take a look at Category:Operating system user templates/Unixen to see userboxes for other great Linux-like systems.