<i>&quot;On desktop Linux, it&#x27;s obvious that, say, a web browser and a PDF viewer should be different applications&quot;</i><p>Not completely obvious. The web browser could use the PDF viewer to render PDF and&#x2F;or the PDF viewer could use the web browser to run JavaScript embedded in a PDF file (<a href="http:&#x2F;&#x2F;www.adobe.com&#x2F;devnet&#x2F;acrobat&#x2F;javascript.html" rel="nofollow">http:&#x2F;&#x2F;www.adobe.com&#x2F;devnet&#x2F;acrobat&#x2F;javascript.html</a>). Both examples are similar to your find|grep example.<p>Yes, that typically would be done by reusing libraries, not by starting processes, but modern browsers run a separate process per tab and some run flash in a separate process. Running a PDF viewer in a separate process would be a logical extension.