When I have an Eclipse update site project, a site.xml file is created which looks like:
<?xml version="1.0" encoding="UTF-8"?>
<site>
<feature url="features/org.eclipse.egit_1.1.0.201109151100-r.jar" id="org.eclipse.egit" version="1.1.0.201109151100-r"/>
<feature url="features/org.eclipse.draw2d_3.7.1.v20110830-1143-4607w3122194102254.jar" id="org.eclipse.draw2d" version="3.7.1.v20110830-1143-4607w3122194102254">
<category name="test_cat1"/>
</feature>
<category-def name="test_cat1" label="Test Catergory 1">
<description>
This be a test
</description>
</category-def>
</site>
But if I have an existing update site (created by for example B3 aggregator), is there an easy way to generate this site.xml file?
The site.xml file is a deprecated repository descriptor. It was replaced by artifacts.jar and content.jar with introduction of p2. For the sake of backwards compatibility, p2 still understands repositories containing only site.xml and some of p2 tooling can take site.xml as an input, but you are unlikely to find anything that goes the other way.