Jenkins System Groovy Script - Enumerate Upstream Projects. If the project is triggered by other projects and you need to enumerate them, this is how I found out to do it in a System Groovy build step

upstream.groovy

def upStreamProjects = build.getParent().getUpstreamProjects()

println "Found ${upStreamProjects.size()} upstream projects"

upStreamProjects.each { p -> println "upstream project: ${p.getName()}" }