jenkins - Downstream project needs to know if it was built by upstream change -

i've got following tree:

project --|             | - project b             | - project c 

when change happens project a, kicks off new build, , installs library local directory. when kicks off project b , project c. however, since library install build system project b , project c think nothing has changed , don't anything.

i set project b , project c clean before building, prefer incremental builds when there changes project b/c.

is there way find out if build triggered upstream project? if there environment variable gets set based upon being set project's first clean before building.

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i trying keep developer cycle low, if project b changes, developer doesn't have wait project clean , build scratch, if project a changes developer project b doesn't care build time, since aren't waiting complete move on asking changes merged.