For his installation Websphere need to use root account. The installation's script don't propose the creation or utilisation of an other user account. This is disturbing because all the objets created by Webphere belong to root.
But it's easy to correct this problem.
Modifications to do
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I take in account that you have created the user and the group that you wanted. After that you just have to connect yourself to the Websphere's administration console to change some parametres in this page : Execution process parameters.
You can go to this page thanks to the menu Servers > Applications server > name_of_your_server and (in the options of this page) Server Infrastructure > Gestion des processus and Java > Processus Execution.
So you can change the user and group wich is going to run the principal process of Websphere ( java ). With that, all objects created by websphere (images, files, ...) are accessible even to other user than root.
Warning : after this you must change the owner of all files and directorys read by Websphere ( $WAS_HOME/ and your WebApp and all other required files)
ref: http://www.berthou.com/us/category/websphere/
