How to get base directory path from my file repository

Hello all,
I want to put all my configuration files inside my file repository and access them using a relative path to my repository base dir.
How can I get the base dir of my repository from inside my file input step? I cannot find such a variable.

Example:
Let's say the base dir of my file repository is defined in c:/PDI_REP.
Let's say I have a conf file in c:/PDI_REP/CONFIG/config.properties
In my file input step I need a variable giving me "c:/PDI_REP" as my base dir. This way I can set the file path to ${base_dir}/conf/config.properties.
Any way to get such a variable (${base_dir}) ?

Thanks.