How to set/get relative path in Jmeter

Sometimes, we need to read content of external file during run time. Instead to use hardcoded path, like D:\input\private.pem, you should use a relative path (using a property). 

You can define it on the command line
-Jjmeter.input.path=%CD%\input

Place your data file in folder "input" (one level under jmeter.bat file) and set path in test: ${__P(jmeter.input.path)}/private.pem