Maven how to find out what settings.xml is in use (Ubuntu)

I am using Ubuntu 16.10 and Maven 3.3.9. I found maven folders in different directories with settings.xml files:

• ~ / .m2 / (I have a cp file settings.xml in this directory)
• / etc / maven
• / usr / share / maven / conf

...

What would be the location for the settings.xml file and how do I know which Maven file is in use?

mvn -X

return. There are no targets for this build. [...]

mvn <goal> -X

doesn't even show anything about the settings.xml file