Ubuntu MATE and VirtualBox
Of late I have been dual booting my office desktop between Slackware 14.1 and Ubuntu MATE 16.04. The Ubuntu system would not use my common VirtualBox directory. I store all virtual machines and related config files in a separate partition rather than $HOME. I have been using this approach in Slackware for many years.
Ubuntu defaults to storing machines in the user’s $HOME/.config directory.
The cause of the problem became obvious with nominal sleuthing. Ubuntu does not establish the expected environment variables, most notably $VBOX_USER_HOME. To complete the journey and retain consistency with the Slackware side, I also configured $VBOX_PROGRAM_PATH and $VBOX_SDK_PATH.
I placed all of the variable declarations in /etc/environment.
VBOX_USER_HOME="/var/vm-images/config" VBOX_PROGRAM_PATH="/usr/lib/virtualbox" VBOX_SDK_PATH="/usr/lib/virtualbox/sdk"
The /etc/environment location seems to work only with PAM dependent apps, such as Light DM and login. Placing the variables in any other container prevented the virtualbox executable from sourcing the variables.