Missing environmental variables in CentOS7?
 

~/.bash_profile is the place for
user specific environment and startup programs.
E.g. Adding a new path to PATH variable.
So PATH was defined before .bash_profile is executed.

Since /etc/environment is empty,
but command env produces a lot of
environmental variables.
Where were those env variables defined?

Thank you.

all over the place, is my guess.
and as you can see from the output.
certain shells have the ability to define persistent environment variables, i think they're called login shells.
everything that a login shell parses. (i hope i'm right with this, it's sort of "felt" knowledge)
good starting point: /etc/bash.bashrc /etc/profile, and the same for your $HOME