Things are a little more complex than just what your shell provides.
There are three main ways to login:
- login from a true terminal (nowadays, mainly the console)
- login from a pseudo-terminal (mainly network connections)
- login from a graphic environment
They all are able to set up the environment before executing your shell, and they all do (HOME, LOGNAME, TERM are probably not set by your shell, and even if you have no start up files, your shell get probably also a PATH).
When login from a true terminal, the process handling the connection will probably set up the TERM environment variable and delegate the rest of the work to the
login program. That program does some verifications (such as preventing login for root on unsecure terminals), ensures that the whole environment excepted TERM is clean, initializes HOME, PATH, SHELL, TERM, MAIL, and LOGNAME and then launch you shell as a login shell. Your shell will then do its own initialization.
When login from a pseudo-terminal, the same things happen with a little twist. Often the environment get more initialization form the process handling the connection that just the TERM environment variable (network protocols often have a way to transfer the environment from the other side) and thus
login is used in a mode in which it doesn't clean the environment, the clean up is done by the program handling the connection.
Graphic environments usually don't delegate to
login but behaves mostly the same. After verifying your credentials, they create a clean environment (with usual environment variable and at least DISPLAY set correctly, they often allow a sysadmin provided script to add things) then launch the start up script for your desktop environment; those may try to get the environment from your login shell and also often provide the possibility to provide a script to complete the set up. So when you launch any program from your desktop, it get an environment which is the combination of what the graphical login program, your desktop environment and your login scripts set.
A last thing, when you launch a terminal emulator, you may get either a login shell (in which case the login script from the shell is executed and thus you get to see the effect of your latest changes in it, but won't see the same environment as the other programs) or not (in which case the login script from the shell isn't executed, you don't see your latest changes, but you get the same environment as the other programs — modified by the interactive initialization script of your shell).