To log into the system.
A greeter provides a login screen interface.
The .profile file is used to put any settings or changes to the login shell environment when you log in. This file is only read once during login.
exit
The purpose of a login is simply to verify a user's identity.
The whoami command.
A shell in Linux is the interpreter that provides a commandline interface (CLI). There are many kinds of shells.
what is ran at login is whatever is listed in your /etc/init.rd files for your user profile.
The Z shell is a Bourne-compatible shell for Linux and Unix systems.
bash (Bourne Again Shell) is the default shell in most Linux distributions. It was created as part of the GNU project.
Shell is the bridge between Kernel and User.
echo $SHELL
Strictly speaking, a "shell" is simply the interface between the user and the rest of the operating system. In that definition not all shells on Linux are command lines. However, most day-to-day usage of the term "shell" does refer to command line user interfaces. Finally, "Linux shell" is not a thing. Linux is an operating system, specifically a kernel, not a shell. Most Linux distributions use BASH, but I personally prefer ZSH.