Type of Shells

Start an interactive shell

bash

Detect type of shell

shopt -q login_shell && echo 'login' || echo 'not-login'

Introduction to dot files

In Unix, files and directories beginning with a period usually contain settings for a specific program/a series ofprograms. Dot files are usually hidden from the user, so you would need to run ls -a to see them.
An example of a dot file is .bash_history, which contains the latest executed commands, assuming the user isusing Bash.
There are various files that are sourced when you are dropped into the Bash shell. The image below, taken from this site, shows the decision process behind choosing which files to source at startup.