Dotfiles are any files that begin with a period. These files are typically configuration for your shell and other command line tools. The dotfiles used by the shell are sourced (executed) when you login. By default, dotfiles are hidden by most operating systems in a GUI like Finder or Windows Explorer.
To see them at the command line using ls, you need to add the -a option:
action@cli-crash-course-197957:~$ ls -a
. .bash_history .bashrc .gemrc hello_world.sh .pam_environment README.md .tmux.conf .viminfo
.. .bash_profile .cache .gitconfig .nitrousboxrc.sample .parts .ssh .VERSION workspace
GitHub has a great resource of other people's dotfiles: https://dotfiles.github.io/
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